This document summarizes different types of gears and their characteristics. It discusses spur gears, helical gears, bevel gears, worm gears, and gear teeth profiles such as cycloidal and involute. Some key advantages and applications are outlined for each gear type. Spur gears provide an exact velocity ratio but are noisy, while helical gears operate more smoothly. Bevel gears can transmit power between perpendicular shafts. Worm gears provide high gear ratios in a compact space and can self-lock. Involute gear teeth profiles allow variable center distances while cycloidal teeth are stronger.

1. Theory of Machine

2. Positive drive Positive drive

3. Advantages
1.It transmits exact velocity ratio.
2.It may be used to transmit large power.
3.It has high efficiency.
4.It has reliable service.
5.It has compact layout.
Disadvantages
1.The manufacture of gears require special tools and
equipment.
2.The error in cutting teeth may cause vibrations and
noise during operation.

4.  Gears are toothed wheel which transmit the power and motion from one
shaft to another by means of successive engagement of teeth.
 Shaft are parallel to each other, two types of gears are mounted on the shaft to
transmit the power : Spur Gear, Helical Gear.
 Shafts are non intersecting and non parallel and non coplanar then power
transmitting gears are: Spiral Gear, Worm and Worm Wheel

5. Gear Terminology:

6. Spur gears are cylindrical and have straight
teeth, and are mounted on parallel shafts.
Used to transmit the rotary motion between
two shaft which are parallel.
It is used at once to create very large gear
reductions
Larger gear: Driven gear- Spur gear / wheel.
Smaller gear: Driver Gear: Pinion.
Motion of the two gears are always opposite to
each other.
SPUR
GEARS

8. (i) Easy to manufacture.
(ii) Simple and Cheaper among all types of gears.
(iii)The spur gears do not create axial load or thrust on the
shaft and the bearings.
(iv) The spur gears are interchangeable.

9. (i)The contact between the two meshing teeth is sudden
line contact. Due to sudden line contact, the
operation of spur gear pair is noisy.
(ii)The spur gears are not suitable for high speed
applications.
(iii)The power transmitting capacity of spur gear pair is
lower than the helical gear pair.

10. (i) The spur gears are used in automobile gear boxes, machine
tool gear boxes, industrial gear boxes, etc.
(ii)The spur gears are mainly used for low speed applications.

11.  The teeth on helical gears are cut at
an angle to the face of the gear.
When two teeth on a helical gear
system engage, the contact starts at
one end of the tooth and
gradually spreads as the gears
rotate, until the two teeth are in full
engagement.

12. This gradual engagement makes helical gears operate
much more smoothly and quietly than spur gears. For
this reason, helical gears are used in almost all car
Transmission
Because of the angle of the teeth on helical gears, they
create a thrust load on the gear when they mesh.
Devices that use helical gears have bearings that can
support this thrust load.
Helical Gear:

13. Fig. Helical Gear:

14.  Advantages of helical gears:
(i)The engagement is gradual, and hence the operation
is smooth and silent.
(ii)Suitable for high speed application.
(iii)The power transmitting capacity of helical gear pair
is higher than that of identical spur gear pair.
(iv)The efficiency of helical gear pair is high (96% to
99%).

15.  Limitations of helical gears:
(i)The helical gears are difficult to manufacture as
compared to spur gears.
(ii)The helical gears are costlier than the spur gears
(iii)The helical gears create axial load or thrust on the
shaft and the bearings.

16. Applications of helical gears
(i)The helical gears are used in high speed stages of
automobile gear boxes, machine tool gear boxes,
industrial gear boxes, etc.
(ii)The helical gears are used in high speed application like
steam and gas turbines.

17. 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 hypoid.
Straight bevel gear teeth actually have the same problem as
straight spur gear teeth -- as each tooth engages, it impacts the
corresponding tooth all at once.

18. Spiral Bevel Gear
These spiral teeth engage just like helical teeth
: the contact starts at one end of gear and
progressively spreads across the whole teeth

19. Advantages of bevel gears:
(i) The bevel gears can transmit the power/motion
between two shafts which are at right angles.
(ii) The efficiency of bevel gear pair is high (95% to
98%).
(iii) The arrangement is very compact.

