# Pulley systems and friction (simple or compound)

• gloo
In summary, the conversation discusses the efficiency and work loss in a pulley system. It is stated that there are two sources of loss - friction and rotational kinetic energy. The friction force scales with tension and the rotational KE does not scale, but it is temporary. It is also mentioned that additional pulleys in a system can increase friction and decrease efficiency. Bending and unending forces on the rope or cable can also be a significant source of loss. It is suggested that using thin line or wire can increase efficiency, but it may cause damage to one's hands.
gloo
This is a two part question:

1. With a simple pulley system using one pulley - say 10000 kg on each side - the ability to pull either one up or down to change the position is theoretically just the friction on the system and any extra length of rope on one side?

2. Would it be more work lost on pulling up a large weight (i.e. 1 million Kg) with a 1 pulley system with equal weight on each side OR would it be more work on a multiple compound pulley system?
--- The goal of my question in part 2 is whether one large weight on a single pulley would mean more friction than a lighter counterweight...but it has more pulleys for the rope to run through?

I do not think that it's either one or the other. It will all depend on the weights, materials and lengths of ropes involved.

gloo said:
Would it be more work lost on pulling up a large weight (i.e. 1 million Kg) with a 1 pulley system with equal weight on each side OR would it be more work on a multiple compound pulley system
There are two sources of "loss" in a non ideal pulley. One is friction, and the other is rotational KE.

The friction scales with the tension, so that will be essentially constant (2 pulleys, 1/2 tension).

The rotational KE does not scale, so it will increase with more pulleys. But it is also temporary, meaning that you get this energy back when you stop.

gloo
gloo said:
OR would it be more work on a multiple compound pulley system?
The friction force for a pulley will be proportional to the 'normal force' on the bearing. That's based on basic friction theory. The final pulley in a system will be supporting the whole of the weight of the load, whether the effort is supplied directly (as with a single pulley) or via another pulley. Any extra pulleys will be supporting some weight and adding to the friction force that the input pull has to provide. So, as you increase the velocity ratio, you will be increasing the loss and the effective Mechanical Advantage will be less and less pro rata. (The efficiency will go down). If you take any system with a very high velocity ratio (such as a screw or a rack and pinion) the losses can be so high that it may not actually work backwards - if you try to swap load and effort. You pays yer money and takes yer pick, depending how weak the person or motor drive happens to be.

gloo
sophiecentaur said:
The final pulley in a system will be supporting the whole of the weight of the load,
Good point, I think you are correct. I was just thinking of the tension being reduced in the rope.

Dale said:
Good point, I think you are correct. I was just thinking of the tension being reduced in the rope.

We would really need to draw an example pulley system and commit to a particular layout but I am pretty sure that you will end up with a series of additional friction forces, one from each additional pulley, which will always come to more than the force from just one. I must get out my pencil tomorrow morning.

sophiecentaur said:
We would really need to draw an example pulley system and commit to a particular layout but I am pretty sure that you will end up with a series of additional friction forces,
Yes, me too. I think my original comment was a mistake. It isn't the tension in the rope that determines the force at the bearing, but the load.

Dale said:
There are two sources of "loss" in a non ideal pulley. One is friction, and the other is rotational KE.

There is a third source that can be very significant. Bending and unending forces on the rope or cable going through the pulleys, I test that regularly on my 6:1 main sheet system. With zero load applied, it takes a lot of force to pull the rope through.

Large diameter pulleys minimize the bending force, but maximize rotational energy.

Dale and sophiecentaur
anorlunda said:
There is a third source that can be very significant. Bending and unending forces on the rope or cable going through the pulleys, I test that regularly on my 6:1 main sheet system. With zero load applied, it takes a lot of force to pull the rope through.

Large diameter pulleys minimize the bending force, but maximize rotational energy.
Thin line or even wire will be higher efficiency - but tear your hands apart.

## 1. What is a pulley system?

A pulley system is a mechanical device that consists of a wheel with a groove around its circumference and a rope or belt that runs through the groove. It is used to change the direction of a force, making it easier to lift or move heavy objects.

## 2. How does a simple pulley system work?

In a simple pulley system, a single pulley is used to change the direction of the force. The rope is looped over the pulley and attached to the object being lifted. When a downward force is applied on one end of the rope, the object is lifted up by the same amount on the other end of the rope.

## 3. What is the difference between a simple and compound pulley system?

A simple pulley system only uses one pulley to change the direction of the force, while a compound pulley system uses multiple pulleys arranged in a specific way to increase the mechanical advantage and make it easier to lift heavy objects.

## 4. How does friction affect the efficiency of a pulley system?

Friction can reduce the efficiency of a pulley system by causing the rope to rub against the pulley, which creates resistance and reduces the amount of force that can be transmitted. To minimize friction, pulleys are often designed with smooth surfaces and lubricants may be used.

## 5. What are some common uses of pulley systems in everyday life?

Pulley systems are commonly used in elevators, cranes, and construction equipment to lift and move heavy objects. They are also used in exercise machines and rock climbing equipment to provide resistance. In addition, pulleys are used in household items such as blinds, curtains, and garage doors to make them easier to open and close.

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