Pulley Structure And Efficiency

Pulley Structure And Efficiency

A pulley is a wheel which rotates on a shaft which is supported by a frame. They can either be used to change the direction of a rope or provide mechanical advantage to make it easier to move or lift an object. In rope systems, pulleys are primarily used to build mechanical advantage systems for raising operations.

The wheel of a pulley, called a sheave, is specially designed to accept a rope without damaging it. Most sheaves are made out of aluminum but stainless steel versions are also available for use with metal cable.

Sheave Diameter

The size of a pulley is determined by the inside diameter of the sheave. This is where a rope or cable contacts the pulley. For example, in a 2 inch pulley, the rope turns around a 2 inch diameter circle inside the pulley. The correct size of the pulley is determined by the rope that is being used. Traditionally, pulleys were very large. It was common for the sheave to be 8 times the size of the diameter of the rope. This is because natural fiber ropes, like manila, can be damaged by bending too tightly. Modern nylon and polyester ropes are much more flexible and do not suffer from this problem.

For modern ropes, we recommend a sheave that is at least 3 times the diameter of the rope. For example, for a half inch rope, a 1.5 inch pulley is recommended. This provides a good balance of efficiency and size. If additional efficiency is needed a slightly larger diameter, such as 4 times the rope diameter, can be used. Larger sheave sizes add additional efficiency but the pulley also becomes much larger and heavier.

Sheave Width

It is also important to pay attention to the width of the sheave. There is no problem using a smaller rope on a wide sheave but a rope larger than the sheave width is not recommended. Even if the rope can fit in the pulley, under a heavy load the rope can flatten and become wider. This will cause the rope to rub against the side plates of the pulley and greatly reduce efficiency.

Bushings and Bearings

The sheave rotates on the shaft which allows it to turn. It does this by using either a bearing or a bushing. These come in a few different forms:

Plastic Bushing: Bushings made out of plastic are the cheapest type. These are good for very light loads but under load they can deform causing the pulley to jam and stop turning. These types of bushings are not recommended.

Oilite Bushing: Oilite is a porous bronze or iron which is impregnated with an oil lubricant. It was originally designed for use in water pumps. These are better than plastic bearings since they will not deform under heavy loads but still have problems. Since they are not sealed, dirt can get inside the bearing and cause increased friction. Pulleys are not designed to be taken apart so cleaning out the dirt can be very difficult. The main advantage of oilite bushings is they are less expensive than sealed ball bearings. For lighter loads and if properly maintained they are a good option.

Sealed Ball Bearing: Bearings are more complicated than bushings and consist of many small balls which minimize friction. One of the main advantage of a sealed ball bearing is there is no maintenance needed. Sealed ball bearings are also much more efficient than oilite bushings.

Efficiency

The efficiency of the pulley is a major consideration when buying a pulley. Pulleys using oilite bushings are approximately 70-80% efficient. The same size pulley using a ball bearing is approximately 90-95% efficient. As the load on a pulley increases, the pulley’s efficiency drops. A badly designed pulley, or one being used in a way it is not designed, can make it more difficult to lift a load than no pulley being used at all.

There are many considerations in choosing a pulley and many of them can not be seen by looking at a picture online. Often they only become noticeable during actual use.

There are some pieces of equipment where it is alright to save money but we recommend purchasing the highest quality pulley within budget. Many pieces of equipment can be improvised but a proper pulley is not one of them.

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