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Journal Bearing
Bearings:
The main function of bearing is to permit constrained relative motion of rigid parts. It is the
machine part which supports the rotating shaft, axles etc. Shaft rotates smoothly in the bearing.
Hence, loss of power due to friction is reduced. Bearing also confines the motion of shaft.
Types of bearings
Bearings are classified into two types. They are
1. Sliding contact bearings (plain bearings).
2. Rolling contact bearings (Anti-friction bearings).
Sliding contact bearings
They are also called as plain bearings. Lubricant oil film is kept between shaft and the
bearing. The mating surfaces are in sliding contact. Hence, it is known as sliding contact bearing.
It is classified into two
1. Based on oil film thickness.
2. Based on direction of load.
1. Basedon thickness of lubricant oil film
It further classified into four types
a) Zero – film bearing
b) Thin – film bearing
c) Thick – film bearing (or) hydro dynamic bearing
d) Externally pressurized bearing (or) hydro static bearing.
Zero – film bearing
It does not use lubricant oil. Metal to metal contact take place. So, its application is limited.
Thin – film bearing
It is also named as boundary lubricated bearing. Even though thin oil film is lying between
mating surfaces, metal to metal contact take place.
Thick – film bearing
It is also named as hydrodynamic bearing. Thick oil film is lying between mating surfaces.
During running position, metal to metal contact does not occur. The relative motion between shaft
and bearing creates positive oil pressure which supports the load. Hence separate pump is not
required. But at the starting position, metal to metal contacts take place.
Externally pressurized bearing

It is also named as hydro static bearing. External pump supplies oil to the bearing at high
pressure. This high pressure oil supports the load. At any position, metal to metal contact does not
occur.
2. Basedon direction of load
a. Radial bearings (or) Journal bearing
b. Thrust bearings
In sliding contact bearing if load acts perpendicular to the axis of shaft, it is called journal bearing.
Shaft: Rotating part is called shaft.
Bearing: Shaft supporting member is called bearing.
Journal: The enclosed portion of shaft by the bearing is called journal.
Journal bearings
Hydrodynamic journal bearings are a bearing operating with hydrodynamic lubrication in
which the bearing surfaces are separated from the journal surface by the lubricant film.
Hydrodynamic journal bearing and a journal rotating in a clockwise direction. Journal
rotation causes pumping of the lubricant (oil) flowing around the bearing in the rotation direction. If
there is no force or load applied to the journal, its position will remain concentric to the bearing
position. However, a loaded journal will be displaced from the concentric position and forms a
converging gap between the bearing and journal surfaces.
The pumping action of the journal forces the oil to squeeze through the wedge shaped gap
generating a pressure. The pressure falls to the cavitations pressure (close to the atmospheric
pressure) in the diverging gap zone where cavitations forms.
The displacement of the shaft centre with respect to the bearing centre is known as
eccentricity. The eccentric position of the shaft is governed by the radial load carried by it and is

adjusted by itself until the load is balanced by the pressure generated in the converging lubricant
film between the bearing and the journal.
The line joining the centre joining the shaft and the sleeve centre is known as the line of
centre. The load carrying capacity depends on the amount of eccentricity (e), angular speed (ω),
viscosity of lubricant (µ), bearing dimensions and the clearance.
Types of Journal Bearings
i. Full Journal bearing
ii. Partial Journal bearing
Full journal bearing
Journal is surrounded by bearing fully. Covered angle is 360o
. It may be represented
as a cylindrical sleeve (bearing) wrapped around shaft. If the wrapping extends around the full 360o
of the journal, it is termed as full journal bearing.
Partial journal bearing
Journal is surrounded by bearing partially. Covered angle is 120o
.
Application of journal bearing [Refer to PSGDB P.NO: 7.30]
 Conveyors, Cam shafts, Motor shafts, Turbine shaft
Materials Used For Sliding Contact Bearings [Refer to PSGDB P.NO: 7.30]
All the desirable characteristics of bearing materials are not to be found to a high degree in
any particular bearing material. Hence the choice of a material for any application must represent a
compromise.
 Tine base Babbit and lead- basebabbit are in widespread use since they satisfy most
requirements for general application.
 Where loads are very high, bronze or brass bearings may be used. Bronze is an alloy of
copper and tin. This bronze bearing is suitable for heavy loads at slow speeds.

 Where high compressive and fatigue strength are required, copper, lead, and tin alloys may
be used.
 Gun metal is an alloy of copper, tin, and zinc. Gun metal bearing is suitable for high speeds.
 Nylon and rubber are used as bearing materials. It is used in water turbine bearings and
water pump bearings.
Properties of good sliding contact bearing materials [Refer to PSGDB P.NO: 7.30]
Required properties of good bearing material are given below
 To reduce wear, co-efficient of friction must be less.
 To maintain the clearance between shaft and bearing, co-efficient of thermal expansion must
be less.
 Low cost
 To adjust the alignment error, young’s modulus must be less.
 To remove the heat generated due to friction, thermal conductivity must be more.
 To prevent rusting, corrosion resistance must be high.
 To withstand hydro-dynamic pressure, compression strength must be high.
 To withstand varying load, fatigue strength must be high.
Lubricant
A lubricant (sometimes referred to as "lube") is a substance (often a liquid) introduced
between two moving surfaces to reduce the friction between them, improving efficiency and
reducing wear. It may also have the function of dissolving or transporting foreign particles and of
distributing heat.
Lubricants perform the following key functions.
 Keep moving parts apart, Reduce friction, Transfer heat, Transmit power, Protect against
wear, Prevent corrosion, Seal for gasses, Stop the risk of smoke and fire of objects
Types of lubricants
 Solid, Semi-solid, Liquid
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in
close proximity, and moving relative to each another, by interposing a substance called lubricant
between the surfaces to carry or to help carry the load between the opposing surfaces. The
interposed lubricant film can be a solid, a liquid, and gas.
Types of Lubrication
 Hydrodynamic Lubrication
 Hydrostatic Lubrication
 Elastro hydrodynamic Lubrication

The regimes of lubrication
As the load increases on the contacting surfaces three distinct situations can be observed with
respect to the mode of lubrication, which are called regimes of lubrication:
 Fluid film lubrication is the lubrication regime in which through viscous forces the load is
fully supported by the lubricant within the space or gap between the parts in motion relative
to one another (the lubricated conjunction) and solid-solid contact is avoided.[2]
 Hydrostatic lubrication is when an external pressure is applied to the lubricant in
the bearing, to maintain the fluid lubricant film where it would otherwise be
squeezed out.
 Hydrodynamic lubrication is where the motion of the contacting surfaces and the
exact design of the bearing is used to pump lubricant around the bearing to maintain
the lubricating film. This design of bearing may wear when started or stopped, as the
lubricant film breaks down.
HYDROSTATIC BEARINGS
Hydrostatic films are created when a high-pressure lubricant is injected between opposing
(parallel) surfaces (pad and runner), thereby separating them and preventing their coming into direct
contact. Hydrostatic bearings require external pressurization. The film is 5–50 micrometers thick,
depending on application.
Though hydrostatic lubrication does not rely on relative motion of the surfaces, relative motion
is permitted and can even be discontinuous. Figure 1 is a schematic of a hydrostatic bearing pad. To
handle asymmetric loads, hydrostatic systems generally employ several evenly spaced pads.
Hydrostatic bearings find application where relative positioning is of extreme importance. They are
also applied where a low coefficient of friction at vanishing relative velocity is required.
Lubrication of bearings
Journal bearing is lubricated for the following reasons.
 To reduce friction
 To reduce wear
 To transfer the heat due to friction

 To prevent the rusting of bearing surface.
 To prevent the damage of bearing surface.
Properties of good lubricant
The essential properties required for lubricant are
 High viscosity index, High flash point, High fire point, High corrosion resistance, Low
freezing point, Low cost.
Factor to be considered for the selection of type of bearing
While selecting the type of bearing, the following factors are considered
 Type of load, Speed of shaft, Space required, Vibrations, Temperature, Stating torque
Advantages of sliding contact bearing
 Low cost
 Silent in operation
 Long life
 Withstands shocks
 Not breaks easily
 Not affected by fatigue load
 Simple design
 Less radial space is enough
 Not damaged by impurities.
Disadvantages of sliding contact bearing
 More friction
 More loss of power
 High maintenance cost
 Large amount of lubricant is required
 Replacement is not easier
 Not operate in inclined position, and Less accuracy
Application of sliding contact bearing
 Diesel engine, Gas engines, Pumps, Compressors, Turbines, Aircraft engines, Conveyors

Rolling contact bearing
In rolling contact bearings, the contact between the bearing surfaces is rolling instead of
sliding as in sliding contact bearings. We have already discussed that the ordinary sliding bearing
starts from rest with practically metal-to-metal contact and has a high coefficient of friction.
It is an outstanding advantage of a rolling contact bearing over a sliding bearing that it has a
low starting friction. Due to this low friction offered by rolling contact bearings, these are called
antifriction bearings.
Advantages and Disadvantages of Rolling Contact Bearings over Sliding Contact Bearings
The following are some advantages and disadvantages of rolling contact bearings over
sliding contact bearings.
Advantages
 Low starting and running friction except at very high speeds.
 Ability to withstand momentary shock loads.
 Accuracy of shaft alignment.
 Low cost of maintenance, as no lubrication is required while in service.
 Easy to mount and erect.
 Cleanliness.
Disadvantages
 More noisy at very high speeds.
 Low resistance to shock loading.
 More initial cost.
 Design of bearing housing complicated.
Types of Rolling Contact Bearings
Following are the two types of rolling contact bearings:
1. Ball bearings; and 2. Roller bearings.
Basic Static Load Rating of Rolling Contact Bearings
The load carried by a non-rotating bearing is called a static load. The basic static load
rating is defined as the static radial load (in case of radial ball or roller bearings) or axial load (in
case of thrust ball or roller bearings) which corresponds to a total permanent deformation of the ball

