Mechanical Advantages of Pulleys

Dating to the Renaissance period, people have agreed that the six "simple machines" are the pulley, the lever, the inclined plane, wheel and axle, the screw, and the wedge. The pulley is the simple machine used to raise and lower a weighted load of any size. For example, a flagpole rope operates from a simple, small pulley. "Mechanical advantage" describes the outcome effect from the use of a pulley or the ratio of load to effort.

Fixed Pulley

• A single line fixed pulley.

  There are three types of mechanical pulleys: a fixed, a movable and the combined or compound pulley. Mechanical advantage is best understood in pulleys by using a simple pulley as an example. The pulley is fixed to a secured stationary surface such as a rafter, a beam or a girder with a length of cable, rope or chain running up, through the pulley and back down.You would have to exert more downward force on the pulley rope than the weight of the object in order to raise it. For example, visualize a stationary 100-pound weight affixed to a rope. A minimum force of more than 100 pounds has to be applied to the free end of the rope to lift the weight. The labor force expended pulling downward has to exceed the weight lifted upward, so one rope length is bearing the full weight and doing all the work, which yields a mechanical advantage of "one."

The Movable Pulley

• The pulley moves with the load.

  The movable pulley attaches directly to the load to be lifted. The cable is attached to a fixed stationary surface on one side of the pulley, and the unattached cable is for pulling on the other side. The force applied is upward instead of downward, and when the cable is taut, the load weight is equal on both cables. Therefore, as the free cable is pulled, the effort is only one-half the total weight as the pulley rises with the load. The mechanical advantage is "two" because two cable lengths are doing the work.

Calculating Pulley Mechanical Advantage

• The method to calculate mechanical advantage is easy to remember and is necessary when rigging the assembly to accomplish the job. The mechanical advantage of a rigging that will require upward pull can be determined by counting the number of rope lengths running between engaged pulleys and those doing the work. Likewise, if the assembly will require downward pull, count the ropes and subtract one to get the mechanical advantage number. The subtraction is necessary because with the fixed pulley, the downward pull equals the load on the other length of rope so the last "pull" rope does not provide any mechanical advantage.