Summerlift™ CL (Cable Lift) Models

Pulley Basics

Official Pulley Inspector

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 movable  pulley  is a pulley that moves with the load. The movable pulley allows the effort to be less than the weight of the load. The movable pulley also acts as a second class lever. The load is between the fulcrum and the effort. The main advantage of a movable pulley is that you use less effort to pull the load. The main disadvantage of a movable pulley is that you have to pull or push the pulley up or down.

If you add a second  pulley, the amount of effort to lift the heavy object is much less . For example, to lift a box weighing 150 N, one would need to exert 150 N of force without the help of pulleys. However, by using just two pulleys, the person would only need to use 50 N of force.

 

A combined pulley makes life easier as the effort needed to lift the load is less than  half the weight of the load. The main advantage of this pulley is that the amount of effort is less than half of the load. The main disadvantage is it travels a very long distance.

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.

 

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