(or roller) and race, at the most heavily stressed contact, equal to 0.0001 times the ball (or roller)
diameter.
In single row angular contact ball bearings, the basic static load relates to the radial
component of the load, which causes a purely radial displacement of the bearing rings in relation to
each other.
Static Equivalent Load for Rolling Contact Bearings
The static equivalent load may be defined as the static radial load (in case of radial ball or
roller bearings) or axial load (in case of thrust ball or roller bearings) which, if applied, would cause
the same total permanent deformation at the most heavily stressed ball (or roller) and race contact
as that which occurs under the actual conditions of loading.
Life of a Bearing
 The life of an individual ball (or roller) bearing may be defined as the number of revolutions
(or hours at some given constant speed) which the bearing runs before the first evidence of
fatigue develops in the material of one of the rings or any of the rolling elements.
 The rating life of a group of apparently identical ball or roller bearings is defined as the
number of revolutions (or hours at some given constant speed) that 90 per cent of a group of
bearings will complete or exceed before the first evidence of fatigue develops (i.e. only 10
per cent of a group of bearings fail due to fatigue).
 The term minimum life is also used to denote the rating life. It has been found that the life
which 50 per cent of a group of bearings will complete or exceed is approximately 5 times
the life which 90 per cent of the bearings will complete or exceed. In other words, we may
say that the average life of a bearing is 5 times the rating life (or minimum life). It may be
noted that the longest life of a single bearing is seldom longer than the 4 times the average
life and the maximum life of a single bearing is about 30 to 50 times the minimum life.
Bearing materials:
High carbon chromium steel (carbon 1% and chromium 1.5%) is widely used as a ball
bearing material.
In contrast to ball bearings, roller bearings are usually made of case hardened steels. The
separator, cage, or retainer is usually a stamping of low carbon steel.
Lubrication of ball and roller bearing:
The ball and roller bearing are lubricated for the following reasons:
 To reduce the friction and wear between the mating parts of bearing.
 To prevent the corrosion on the bearing surfaces.
 To dissipate heat.

Unit II
BELT DRIVE
The belts are used to transmit power from one shaft to another by means of pulleys which
rotate at the same speed or at different speeds. The amount of power transmitted depends upon
the following factors:
 The velocity of the belt.
 The tension under which the belt is placed on the pulleys.
 The arc of contact between the belt and the smaller pulley.
 The conditions under which the belt is used.
It may be noted that
 The shafts should be properly in line to insure uniform tension across the belt section.
 The pulleys should not be too close together, in order that the arc of contact on the
smaller pulley may be as large as possible.
 The pulleys should not be so far apart as to cause the belt to weigh heavily on the shafts,
thus increasing the friction load on the bearings.
 A long belt tends to swing from side to side, causing the belt to run out of the pulleys,
which in turn develops crooked spots in the belt.
 The tight side of the belt should be at the bottom, so that whatever sags is present on the
loose side will increase the arc of contact at the pulleys.
 In order to obtain good results with flat belts, the maximum distance between the shafts
should not exceed 10 meters and the minimum should not be less than 3.5 times the
diameter of the larger pulley.
Selection of a Belt Drive
Following are the various important factors upon which the selection of a belt drive
depends:
 Speed of the driving and driven shafts,
 Speed reduction ratio,
 Power to be transmitted,
 Centre distance between the shafts,
 Positive drive requirements,
 Shafts layout,
 Space available, and

 Service conditions.
Types of Belt Drives
The belt drives are usually classified into the following three groups:
1. Light drives. These are used to transmit small powers at belt speeds up to about 10
m/s as in agricultural machines and small machine tools.
2. Medium drives. These are used to transmit medium powers at belt speeds over 10 m/s
but up to 22 m/s, as in machine tools.
3. Heavy drives. These are used to transmit large powers at belt speeds above 22 m/s as
in compressors and generators.
Types of Belts
Though there are many types of belts used these days, yet the following are important
from the subject point of view:
Flat belt:
The flat belt is mostly used in the factories and workshops, where a moderate amount of
power is to be transmitted, from one pulley to another when the two pulleys are not more than 8
meters apart.
V- belt:
The V-belt is mostly used in the factories and workshops, where a great amount of power
is to be transmitted, from one pulley to another, when the two pulleys are very near to each other.
Circular belt or rope:
The circular belt or rope is mostly used in the factories and workshops, where a great
amount of power is to be transmitted, from one pulley to another, when the two pulleys are more
than 8 meters apart.
If a huge amount of power is to be transmitted, then a single belt may not be sufficient. In
such a case, wide pulleys (for V-belts or circular belts) with a number of grooves are used. Then
a belt in each groove is provided to transmit the required amount of power from one pulley to
another.
Material used for Belts
The material used for belts and ropes must be strong, flexible, and durable. It must have a
high coefficient of friction. The belts, according to the material used, are classified as follows:

1. Leather belts. The most important material for flat belt is leather. The best leather
belts are made from 1.2 meters to 1.5 meters long strips cut from either side of the back bone of
the top grade steer hides.
The leather may be either oak-tanned or mineral salt. The belts are specified according to
the number of layers e.g. single, double or triple play and according to the thickness of hides
used e.g. light, medium or heavy.
The leather belts must be periodically cleaned and dressed or treated with a compound or
dressing containing neat’s foot or other suitable oils so that the belt will remain soft and flexible.
2. Cotton or fabric belts:
Most of the fabric belts are made by folding cotton duck to three or more layers
(depending upon the thickness desired) and stitching together. These belts are woven also into a
strip of the desired width and thickness. The cotton belts are cheaper and suitable in warm
climates, in damp atmospheres and in exposed positions. Since the cotton belts require little
attention, therefore these belts are mostly used in farm machinery, belt conveyor etc.
3. Rubber belt.
The rubber belts are made of layers of fabric impregnated with rubber.Composition and
have a thin layer of rubber on the faces. These belts are very flexible but are quickly destroyed if
allowed to come into contact with heat, oil or grease. One of the principle advantages of these
belts is that they may be easily made endless. These belts are found suitable for saw mills, paper
mills where they are exposed to moisture.
4. Balata belts
These belts are similar to rubber belts except that balata gum is used in place of Rubber.
These belts are acid proof and water proof and it is not affected by animal oils or alkalies. The
balata belts should not be at temperatures above 40°C because at this temperature the balata
begins to soften and becomes sticky. The strength of balata belts is 25 per cent higher than
rubber belts.
Working Stresses in Belts
The ultimate strength of leather belt varies from 21 to 35 MPa and a factor of safety may
be taken as 8 to 10. However, the wear life of a belt is more important than actual strength. It has
been shown by experience that under average conditions an allowable stress of 2.8 MPa or less
will give a reasonable belt life. An allowable stress of 1.75 MPa may be expected to give a belt
life of about15 years.

Belt Speed
A little consideration will show that when the speed of belt increases, the centrifugal
force also Increases which tries to pull the belt away from the pulley. This will result in the
decrease of power transmitted by the belt. It has been found that for the efficient transmission of
power, the belt speed20 m/s to 22.5 m/s may be used.
Coefficient of Friction between Belt and Pulley
The coefficient of friction between the belt and the pulley depends upon the following factors:
1. The material of belt;
2. The material of pulley;
3. The slip of belt; and
4. The speed of belt.
Open belt drive
The open belt drive, as shown in Fig below is used with shafts arranged parallel and
rotating in the same direction. In this case, the driver A pulls the belt from one side (i.e. lower
side RQ) and delivers it to the other side (i.e. upper side LM). Thus the tension in the lower side
belt will be more than that in the upper side belt. The lower side belt (because of more tension) is
known as tight side whereas the upper side belt (because of less tension) is known as slack side,
as shown in Fig, below
Crossed or twist belt drive
The crossed or twist belt drive, as shown in Fig. 18.5, is used with shafts arranged
parallel and rotating in the opposite directions. In this case, the driver pulls the belt from one side
(i.e. RQ) and delivers it to the other side (i.e. LM). Thus, the tension in the belt RQ will be more

than that in the belt LM. The belt RQ (because of more tension) is known as tight side, whereas
the belt LM (because of less tension) is known as slack side, as shown in figure
A little consideration will show that at a point where the belt crosses, it rubs against each
other and there will be excessive wear and tear. In order to avoid this, the shafts should be placed
at a maximum distance of 20 b, where b is the width of belt and the speed of the belt should be
less than15 m/s.
Quarter turn belt drive
The quarter turns belt drive (also known as right angle belt drive) as shown in Fig (a), is
used with shafts arranged at right angles and rotating in one definite direction. In order to prevent
the belt from leaving the pulley, the width of the face of the pulley should be greater or equal to
1.4 b, where b is width of belt.
In case the pulleys cannot be arranged as shown in Fig. below or when the reversible
motion is desired, then a quarter turn belt drive with a guide pulley, as shown in Fig. below
may be used.

Belt drive with idler pulleys
A belt drive with an idler pulley (also known as jockey pulley Drive) as shown in Fig.
Below, is used with shafts arranged parallel and when an open belt drive cannot be used due to
small angle of contact on the smaller pulley. This type of drive is provided to obtain high
velocity ratio and when the required belt tension cannot be obtained by other means.
When it is desired to transmit motion from one shaft to several shafts, all arranged in
parallel, a V Belt drive with many idler pulleys, as shown in Fig. above may be employed.
Compound belt drive
A compound belt drive as shown in Fig. below is used when power is transmitted from
one shaft to another through a number of pulleys.
Stepped or cone pulley drive
A stepped or cone pulley drive, as shown in Fig. below, is used for changing the speed of
the driven shaft while the main or driving shaft runs at constant speed. This is accomplished by
shifting the belt from one part of the steps to the other.

