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Machine Design

Clamp Collars Hold Tight Without Harming Shafts

Shaft collars are one of the simplest, and consequently most overlooked, components in the power-transmission industry. However, their use is also one of the most widespread. They act as mechanical stops, locating components, and bearing faces. Collars are also used as accessories in power-transmission equipment, such as motors and gearboxes.

Set-screw shaft collars are an industry standard and derive all of their holding power from the screw as it tightens onto the shaft. However, set-screw designs have several drawbacks. First, the amount of holding power depends on the material and condition of the shaft. For maximum holding power, the shaft must be a softer material than that of the set screw. This limits design options.

The softer shaft material points to another fault in set-screw collars. The softer material allows impingement of the screw point into the shaft, binding the screw and collar to the shaft. Unfortunately, this damages the shaft. The screw creates a raised burr on the shaft surface, making it difficult to remove the collar from the shaft. Small angular or lateral adjustments are almost impossible because the screw point will be drawn back to the center of the original impingement.

Clamp-style shaft collars are a better alternative. They are available in one and two-piece designs. Each type uses socket-head cap screws to create compressive forces that lock the collar onto the shaft. There is no shaft damage because there are no set screws.

Clamp-style collars are easily removed, infinitely adjustable, and tighten with a nearly uniform force distribution around the shaft. This results in holding strength that is up to twice that of set-screw collars. And clamp-style collars hold tight on all shaft materials, because their holding power does not depend on screw impingement.

Although clamp collars work well under relatively constant loads, shock loads can shift the collar position on the shaft. Applications with high shock loads call for an undercut on the shaft and a two-piece clamp collar to create a positive stop that is more resistant to shock loads.

Two-piece collars have slightly more holding strength than one-piece designs. This is because one-piece collars use some of their seating torque to bend the collar around the shaft, while two-piece collars use their full seating torque to clamp onto the shaft.

Two-piece collars also have a distinct advantage in installation and assembly. While set-screw collars and one-piece clamp collars must slide over the end of a shaft, two-piece collars can be disassembled and installed without removing other components from the shaft. In the case of an undercut shaft, a one-piece collar would have to be pried open to fit over the shaft and a set-screw collar could not be installed.

This information supplied by Ruland Manufacturing Co. Inc., Watertown, Mass.
For more information, Circle 634

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