1. INTRODUCTI
ON TO GEAR’S:
Gear, toothed wheel or cylinder used to transmit rotary or reciprocating
motion from one part of a machine to another. Two or more gears, transmitting
motion from one shaft to another, constitute a gear train. At one time various
mechanisms were collectively called gearing. Now, however, the word gearing is used
only to describe systems of wheels or cylinders with meshing teeth. Gearing is chiefly
used to transmit rotating motion, but can, with suitably designed gears and flat-
toothed sectors, be employed to transform reciprocating motion into rotating motion,
and vice versa.
THERE ARE BASICALLY TWO TYPES OF GEARS IN GENERAL
USAGE.
Simple Gears
The simplest gear is the spur gear, a wheel with teeth cut across its edge
parallel to the axis. Spur gears transmit rotating motion between two shafts or other
parts with parallel axes. In simple spur gearing, the driven shaft revolves in the
opposite direction to the driving shaft. If rotation in the same direction is desired, an
idler gear is placed between the driving gear and the driven gear. The idler revolves in
the opposite direction to the driving gear and therefore turns the driven gear in the
same direction as the driving gear. In any form of gearing the speed of the driven
shaft depends on the number of teeth in each gear. A gear with 10 teeth driving a gear
with 20 teeth will revolve twice as fast as the gear it is driving, and a 20-tooth gear
driving a 10-tooth gear will revolve at half the speed. By using a train of several
gears, the ratio of driving to driven speed may be varied within wide limits. Internal,
or annular, gears are variations of the spur gear in which the teeth are cut on the inside
of a ring or flanged wheel rather than on the outside. Internal gears usually drive or
are driven by a pinion, a small gear with few teeth. A rack, a flat, toothed bar that
moves in a straight line, operates like a gear wheel with an infinite radius and can be
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2. used to transform the rotation of a pinion to reciprocating motion, or vice versa. Bevel
gears are employed to transmit rotation between shafts that do not have parallel axes.
These gears have cone-shaped bodies and straight teeth. When the angle between the
rotating shafts is 90°, the bevel gears used are called miter gears.
Helical Gears
These have teeth that are not parallel to the axis of the shaft but are
spiraled around the shaft in the form of a helix. Such gears are suitable for heavy
loads because the gear teeth come together at an acute angle rather than at 90° as in
spur gearing. Simple helical gearing has the disadvantage of producing a thrust that
tends to move the gears along their respective shafts. This thrust can be avoided by
using double helical, or herringbone, gears, which have V-shaped teeth composed of
half a right-handed helical tooth and half a left-handed helical tooth. Hypoid gears are
helical bevel gears employed when the axes of the two shafts are perpendicular but do
not intersect. One of the most common uses of hypoid gearing is to connect the drive
shaft and the rear axle in automobiles. Helical gearing used to transmit rotation
between shafts that are not parallel is often incorrectly called spiral gearing.
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3. Another variation of helical gearing is provided by the worm gear, also
called the screw gear. A worm gear is a long, thin cylinder that has one or more
continuous helical teeth that mesh with a helical gear. Worm gears differ from helical
gears in that the teeth of the worm slide across the teeth of the driven gear instead of
exerting a direct rolling pressure. Worm gears are used chiefly to transmit rotation,
with a large reduction in speed, from one shaft to another at a 90° angle.
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4. GEAR NOMENCLATURE:
Pitch circle-It is an imaginary circle which by pure rolling action, would give the
same motion as the actual gear.
Pitch circle diameter-It is the diameter o the pitch circle. The size of the gear is
usually specified by the pitch circle diameter. It is also called the pitch diameter.
Pitch point-It is common point of contact between two pitch circles.
Pitch surface-It is the surface of the rolling discs which the meshing gears have
replaced at the pitch circle.
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 to the pitch point. It is
usually denoted by . The standard pressure angles are 141/2ɸ O
and 20O
.
Addendum-It is the radial distance of a tooth from the pitch circle to the top of the
tooth.
Dedendum-It is the radial distance of a tooth from the pitch circle to the bottom of
the tooth.
Addendum circle-It is the circle drawn through the top of the teeth and is concentric
with the pitch circle.
Dedendum circle-It is the circle drawn through the bottom of the teeth. It is also
called root circle.
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.