.
Fast and loose pulley drive
A fast and loose pulley drive, as shown in Fig. above , is used when the driven or
machine shaft is to be started or stopped whenever desired without interfering with the driving
shaft. A pulley which is keyed to the machine shaft is called fast pulley and runs at the same
speed as that of machine shaft. A loose pulley runs freely over the machine shaft and is incapable
of transmitting any power. When the driven shaft is required to be stopped, the belt is pushed on
to the loose pulley by means of sliding bar having belt forks.
V-Belt Drive
 V-belt is mostly used in factories and workshops where a great amount of power is to be
transmitted from one pulley to another when the two pulleys are very near to each other.
 The V-belts are made of fabric and cords moulded in rubber and covered with fabric and
rubber as shown in Fig. (a).
 These belts are molded to a trapezoidal shape and are made endless.
 These are particularly suitable for short drives.
 The included angle for the V-belt is usually from 30° to 40°.
 The power is transmitted by the *wedging action between the belt and the V-groove in the
pulley or sheave.
 A clearance must be provided at the bottom of the groove as shown in Fig. (b), in order to
prevent touching of the bottom as it becomes narrower from wear.

 The V-belt drive may be inclined at any angle with tight side either at top or bottom. In order
to increase the power output, several V-belts may be operated side by side.
 It may be noted that in multiple V-belt drive, all the belts should stretch at the same rate so
that the load is equally divided between them.
 When one of the set of belts breaks, the entire set should be replaced at the same time.
 If only one belt is replaced, the new unworn and un-stretched belt will be more tightly
stretched and will move with different velocity.
Types of V-belts and Pulleys
According to Indian Standards (IS: 2494 – 1974), the V-belts are made in five types i.e.
A, B, C,D and E. The pulleys for V-belts may be made of cast iron or pressed steel in order to
reduce weight. The dimensions for the standard V-grooved pulley according to IS: 2494 – 1974.
Advantages and Disadvantages of V-belt Drive over Flat Belt Drive
Following are the advantages and disadvantages of the V-belt drive over flat belt drive:
Advantages
 The V-belt drive gives compactness due to the small distance between centers of pulleys.
 The drive is positive, because the slip between the belt and the pulley groove is
negligible.
 Since the V-belts are made endless and there is no joint trouble, therefore the drive is
smooth.
 It provides longer life, 3 to 5 years.
 It can be easily installed and removed.
 The operation of the belt and pulley is quiet.
 The belts have the ability to cushion the shock when machines are started.
 The high velocity ratio (maximum 10) may be obtained.

 The wedging action of the belt in the groove gives high value of limiting *ratio of
tensions. Therefore the power transmitted by V-belts is more than flat belts for the same
coefficient of friction, arc of contact and allowable tension in the belts.
 The V-belt may be operated in either direction, with tight side of the belt at the top or
bottom. The center line may be horizontal, vertical or inclined.
Disadvantages
 The V-belt drive cannot be used with large centre distances, because of larger weight per
unit length.
 The V-belts are not so durable as flat belts.
 The construction of pulleys for V-belts is more complicated than pulleys of flat belts.
 Since the V-belts are subjected to certain amount of creep, therefore these are not suitable
for constant speed applications such as synchronous machines and timing devices.
 The belt life is greatly influenced with temperature changes, improper belt tension and
 Mismatching of belt lengths.
 The centrifugal tension prevents the use of V-belts at speeds below 5 m/ s and above 50
m / s.
Materials of V-belts:
V-belts are made of cotton fabric and cords molded in rubber and covered with fabric and
rubber.

CHAIN DRIVE
 The chain drive is an intermediate between belt and gear drives. It has the major
advantages of both belt and gear drives.
 Chain drives are used for velocity ratios less than 10 with velocities up to 25 m/s and
power ratings up to 125 kW.
 Chain drives are popularly used in the transportation industry such as bi cycles, motor
cycles and automobile vehicles.
 They also find wide applications in agricultural machinery, metal and wood working
machines, textile machinery, and materials handling machinery.
Advantages and Disadvantages chain drive compared with belt and gear drives
Advantages:
 They can be used for long as well as short Centre distances.
 They are more compact than belt and gear drives.
 There is no slip between chain and sprocket.
 Higher efficiency up to 98%
 They transmit more power than belt drives.
 They can be operated under adverse temperature and atmospheric conditions.
Disadvantages:
 They required precise alignment of shaft than the belt drives.
 They required proper maintenance and slack adjustment compare with belt drives.
 Noisy operation.
 They require the take up devices.
 More complicated design.
TYPES OF CHAIN DRIVES:

The common types of chain are:
1. Link chains(welded chains)
2. Transmission chains(roller chains) and
3. Silent chain (inverted tooth chains).
TRANSMISSION CHAINS AND SPROCKETS
Transmission or roller chains:
A roller chain provides a readily available and efficient method for transmitting power
between parallel shafts. That’s why the roller chains are also called as transmission chains. A
roller chain consists of an endless chain running over two sprockets-driver and driven.
Sprocket is a wheel with teeth of a special profile. Smaller sprocket is called pinion and
bigger one is called wheel. In general sprockets are made of low carbon or medium carbon
steels. But some stainless steel is also used for sprocket. Typical roller chain on sprocket is
shown in figure.
Construction of roller chains:
Roller Chain consists of alternate links made of inner and outer link plates. The outer
plates are known as pin link or coupling link whereas the inner plates are3 called roller link. The
other parts of a roller chain are pin, bushing and roller.
Specification of a roller chain
Roller chain is specified by three dimensions- pitch, width and diameter.
Pitch: It is the distance from Centre to Centre of adjacent pins.
Width: it is nominal width of the link.
Diameter: It refers to the actual outside diameter of the roller.
Roller chains are available in single-row or multi-row construction such as simplex, duplex or
triplex standards. (Refer data book, page no: 7.71)
Chain Materials
 Link plates are made of cold-rolled, medium-carbon or alloy steels such as C45,
C50 and 40 Crl.
 Pins, bushings and rollers are made of carburizing steel such as C15, C20, and 30
Ni4 Crl.

Chordal (or polygonal) action:
 Chordal (or polygonal) action, which causes a fluctuation in chain speed and chain
tension each time a chain link engages a sprocket tooth, may be serious limiting design
factor in chain performance, especially in high speed applications.
 As shown in figure for roller chain, chordal action is a kinematic consequence of the fact
that the line of approach of the chain is not tangent to the pitch circle of the sprocket; it is
collinear with a chord of the pitch circle.
 Therefore, as the sprocket rotates, the link makes first contact with the sprocket when the
link centerline is below the tangent to the pitch circle, which has the radius rp.
 As a consequence, the link centerline is caused to rise from rch to rp, and then fall back to
rch, a behavior known as chordal action.
 Chordal action causes the sprocket pitch radius to cylindrically fluctuate, resulting in a
cyclic fluctuation in chain speed. Thus even if the drive sprocket rotates at constant
angular velocity, the driven sprocket experiences a speed fluctuation.
SILENT (OR) INVERTED-TOOTH CHAIN
Inverted-tooth chains are also called as silent chains because of their relatively quiet
operation. Silent drives are often selected for high-power, high-speed and smooth operation.
Silent chains have inward-pointing teeth that engage the sprocket.

Types of silent chains:
Depending upon the type of joint between links, the silent chains are classified into:
i. Reynold chain: In Reynolds chain, the links are connected by pins resulting in sliding
friction.
ii. Morse chain: In Morse chain, the rocket pins are used.
Advantages:
 It can be used for high-speed and high-power applications.
 They operate much smoother and quieter than roller chains.
 More reliable due to laminated construction.
Disadvantages:
 More heavier
 More complex
 More expensive
 More difficult to manufacture
 Required more careful maintenance
Due to the above reasons, the silent chains have limited applications.

Unit III
GEAR DRIVE
 A gear is a rotating machine part having cut teeth, which mesh with another toothed part
in order to transmit motion or power between the shafts.
 In precision machines, in which a definite velocity ratio is of importance (as in watch
mechanism), the only positive drive is by gears or toothed wheels. A gear drive is also
provided, when the distance between the driver and the follower is very small.
Advantages and Disadvantages of Gear Drives
The following are the advantages and disadvantages of the gear drive as compared to other
drives, i.e. belt, rope and chain drives:
Advantages:
 It transmits exact velocity ratio.
 It may be used to transmit large power.
 It may be used for small centre distances of shafts.
 It has high efficiency.
 It has reliable service.
 It has compact layout.
Disadvantages:
 Since the manufacture of gears requires special tools and equipment, therefore it is costlier
than other drives.
 The error in cutting teeth may cause vibrations and noise during operation.
 It requires suitable lubricant and reliable method of applying it, for the proper operation of
gear drives.
Classification of Gears
The gears or toothed wheels may be classified as follows:
1. According to the position of axes of the shafts:
The axes of the two shafts between which the motion is to be transmitted, may be
(a) Parallel,
(b) Intersecting, and
(c) Non-intersecting and non-parallel.
Spur Gear:

 The two parallel and co-planar shafts connected by the gears.
 These gears are called spur gears and the arrangement is known as spur gearing. These
gears have teeth parallel to the axis of the wheel.
Helical Gear:
 Another name given to the spur gearing is helical gearing, in which the teeth are inclined to
the axis.
 The (a) single and (b) double helical gears connecting parallel shafts are shown in Fig. below.
 The object of the double helical gear is to balance out the end thrusts that are induced in single
helical gears when transmitting load. The double helical gears are known as herringbone
gears.
Bevel Gear:
 The two non-parallel or intersecting, but coplanar shafts connected by gear are shown in
Fig(c). Below.
 These gears are called bevel gears and the arrangement is known as bevel gearing.

