Shaft collars are one of the simplest components in machine design but are also one of the most indispensable. Shaft collars can be found in virtually any type of machinery and are frequently accessories to other components. Capable of fulfilling many roles, shaft collars often hold bearings and sprockets on shafts, situate components in motor and gearbox assemblies, and serve as mechanical stops.
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Set Screw Shaft Collar
The first mass produced shaft collars were solid ring collars which used protruding square headed set screws. Unsurprisingly, these collars were suboptimal and posed a safety risk to workers who could catch their clothing on the protruding screw. Set screw collar technology has advanced in the decades since, and they are now manufactured with a recessed set screw.
Set screw collars derive their holding power from their set screw, which is tightened onto the shaft. Ideally, the screw impinges, or “bites”, into the shaft when it is tightened, assuming the shaft is of a softer material than the screw. A shaft that is harder than the set screw will not support the impingement and the collar’s holding power will be greatly reduced. It is important that designers know the Rockwell rating of the shaft and screw to determine if a set screw shaft collar is suitable.
Unfortunately, the set screw cannot bite into the shaft without permanently marring it. Other than being aesthetically displeasing, this can also result in functional problems. The impingement of the screw causes an eruption of material around the screw point resulting in a raised burr, making it difficult to remove the collar or further refine its position. Small angular and lateral adjustments are nearly impossible to make since the screw point will always be drawn to its original location.
Clamp Style Shaft CollarsOne and Two piece clamp style Shaft Collars
An improvement over the set screw collar, clamp style shaft collars do not mar the shaft. While no one can say who invented the clamp style collar, Ruland has been manufacturing them since the 1930s.
Clamp style collars solve many of the problems that exist with the set screw collar and are available in one and two-piece designs. They utilize compressive forces to lock the collar onto the shaft. Since this does not damage the shaft, clamp style collars are easily removed, indefinitely adjustable, and work well on virtually any shaft. In addition, when the clamp screws are tightened correctly, the clamping forces are distributed nearly uniformly around the circumference of the shaft (as shown in the image above). This is significantly more secure than the point contact of set screw collars, as much as doubling the holding power of the clamping shaft collar.
Clamping force of one and two piece shaft collars
Although clamp style collars work very well under relatively constant loads, shock loads can cause the collar to shift its position on the shaft. One option for applications with this this type of loading is to make an undercut on the shaft and use a two-piece clamping shaft collar to create a positive stop that is more resistant to shock loads.