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6. BLANK PREPARATION
MATERIAL TO BE USED
The material used for blank preparation should have following properties
1) The material should have high strength to weight ratio.
2) High yield strength and hardness
3) Simple, uniform section
4) Smoothness
5) Ductility
6) Fatigue
7) The material should have low cost.
A variety of cast iron, powder-metallurgy materials, nonferrous alloys, nonmetallic
materials are used in gears.
The blank is prepared by general metal forming process. The processes are
1) Forging
2) Rolling
3) Metal Extrusion Process
FORGING
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7. Forging, process of shaping iron and other malleable metals by hammering or
pressing them after making them plastic by application of heat. Forging techniques
are useful in the working of metal because the metal can be given the desired form,
and the process improves the structure of the metal, particularly by refining the grain
size of the metal.
Forged metal is stronger and more ductile than cast metal and exhibits greater
resistance to fatigue and impact.
The metal is heated above recrystallization temperature but below melting point.
Then it is forged by using any of the three types of hammer . After preparation of
blank stress relieving tempering is done. `The last stage of bank preparation is
inspection .
As after forging the surface finish is not up to the mark so the forged blank has
to be machined well. So that the required surface finish is obtained.
ROLLING
Cylindrical blanks are prepared by roll forming. In this type three rollers are arranged
and material is place in between them. So that a blank of required surface finish is
obtained .
EXTRUSION
Extrusion is a process of forcing substances, especially metals or thermoplastics,
through a die to produce various shapes of uniform cross section widely used in
industry and constructions. Hot extrusion being more common than cold extrusion.
The manufacturing of cylindrical hollow blanks are done by such extrusion process.
GEAR CUTTING AND TOOTH HOBBING
Three methods of cutting gear are
• Forming
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8. • Form cutting
• Generation
BASIC CHARECTERISTICS OF GEAR CUTTING MACHINES
Any gear cutting machine which works by any one of these methods must have
o A cutting tool of suitable shape, hardness and sharpness
o A means for properly holding the work piece and correctly aligning it with the
cutting tool, and
o A precision indexing device for accurately spacing the gear teethes.
o An accurately controlled means of providing relative role between the work and the
tool during cutting.
1) FORMING
In the forming method the gear tooth takes its shape directly from the shape of
the cutting tool. It is the oldest of the gear cutting methods yet is still widely and
efficiently used. Many spur gears-especially those with involute teeth profiles are cut
with brown and sharp forming milling cutters, which first were patented in 1864.
The accuracy of the form tooth profile depends upon the accuracy with which
the cutter was made is of the finish cutting edges must be identical to every other
The Gleason FORMATE of helix form gear finishing methods are forming
methods
2) FORM CUTTING
The Gleason straight bevel gear planner, introduced in the year 1875, used the
form cutting method. The reciprocating planning tool cut with its rounded point only,
as the tool slide arm traveled along a master former to trace out the desired tool
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9. profile curvature. The accuracy of the profile depends up on the skill and accuracy
with which the master template was made.
By today’s standard form cutting is the slow and cumbersome method. In 1875,
however, it represented tremendous improvement in the speed and quality of
production over the existing practice of hand file finishing cast gear teeth. Form
cutting is still used today for some larger group of 1 DP an coarser, although no new
Gleason machine of this type have been built since about 1940.
The surface of form cut teeth are composed of adjacent groups and ridges left by
the grounded point of the cutting tool. These surfaces are not as smooth as generated
surfaces, which are composed of adjacent flats. Only straight bevel gears are formed
by form cutting.
3) GENERATION
The generation of the gear tooth profile requires accurately controlled relative
role between the utter and the work. In the Gleason machine, relative role is provided
by ratio of roll gears. A generated profile shape does not depend on the shape of the
generating tool’s cutting edges. Since this is so, the cutting edges of any shape and it
is most convenient and least expensive to make them straight.
A generated tooth surface is composed of a series of adjacent flats, which can be
placed as close together as desired to control the finish of the surface. Naturally, the
closer and narrower the flats are, the smoother the finish will be.
TOOTH HOBBING
A gear-cutting worm made into a gear generating tool by a series of
longitudinal slots or gashes machined into it to form the cutting teeth
Available with one thread, two thread, or more
It can produce a variety of gears at high rate with good dimensional accuracy
Extensively used in the industry
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10. Hobbing is perhaps the most widely used method of cutting spur and helical gears.
