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Massmudman -> RE: What size winch? (10/13/2008 6:32:54 PM)
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I will try this so everyone can be on the same page. [:D]
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.
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
[image]http://www.the-office.com/summerlift/pulley1.gif[/image]
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.
[image]http://www.the-office.com/summerlift/pulley2.gif[/image] 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. [image]http://www.the-office.com/summerlift/pulley3.jpg[/image]
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.
[image]http://www.the-office.com/summerlift/pulley4.gif[/image]
BASIC PULLEY PHYSICS
[image]http://www.the-office.com/summerlift/pulley001.gif[/image]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!
[image]http://www.the-office.com/summerlift/pulley002.gif[/image]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!
[image]http://www.the-office.com/summerlift/pulley003.gif[/image]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.
[image]http://www.the-office.com/summerlift/pulley004.gif[/image]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.
When designing the system you will utilize to raise and lower a TV, etc., think about the load you plan to lift.
1. How heavy is it?
2. Can pulley positions be placed to take advantage of leverage?
Particularly important to remember is that both sides of your platform MUST be raised with identical pulley layouts.
If this isn't done, one side will move faster than the other --
[image]http://www.the-office.com/summerlift/pulley5.gif[/image]
Learn more about these models:
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