When the pulley cannot rise or fall, it is termed a fixed pulley, otherwise it is considered movable.
The fixed pulley, sometimes known as a gin wheel, can only change the direction of a force, and gives no mechanical advantage, but when used in conjunction with a movable pulley a mechanical advantage is gained.
[Fig. 113] is an illustration of the single movable pulley. The rope is connected to the beam at A, and passes round the pulley B and over the fixed pulley C Now, if power be exerted on the free end of the rope sufficient to move the machine, it will be seen that for every one inch W rises, P will descend two inches, therefore the mechanical advantage is 2. In other words, a workman pulling at P with a force equal to 100 lbs., will lift W weighing 200 lbs., but the weight will rise at only half the speed at which P falls.
Various combinations of pulleys are possible, but the most common in use on buildings is shown in [fig. 114], which is an illustration of the system of pulleys known as block and tackle.
Fig. 113
Fig. 114
The power obtained by this machine is calculated as follows:
Divide the weight by double the number of pulleys in the lower block; the quotient is the power (in the same units as the weight) required to balance the load.