## What is called a Winch?

*Therefore, a crab winch is a lifting machine which is used for lifting of heavy loads by the application of small effort in the places where other lifting devices can not be used.*

### Use of a Crab Winch

The main uses of a crab winch are as follows –

- It is used for lifting of heavy weights and for other heavy duty purposes.
- It is also used for erection of pillars.
- Crab winch is also used for push, pull, lift or lower of heavier loads.
- While mounted on a mono rails, crab winch is used in workshops, foundries and small factories for lifting of moderate loads instead of using of EOT cranes.
- It is used for positioning and leveling and fixing of heavy loads like machine beds.
- In mines, winches are extensively used for lifting of loads through a hole.

## Types of Crab Winch

A crab winch is of two types –

- Single purchase crab winch.
- Double purchase crab winch.

### Single Purchase Crab

A single purchase crab winch consists of one set of pinion gear and spur gear arrangement. The pinion is attached with the crank and handle for application of effort. Gear is assembled with the grooved drum for lifting of load. Load is attached with the rope wound around the grooved drum.

Consider about a single purchase crab winch as shown in figure.

- It consists of the two supports A_1 \ \text {and} \ A_2 at two ends which are joined rigidly by bars B_1 \ \text {and} \ B_2 .
- In between end supports, an axle ( A ) is fitted on which a small toothed wheel called a pinion ( M ) is mounted.
- Axle ( A ) can be rotated by a handle ( H ) fixed at one or both end supports.
- In between supports another big toothed wheel or spur gear ( N ) is fitted which mounted on a drum ( B ) .
- Teaths of spur gear ( N ) and pinion ( M ) mesh with each other.
- A rope is wound on the drum ( B ) whose free end is attached with the lifted weight ( W ) .

### Velocity Ratio of Single Purchase Crab

Assume that –

- Number of teeth in pinion ( M ) is ( T_1 ) .
- Number of teeth in spur gear ( N ) is ( T_2 ) .
- Length of the handle ( H ) is ( R ) .
- Radius of drum ( B ) is ( r ) .

In one complete revolution of handle ( H ) , distance moved by the *effort* is ( 2 \pi R )

- Spur gear ( N ) has ( T_2 ) teeth and pinion ( M ) has ( T_1 ) teeth.
- Hence, when pinion ( M ) makes one revolution, the spur gear ( N ) together with drum ( B ) makes \left ( \frac {T_1}{T_2} \right ) of a revolution.

In this way, for one complete rotation of the handle ( H ) , distance moved by the load will be \left [ \left ( \frac {T_1}{T_2} \right ) \times 2 \pi r \right ] .

Therefore, *velocity ratio* of single purchase crab will be –

\text {Velocity Ratio} = \left ( \frac {\text {Distance moved by effort}}{\text {Distance moved by load}} \right )

= 2 \pi R \div \left [ \left ( \frac {T_1}{T_2} \right ) \times 2 \pi r \right ]

= \left [ \left ( \frac {R}{r} \right ) \times \left ( \frac {T_2}{T_1} \right ) \right ]

### Double Purchase Crab

A double purchase crab winch consists of two sets of pinion gear and spur gear arrangement. One set is assembled with the crank handle for application of effort. Other set is assembled with the grooved drum for lifting of load. Load is attached with the rope wound around the grooved drum.

Consider about a double purchase crab winch as shown in figure.

- It consists of two supports joined rigidly by bars ( or stays ) B_1 \ \text {and} \ B_2 .
- Two axles A_1 \ \text {and} \ A_2 are fitted between the two end supports.
- A small pinion ( P_1 ) is mounted on the axle A_1 .
- Axle ( A_1 ) can be rotated by a handle ( H ) fixed at one or both end supports.
- Another pinion ( P_2 ) and a small spur gear ( S_1 ) are mounted on the axle ( A_2 ) .
- In between the supports, a large toothed wheel or spur gear ( S_2 ) is mounted on a drum.
- Teeth of spur gear ( S_2 ) meshes with the teeth of pinion ( P_2 ) . In same manner, teeth of spur gear ( S_1 ) meshes with the teeth of pinion ( P_1 ) .
- A rope is wound on the drum ( B ) whose free end is attached with the lifted weight ( W ) .

### Velocity Ratio of Double Purchase Crab

Assume that –

- Number of teeth in pinion ( P_1 ) is ( T_1 ) .
- Number of teeth in pinion ( P_2 ) is ( T_2 ) .
- Number of teeth in spur gear ( S_1 ) is ( T_3 ) .
- Number of teeth in spur gear ( S_2 ) is ( T_4 ) .
- Length of the handle ( H ) is ( R ) .
- Radius of drum ( B ) is ( r ) .

In one complete revolution of handle ( H ) , distance moved by the *effort* is ( 2 \pi R )

- Spur gear ( S_1 ) has ( T_3 ) teeth and pinion ( P_1 ) has ( T_1 ) teeth.
- Hence, when pinion ( P_1 ) makes one revolution, the spur gear ( S_1 ) will make \left ( \frac {T_1}{T_3} \right ) of a revolution.

Similarly –

- Spur gear ( S_2 ) has ( T_4 ) teeth and pinion ( P_2 ) has ( T_2 ) teeth.
- Hence, when pinion ( P_2 ) makes one revolution, the spur gear ( S_2 ) will make \left ( \frac {T_2}{T_4} \right ) of a revolution.

Pinion ( P_2 ) and spur gear ( S_1 ) are mounted on same axle. So their revolution will be same.

Therefore, for one complete rotation of the handle ( H ) , distance moved by the *load* ( W ) will be \left [ 2 \pi r \times \left ( \frac {T_1}{T_3} \right ) \times \left ( \frac {T_2}{T_4} \right ) \right ] .

Hence, *velocity ratio* of double purchase crab will be –

\text {Velocity Ratio} = \left ( \frac {\text {Distance moved by effort}}{\text {Distance moved by load}} \right )

= 2 \pi R \div \left [ 2 \pi r \times \left ( \frac {T_1}{T_3} \right ) \times \left ( \frac {T_2}{T_4} \right ) \right ]

= \left [ \left ( \frac {R}{r} \right ) \times \left ( \frac {T_3}{T_1} \right ) \times \left ( \frac {T_4}{T_2} \right ) \right ]