All of the above tackles/purchases are designed to increase the power utilised to the effect achieved. Because of friction between the ropes and sheaves, the actual (theoretical) weight lifted can be roughly calculated as being:

Weight + (1/10 x weight) for each sheave.

The mechanical advantage gained can be found by counting the number of parts of rope at the moving block. The static block is the one attached to a lifting point while the moving block is the one actually attached to the weight being lifted. A tackle is roved to advantage when the hauling part of the rope is rove around the moving block. It is rove to disadvantage when the hauling part is rove around the static block. The power gained is calculated by dividing the calculated weight by the number of parts of rope at the moving block.

Example 1: A weight of 10 kg has to lifted with a double purchase rove to advantage. What is the theoretical weight that has to be lifted and what is the minimum force required to lift it?

There are two double blocks, therefore there are four sheaves. Also, since it is rove to advantage, it has five parts of rope at the moving block.

Theoretical weight = 10 + (4/10 x10) = 14 kg
Power gained = 14/5 = 2.8 kg

This means that in order to pick up the 10 kg weight a minimum force greater than 2.8 kg is needed.

Example 2: A weight of 10 kg has to be lifted with a double purchase rove to disadvantage. What is the theoretical weight that has to be lifted, and what is the minimum force required to lift it?

There are two double blocks, therefore it has 4 sheaves and since it is rove to disadvantage, it has four parts of rope at the moving block.

Theoretical weight = 10 + (4/10 x 10) = 14 kg
Power gained = 14/4 = 3.5 kg

This means that in order to pick up the 10 kg weight, a minimum force greater than 3.5 kg is needed.