 The bevel gears, like spur gears may also have their teeth inclined to the face of the bevel, in
which case they are known as helical bevel gears.
Spiral Gear:
 The two non-intersecting and non-parallel i.e. non-coplanar shafts connected by gears is
shown in Fig (d).
 These gears are called skew bevel gears or spiral gears and the arrangement is known as skew
bevel gearing or spiral gearing.
2. According to the peripheral velocity of the gears:
The gears, according to the peripheral velocity of the gears, may be classified as :
(a) Low velocity,
(b) Medium velocity, and
(c) High velocity.
 The gears having velocity less than 3 m/s are termed as low velocity gears and gears having
velocity between 3 and 15 m/s are known as medium velocity gears.
 If the velocity of gears is more than 15 m/s, then these are called high speed gears.
3. According to the type of gearing:
The gears, according to the type of gearing, may be classified as:
(a) External gearing,
(b) Internal gearing, and
(c) Rack and pinion.

 In external gearing, the gears of the two shafts mesh externally with each other as shown in
Fig. above.
 The larger of these two wheels is called spur wheel or gear and the smaller wheel is called
pinion.
 In internal gearing, the gears of the two shafts mesh internally with each other as shown in
Fig. above.
 The larger of these two wheels is called annular wheel and the smaller wheel is called pinion.
 Sometimes, the gear of a shaft meshes externally and internally with the gears in a straight
line, as shown in Fig. above. Such a type of gear is called rack and pinion.
4. According to the position of teeth on the gear surface:
The teeth on the gear surface may be
(a) Straight,
(b) Inclined, and
(c) Curved.
Terms used in Gears
The following terms, which will be mostly used in this chapter, should be clearly understood at this
stage. These terms are illustrated in Fig. 28.6.
1. Pitch circle. It is an imaginary circle which by pure rolling action, would give the same
motion as the actual gear.
2. Pitch circle diameter. It is the diameter of the pitch circle. The size of the gear is usually
specified by the pitch circle diameter. It is also called as pitch diameter.
3. Pitch point. It is a common point of contact between two pitch circles.
4. Pitch surface. It is the surface of the rolling discs which the meshing gears have replaced at
the pitch circle.

5. Pressure angle or angle of obliquity. It is the angle between the common normal to two gear
teeth at the point of contact and the common tangent at the pitch point. It is usually denoted by
. The standard pressure angles are 14 1/2and 20°.
6. Addendum. It is the radial distance of a tooth from the pitch circle to the top of the tooth.
7. Dedendum. It is the radial distance of a tooth from the pitch circle to the bottom of the tooth.
8. Addendum circle. It is the circle drawn through the top of the teeth and is concentric with the
pitch circle.
9. Dedendum circle. It is the circle drawn through the bottom of the teeth. It is also called root
circle.
10. Module. It is the ratio of the pitch circle diameter in millimeters to the number of teeth. It is
usually denoted by m. mathematically,
Module, m = D / T
12. Clearance. It is the radial distance from the top of the tooth to the bottom of the tooth, in a
meshing gear. A circle passing through the top of the meshing gear is known as clearance
circle.
13. Circular pitch. It is the distance measured on the circumference of the pitch circle from a
point of one tooth to the corresponding point on the next tooth. It is usually denoted by pc.
Mathematically,
Circular pitch, pc = πD/Z

Where
D = Diameter of the pitch circle, and
Z = Number of teeth on the wheel.
A little consideration will show that the two gears will mesh together correctly, if the
two wheels have the same circular pitch.
Note: If D1 and D2 are the diameters of the two meshing gears having the teeth Z1 and Z2
respectively; then for them to mesh correctly,
14. Diametral pitch. It is the ratio of number of teeth to the pitch circle diameter in millimeters.
It denoted by pd. mathematically,
15. Total depth. It is the radial distance between the addendum and the dedendum circle of a
gear. It is equal to the sum of the addendum and dedendum.
16. Working depth. It is radial distance from the addendum circle to the clearance circle. It is
equal to the sum of the addendum of the two meshing gears.
17. Tooth thickness. It is the width of the tooth measured along the pitch circle.
18. Tooth space. It is the width of space between the two adjacent teeth measured along the pitch
circle.
19. Backlash. It is the difference between the tooth space and the tooth thickness, as measured on
the pitch circle.
Forms of Teeth:
Following are the two types of teeth commonly used.
1. Cycloidal teeth and
2. Involute teeth.
The following four systems of gear teeth are commonly used in practice:
1. 1 14 /2° Composite system,
2. 14 1/2° Full depth involute system,
3. 20° Full depth involute system, and
4. 20° Stub involutes system.

 The 14 1/2° composite system is used for general purpose gears. It is stronger but has no
interchangeability. The tooth profile of this system has cycloidal curves at the top and bottom
and involute curve at the middle portion.
 The tooth profile of the 14 1/2° full depth involute system was developed for use with gear
hobs for spur and helical gears.
 The tooth profile of the 20° full depth involute system may be cut by hobs. The increase of the
pressure angle from 14 1/2° to 20° results in a stronger tooth, because the tooth acting as a
beam is wider at the base.
 The 20° stub involute system has a strong tooth to take heavy loads.
Gear Materials:
 The material used for the manufacture of gears depends upon the strength and service
conditions like wear, noise etc.
 The gears may be manufactured from metallic or non-metallic materials.
 The metallic gears with cut teeth are commercially obtainable in cast iron, steel and bronze.
 The nonmetallic materials like wood, rawhide, compressed paper and synthetic resins like
nylon are used for gears, especially for reducing noise.
 The cast iron is widely used for the manufacture of gears due to its good wearing properties,
excellent machinability and ease of producing complicated shapes by casting method.
 The cast iron gears with cut teeth may be employed, where smooth action is not important.
 The steel is used for high strength gears and steel may be plain carbon steel or alloy steel.
 The steel gears are usually heat treated in order to combine properly the toughness and tooth
hardness.
Design Considerations for a Gear Drive
In the design of a gear drive, the following data is usually given:
1. The power to be transmitted.
2. The speed of the driving gear,
3. The speed of the driven gear or the velocity ratio, and
4. The centre distance.
The following requirements must be met in the design of a gear drive:
(a) The gear teeth should have sufficient strength so that they will not fail under static loading
or dynamic loading during normal running conditions.
(b) The gear teeth should have wear characteristics so that their life is satisfactory.
(c) The use of space and material should be economical.

(d) The alignment of the gears and deflections of the shafts must be considered because they
effect on the performance of the gears.
(e) The lubrication of the gears must be satisfactory.
Causes of Gear Tooth Failure:
The different modes of failure of gear teeth and their possible remedies to avoid the failure are
as follows:
Bending failure:
 Every gear tooth acts as a cantilever.
 If the total repetitive dynamic load acting on the gear tooth is greater than the beam strength
of the gear tooth, then the gear tooth will fail in bending, i.e. the gear tooth will break.
 In order to avoid such failure, the module and face width of the gear is adjusted so that the
beam strength is greater than the dynamic load.
Pitting:
 It is the surface fatigue failure which occurs due to many repetitions of Hertz contact stresses.
 The failure occurs when the surface contact stresses are higher than the endurance limit of the
material.
 The failure starts with the formation of pits which continue to grow resulting in the rupture of
the tooth surface.
 In order to avoid the pitting, the dynamic load between the gear tooth should be less than the
wear strength of the gear tooth.
Scoring:
 The excessive heat is generated when there is an excessive surface pressure, high speed or
supply of lubricant fails.
 It is a stick-slip phenomenon in which alternate shearing and welding takes place rapidly at
high spots.
 This type of failure can be avoided by properly designing the parameters such as speed,
pressure and proper flow of the lubricant, so that the temperature at the rubbing faces is within
the permissible limits.
Abrasive wear:
 The foreign particles in the lubricants such as dirt, dust or burr enter between the tooth and
damage the form of tooth.

 This type of failure can be avoided by providing filters for the lubricating oil or by using high
viscosity lubricant oil which enables the formation of thicker oil film and hence permits easy
passage of such particles without damaging the gear surface.
Corrosive wear:
 The corrosion of the tooth surfaces is mainly caused due to the presence of corrosive elements
such as additives present in the lubricating oils.
 In order to avoid this type of wear, proper anti-corrosive additives should be used.
Design of Shaft for Spur Gears
In order to find the diameter of shaft for spur gears, the following procedure may be followed.
1. First of all, find the normal load (FN), acting between the tooth surfaces. It is given by
Ft = Tangential Load, And
α = Pressure Angle
A thrust parallel and equal to FN will act at the gear centre as shown in Fig.
2. The weight of the gear is given by
FG = 0.001 18 ZG.b.m2
(in N)
Where
ZG = No. of teeth on the gear,
b = Face width in mm, and
m = Module in mm.
3. Now the resultant load acting on the gear,

4. If the gear is overhung on the shaft, then bending moment on the shaft due to the resultant
load,
M = FR × x
Where
x = Overhang i.e. the distance between the centre of gear and the centre of
bearing.
5. Since the shaft is under the combined effect of torsion and bending, therefore we shall
determine the equivalent torque. We know that equivalent torque,
6. Now the diameter of the gear shaft (d ) is determined by using the following relation, i.e.
Note: Proceeding in the similar way as discussed above, we may calculate the diameter of the pinion
shaft.
Design of Arms for Spur Gears
 The cross-section of the arms is calculated by assuming them as a cantilever beam fixed at the
hub and loaded at the pitch circle.
 It is also assumed that the load is equally distributed to all the arms. It may be noted that the
arms are designed for the stalling load.
 The stalling load is a load that will develop the maximum stress in the arms and in the teeth.
This happens at zero velocity, when the drive just starts operating.
 The stalling load may be taken as the design tangential load divided by the velocity factor.

