Difference Between a Sheave and a Pulley
You may have heard of a sheave and you most definitely have heard of a pulley. Are there any differences between these two items? How do they work together, if at all? Chances are pretty good that if you’ve ever been on a ship or a sailboat, you’ve seen both of these items at work and not realized it. Let’s take a deeper dive into the sheave and the pulley!
What is a Sheave?
A sheave is the wheel that’s located inside a pulley mechanism that the rope moves through. If you’ve tried to lift a heavy weight that’s tied with a rope with a pulley, you’ll no doubt have seen this rope maneuver through a sheave above. The more sheaves used in the process, the less force that you will need to move the heavy object, although there are drawbacks to this which we’ll cover below.
More sheaves do cut down the force required to lift the object, but it also adds friction to the mix. More friction causes more force needed to move the object. More friction is added with each additional sheave that you employ, eventually making the adding of more sheaves an exercise in diminishing returns. You can set up multiple sheaves and ropes in a compound pulley which will lessen the friction created and decrease the force needed to move the object.
What is a Pulley?
A pulley is a system that is used to move or lift an object with less force than you would normally need to move it under normal circumstances. A simple pulley system uses a single rope and a sheave to do the job. There are more intricate pulley systems that utilize multiple ropes and sheaves in various configurations and setups to lift extremely heavy objects with little effort.
How does a Sheave and a Pulley Work Together?
Let’s paint a scenario here and imagine yourself in your garage working on your car. Let’s say that you’re a maverick, and you have one of those fancy new Tesla model cars. Your battery isn’t working, and you refuse to drive the 500 miles in your regular gas-powered car to consult with your nearest Tesla repair facility. You decide to go ahead and take the entire battery out of the car yourself. There’s a slight problem here as the battery pack weighs an astronomical 2,000 pounds!
Through hard work and common sense planning, you manage to separate the battery pack housing from the car. But how do you lift the battery pack off the ground and onto your bench? Sure, you could use a multitude of jack stands, but that would take way too long and way too many jack stands that you have. You decide to go ahead and use the pulley and sheave method.
You set up a system of ten sheaves interconnected with one very long piece of cabling. You fasten the cabling around the battery pack and manage to lift the battery pack with little physical effort on your behalf. You do manage to pull on a lot of cabling to get the battery pack where it needs to be height-wise to place onto the bench. How did this all work out?
The rope acts as a force transmitter of sorts as that’s the medium you’re using to apply force. If the battery pack was on the ground, you would have to pull with 2,000 pounds of force to move it towards you. Using a one-rope / one sheave method would change the force direction from a pullback to a pull-down which is a little more manageable. The force needed would not change however, so you would need to employ more sheaves. With each additional sheave, the force is reduced, however more rope is needed to match the work required to move the object.
In this particular case, the battery pack weighed 2,000 pounds and ten sheaves were used in the pulley system devised. This means that you would only need to exert 200 pounds of force to move the battery pack. Suppose that you would need three feet of rope to lift the battery pack from the ground to the bench without any pulleys.
Therefore, to use a pulley system with ten sheaves, you would need to multiply ten by three to get 30 feet of rope that would be needed to exert the same amount of force needed to lift the battery pack three feet in the air. So in essence, little force is needed, but you do need to pull a lot of rope to move the battery pack a fraction of the space.
What is a Vet Belt Pulley
A Vet Belt Pulley or a V-Belt is a system of pulleys that are interconnected together with a third hub in the middle connecting two belts together. You see vet belt pulleys used inside car engines for timing belts and serpentine belts, they are also used in other applications as well
Motion Systems for Pulley Systems
Motion Systems have been providing custom pulley solutions since 1973. If you have any questions about pulley systems, please contact Motion Systems today for more information.