The hob is a cutting tool, cylindrical is shape, with the teeth wrapped in a spiral
around the cylinder. The hob looks like a worm gear with axial slots to provide
cutting surfaces and with the teeth relieved back from the cutting edges to provide
cutting clearance. An axial section of a hob can be thought of as a rack.
Because the involute curve decrease as the gear size increases, the involute curve on a
rack has decreased to zero and the hob tooth, along he pressure angle, is essentially a
straight line. A gear to be cut is rotated about its own axis and therefore the hob tooth
will produce the proper involute regardless of the size of the gear to be cut. The hob is
rotated about its own axis in a timed relationship to the work piece so that, for a single
lead hob, one rotation of the hob equals a moment of one pitch of the gear.
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12. Case hardening is the change of surface composition under the
combined action of Chemical and heat thermal treatments. E.g. carburization,
nitriding, cyaniding, carbunitiding etc.
Carburization of steel involves a heat treatment o the metallic surface
using a source of carbon. Early carburization used a direct application of a charcoal
packed into the metal (initially referred to as case hardening), but modern techniques
apply carbon bearing gases or plasma (such as carbon dioxide or methane). The
process depends primarily upon ambient gas composition and furnace temperature,
which must be carefully controlled, as the heat may also impact the microstructure of
the rest of the material. The application where great control of over gas composition is
desired carburization may take place in a very low pressure in a vacuum chamber.
The carbon content of case hardening steel is low, usually about 0.15
to 0.20 %. Case hardening steel also contains Ni, Cr, Mo, Mn, etc. It is suitable for
carburizing and quenching.
For such hardening the process followed is as follows
CARBURIZINGQUENCHINGCLEANING
TEMPERINGSHOT BLASTINSPECTION.
Case hardened steel is usually formed by diffusion of carbon
(carburizing) into the outer layer of the steel at high temperature. And putting carbon
into the surface of steel makes it high-carbon steel like S45C, which can be hardened
by heat treatment.
Surface hardness is about 55~60 HRC.
Depth of surface hardening is about 1.0 mm.
Carburizing and quenching produces a hard, wear resistant
surface over a strong tough core. Some special purpose steel gears are
case hardened by either carbo-nitriding or nitriding. Other special purpose
gears, such as those used in chemical or food processing equipment, are
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13. made of stainless steel or nickel based alloy or carburized because of their
corrosion resistance, their ability to satisfy sanitary standards or both.
WORK MATERIAL PROPERTIES
EFFECTS OF CARBURIZING
Mechanical 1)Increased surface hardness. 2)Increased
wear resistance. 3)Increased
fatigue/tensile strength.
Physical 1)Grain growth may occur 2)Change in
volume may occur
Chemical 1)Increased surface carbon content.
CONVENTIONAL MACHINING
TURNING
This operation involves grabbing or holding the job or work piece on the lathe
or turning machine and rotating it at a very high speed and removing material from its
surface (may it be internal or exterior) using a tool. This is a very extensive method
used rigorously during gear preparation. Starting from blank preparation to thread
formation etc. this method is used.
MILLING
Milling is a form-cutting process limited to making single gears for prototype or very
small batches of gears as it is a very slow and uneconomical method of production.
An involute form-milling cutter, which has the profile of the space between the gears,
is used to remove the material between the teeth from the gear blank on a horizontal
milling machine. The depth of cut into the gear blank depends on the cutter strength,
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14. set-up rigidity and machineability of the gear blank material.
GEAR PLANING OR RACK GENERATION
This is used for mid volume production. A rack, which may be considered to
be a gear of infinite radius, is used as the cutter. It is constructed of hardened steel
with cutting edges round the teeth boundaries. The rack which is given a reciprocal
lateral motion equal to the pitch line velocity of the gear is slowly fed to the slowly
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15. rotating gear blank. In this way, the material between the teeth is removed and the
involute teeth are generated.
GEAR SHAPING
The cutter is a circular pinion-shaped cutter with the necessary rake angles
to cut as shown. Both the gear blank and cutter are set in a vertical plane and rotated
such as that the two are like gears in mesh. Gear shaping is faster than gear planing
because the cutting process is continuous and the cutter does not have to be stepped
back.
