2. Gears are used to transmit the
motion from one shaft to another
by the direct contact without
using any connector link.
Here the surfaces of the body
make the tangential contact and
the bodies will have either a
rolling or sliding motion along a
tangent and no motion is possible
along the common normal
because either that will break the
contact and one body will tend to
penetrate into the other.
4. SPUR GEARS
Parallel and coplanar shafts connected
by spur gears .
Spur gears have straight teeth which are
parallel to the axis of the teeth.
Simple in design, economy of
manufacture,maintainence and absence
of end thrust.
Slow speed
At the time of engagement the contact
extends along the entire width along a
line parallel to axis which results in the
sudden application of the load , high
impact stresses and excessive noise at
the high speed.
5. Helical Gears
Helical gears have teeth inclined to the axis of the shafts in the form of
helix.
They can take higher loads than similar types of spur gears. The motion
of the helical gears is smoother and quieter than motion of spur gears.
The angle of helix on both the gears should be equal in magnitude but
opposite in direction.
Right hand pinion meshes with left hand gear.
They are generally high speed gears.
At the beginning of engagement,contact occurs only at the point of
leading edge of the curved teeth. As the gears rotate,the contact
extendsalong a diagonal line across the teeth. Thus, the load application
is gradual which results in low impact stresses at higher velocity than the
spur gears and have greater load carrying capacity and reduction in
noise.
6. Herringbone Gear
Herringbone Gear resemble two helical gears
placed side by side.
Disadvantage is that they can not be cut with
simple gear hobbing machine.
7. Straight Bevel Gears
The teeth are straight and radial to the point of intersection and vary
in their cross section throughout the length.
They are used to connect the shafts at right angles at low speeds.
Gears of the same size connecting the shafts at right angles are
called as mitre gears.
8. Spiral Bevel Gears
When the teeth of the bevel are inclined to the
face at an angle then they are called as spiral
bevel gears.
they are smoother in action as there is gradual
load application and low impact stresses.
They are used to the drive to the differential of
the automobile.
9. Worm Gears
They are used to transmit the
power at 90 degree and where
the high reductions are required.
In worm gears one gear has
screw threads. Due to this worm
gears are quiet , vibration free,
give smooth output.
Worm gears and shafts are
almost at right angles to each
other.
10. Rack and Pinion
A rack is a toothed bar or rod that can
be thought of as a sector gear with an
infinitely large radius of curvature.
Pinion is s spur gear.
Torque is converted to linear force by
meshing a rack with pinion. when
pinion turns ; rack moves in the
straight line.
Such a mechanism is used in
automobiles to convert the rotation of
the steering wheel into the left to
right motion of the rod.
15. Nomenclature
Pitch Cylinders:
Pitch Circles:
Pitch Diameter
Pitch Surface
Pitch Point
Line of centers
Pinion
Rack
Pitch Line
16. Nomenclature
Circular Pitch:
Diametral Pitch: ratio of the number of teeth to the length of the
pitch circle diameter.
Module: It is the ratio of the pitch diameter in mm to number of
teeth.
Gear ratio: Ratio of the number of teeth on gear to pinion
Velocity ratio:
Ratio of the angular velocity of the follower to the angular velocity
of the driving gear.
17. Nomenclature
Addendum Circle:
Addendum: radial height of tooth above the pitch circle. Standard
value is one module.
Dedendum/root circle:
Circle passing through the roots of the teeth.
Dedendum: radial depth of the teeth below the pitch circle.
Clearance: radial distance between the addendum and dedendum
of the mating teeth.
Addendum= PCD+2m
18. Nomenclature
Full depth of the teeth: Addendum+ Dedendum
Working Depth of teeth: The maximum depth to which the tooth
penetrates into the tooth space of the mating gear is the working depth
of the gear. It is the sum of the addendums of the two gears.
Space width: it is the width of the tooth space along the pitch circle.
Tooth Thickness: It is the thickness of the tooth measured along the
pitch circle.
19. Nomenclature
Backlash:
It is the difference between the space width and
the tooth thickness along the pitch circle.
Face width:
Length of tooth parallel to gear axis .
Top land: Surface of the top of the tooth.
Bottom land: Surface pf the bottom of the tooth between the adjacent
fillets.
Face and Flank:
25. Law of Gearing
It is the condition which must be fulfilled by the gear tooth
profiles to maintain the constant angular velocity ratio
between the gears.