Design for the rim
The thickness of the rim for the pinion may be taken as 1.6 m to 1.9 m, where m is the
module.
Let us take thickness of the rim for pinion,
tRP = 1.6 m.
The thickness of the rim for the gear (tRG) is given by
tRG = m

HELICAL GEAR:
 A helical gear has teeth in form of helix around the gear.
 Two such gears may be used to connect two parallel shafts in place of spur gears.
 The helixes may be right handed on one gear and left handed on the other.
 The object of the double helical gear is to balance out the end thrusts that are induced in single
helical gears when transmitting load. The double helical gears are known as herringbone
gears.
Terms Used In Helical Gears:
The following terms in connection with helical gears, as shown in fig. 29.1, are important
from the subject point of view.
Helix angle: it is a constant angle made by the helices with the axis of rotation.
Axial pitch: it is the distance, parallel to the axis, between similar faces of adjacent teeth. it is the
same as circular pitch and is therefore denoted by pc. The axial pitch may also be defined as the
circular pitch in the plane of rotation or the diametral plane.
Normal pitch: it is the distance between similar faces of adjacent teeth along a helix on the pitch
cylinders normal to the teeth. It is denoted by pn. The normal pitch may also be defined as the circular
pitch in the normal plane which is a plane perpendicular to the teeth. Mathematically, normal pitch,
Pn = Pc Cos β
Herringbone or double helical gear:
Herringbone or double helical gear can be two helical gears with opposing helix angle stacked
together. As a result, two opposing thrust load s cancel and the shafts are not acted upon by any thrust
load.
The advantages of elimination of thrust load in Herringbone a gear is obliterated by
considerably higher machining and mounting costs. This limits their applications to very heavy power
transmission.

Crossed helical gears:
Crossed helical gears are used for transmitting power between two non- parallel, non-
intersecting shafts.
Common application is distributor and pump drive from cam shafts in automotive engines.
Formative or Equivalent Number of Teeth for Helical Gears
The formative or equivalent number of teeth for a helical gear may be defined as the number
of teeth that can be generated on the surface of a cylinder having a radius equal to the radius of
curvature at a point at the tip of the minor axis of an ellipse obtained by taking a section of the gear in
the normal plane. Mathematically, formative or equivalent number of teeth on a helical gear,
TE = Z / cos3
β
Z = Actual number of teeth on a helical gear, and
β = Helix angle.
Designfor the pinion shaft:
Let dP = Diameter of the pinion shaft.
The axial load of the pinion,
Fa = Ft tan β (Pag: No: 8.57)
Bending moment on the pinion shaft due to the tangential load,
M1 = Ft × Overhang

Bending moment on the pinion shaft due to the axial load,
M2 = Fa x
Dp is the pitch circle diameter of the pinion.
Since the bending moment due to the tangential load (i.e. M1) and bending moment due to the axial
load (i.e. M2) are at right angles, therefore resultant bending moment on the pinion shaft,
Equivalent twisting moment,
Now the diameter of the pinion shaft (dp) is determined by using the following relation, i.e.
NOTE: Design of Arms for helical gear is same as that of Spur Gears.
Helical Gear:
1. What is a herringbone gear? Where they are used?
2. Explain the following terms used in helical gears :(a) Helix angle; (b) normal pitch; and (c)
axial pitch.
3. Define formative or virtual number of teeth on a helical gear. Derive the expression used to
obtain its value.

BEVEL GEAR:
 The bevel gears are used for transmitting power at a constant velocity ratio between two shafts
whose axes intersect at a certain angle.
 The pitch surfaces for the bevel gear are frustums of cones.
Classification of Bevel Gears
The bevel gears may be classified into the following types, depending upon the angles between the
shafts and the pitch surfaces.
a) Straight bevel gears: The axes intersect, generally at right angles. The teeth are straight and
converge at the apex of the pitch cone. Like spur gears straight bevel gears are noisy.
b) Zero bevel gears: The axes intersect and the teeth are curved. (Spiral shaped, with zero spiral
angle).
c) Spiral bevel gears: The axes intersect and the teeth are curved and oblique. Like helical gear,
spiral bevel gears are less noisy.
d) Skew bevel gears: The axes are non-parallel and non-intersecting and the teeth are straight.
e) Hypoid gears: The axes are non-parallel and non-intersecting and the teeth are curved. These
gears are used in automobile differential unit. Because of pinion offset, the drive shaft can be
placed beneath the floor level
1. Mitre gears: When equal bevel gears (having equal teeth and equal pitch angles) connect
two shafts whose axes intersect at right angle, as shown in Fig. 30.2 (a), then they are known
as mitre gears.
2. Angular bevel gears: When the bevel gears connect two shafts whose axes intersect at an
angle other than a right angle, then they are known as angular bevel gears.
3. Crown bevel gears: When the bevel gears connect two shafts whose axes intersect at an
angle greater than a right angle and one of the bevel gears has a pitch angle of 90º, then it is
known as a crown gear. The crown gear corresponds to a rack in spur gearing, as shown in
Fig. 30.2 (b).

4. Internal bevel gears. When the teeth on the bevel gear are cut on the inside of the pitch
cone, then they are known as internal bevel gears.
Terms used in Bevel Gears:
1. Cone centre: It is the apex of the pitch cone. It may be defined as that point where the axes of
two mating gears intersect each other.
2. Cone distance: It is the length of the pitch cone element. It is also called as a pitch cone
radius.
3. Pitch angle: It is the angle made by the pitch line with the axis of the shaft. It is denoted by
‘θP’.
4. Back cone distance: It is the length of the back cone. It is denoted by ‘RB’. It is also called
back cone radius
5. Face angle: It is the angle subtended by the face of the tooth at the cone centre. It is denoted
by ‘φ’. The face angle is equal to the pitch angle plus addendum angle.
6. Root angle: It is the angle subtended by the root of the tooth at the cone centre. It is denoted
by ‘θR’. It is equal to the pitch angle minus dedendum angle.
7. Crown height: It is the distance of the Crown Point (C) from the cone centre (O), parallel to
the axis of the gear. It is denoted by ‘HC’.

Design of a Shaft for Bevel Gears
In designing a pinion shaft, the following procedure may be adopted:
1. First of all, find the torque acting on the pinion. It is given by
Where
P = Power transmitted in watts, and
NP = Speed of the pinion in r.p.m.
2. Find the tangential force (WT) acting at the mean radius (Rm) of the pinion. We know that
T = Ft x Dp/2
3. Now find the axial and radial forces (i.e. WRH and WRV) acting on the pinion shaft as
discussed above.
4. Find resultant bending moment on the pinion shaft as follows:
The bending moment due to Fa and Fr is given by
M1 = Fr× Overhang – Fa x Rp (Pinion radius)
And bending moment due to FT,
M2 = Ft × Overhang.
∴ Resultant bending moment,
5. Since the shaft is subjected to twisting moment (T) and resultant bending moment (M),
therefore equivalent twisting moment,
3. Now the diameter of the pinion shaft may be obtained by using the torsion equation. We
know that
4. The same procedure may be adopted to find the diameter of the gear shaft.

Worm Gear:
 The worm gears are widely used for transmitting power at high velocity ratios between non-
intersecting shafts that are generally, but not necessarily, at right angles.
 It can give velocity ratios as high as 300: 1, but it has a lower efficiency.
 The worm gearing is mostly used as a speed reducer, which consists of worm and a worm
wheel or gear.
 The worm (which is the driving member) is usually of a cylindrical form having threads of the
same shape as that of an involute rack. The threads of the worm may be left handed or right
handed and single or multiple threads.
 The worm wheel or gear (which is the driven member) is similar to a helical gear with a face
curved to conform to the shape of the worm.
 The worm is generally made of steel while the worm gear is made of bronze or cast iron for
light service.
Types of Worms
The following are the two types of worms:
1. Cylindrical or straight worm, and
2. Cone or double enveloping worm.
 The cylindrical or straight worm, as shown in Fig. (a) is most commonly used. The shape of
the thread is involute helicoids of pressure angle 14 ½° for single and double threaded worms
and 20° for triple and quadruple threaded worms.
The cone or double enveloping worm, as shown in Fig. (b), is used to some extent, but it requires
extremely accurate alignment.
Types of Worm Gears
The following three types of worm gears are important from the subject point of view:

1. Straight face worm gear, as shown in Fig. (a),
2. Hobbed straight face worm gear, as shown in Fig. (b), and
3. Concave face worm gear, as shown in Fig. (c).
 The straight face worm gear is like a helical gear in which the straight teeth are cut with a
form cutter. Since it has only point contact with the worm thread, therefore it is used for light
service.
 The hobbed straight face worm gear is also used for light service but its teeth are cut with a
hob, after which the outer surface is turned.
 The concave face worm gear is the accepted standard form and is used for all heavy service
and general industrial uses. The teeth of this gear are cut with a hob of the same pitch
diameter as the mating worm to increase the contact area.
Terms used in Worm Gearing:
Axial pitch (Pa): It is also known as linear pitch of a worm. It is the distance measured axially from
a point on one thread to the corresponding point on the adjacent thread on the worm.
Lead: It is the linear distance through which a point on a thread moves ahead in one revolution of the
worm. For single start threads, lead is equal to the axial pitch, but for multiple start threads, lead is
equal to the product of axial pitch and number of starts. Mathematically,
Lead, L= Pa. n
Where
Pa = Axial pitch; and
n = Number of starts.
Lead angle: It is the angle between the tangent to the thread helix on the pitch cylinder and the plane
normal to the axis of the worm. It is denoted by γ.

