Selecting The Correct Pulley For Your Application
Overwhelmed by the breadth and depth of our line? Confused about the advantages of different materials or construction techniques? A little rusty on your pulley anatomy? Use this quick guide to help ease your selection woes!
Sideplates and Beckets: Sideplates form the body of the pulley, and together with the axle are primarily responsible for its strength. Beckets are installed in multi-sheave pulleys to provide a lower tie-in point.
Most pulleys have sideplates that rotate independently around the axle to make rigging easier. This allows the pulley to be rigged onto a rope mid-strand, rather than only at the ends. Sideplates come in all shapes, sizes and colors depending on pulley function. Anodized Aluminum (AA) is lightweight and durable, but not as strong. Stainless Steel (SS) is very strong and resistant to the elements, but heavy.
Sheaves: The sheave (the wheel of the pulley) affects pulley performance in two ways: rope capacity and efficiency. Sheave widthis the major factor in how large a rope a pulley can handle, wider sheaves handle larger ropes. sheave diameter, along with bearing efficiency, determines the overall efficiency of the piece. Generally speaking, larger sheave diameters produce greater pulley efficiencies. Steel (SS): Strong, but heavy. Excellent for use with steel cable.
Aluminum (AA): Light weight, but soft. Glass Filled Celcon (GFC): Self-Lubricating, no bushing or bearing is needed. Weaker and softer than the other sheaves. Nylon: Lightweight, self-lubricating, stronger than GFC.
Bushings vs. Needle vs. Ball Bearings: The bushing or bearing is the load bearing member between the sheave and axle. A bushing spreads the load over a larger area of axle. This results in greater transfer and very little wear and extremely long life. However, the greater area in contact results in increased friction, decreasing efficiency.
Bearings decrease the amount of surface area in contact with the axle, which creates a more efficient transfer. Load on the transfer point is increased, which results in faster wear, shortening the life of the pulley. Needle bearings are designed for slow pulls with heavy weight, while ball bearings are better suited for high speed applications.
Axles: In contact with all the other parts of the pulley, the axle is primary determinant of the overall strength. Our special corrosion resistant nuts round out the package, ensuring a lifetime of resistance to the elements. Steel (SS): High strength, but heavy. Aluminum (A): Lighter, but not as strong. Hardened and Ground (HG): Longest lifetime of any axle. Zinc-Plated Steel (ZPS): A less expensive option.
So what is the best pulley for my application?
Generally speaking, we recommend users get a pulley with a bushing because the pulley will tend to last longer. We use sealed bearings in all our pulleys but a bushing will still tend to outlast most bearing pulleys. But bearings are more efficient, right? Yes, bearings are more efficient but in a real world situation, most people cannot tell the difference between a pulley with a bushing over a pulley with a bearing.
For example, when comparing a CMI RP102 pulley with a bushing to a CMI RP103 pulley with a bearing, the RP102 yields a 91.4% efficiency rating vs. the RP103 which has a 95.6% efficiency rating. The difference in efficiency over the two basically identical pulleys is only 4.2%
Pulleys with bushings also tend to be a little less expensive than pulleys with bearings. So for those reasons, we usually recommend our customers opt for pulleys with bushings over pulleys with bearings. Of course there are circumstances where the user must use a pulley with a bearing. Ziplines are a good example of that.
Still confused about which pulley is best for your needs. Just give our friendly sales staff a call and let them help you figure out what pulley is best for your needs.