Power Transmission Devices

• Belt drive, • chain drive,• gear drive

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

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

• 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 efficiency of spur gear pair is high

(96% to 99%).(v) The spur gears are interchangeable.

•

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.

•

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.

• 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

• 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.

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%).

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.

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.

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

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

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.

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.

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

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.

Limitations of worm gear drives

1. 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%.

2. In worm gear drive, due to frictional losses, considerable amount of heat is generated. Therefore, effective cooling is necessary.

3. The power transmitting capacity of the worm gear drive is comparatively low.

4. The worm gear pair is costly as compared to other gear pairs.5. The worm as well as worm gears are difficult to manufacture.6. The worm gears are non-interchangeable.

Applications of worm gear drives1. 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 nonintersecting and perpendicular axesshafts are hypoid bevel gears, skew bevel gears andspiral or crossed helical gears.

Gear drive

Types of gears

• Parallel axis• Intersecting• Non-intersecting and perpendicular• Non-intersecting and non-perpendicular

Parallel axis

Parallel gears also classified

Intersecting axes gears

Non-intersecting and perpendicular

Non-intersecting and non-perpendicular

SIMPLE GEAR TRAIN

Velocity ratio