Tooth pressure angle. It is measured in a plane containing the axis of the worm and is equal to one-
half the thread profile angle.
Normal pitch: It is the distance measured along the normal to the threads between two corresponding
points on two adjacent threads of the worm.
Helix angle: It is the angle between the tangent to the thread helix on the pitch cylinder and the axis
of the worm.
Velocity ratio: It is the ratio of the speed of worm (Z) in r.p.m. to the speed of the worm gear (z) in
r.p.m.
Thermal Rating of Worm Gearing
In the worm gearing, the heat generated due to the work lost in friction must be dissipated in order to
avoid over heating of the drive and lubricating oil. The quantity of heat generated (Qg) is given by
Qg = P (1 – η)
P = Power transmitted in watts, and
η = Efficiency of the worm gearing.
The heat generated must be dissipated through the lubricating oil to the gear box housing and then to
the atmosphere. The heat dissipating capacity depends upon the following factors:
1. Area of the housing (A),
2. Temperature difference between the housing surface and surrounding air (t2 – t1), and
3. Conductivity of the material (K).
Mathematically, the heat dissipating capacity,
Qd = A (t2 – t1) hcr
Force acting on worm gear:
1. Tangential force.
2. Axial force or thrust on the worm.
3. Radial or separating force on the worm.
Questions
1. How the bevel gears are classified? Explain with neat sketches.
2. Sketch neatly the working drawing of bevel gears in mesh.

3. For bevel gears, define the following :(i) Cone distance; (ii) Pitch angle; (iii) Face angle;
(iv) Root angle; (v) Back cone distance; and (vi) Crown height.
5. What are the various forces acting on a bevel gear?
6. Write the procedure for the design of a shaft for bevel gears.
Worm gear:
1. Discuss, with neat sketches, the various types of worms and worm gears.
2. Define the following terms used in worm gearing : (a) Lead; (b) Lead angle; (c) Normal
pitch; and
(d) Helix angle.
3. What are the various forces acting on worm and worm gears?
4. Write the expression for centre distance in terms of axial lead, lead angle and velocity ratio.

Unit V
Gear Box
Machine tools like lathe, milling machines, etc., require a wide range of spindle speeds.
Because a machine tool is adoptable for cutting different types of meals having different
properties using varying grades of cutting tools on work-pieces of different diameters.
Thus the provision of variable spindle speeds is necessary in order to meet different
requirements. The various methods used for obtaining different speeds of machine tool spindle
are as follows:
i. By using a gear box mechanism,
ii. By using a cone pulley arrangement,
iii. By using a variable speed electric motor, and
iv. By hydraulic operation.
Among these methods, the gear box method is very popularly used. In this chapter, we shall
discuss the design of gear boxes, in detail, in the following sections.
REQUIREMENTS OF A SPEED GEAR BOXES
A speed gear box should have the following requirements:
 It should provide the designed series of spindle speeds.
 It should transmit the required amount of power to the spindle.
 It should provide smooth silent operation of the transmission.
 It should have simple construction.
 Mechanism of speed gear boxes should be easily accessible so that it is easier to carry out
preventive maintenance.
THE SPEEDS IN MACHINE TOOL GEAR BOXES ARE IN GEOMETRIC
PROGRESSION. WHY?
The speeds in gear boxes can be arranged in arithmetic progression (A.P.), geometric
progression (G.P.), harmonic progression (H.P.), and logarithmic progression (L.P.). However,
when the speeds are arranged in G.P., it has the following advantages over the other
progressions.
1. The speed loss in minimum.
i.e., Speed loss = Desired optimum speed  Available speed
2. The number of gears to be employed is minimum.

3. G.P. provides a more even range of spindle speeds at each step.
4. The layout is comparatively very compact.
5. Productivity of a machining operation, i.e., surface area of the metal removed in unit
time, is constant in the whole speed range.
6. G.P. machine tool spindle speeds can be selected easily from preferred numbers. Because
preferred numbers are in geometric progression.
METHODS FOR CHANGING SPEED IN GEAR BOXES
The two important methods widely used are:
1. Sliding mesh gear box, and
2. Constant mesh gear box.
Sliding MeshGear Box
It is the oldest and simplest from of gear box. Sliding type gear boxes are quite
commonly used in general purpose machine tools. In order to mesh gears on the main shaft with
appropriate gears on the spindle shaft for obtaining different speeds, they are moved to the right
or the left. It derives its name from the fact that the meshing of the gears takes place by sliding of
gears on each other.
Constant MeshGear Box
It derives its name from the fact that all the gears whether of the countershaft or the main
shaft are in constant mesh with each other. It is also known as a silent or quite gear box. It gives
a quieter operation and makes gear changing easier by employing helical gears for the constant
mesh. In order to connect the required gear wheel by means of teeth on the side of the gear
wheel, a separate sliding member is employed.
PREFERRED NUMBERS
Preferred numbers are the conventionally rounded off values derived from geometric
series. There are five basic series, denoted as R 5, R 10, R 20, R 40 and R 80 series. The symbol
‘R’ is used as a tribute to French engineer Charles Renard, who introduced the preferred numbers
first. Preferred numbers assist the designer in avoiding the selection of sizes in an arbitrary
manner.
Step ratio or series ratio or progression Ratio
When the spindle speeds are arranged in geometric progression, then the ratio between
the two adjacent speeds is known as step ratio or progression ratio. It denoted by ϕ.

Structural formula:
N= number of speeds available at the spindle.
P1, P2….= stage numbers in the gear box, and
X1,X2….= Characteristic of stage.
The structural formula is given as
n= P1(X1). P2(X2). P3(X3)
X1= 1, X2= P1, X3= P1.P2, X3= P1.P2.P3
GEAR BOX HOUSING
The gearbox housing is a member of the gearbox but it is a non-rotating member. It lies at
the center of the gearbox and joins all other elements of the gearbox like flanges, bearings and
spiral bevel gears. The base to adjust the gears with a certain tooth bearing and backlash are the
angle of the housing and the offshoot of the holes. This ensures the best possible running of the
gears and standard attribute of transmission.
Gearbox Housings as Cast
Gearbox housings as cast, which are also known as cast gearbox housings are the gear
housings that are made by a process called metal-casting. In other words, we can say that the cast
gearbox housings are made by using molds. These types of gearbox housings are very sturdy and
durable in nature. These housings are available in various types of seals like lip seal, double lip
seal, taconite seal, etc.
Gearbox Housings as Fabricated
The fabricated gearbox housings are manufactured by using various industrial standard
materials like cast iron, modular cast iron, steel, and so. Irrespective of the cast gearbox
housings, these gearbox housings are fabricated in hi-tech machining centers. These housings are
ideal for heavy industrial gears.

Working and Utility of Gearbox Housing
An oil outlet opening is present in the Gearbox Housing. A gearbox Housing also
consists a method which assists it to work upon the fluid level of the gearbox. The gearbox
housing contains such a device which can work from two different positions.
In the first working position, the height of the fluid level with in the gearbox housing is
predetermined during its filling and operation. This predetermination is done by an upper
opening of the device. In the second working position, primarily the provision is made to drain-
out the gearbox oil.
For this purpose, an oil drain opening of the device is released. This draining opening is
adjusted properly beneath the upper opening. This adjustment allows to drain-out a major portion
of oil disposed in the gearbox. The device which is used to open the drain opening is used from
outside of the gear box housing.
Structure of Gearbox Housing
The material that is used most commonly to manufacture gearbox housing is nylon.
Through the gear housing surfaces and thermal condition of the surrounding air, most of the heat
is dispelled by the process of radiation. Dispelling of heat through heat radiation depends on
some other factors also, and these factors are: surrounding structure within the gearbox and the
various components which are present.
In a gearing housing, there is a bearing pocket. An extra bearing is installed at the center
of the bearing pocket. This ensures the permanent seal and proper alignment of the bearing.
Applications of Gearbox Housing
Gear housing has a lot of different applications. Some of these applications are:
 In aerospace transmission.
 In motor sport transmission.
 In other different kinds of transmission.
 In transmission of heat in robots, electric motors, solar gas turbine motors.
 It forms a complete wet oil slum in the engines. This helps the engine to drive out the
residues from inside.

