Hold it right there

July 12, 2007
Shaft collars keep assemblies and components in place on shafts.

William Hewitson
Director of Manufacturing and Engineering
ruland Manufacturing Co. Inc.
Marlborough, Mass.


If there's a gear or component mounted on a shaft, there's a good chance a shaft collar is holding it in place. These widely used devices also serve as mechanical stops and bearing faces, and can be found in millions of gearboxes. That's because of their simplicity and reliability. But engineers and designers should be aware of the differences among them and ways to make the best use of shaft collars.

A setscrew collar is simply a ring with a recessed setscrew, often a socket screw, which goes perpendicularly through one side of the ring. The collar slides on a shaft and then stays in place when the screw is tightened. But how much force holds the collar in place depends on what materials the shaft and screw are made of. For maximum holding power, the shaft must be softer than the screw. This lets the screw dig into the shaft, letting the collar resist torque and axial loads and not simply slide along the shaft.

Unfortunately, the screw digging into the shaft raises a burr between the shaft and collar, making it difficult to remove or adjust the collar. Loosening the screw to move or twist the collar just a bit is next to impossible because the screw tends to drive back into its original hole when retightened, putting the collar right back where it started.

An alternative to setscrew collars are one and two-piece clamping collars. They use sockethead screws recessed within the collar's circumference to create compressive forces that lock the collar in place on the shaft. There are no setscrews, so there's no damage to the shaft. And workers can easily remove and adjust collars.

With clamping collars, shaft material is a relatively small factor in how the collar performs. Tightening the clamp screwpulls the collar onto the shaft, creating anearly uniform ring of forces holding itto the shaft. Setscrew collars, on the other hand, rely on forces at only two diametrically opposed points to keep them in place. So clamping collars have up to twice the holding power of setscrew collars, depending on shaft size and conditions.

When comparing one and twopiece clamping collars, two-piece versions often come out on top. That's because one-piece collars must divert some of the screw's seating torque from clamping to bending the collar around the shaft. Two-piece collars, with a built-in hinge can apply all the seating torque as clamping forces on the shaft.

Two-piece collars are also easier to install. They can be installed in position without removing other components from the shaft. Setscrew and one-piece clamp collars, on the other hand, must be slid over one end of the shaft. And in the case of an undercut shaft, one-piece collars would have to be pried open to fit over the shaft, and setscrew collars could never be installed properly.

And finally, it's easier to balance two-piece collars (by adding an opposing screw). Collars that rotate at high rpm, as may be the case on split-hub shafts, must often be balanced.

But although clamp collars work well under relatively constant loads, shock loads can move the collar around on the shaft. This is because a relatively small mass can generate high forces in an impact. (Think of a hammer hitting a nail.) To resist shock loads, workers can make an undercut on the shaft and use a two-piece clamp.

There are a couple other methods to make sure collars don't shift under shock loads. Stacking several collars, for example, adds frictional forces from more contact with the shaft and more clamping force. The same effect comes from extra-wide collars held in place with several screws. And adding bumpers to a collar lets the bumper absorb some of the shock load and can reduce noise due to impacts.

A variation on clamping collars adds threads to the inside bore. Such collars are used on threaded shafts, especially if the collars will see high axial loads or need to be precisely located, preloaded, or both. To resist high axial loads, threaded collars hold a distinct advantage over smooth bore collars. It is almost impossible to move a threaded collar axially on its shaft without breaking the shaft.

Bearing locknuts, which are threaded collars intended solely to mate with bearings, have precisely controlled tolerances for face runout to the threads to ensure even pressure on the entire bearing face and accurate preloads. Typically, spanner wrench slots machined into the collar's outer diameter simplify access and adjust this preload. As with other threaded collars, once you establish preload, the locknut is secured by tightening its screw.

Holding power is important for collars on split hubs. Split hubs connect components such as gearboxes, sprockets, encoders, and couplings to shafts. Shaft collars can serve as inexpensive clamps on split hubs, but some of the collar's clamping forces will be spent compressing the split hub. Thin fingers on the split hub and close tolerances between the shaft, split hub, and collar and help minimize this effect.

Screw size and quality are also critical to performance. Size is easily determined, but quality differences are sometimes overlooked. Better screws generally have welldefined, cleanly cut threads, are made of materials with high tensile strength, and are topped with properly sized and shaped heads that reduce frictional drag on the collar pocket. Screws should also have forged rather than broached sockets. With so many attributes to consider, the ultimate suitability of screws is best determined empirically.

Material strength and collar design are also key in turning screw torque into holding power. The collar should be of a material strong enough to withstand the recommended screw torque. Weaker materials can crack. The threads, bottom, and side of the counterbore can also deform under torque, leading to failure.

It is a common misconception that larger ODs make collars stronger. Larger ODs do let clamp screws be buried within the OD rather than left protruding. But unless collar width is increased to accommodate a larger screw, increasing OD relative to bore yields no benefits and can reduce holding power. This is because it takes some screw torque to bend the collar around the shaft, leaving less to clamp onto the shaft. To beef up clamping forces, use a larger screw or move the screw away from the centerline of the shaft to create more leverage and more holding power. Increasing the collar OD without changing screw size or location only means too much material must be bent before any force is applied to the shaft. Some force is always expended this way regardless, but it can be reduced and holding power increased by having the collar OD no larger than necessary.

Holding power can be further increased by having a back cut opposite the clamp-cut in the bore of a onepiece clamp collar. This thins the material at the hinge point of the collar, letting it bend easier. A two-piece collar further reduces the amount of material to bend and has a second screw to transmit torque, but the result is only a 2% increase in holding power over properly designed one-piece clamp collars. This small increase may not be worth the added cost of the two-piece design, especially if it complicates design and installation such as the need to get to both screws when the collar is in an enclosure.

Surface treatments on the collar and screws also contributes to collar holding power. Most collars are steel with a black-oxide finish. Black oxide makes it easier to install screws by eliminating stick-slip. Alternative surface treatments such as zinc resist corrosion better than black oxide but reduces the collar's holding power.

Shaft collars are sometimes used to align other shaft components with a precisely machined bearing face that is perpendicular to the shaft. A precise face-tobore relationship ensures components are square to the shaft and won't shift or tilt easily, especially when subjected to shock loading. It also eliminates spot loading in component-to-shaft contact. Misaligned components and spot loading prematurely wear components, especially bearings, and lead to excessive noise, slippage, whipping, and even total failure.

Shaft collars come in a variety of sizes and styles.

Two-piece clamping collars can be installed on shafts without having to remove the shaft or other components on it.

One-piece clamping collars provide high levels of holding power without damaging the shaft. They can fit shafts with diameters from 0.125 to 6.0 in. This stainless-steel version is suitable for corrosive environments.

Shaft collars can have threaded bored, letting them be precisely positioned and able to withstand high axial loads.

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