The document discusses different types of power transmission devices including belt drives, chain drives, and gear drives. It focuses on different types of gears, providing details on spur gears, helical gears, bevel gears, worm gears, and their advantages and applications. Key points covered include how helical and bevel gears allow power transmission between non-parallel shafts, how worm gears enable high gear reductions, and the law of gearing which specifies gears must maintain contact through the pitch point.

2. • Belt, Rope and Chain
Previous lecture

3. Today’s lecture
• Gear Drive

4. Power Transmission Devices
 Belt drive,
 Chain drive,
 Gear drive

5. Gear drive
Positive drive Positive drive

6. 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.

7. Gears 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

9. Spur gears are the most common type of
gears.
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

11. Advantages of spur gears
(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.

12. Limitations of spur gears
(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.
•

13. Applications of spur gears
(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.

14.  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.
Helical Gear

15.  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.

17. 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%).

18. 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.

19. 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.

20. 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.
Bevel gears

21. 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

22. 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.

23. 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

24. 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.

25. Worm gears are used when large gear
reductions are needed. It is common for
worm gears to have reductions of 20:1, and
even up to 300:1 or greater.
 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

26. 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.

27. 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.

28. 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.
The other gears used for connecting two non
intersecting and perpendicular axes
shafts are hypoid bevel gears, skew bevel gears and
spiral or crossed helical gears.

29. Types of gears
 Parallel axis
 Intersecting
 Non-intersecting and perpendicular
 Non-intersecting and non-perpendicular

31. Parallel axis

33. Parallel gears also classified

34. Intersecting axes gears

35. Non-intersecting and
perpendicular

36. Non-intersecting and non-
perpendicular

37. SIMPLE GEAR TRAIN

38. Velocity ratio

42. Law of Gearing
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 O1 and 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 v1 and v2 be 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.

43. Law of Gearing
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.

44. Forms of Teeth
 Cycloidal Teeth
 Involute teeth

45. Cycloidal Teeth
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.

46. Involute teeth
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.

47. Comparison Between Involute and
Cycloidal Gears
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.

48. Next lecture
• Friction
• Screw friction

49. Contact details:
Rohit Kumar Singla
Email ID: rohit.kumar@thapar.edu