Putting the squeeze on

June 1, 2006
They're one of the simplest components in power transmission. They're also one of the most indispensable. Among their many roles, shaft collars hold sprockets

They're one of the simplest components in power transmission. They're also one of the most indispensable. Among their many roles, shaft collars hold sprockets and bearings on shafts, situate components in motor and gearbox assemblies, and serve as mechanical stops.

Like their predecessors, clamp-style collars are relatively simple in concept. They come in one and two-piece varieties, both using socket-head cap screws to create the compressive forces that close the collar onto the shaft and lock it in place. This design makes them easy to install and remove, and infinitely adjustable.

Another advantage with clamp-style collars is that they work well on virtually any shaft. This has to do with the fact that holding power does not depend on the impingement of a screw, taking shaft material largely out of the equation in how the collar performs. Tightening a few clamp screws closes the collar onto the shaft for a nearly uniform distribution of forces around the shaft's circumference. This uniform clamping is mechanically more secure than point contact, as much as doubling holding power depending on shaft size and condition.

Overcoming shock

Clamp-style collars are extremely dependable under relatively constant loads. Align and lock them, and they'll pretty much do the rest on their own. When confronted with shock loads, however, they may need a little help to stay in place. Think of a hammer hitting a nail — even relatively small masses can create very high forces during impact, in contrast with statically or gradually applied loads.

One way to deal with shock loading is to add an undercut on the shaft and mount the collar on that. The sides of the undercut create positive stops in both (axial) directions. Applications like this take advantage of two main benefits of two-piece collars, namely their holding power and ease of assembly.

One-piece collars sacrifice some of their clamping ability because the screw uses a portion of its seating torque to bend the collar around the shaft. Two-piece collars, in contrast, use their full seating torque to apply clamping forces to the shaft. Two-piece collars also install and assemble more easily. While one-piece designs must slide over the end of a shaft, two-piece collars can be disassembled and installed in position without removing other components. With an undercut shaft, a one-piece collar would have to be pried open to accommodate the shaft's diameter.

Other ways of dealing with shock include stacking multiple collars and adding bumpers or pads. Multiple collars increase load capability two ways: They achieve greater clamping force (afforded by multiple screws), and offer more shaft contact, which increases friction. Double or otherwise extra-wide collars with multiple screws have the same advantages as a stack of collars. Bumpers and pads alleviate shock by absorbing some of the load and also serve to reduce noise caused by impact.

As we'll soon discuss, yet another solution to shock is to thread the inner bore of the collar.

Holding power

Choosing the best shaft collar is a matter of matching one or more performance factors with application requirements. In many applications, the collar's holding power is paramount. Other important factors include weldability, inertia, conductivity, corrosion resistance, and collar-face precision as it relates to the bore.

Holding power is especially important when collars are used as mechanical stops or on split hubs. (Split hubs are interfaces for connecting components such as gearboxes, sprockets, encoders, and couplings to shafts.) Collars are effective split-hub clamps, but the application is particularly demanding since a portion of the clamping force is expended closing the hub, which reduces the forces applied to the shaft. Maintaining close tolerances between the shaft, hub, and collar (and keeping hub finger thickness as small as possible) helps minimize the amount of force lost on the hub itself.

Holding power depends on several design and manufacturing variables, including bore size and concentricity. It also depends on fundamental mechanics, a function of the amount of screw torque indirectly transmitted to the shaft by frictional forces between the shaft and bore.

Screw size and quality also factor into holding power. The former is easily determined, but quality differences are sometimes overlooked, even with all the standards for screw specification today. Better screws have better thread quality, tensile material strength, and closely held head geometry and size tolerances, which eliminates frictional drag on the collar socket. In general, forged screws are superior to those that are broached. But with so many attributes to consider, comparisons are best made empirically.

Bigger not better

One common misconception is that a larger outer diameter makes a collar stronger. A large outer diameter does mean clamp screws aren't left protruding; they can be buried in the collar. Unless width is also increased to accommodate a larger (again, protruding) screw, a larger diameter relative to the bore size has no performance benefit. In fact, it can even work to reduce holding power. Increasing the outer diameter of the collar without making any changes to the screw size or location only creates a condition where excess material must be elastically deformed before forces are applied to the shaft. Some forces are expended this way regardless, but they can be reduced and holding power increased by having the collar outer diameter no larger than necessary.