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BEARINGS
What is a bearing?
Bearing is a machine member, used to support the
axles and power transmitting shafts, directs the motion
of shafts and also reduce friction between contact
surfaces, while carrying the load.
Classify the bearings.
Based on nature of contact between bearing surfaces.
 Sliding contact bearing.
 Rolling contact (or) Antifriction bearing.
Based on load applied.
 Radial bearing (Circumferentially loaded)
 Thrust bearing (Axially loaded)
What are the types of sliding contact bearings?
 Zero film bearing.
 Thin film bearing.
 Thick film (or) Hydrodynamic bearing.
 Externally pressurized (or) Hydrostatic
bearing.
 Pivot bearing.
 Collar bearing.
What are the bearing materials?
 Aluminium alloy,
 Copper alloy,
 Babbit,
 Cast Iron Steel,
 Silver etc.
What is Babbit?
Babbit is the alloy of tin, lead, copper and
antimony.
List the desirable properties of bearing materials.
 High compressive strength
 Sufficient fatigue strength
 Conformability
 Embed ability
 Bond ability
 Corrosion resistance
 Thermal Conductivity
 Thermal Expansion
What is meant by journal bearing?
A sliding contact bearing that supports load in a radial
direction and there is sliding action along the
circumference of circle is called as circle journal
bearing. It consists of two parts. 1. Shaft. 2. Sleeve
(or) Bearing.
Differentiate between full journal bearing and
partial journal bearing.
In full journal bearing, the Shaft (journal) is
fully covered by bearing where as in partial journal
bearing, the shaft is partly covered by the bearing.
What is Hydro static bearing?
Bearings which can support steady loads
without any relative motion between the journal and
the bearing are called as hydro static (or) externally
pressurized lubricated bearing. This is achieved by
forcing externally pressurized lubricant between the
members.
What are the assumptions made in the theory of
hydrodynamic lubricated bearings?
 The lubricant obeys Newton’s law of viscous
flow.
 The pressure is assumed to be constant
throughout the film thickness.
 The lubricant is assumed to be incompressible.
 The viscosity is assumed to be constant
throughout the film.
 The flow in one dimensional i.e., side leakage
is neglected.
What is lubricant and why is it employed?
Lubricants are used in bearings to reduce
friction between the rubbing surfaces and to carry
away the heat generated by friction. It also protects the
bearing against corrosion.
Specify the types of lubricant with example.
Liquid lubricants - Mineral and synthetic oils.
Semisolid lubricants - Grease,
3. Solid lubricants - Graphite
What are the desirable properties of lubricant?
 Viscosity,
 Oiliness,
 Density,
 Viscosity index,
 Flash point,
 Fire point,

TWO MARKS
 Power point (or) Freezing point.
Define viscosity and Viscosity Index.
Viscosity is the property of fluid which resists the
flow of one layer of fluid from its adjacent layer. It is
defined as force required to resists the layer of unit area
running with unit velocity relative with its adjacent layer,
when these two layers are separated by unit distance.
Viscosity Index is the term used to denote the
degree of variation of viscosity with temperature.
What are the materials for non-metallic Bearing?
 Carbon-graphite,
 rubber,
 Wood and plastics.
List the terms used in journal bearing.
 Diametral clearance,
 Clearance ratio,
 Eccentricity,
 Minimum oil film thickness,
 Attitude (or) eccentricity ratio.
Define Diametral clearance and Diametral
clearance ratio.
 Diametral clearance is the difference between
diameters of bearing and journal.
 Diametral clearance ratio is the ratio of
diametral clearance to the diameter of the
journal.
Define eccentricity and attitude.
 Eccentricity is the radial distance between
centre of the bearing and the displaced centre
of bearing under load.
 Attitude (or) eccentricity ratio is the ratio of the
eccentricity to the radial clearance.
What is long and short bearing.
It the ratio of length to diameter of journal is
less than 1, then it is short bearing, on the other hand,
if l/d is greater than 1 then the bearing is known as
long bearing.
What is meant by square bearing?
When the length of the journal (l) is equal to
the diameter of the journal (d), then the bearing is
called square bearing.
Expand the following: SAE, AFBMA and SKF.
SAE - Society of Automotive Engineers
AFBMA - Anti Friction Bearing Manufacturing
Association
SKF - SKEFKO
Define bearing characteristic number.
The term ZN/P is called as bearing
characteristic number. Where,
Z = Absolute viscosity
N = Speed of journal
P = Bearing pressure.
Define Bearing modulus.
The value of co-efficient of friction varies with
the variation of bearing characteristic number (ZN/P).
The value (ZN/P) for which the value of  is minimum
is identified as bearing modulus.
How lubricant oil is designated?
SAE followed by grade number.
Define Summerfield number.
It is the dimensionless parameter used is design
of journal bearing.
S = (ZN/P) (D/C)2
Define kinematic viscosity
Kinematic viscosity = (Absolute viscosity /
Density)
Define Anti friction bearing.
The contact between the bearing surfaces is
rolling and it has a very low friction, then the bearing
is called as rolling contact bearing (or) Anti friction
bearing.
Specify the materials by which the rolling contact
bearings are made.
High carbon chromium steel.
What are the types of rolling contact bearings.
Based on type of rolling element.
a. Ball bearing b. Roller bearing.
Based on load to be carried.
a. Radial bearing. b. Angular contact
bearing c. thrust bearing.

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What are the components of rolling contact
bearings?
1. Outer race 2. Inner race 3. Rolling
element 4. Cage or Separator
List the factors should be considered when selecting
roller bearing.
 Space availability
 Type and amount of load
 Speed
 Alignment
Enumerate the advantages of rolling contact
bearing over sliding contact bearing.
 Low starting and running friction except at
very high speeds.
 Ability to withstand momentary shock
loads.
 Accuracy of shaft alignment.
 Low cost of maintenance as no lubrication
is required while in service.
 Small overall dimensions.
 Reliability of service.
 Cleanliness
 Easy to mount and erect.
List the disadvantages of rolling contact bearing.
 More noisy at very high speeds.
 Low resistance to shock loading.
 More initial cost.
 Design of bearing housing complicated.
What is nominal life and average life of rolling
contact bearing?
The nominal life of rolling contact bearing is
defined as the number of revolutions which the bearing
is capable of enduring before the first evidence of
fatigue that is developed in the bearing material of
either rings or rolling element. The average life of
bearing is defined as the summation of all bearing
lives in a series of life tests and is divided by the
number of life tests. Usually this average life is
approximately equal to five times the nominal life.
Define basic static load rating.
The basic static load rating is defined as the
static radial load or axial load which corresponds to a
total permanent deformation of the ball and race, at the
most heavily stressed contact equal to 0.0001 times the
ball diameter.
Define Equivalent load.
Equivalent load is defined as that constant
stationary radial or axial load which, if applied to a
bearing with rotating inner ring and stationary outer
ring, would give the same life as that which the
bearing will attain under the actual condition of load
and rotation.
P = (X Fr + Y Fa) S
Where, P = Equivalent load
Define dynamic load rating.
It is defined as the constant stationary radial
load or constant axial load which a group of apparently
identical bearing with stationary outer ring can endure
for a rating life of one million revolutions with only
10% of failure.
How are rolling bearings designated?
According to AFBMA & ISO
– BC –
Bore Dia. Type of bearing Type of duty
According to SKF
SKF _ _ _ _
Last two digits X 5 = bore diameter.
What are modes of failure of rolling contact
bearings?
 Fatigue pitting or spalling of contact
surfaces
 Abrasive wear of rubbing surfaces
 Indenting of working surfaces
 Scoring of working surfaces
 Breakdown of retainers.
Name the assembly methods of rolling elements in
the bearings.
 Eccentric displacement method
 Filling notch method
List the factors contributing to friction in rolling
contact bearing.
 Rolling resistance
 Sliding between rolling elements & race

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 Sliding between rolling elements & cage
 Losses due to churning of lubricant.
State the merits of hydrostatic bearing?
The hydrostatic bearing steady loads without
any relative motion between the journal and the
bearing
Name the type of lubricant used in journal
bearing?
 Graphite
 Grease
 Mineral oil and synthetic oil
What is the application of thrust bearing? [AUT
It is mainly used in turbines and propeller
shafts.
BELT
Name four types of belts.
Flat belts 2.V- belts 3.Ribbed belts 4.Toothed
timing belts
Define creep and slip in belts.
A condition that occurs in flat belt drives that
causes the belt to move forward slightly on the driving
pulley, which causes the driven pulley to rotate at a
slower speed.
A condition that occurs in flat belt drives when
the load causes the belt to slide out of proper position
on the pulley.
What are the materials used for belt drives?
1. Leather 2.fabric and cotton 3.Rubber 4.Balata
5.Nylon
How the ends of flat belt joined?
A device used to join a flat belt into an endless
loop. Common types of belt joints include lacing
and hooks.
What is law of belting?
Law of belting states that the center line of the
belt as it approaches the pulley must lie in a plane
perpendicular to the axis of that pulley or must lie in
the plane of the pulley otherwise the belt will runoff
the pulley.
What is whipping?
It the center distances between the two pulleys are
too long then the belt begins to vibrate in a direction
perpendicular to the direction of motion of the belt.
This phenomenon is called whipping.
Why slip is less in the case of V-belts when
compared with flat belts?
The slip is less due to the wedging action in the
grooved pulley.
How will you determine the number of belts
required in the design of v-belt drives?
No. of belts =Total power transmitted/Power
transmitted per belt
When do you prefer a rope drive?
Large power is transmitted through long
distance (upto150m)
When do you prefer a chain drive to a belt or rope
drive?
Chain drives are preferred for velocity ratio
less than 10, Chain velocities up to 25 m/s and for
power ratings up to 125 KW.
Sketch the cross section of a V-belt and label its
important parts.
What are the five parts of roller chain?
 Pin link, Roller link, Pins, Bushes & Roller
Give the relationship of ratio of tensions in a V-belt
drive.
 T1/T2 = e μα.cosecβ
What is a silent chain? In what situations, silent
chains are preferred?
 Inverted tooth chains are called silent chains
because of their relatively quiet operation.
 They are preferred for high-power, high speed
& smooth operation.

