•Gear & wheel
•Advantages & disadvantages of gear.
•Types of gear
Gear are used to transmit motion from one
shaft to another or between a shaft and a slide .
Gears use no intermediate link .
It transmit motion by direct contact .
Two bodies have either a rolling or a
sliding motion along the tangent .
No motion is possible
common normal .
If power transmitted between two shaft is
small, motion between them may be
obtained by using two pin cylinder or disk
1 and 2 as shown in figure given below .
As the power transmitted increases, slip
occurs between the discs and the motion
no longer remains definite .
To remove slipping the wheel provided
A toothed wheels is called gears
The motion of gears is equivalent to the
friction wheels .
transmit exact velocity ratio .
may be used to transmit large powers
has high efficiency.
has reliable service .
has compact layout
The manufacture of gears require
special tools and equipment
The error in cutting teeth may
cause vibration and noise during
Teeth is parallel to axis of
Transmit power from one
shaft to another parallel
Used in Electric
sprinkler, windup alarm
clock, washing machine
and clothes dryer
An external gear is one with
the teeth formed on the outer
surface of a cylinder or cone.
Conversely, an internal gear is
one with the teeth formed on the
inner surface of a cylinder or
The teeth on helical gears are cut at an angle to the
face of the gear
This gradual engagement makes helical gears
operate much more smoothly and quietly than spur
One interesting thing about helical gears is that if
the angles of the gear teeth are correct, they can be
mounted on perpendicular shafts, adjusting the
rotation angle by 90 degrees
A rack and pinion is a type of linear
actuator that comprises a pair
of gears which convert rotational motion
into linear motion. A circular gear called
the pinion engages teeth on a linear
"gear" bar called "the rack“ .
rotational motion applied to the pinion
causes the rack to move linearly, thereby
translating the rotational motion of the
pinion into the linear motion of the rack.
is Special case of a
spur gear where it is
made of infinite
diameter so that the
pitch surface is a
It is used in a lathe
in which the rack
transmits motion to
the saddle .
A double helical gear is
equivalent to a pair of helical
gears secured together, one
having a right hand helix and
the other a left hand helix
The teeth of the two rows are
separated by a groove used
for tool run out.
Axial thrust of the two rows
of teeth cancel each other .
These can be run at high speeds with
less noise and vibration
If the left and right inclinations of a
double helical gear meet at a common
apex and there is no groove in between,
the gear is known as herringbone gears.
Bevel gears are useful when the direction of a
shaft's rotation needs to be changed
They are usually mounted on shafts that are 90
degrees apart, but can be designed to work at other
angles as well
The teeth on bevel gears can be straight, spiral or
applications, automobiles, printing presses, cooling
towers, power plants, steel plants, railway track
inspection machines, etc.
teeth are straight, radial to the point
of intersection of the shaft axes and vary in
cross section throughout their length .
Miter gears are mating bevel gears with
equal numbers of teeth and with axes at
Use – roller shutter door.
spiral bevel gear is
gear with helical teeth .
There exists an axial
bevel gears –spiral
bevel gears with curved
teeth but with a zero
degree spiral angle .
Gears are the most common means of
transmitting power in mechanical engineering.
Gear or wheel. The larger of two interacting
Pinion. The smaller gear in a pair.
Base circle : It is a circle from which involute form is
generated. Only the base circle on a gear is fixed and
Base pitch (pb): It is the distance from one face of a
tooth to the corresponding face of an adjacent tooth on
the same gear, measured along the base circle.
Sometimes called the 'normal pitch'.
Pitch is the distance between a
point on one tooth and the
corresponding point on an
Pitch circle. A circle, centered on
and perpendicular to the axis, and
passing through the pitch point.
Sometimes also called the 'pitch
line', although it is a circle.
Pitch diameter (D). Diameter of
a pitch circle. The nominal gear
size is usually the pitch diameter.
Circular pitch (p). The distance from one face of a
tooth to the corresponding face of an adjacent tooth
on the same gear, measured along the pitch circle.
Diametral pitch (Pd). The ratio of the number of
teeth to the pitch diameter.
Module (m). The module of a gear is equal to the
pitch diameter divided by the number of teeth.
Addendum : The radial distance from the pitch
surface to the outermost point of the tooth.
Dedendum : The radial distance from the depth of
the tooth to the pitch surface.
A point of contact is any point at which two tooth profiles touch
The pitch point is the point of tangency of two pitch circles and
is on the line of centers.
The line of action is
also called path of
It is the straight line
passing through the
pitch point and tangent
to both base circles.
Bottom land is the
surface at the bottom
of a gear tooth space
adjoining the fillet.
Top land is the
surface of the top of a
A gear train is two or more gear working together
by meshing their teeth and turning each other in a
system to generate power and speed
It reduces speed and increases torque
Electric motors are used with the gear systems to
reduce the speed and increase the torque
Simple gear train
Compound gear train
Reverted gear train
Planetary or epicyclic gear train
Simple Gear Train
The most common of the gear train is the gear pair
connecting parallel shafts. The teeth of this type can
be spur, helical or herringbone.
Only one gear may rotate about a single shaft axis.
d arrangement is
Two or more gears
may rotate about a
the axes of the first and the last wheels
of a compound gear coincide, it is called a
reverted gear train .
an arrangement is used in clocks
and in simple lathes where back gear is
used to give a slow speed to the chuck.
A gear train having a relative motion
of axes is called a planetary or an
epicyclic gear train
Important application of epicyclic
gears are in transmission, computing
devices, wrists watches, hoists, back
gear of lathe etc.