Shaping is practically the only method available for cutting teeth close to a
shoulder and is the only available method for generating internal spur gears. The
fellows process has been widely accepted since it development in 1900 and still
commonly used despite recently increased competition from hobbing, shaving and
grinding techniques. The Fellows gear shaper can be specially equipped to cut helical
gears, helical or spur racks and shaft ends with splined or solid keys
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16. Fig-Gear shaping
CYLINDRICAL GRINDING
The cylindrical grinder is a type of grinding machine used to shape the
outside of an object. The cylindrical grinder can work on a variety of shapes; however
the object must have a central axis of rotation. This includes but is not limited to such
shapes as a cylinder, an ellipse, a cam, or a crankshaft.
Basically 5 kinds of cylindrical grinding operations are carried out during
gear machining.
Outside diameter grinding-OD grinding is grinding occurring on
external surface of an object between the centers. The centers are end units
with a point that allow the object to be rotated. The grinding wheel is also
being rotated in the same direction when it comes in contact with the object.
This effectively means the two surfaces will be moving opposite directions
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17. when contact is made which allows for a smoother operation and less chance
of a jam up.
Inside diameter grinding-ID grinding is grinding occurring on the
inside of an object. The grinding wheel is always smaller than the width of the
object. The object is held in place by a collet, which also rotates the object in
place. Just as with OD grinding, the grinding wheel and the object rotated in
opposite directions giving reversed direction contact of the two surfaces where
the grinding occurs
Plunge grinding-A form of OD grinding, however the major difference is
that the grinding wheel makes continuous contact with a single point of the
object instead of traversing the object
Creep feed grinding-Creep Feed is a form of grinding where a full
depth of cut is removed in a single pass of the wheel. Successful operation of
this technique can reduce manufacturing time by 50%, but often the grinding
machine being used must be designed specifically for this purpose. This form
occurs in both cylindrical and surface grinding.
Centerless grinding-It is a form of grinding where there is no collet or
pair of centers holding the object in place. Instead, there is a regulating wheel
positioned on the opposite side of the object to the grinding wheel. A work
rest keeps the object at the appropriate height but has no bearing on its rotary
speed. The work blade is angled slightly towards the regulating wheel, with
the work piece centerline above the centerlines of the regulating and grinding
wheel; this means that high spots do not tend to generate corresponding
opposite low spots, and hence the roundness of parts can be improved.
Centerless grinding is much easier to combine with automatic loading
procedures than centered grinding; through feed grinding, where the regulating
wheel is held at a slight angle to the part so that there is a force feeding the
part through the grinder, is particularly efficient.
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18. Fig. Cylindrical grinding process and machine.
HARDENING
Martensite is a very hard phase(VHNFe3C=800, VHNMart= 880). It can be produced
only if the transformation of austenite to mixture of ferrite and carbide is avoided. In
most of the cases it is possible by faster cooling (quenching) of the steel.
Hardening consists of heating the steel to proper austenitising temperature, soaking
of this temperature to get fine–grained and homogenous austenite, and then cooling
the steel at a rate faster than its critical cooling rate. Such cooling is called quenching.
Normally, carbon steels are quenched in water, alloy steels in oil (as critical cooling
rate of alloy steels is much less), etc.
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19. OBJECTIVE OF HARDENING
Hardening is done to all tools, heavy – duty carbon steel machine parts and almost all
machine parts made of alloy steels.
1. Main aim of hardening tools is to induce high hardness. The cutting property
of the tool is directly proportional to the hardness of the steel.
2. Many machine parts and all tools are also hardened to achieve high wear
resistance. Higher is the hardness, higher is the wear and abrasion resistance.
For example, spindles, gears, shafts, cams, etc.
3. Develop high yield strength with good toughness and ductility, so that higher
working stresses are allowed.
DEFECTS IN HARDENED PRODUCTS
1. Mechanical properties not up to the specification
2. Soft spot
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20. 3. Quench crack
4. Distortion and warpage
5. Change in dimensions
6. Oxidation and decarburization
7. Overheating
GRINDING OF BEARING SETTING DIAMETER
The gear has its primary purpose to transmit power from one source to another.
During this process the gears are required to be held by a bearing which holds it to be
allowed to be rotated.
The accuracy required here is very high because the entire mechanism of transmission
of rotation rests on the bearing. Hence it is maintained in microns. The bearing setting
is formed by the method of grinding.