20. Limitations of bevel gears
(i) The bevel gears are difficult to manufacture.
(ii)The cost of bevel gears is higher than spur and helical
gears.
(iii)The bevel gears are non-interchangeable
Applications of bevel gears
(i)The bevel gears are used in differential gear box of
automobiles.
(ii)They are also used in industrial gear boxes where the
input and output shafts are required at right angles.

21.  Many worm gears have an interesting
property that no other gear set has: the
worm can easily turn the gear, but the gear
cannot turn the worm. This is because the
angle on the worm is so shallow that when
the gear tries to spin it, the friction
between the gear and the worm holds the
worm in place.
 This feature is useful for machines such as
conveyor systems, in which the locking
feature can act as a brake for the conveyor
when the motor is not turning
 Worm gears:

22. Advantages of worm gears
(i) The worm gear drives are compact as
compared to spur, helical, or bevel gear
drives.
(ii) They can be used for high reduction
ratios, as high as 70:1 in small space
(iii)The operation of worm gear drive is
smooth
and silent.
(iv)The provision of self locking can also
be made.
(V) Tooth engagement occurs without
shock hence operation in quieter.

23.  Limitations of worm gear drives:
The efficiency of worm gear drive is low (45% to 95%). The
efficiency of worm gear drive decreases with increase in
reduction ratio. For the reduction ratio of 70:1, the efficiency is in the range of
45% to 50%.
1. In worm gear drive, due to frictional losses, considerable
amount of heat is generated. Therefore, effective cooling is
necessary.
2. The power transmitting capacity of the worm gear drive is comparatively
low.
3. The worm gear pair is costly as compared to other gear pairs.
4. The worm as well as worm gears are difficult to manufacture.
5. The worm gears are non-interchangeable.

24. Applications of worm gear drives
1. The worm gear drives are used in lifting or hoisting devices,
cranes, lifts, conveyors, etc.
2. They are used in all applications where high reduction ratio
is required.
3. The other gears used for connecting two non intersecting
and perpendicular axes.
4. shafts are hypoid bevel gears, skew bevel gears and
spiral or crossed helical gears.

25. Parallel axis
Intersecting
Non-intersecting and perpendicular
Non-intersecting and non-perpendicular

38. The common normal at the point of contact between a pair of
teeth must always pass through the pitch point.
Let TT be the common tangent and MN be the common normal to the
curves at the point of contact Q. From the centres O1and O2, draw O1M
and O2N perpendicular to MN. A little consideration will show that the
point Q moves in the direction QC, when considered as a point on wheel
1, and in the direction QD when considered as a point on wheel 2.
Let v1and v2be the velocities of the point Q on the wheels
1 and 2 respectively. If the teeth are to remain in contact,
then the components of these velocities along the
common normal MN must be equal.

39. Also from similar triangles O1MP and O2NP,
Therefore in order to have a constant angular velocity ratio for all
positions of the wheels, the point P must be the fixed point (called pitch
point) for the two wheels. In other words, the common normal at the
point of contact between a pair of teeth must always pass through the
pitch point. This is the fundamental condition which must be satisfied
while designing the profiles for the teeth of gear wheels. It is also known
as law of gearing.

40.  CycloidalTeeth
 Involuteteeth

41. A cycloid is the curve traced by a point on the circumference of a circle
which rolls without slipping on a fixed straight line. When a circle rolls
without slipping on the outside of a fixed circle, the curve traced by a point
on the circumference of a circle is known as epi-cycloid. On the other
hand, if a circle rolls without slipping on the inside of a fixed circle, then
the curve traced by a point on the circumference of a circle is called hypo-
cycloid.

42. An involute of a circle is a plane curve generated by a point on a
tangent, which rolls on the circle without slipping or by a point on
a taut string which in unwrapped from a reel as shown in Fig.

43.  Advantages of involute gears:
1. Centre distance for a pair of involute gears can be varied.
2. In involute gears, the pressure angle, from the start of the
engagement of teeth to the end of the engagement, remains
constant.
3. Smooth running and less wear of gears.
4. The involute teeth are easy to manufacture than cycloidal teeth
 Advantages of cycloidal gears:
1. The cycloidal gears are stronger than the involute gears
2. The interference does not occur at all.