# Torque Needed to Turn a Capstan

• Mech13
In summary, the conversation discusses using a motor to lift a weight using a spool and the confusion surrounding the moments involved in the situation. The person assumes that the only torques to consider are the torque from the motor and the torque caused by the weight of the object. They also mention using a mechanical lock to keep the weight suspended and looked into winches for this purpose. However, they found that the capacity of a winch decreases as the wire rope is fully wound, which goes against their initial assumption. The conversation also touches on the differences between a capstan and a winch, with the capstan having a single layer of rope and friction being multiplied by e^theta. The rope also creeps up the slightly tapered capstan
Mech13
Hello,

I am to use a motor to turn a spool and lift a weight.
The bit that's confusing me is the moments active in this situation.
I would assume that the only torques to consider are the torque exerted by the motor and the torque caused by the weight of my object. I would think that the capstan equation applies in this situation and that the torque needed to pull up the weight would be RSpool*(Weight/e^(Coefficient of Friction*Theta)). However, I'm not sure if I'm missing something. I would like to raise the weight and keep it suspended with some kind of mechanical lock on the shaft. I looked at winches which, unless I'm mistaken, are the same thing I'm describing, but in McMaster the capacity of a winch is shown as less when the wire rope is fully wound. This goes against what I assumed which was that when fully wound it should be easier to hold a larger weight. Can anyone shed some light on my confusion or point out flaws in my thinking? My apologies if this was hard to follow I have included a rudimentary picture bellow. Let me know if any clarification is needed. Thank you.

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Winch capacity falls as the wire rope is wound in. That is because each layer of wire on the drum increases the radius of the drum and therefore the torque needed for the same rope tension.

A capstan has a single layer of rope. Friction to the capstan drum being multiplied by e^theta. The rope creeps up the slightly tapered capstan drum as more rope is pulled onto and past the drum.

## 1. What is a capstan?

A capstan is a cylindrical device with a vertical axis used to turn ropes, cables, or lines. It is commonly found on ships, cranes, and other heavy machinery.

## 2. What is torque?

Torque is a measure of the force that can cause an object to rotate about an axis. It is typically measured in units of Newton-meters (Nm) or foot-pounds (ft-lb).

## 3. How does a capstan work?

A capstan works by using friction and leverage to turn the object it is attached to. The rope or line is wrapped around the capstan's cylindrical surface, and as the capstan is turned, the friction between the rope and the capstan creates a force that can move heavy objects or lift weights.

## 4. How do you calculate the torque needed to turn a capstan?

The torque needed to turn a capstan can be calculated by multiplying the force applied to the capstan by the radius of the capstan. This can be expressed by the formula: T = F x r, where T is torque, F is force, and r is the radius of the capstan.

## 5. What factors affect the torque needed to turn a capstan?

The torque needed to turn a capstan is affected by several factors, including the diameter and material of the capstan, the weight of the object being lifted or moved, and the coefficient of friction between the rope and the capstan's surface. The angle of the rope's wrap around the capstan can also affect the required torque.

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