TEMPERING
DEFINITION
Heating the steel to a tempering up to lower critical, soaking followed by slow
cooling. Temperature of tempering is decided by final required hardness and type of
steel.
ADVANTAGE OVER QUENCHING
As quenched steels have very limited applications due to the following reasons:
1. Martensite, generally a hard phase but very brittle
2. Possesses high internal stresses, relieve of internal stresses during use may
develop distortion and cracking
3. Martensite and retained austenite both unstable, decompose in to stable
phases, dimensional instability
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21. Tempering is done to address these limitations of as quenched steels.
OBJECTIVE
1. To relieve internal stresses
2. To restore ductility and toughness, however there is loss of strength
3. To stabilize dimension
4. To improve magnetic properties, as austenite is non-magnetic
STRUCTURE OF AS QUENCHED AND TEMPERED
STEEL
As quenched: Martensite (lath or twinned), Retained Austenite, Cementite and
undissolved alloy carbides
Tempered steel: Carbides (iron and alloy) embedded in a matrix of ferrite.
STAGES OF TEMPERING
Tempering involves heating. This allows diffusion of carbon and other elements as a
result changes in structure. This occurs in four district but overlapping stages:
1. First stage (up to 200C)- C atoms diffuse out from Mart., loss of tetragonality.
Precipitation of ε-carbide (Fe2.4C)
2. Second stage (200-300C)- Transformation of R Austenite (loss of C) to
Bainite/ Martensite
3. Third stage(200-350C)- Complete loss of tetragonality of Martensite,
Dissolution of ε-carbide, formation of rods or plate of Martensite.
4. Fourth stage (350-700C) - Sherardizing and coarsening of cementite and
recrystallisation of ferrite.
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22. 5. Secondary Hardening or fifth stage of tempering. Tempering involves heating.
This allows diffusion of carbon and other elements as a result changes in
structure. This occurs in four district but overlapping stages:
1. First stage (up to 200C)- C atoms diffuse out from Mart., loss of tetragonality.
Precipitation of ε-carbide (Fe2.4C)
2. Second stage (200-300C)- Transformation of R Austenite (loss of C) to
Bainite/ Martensite
3. Third stage(200-350C)- Complete loss of tetragonality of Martensite,
Dissolution of ε-carbide, formation of rods or plate of Martensite.
4. Fourth stage (350-700C)- Sherardizing and coarsening of cementite and
recrystallisation of ferrite.
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23. Fig. Effect of tempering on 0.45% carbon and 0.1%
carbon steel
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24. INSPECTION:
Inspection is a very crucial and mandatory procedure carried out during gear
manufacturing. It is done at almost all levels or steps of gear production to ensure the
quality of gear is up to the mark. Along with the final gear inspection, there are
inspections or checking carried out at every alternate step like case carburizing,
hardening etc.
There are various ways of inspections done.
Magnetic inspections- Here the job is magnetically charged and powdered
metal is poured over it. The powdered metal flows over the surface of the
product and penetrates over the crack. Thereby the cracks are identified. Suck
type of methods are useful in identifying cracks at a laminar or surface profile.
Dye checking- It is a very important method for understanding the points of
gear engagement and pressure which is applied to the teethes. Here we have a
gear for testing which is covered in a dye. The product to be tested in engaged
to gear in a condition similar to that of the working condition. The engagement
of teethes shows the points or area of contact and the pressure distribution. If
any fault is found during contact or the colour is not transmitted then the error
is found.
Visual- It is one of the basic ways of inspecting the product. Simply by visual
checking of the gear the major discrepancies of the gear can be sought out and
relieved. It is done after every process to ensure the process is going in a right
direction.
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25. CONLUSION:
Gears are a very vital part in an aerospace engine as well as for others
products. And the understanding of its manufacturing process gives us an intricate
idea about the care taken in producing a different gear. It is though a production of a
small part, but it still involves a great deal of process, a lot of care and detailed
accuracy. To be able to sustain the adverse conditions is a very challenging task, and
the gear has to be made to face it.
This project makes the reader to be able to differentiate between an
automobile and an different manufacturing process. The various steps involved in
each gear making helps to understand the features of such gears. If the reader finds
this report to be relevant for understanding the manufacturing process of gears then
the report can be deemed to be successful.
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