Pulley Basics

The Summerlift Cable lift models permit cabinet builders to take
advantage
of cables & pulleys & strategic leverage to accomplish load
lifts. This
provides the cabinet maker with maximum design latitude.


WHAT IS A PULLEY?


A pulley is a simple machine made with a rope, belt or chain wrapped

around a wheel. The pulley is usually used to lift a


heavy object (load).



WHAT DOES A PULLEY DO?




A pulley changes the direction of the force, making it easier to

lift things



ARE ALL PULLEYS THE SAME?




No, they are not. There are three types of pulleys:



*A FIXED PULLEY

*A MOVABLE PULLEY and

*A COMBINED PULLEY

A single pulley changes the direction of the lifting force.
For
example, if you are lifting a heavy object with a single pulley
anchored to
the ceiling, you can pull down on the rope to lift the object instead

of pushing up. The same amount of effort is needed as without a
pulley, but
it feels easier because you are pulling down.
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A fixed pulley is the only pulley that when used individually, uses more effort than the load to lift the load from the ground. The fixed pulley when attached to an unmovable object e.g. a ceiling or wall, acts as a first class lever with the fulcrum being located at the axis but with a minor change, the bar becomes a rope. The advantage of the fixed pulley is that you do not have to pull or push the pulley up and down. The disadvantage is that you have to apply more effort than the load

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BASIC PULLEY PHYSICS

This figure shows a single pulley with a weight on one end of the rope. The other end is held by a person who must apply a force to keep the weight hanging in the air (in equilibrium). There is a force (tension) on the rope that is equal to the weight of the object. This force or tension is the same all along the rope. In order for the weight and pulley (the system) to remain in equilibrium, the person holding the end of the rope must pull down with a force that is equal in magnitude to the tension in the rope. For this pulley system, the force is equal to the weight, as shown in the picture. The mechanical advantage of this system is 1!

In the second figure, the pulley is moveable. As the rope is pulled up, it can also move up. The weight is attached to this moveable pulley. Now the weight is supported by both the rope end attached to the upper bar and the end held by the person! Each side of the rope is supporting the weight, so each side carries only half the weight (2 upward tensions are equal and opposite to the downward weight, so each tension is equal to 1/2 the weight). So the force needed to hold up the pulley in this example is 1/2 the weight! The mechanical advantage of this system is 2; it is the weight (output force) divided by 1/2 the weight (input force).

Each figure below shows different possible pulley combinations with
both fixed
and moveable pulleys. The mechanical advantage of each system is easy
to determine.
Count the number of rope/cable segments on each side of the
pulleys, including
the free end. If the free end is to be pulled down, subtract 1 from
this number.
This number is the mechanical advantage of the system! To compute
the amount
of force necessary to hold the weight in equilibrium, divide the
weight by the
mechanical advantage!

Here there are 3 sections of rope. Since the applied force is downward, we subtract 1 for a mechanical advantage of 2. It will take aforce equal to 1/2 the weight to hold the weight steady.

This figure has the same two pulleys, but the rope is applied differently and it is pulled upwards. The mechanical advantage is 3, and the force to hold the weight in equilibrium is 1/3 the weight.