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What Is a Compound Pulley?

Cranes used to lift heavy loads commonly employ compound pulleys.
Three fixed pulleys hang from a metal ring on a boat.
Pulleys often have a groove around their circumference that guides a chain, cable, belt or rope.
A simple rope pulley.
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  • Written By: Lori Kilchermann
  • Edited By: Jenn Walker
  • Last Modified Date: 22 October 2014
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A compound pulley system uses a fixed pulley and a movable pulley so that an operator is able to move a heavy load with less physical effort. The multiple pulley system is able to multiply the strength and pulling power of the winch, lowering the strain on the winch and the object being pulled. Such a system can be found on many heavy lifting cranes and overhead winching machines. The block and tackle is also a type of compound system.

In the most simple terms, the weight reduction of a given load is reduced by the number of pulleys used in a compound pulley system. With this in mind, a system using four pulleys to lift an object will feel like the user is actually lifting one-fourth of the item's weight. This is why a crane is able to operate with a smaller cable and winch, saving valuable space on the machine as well as allowing the crane to be physically smaller and lighter, while still remaining operational and able to lift greater amounts of weight.

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The compound system also affects the ability to pull. When using a winch to recover a vehicle that is stuck in deep mud or has become trapped in a steep ditch, for example, the winch operator will often use a compound pulley system to help ease the vehicle out while exerting minimal strain on the winch and cable. By using this type of system, a much smaller winch can often do the job of a much larger device while remaining cool and drawing much less power.

On battery-powered winch motors, this means extended pulling power from a single battery. It also means less chance of damaging the winch motor due to over-heating of the electrical circuits from straining and stressing the motor. With a hydraulic winch motor, there is much less chance of breaking a fluid line or damaging the pump due to a difficult pull. The pull will require a little more time when using a compound system, although most users agree that the benefits outweigh the problems and repairs. The system can assist with heavy tasks and allow work to be completed with much less strain placed upon any system.

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Agni3
Post 2

Sometimes we get all caught up in the high tech gadgets of today and totally forget how to function with the basics. Simple machines can do amazing things when you really think about it.

For instance, I used to work in the theatre and we had a very high, very scary catwalk. (Very high and very scary for someone terrified of heights, anyway.) Now, a lot of the lights that are used in theatres are pretty big, and we had perhaps fifty plus mounted up in this catwalk at any given time. How crazy would it have been to try to lug all those babies up the very high, very scary ladder?

A pulley system gave us a huge mechanical advantage. Plus, it meant I only had to go up that ladder once to work and then back down once. (I was really more terrified of the ladder than the catwalk itself.) As long as I had a helper on the ground, we could hoist those huge things up, and lower them down all day long. We were also known to send a Mountain Dew or two up that way, as well

nanny3
Post 1

My granddad was a pretty amazing man, and he owned his own mechanic’s shop. I always thought he was pretty wonderful because he had to quit school in the fourth grade to take care of his siblings and mother when his father died.

He ended up being a self-educated man who ingested novels like most people do water, and he was also an entrepreneur.

Pa had a very lucrative mechanic’s business, and I remember well the lifting pulley he had mounted in his shop to help raise motors out of cars. In case you have never had the pleasure of seeing a car motor outside of the vehicle – those things are huge and very heavy. This too was before the advent of the ultra-tiny car.

As a one man operation, he needed this pulley to help him get them out, and it still wasn’t easy. Now mechanics have lifts and all sorts of gizmos; then, my granddaddy made the best out of common sense and simple machines.

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