Pulley Questions And Answerssatadmin
A pulley is a wheel which rotates on a shaft designed to reduce friction. 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.
We receive a lot of questions about pulleys and here are the answers to some of them. If you have additional questions please ask us in the space at the bottom of the page and we will answer them for you.
The first step is to determine how the pulley will be used. Will it be used for lifting equipment or people? Will the pulley add mechanical advantage or simply change the direction of pull? How heavy is the load? How high do you need to lift? What type and size of rope will it be used with? How many people are available to pull on the rope? These questions will help determine the most suitable pulley design.
A pulley is designed using a sheave which rotates around an axle. In order to reduce friction, a bearing or bushing is used. A bushing is a smooth surface designed to reduce friction. A bearing uses small metal balls which rotate and reduce friction. Bearings are much more efficient than bushings and are commonly used in high quality pulleys. Bushings are usually used in less expensive pulleys and in pulleys designed to withstand very high loads such as arborist rigging blocks.
Omni-Block Pulleys are a special design created by the manufacturer Rock Exotica. The Omni-Block combines a pulley and swivel into one piece. This design allows the pulley to rotate freely to keep a rope from rubbing on the side of the pulley creating additional friction and making it more difficult to lift a load. The Omni-Block Pulley also features an opening side plate which allows a rope to be inserted or removed without having to disconnect the pulley.
The pulley must be strong enough to be able to lift a load but also not to damage the sheave preventing the pulley wheel from turning smoothly. Most pulleys have a working load limit (WLL) printed on them. The WLL is recommended by the manufacturer based on the design, materials used, and the expected use. Going over the WLL might not cause the pulley to break but it can cause damage to the internals.
Efficiency is a measure of work output to work input. A perfect pulley would have the same force entering the pulley as exiting the pulley. In other words, 100 kg of force entering the pulley would result in 100 kg leaving the pulley. If a pulley is only 80% efficient then there will be a 20% loss of force using the pulley. 100 kg of force entering the pulley would result in 80 kg of force leaving the pulley.
The only real way to know is to test it yourself. Many things influence the efficiency of the pulley including the design and the rope used. The manufacturer sometimes gives a guideline but this is a number obtained in a laboratory under ideal conditions. The real world efficiency will usually be lower than this number.
Some resources show using a carabiner by itself in place of a pulley in a hauling system. We do not recommend doing this and instead recommend using a pulley in any system which needs a reduction in friction. Pulling a rope around a carabiner introduces an approximately 50% loss in efficiency. This is because of both the friction involved and the very tight bend radius of the rope. An inexpensive basic pulley will result in an approximately 30% loss of efficiency which is still considerably better than a carabiner alone.
Small pulleys are less efficient than larger pulleys but have some important uses. A smaller pulley weighs less which is important if the equipment needs to be carried a long distance such as for mountain rescue. The sheave diameter is also less important if the pulley is being used as a trolley or to redirect a rope since the rope does not wrap completely around the sheave. In this case, the efficiency of the design is more important than the diameter.
A pulley needs to be large enough that the rope can naturally bend around the pulley sheave. As rope technology became better, smaller pulleys were able to be used while keeping similar efficiency. For most synthetic fibers such as nylon and polyester the pulley needs to be around 3 times the rope diameter. For natural fiber ropes such as manila the pulley sheave might have to be 5 times or larger than the rope diameter.
A rope can move in both directions through a pulley. In order to make the rope only move in one direction some type of progress capture must be added. Some pulleys have mechanical captures attached to them. If the pulley does not have this, a progress capture can be created by using a friction hitch. Care must be taken to prevent the friction hitch from being pulled into the pulley. This can cause the pulley to jam as well as push the side plates of the pulley apart destroying the pulley and causing the load to fall. In order to help prevent this, prusik minding pulleys (PMP) are designed with squared bottom sideplates to catch the hitch and hold it in place.
We do not recommend doing this. A pulley is designed to hang straight so the rope does not rub against the side plate. When only one side of a double pulley is used, the pulley will be canted off center. This can create a situation where the rope rubs against the side plate. This adds friction reducing the efficiency of the pulley. Extended use in this manner can create wear which sharpens the side plate and potential damage to the rope.
Yes, there is no lower limit on rope size with a pulley. The only concern is that the rope isn’t so small it can get trapped between the sheave and the side plate.
You should not do this. A larger rope can rub on the side plates and the friction will cause a loss in efficiency.
The pulley size should be matched with the rope being used. A larger pulley sheave diameter will make the system more efficient requiring less force to be used to move a load. For modern rope made from synthetic materials (nylon, polyester, technora, etc..) we recommend the pulley sheave should be at least 3 times the diameter of the rope being used. For example, a 10mm rope should use a pulley with a sheave no less than 30mm in diameter.