Weston's Differential Pulley Block:

Figure illustrates a Weston's Differential Pulley Block. It consists of upper block A comprising two pulleys 1 and 2 that rotate together and their frame is suspended from fixed supports. The lower block contains only one pulley which moves up and down and its frame supports the load to be lifted. A single rope or string or chain passes around the pulleys as illustrated. If chain is utilized, it is better the upper block pulleys are provided with the needed teeth.

The effort P is applied to the chain passing out over the bigger pulley of the upper block.

Consider D be the diameter of pulley 1, and d be the diameter of pulley 2.

As the effort is applied, the string unwinds from pulley 1 and it winds on pulley 2. This results in shortening of the string on the load side.

For one of rotation of pulleys 1 and 2, net shortening of string

= π D - π d = π (D - d ) .

This shortening of the string shall result in the lifting of the load by a distance

= π (D - d ) /2

V R = π D / (π (D - d )/2)=      2D /D - d

If T₁ and T₂ are the member of teeth on pulley 1 and 2, respectively,

V R = 2T₁ /T₁ - T₂