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How the ends of flat belt joined?
 A device used to join a flat belt into an endless
loop. Common types of belt joints include
lacing and hooks.
Why the face of pulley is crowned?
 Crowned pulleys are designed to assist in
tracking belting in conveyor systems. The
center of the pulley has a larger diameter than
the outer edges of the pulley, thus the "crown."
Define maximum tension in a belt.
 Tension on tight side of the belt + Centrifugal
tension
Why tight side of the flat belt should be at the
bottom side of the pulley?
 Because the driving pulley pulls the belt from
bottom side and delivers it to the upper side. So
it is obvious that the bottom side of the belt is
tight.
CHAIN DRIVE
What are the different types of chains?
 Link or welded load chains
 Transmission or roller chains
 Silent or inverted tooth chains
What is chordal action in chain drives?
When chain passes over a sprocket, it moves as a
series of chords instead of a continuous arc as in the
case of a belt drive. It results in varying speed of chain
drive. This phenomenon is known as chordal action.
How can we reduce chordal action?
In order to reduce the variation in chain speed,
the number of teeth of teeth on the sprocket should
be increases.
What are the possible ways by which a chain drive
may fail?
Backsliding, Fatigue, Impact and galling.
What is back sliding in chain drives?
The wear of chain results in the elongation of the
chain. In other words the pitch length is increased.
This makes the chain to ride out on the sprocket teeth
resulting in faulty engagement this is known as
backsliding of chain.
What is galling of roller chains?
Galling is stick slip phenomenon between the
pin and bushing. When the load is heavy and the speed
is high, the high spots (joints) of the contacting
surfaces are welded together .This is galling of roller
chains.
State an advantage and disadvantage of helical
gear.
 Advantage: Produce less noise than spur gears
 Dis Advantage: Subjected to axial thrust loads
Why is tangential component of gear tooth force
called useful component?
 Because it transmits power.
Compare the contact between mating teeth of spur
and helical gears.
 In spur gears the line of contact is parallel to
the axis of rotation. The total length of contact
line is equal to the face width.
 In helical gears the line of contact is diagonal
across the face of the tooth. The total length of
contact line is greater than the face width. This
lowers the unit loading & increases load
carrying capacity.
GEAR DRIVE
What is the advantage of helical gear over spur
gear?
 Helical gears produce less noise than spur
gears.
 Helical gears have a greater load capacity than
equivalent spur gears.
Why is a gear tooth subjected to dynamic loading?
 Inaccuracies of tooth spacing, Irregularities in
tooth profiles, Misalignment between bearings.
What are the commonly used gear tooth profiles?
 Involute & Cycloidal
When do we employ crossed helical gear?
A pair of crossed-helical gears also known as
spiral gears is used to connect and transmit motion
between two non-parallel and non- intersecting shafts.
As the contact between the mating teeth is always a

TWO MARKS
point, these gears are suitable only for transmitting a
small amount of power.
Mention two characteristics of hypoid gear.
They are similar in appearance to spiral-bevel
gears. Their pitch surfaces are hyperboloids rather than
cones. Axis of pinion is offset from the axis of the
gear.
Usually worm is made of hard material and worm
gear is made of softer material – justify.
 A material strength is set so that an amount of
wear of the worm becomes larger that of the
worm wheel.
When is bevel gear preferred?
 They are used to transmit power between two
State the use of bevel gears.
 They are used to transmit power between two
State the advantage of worm gear drive in weight
lifting machine.
The worm gear drives are irreversible. It means
that the motion cannot be transmitted from worm
wheel to the worm. This property of irreversible is
advantageous in load hoisting applications like cranes
and lifts.
Why is the crossed helical gear drive not used for
power transmission?
As the contact between the mating teeth of
crossed helical gears is always a point, these gears are
suitable only for transmitting a small amount of power.
That’s why mostly these gears are not used for power
transmission.
Why is the efficiency of a worm gear drive
comparatively low?
 Because of power loss due to friction caused by
sliding.
Give any two advantages of gear drives.
 It transmits exact velocity ratio.
 It has reliable service
What are the disadvantages of gear drives?
The error in cutting teeth may cause vibrations and
noise. The manufacturing requires special tools and
equipment so it is costlier than other drives.
How will you classify the gears according to the
position of the axes of the shafts?
Parallel, Intersecting, Non parallel and non
intersecting
What is herring bone gears?
The double helical gears are called herringbone
gears.
To balance out the end thrust and to reduce
noise.
What are the forces which affects the spur gears?
Radial force and Tangential force.
What is pitch circle?
It is an imaginary circle which by pure rolling
action would give the same motion as the actual
gear.
What is pressure angle?
It is the angle between the common normal to two
gear teeth at the point of contact and the common
tangent at the pitch point.
Define circular pitch.
The distance measured on the circumference of the
pitch circle from a point of one tooth to the
corresponding point on the next tooth.
Define module.
The ratio between the pitch circle diameters to the
number of teeth on the gear.
Give the relation between module and circular
pitch.
Circular pitch = πD/T=πm
What is backlash in gears?
The difference between the tooth space and tooth
thickness as measured on the pitch circle.
What is law of gearing?
In order to have a constant velocity ratio, The
common normal at the point of contact between a
pair of teeth must always pass through the pitch
point.
What is the disadvantage of involute gears?

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The interference occurs with pinions having less
number of teeth.
Give the merits of cycloidal teeth gears.
This is stronger than involute gears.
The interference does not occur.
What is interference?
The phenomenon when the tip of a tooth undercuts
the root on its mating gear is known as
interference.
What are the materials used for Spur gear
generation?
Metallic gears – Cast iron, steel and bronze.
Nonmetallic-Nylon, wood.
What is the objective of using cast iron as the
material for gear?
Due to its good wearing properties, ease of
producing complicated shapes by casting.
What are the causes of dynamic tooth load on
gears?
Inaccuracies of tooth spacing
Irregularies in tooth profile
Deflection of tooth under load.
What are the causes of gear tooth failure?
Bending failure
Pitting
Scoring
Adhesive wear and
Cohesive wear.
What are the requirements must be met while
designing gears?
Strength of gear teeth, Wear characteristics,
economic material, and possibility of lubrication.
What are the types of helical gears?
Parallel helical, Crossed helical or spiral gears
Why do you prefer helical gears than spur gears?
Less noise and greater load capacity.
What are the applications of helical gears?
Automobiles, Turbines and high speed
applications.
What is the major disadvantage of single helical
gears?
These are subjected to axial thrust load.
Define helix angle.
The angle between the tooth axis and the plane
containing the wheel axis.
Define lead.
The distance advanced by each tooth per
revolution measured along the axis parallel to the
shaft.
Define normal diametral pitch.
The reciprocal of the normal module is called
diametral pitch.
What is transverse pressure angle?
The pressure angle measured in transverse plane is
known as Transverse pressure angle.
Differentiate double helical and herringbone gears.
When the is groove in between the gears the gears
are double helical gears and when there is no
groove between the gears then tat is known as
herring bone gears.
What are spiral or skew gears?
Pair of crossed helical gears is known as spiral
gears.
When do we use spiral gears?
To connect and transmit motion between two non
parallel and non intersecting shafts.
What is bevel gear?
For transmitting power between two intersecting
shafts.
What are the types of bevel gears?
Straight bevel, Spiral bevel, Zerol bevel, Hypoid
gears.
What is straight bevel gear?
If the teeth of the bevel gears are parallel to the
lines generating the pitch cones then they are
called straight bevel gears.
What is Spiral bevel gear?

TWO MARKS
If the teeth of the bevel gears are inclined at an
angle to the face of the bevel then they are called
straight bevel gears.
Define Zerol bevel gears?
Spiral bevel gears with curved teeth but a zero
degree spiral angle is known as Zerol bevel gears.
Define crown gears?
A bevel gear having pitch angle of 900
Define internal bevel gear.
When the pitch angle of a bevel gear exceeds 900
it
is called internal bevel gear.
Define mitre gears?
When the two meshing gears have a shaft angle of
900
and have the same number of teeth, they are
called mitre gears.
What are the forces acting on bevel gear?
Tangential axial and radial forces.
Where do we use worm gears?
Worm gears are used as a speed reducer in
materials handling equipment machine tools and
automobiles.
What is irreversibility in worm gears?
The motion cannot be transmitted from worm
wheel to worm. This is called irreversibility.
How do you specify a pair of worm gears?
It is specified as (z/Z/q/mx)
Where z – No of starts
Z- No of teeth on worm wheel
Q- Diameter factor
mx- Axial module
Why phosphor bronze is widely used for worm
drives?
It has high anti friction properties to resist seizure.
List the main types of failure in worm gear drive.
Seizure, pitting and rupture.
In worm gear drive only the wheel is designed why?
The worm is stronger than wheel, therefore only
the worm wheel is designed.
Why dynamic loading is is rarely considered in the
design of worm gear drives?
Due to the sliding action between the worm and
worm wheel.
What are the various losses in the worm gear?
 Losses due to friction in sliding
 Losses due to churning and splashing of
lubricating oil.
In which gear drive, self locking is available?
A self-locking gear cannot be driven by the load in
the opposite direction from its intended direction
of rotation.
When worm gearing is self-locking or irreversible,
this means that the worm gear cannot drive the
worm.
It is usually impractical to design irreversible
worm gearing with any security.
GEAR BOX
What are the points to be considered while
designing a sliding mesh type of multi-speed gear
box?
 i) The transmission ratio in a gear box is
limited by 1/4 < i < 2
 ii) Speed ratio of any stage should not be
greater than 8.
Which type of gear is used in constant mesh gear
box? Justify.
 Helical gears are used in constant mesh gear
boxes to provide quieter and smooth operation.
Compare sliding mesh and synchromesh gear box.
 Sliding mesh gear box: It derives its name
from the fact that the meshing of the gears take
place by sliding of gears on each other. With
sliding mesh gear box, double de-clutching is
necessary to bring the two sets of dog teeth to
the same speed so that they can be slid into
engagement quietly.
 Synchromesh gear box: To eliminate the need
to de-clutch, the synchromesh gear box was
introduced. The basic gear box is laid out in the
same manner as the constant mesh, but with the
addition of a cone clutch fitted between the dog
and gear members.
Where is multi-speed gear boxes employed?

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