Another way to reduce force loss — further increasing holding power — in one-piece designs is by adding a back cut opposite the clamp-cut in the bore. This reduces the amount of material at the collar's hinge point. Use of a two-piece collar reduces the amount of material to bend even more with the added advantage of a second screw to transmit torque.

Still, this only increases holding power about 2% over a properly designed one-piece clamp collar. This small increase may not be worth the added cost of a two-piece design, especially if it complicates design and installation. For example, it may be difficult to provide access to both screws when a two-piece collar is used in an enclosure.

Though the holding power gained by a two-piece collar may not be significant — and one-piece designs are often more convenient — two-piece collars benefit from easy disassembly and removal from the shaft, and they usually come off without disturbing other components. Moreover, two-piece collars can be balanced more easily by the opposing screws. Balancing is sometimes necessary where the collar rotates at high rpm, as may be the case with split-hub applications.

Align and locate

In addition to their holding power, shaft collars are distinguished by their ability to locate and align other shaft components with a precisely machined bearing face. To maximize this quality, shaft collars may be single-point faced at the same time that the bore is finished. This results in low runout (TIR less than or equal to 0.002) when the collar is mounted on the shaft — important when interfacing with other precision components.

Some manufacturers mark which collar face is perpendicular with a circular groove. This groove is also a key indicator of orientation for proper assembly of two-piece devices.

Clamp-style collars, because of their easy adjustment, are often used to locate components (sprockets, gears, pulleys, and ball bearing units) on a shaft. In these cases, the ability to retain axial loads is important, but perpendicularity of the collar face to the shaft is just as critical.

A precise face-to-bore relationship keeps components perpendicular to the shaft, preventing shifting or tilting. This, in turn, prevents premature wear and problems such as noise, slippage, and whipping — potential problems where sprockets or pulleys are concerned.

Face perpendicularity also ensures even pressure at the interface with the mounted component, eliminating spot loading and its propensity to shorten component life. Where collars are used against bearings, for example, squared collars keep bearing loads evenly distributed, maximizing lifetime. Properly squared collars also protect machinery that sees moderate axial shock loading. For example, on linear actuators where collars are used as mechanical stops, face perpendicularity makes for even force distribution across the collar, minimizing impact pressure and preventing the collar from shifting position.

Nice threads

Threaded-bore collars are made in all collar types, but are most common in one and two-piece clamp styles. (Setscrew styles do major and permanent damage to threaded shafts on which they're installed by impinging the screw into the threads.) Clamp-style threaded collars open possibilities for many applications using threaded shafts, but stand out particularly in two areas: applications with high axial loads and those requiring fine location or preload adjustments.

For high axial loads, threaded collars hold a distinct advantage over smooth-bore types. The latter rely entirely on friction for resistance to axial loads, making them susceptible to movement upon impact. In contrast, threaded collars have a positive mechanical stop created by the interface of collar and shaft threads, making the collar virtually impossible to move axially without breaking the shaft itself.

For fine adjustments and preloading components such as bearings, the collar is threaded into location and locked into place with tightening. Bearing locknuts are specially designed threaded collars intended solely to mate with bearings. These locknuts have an even more precisely controlled tolerance for face runout relative to the threads, ensuring even pressure on the entire bearing face and precise control of preload. Typically, spanner wrench slots are machined into the outer diameter of the locknut for easy access and precise preload adjustment.

Special thanks to Robert Ruland, and credits to Fred F. Ruland. For more information call (508) 485-1000 or e-mail the editor at [email protected].

Collar evolution

The first mass-produced collars were used primarily on line shafting in early mills. They were solid ring types, employing square-head setscrews that protruded from the collar. Protruding screws proved to be a problem because they could catch on workmen's clothing and pull them into machinery.

Shaft collars saw few improvements until the early 1900s when Howard T. Hallowell, the founder of the fastener manufacturer SPS Technologies, Jenkintown, Pa., created the first recessed head socket setscrew collar. Hallowell was awarded a patent for his safety set collar, which soon became the industry standard — and in time, copied by others. The invention was the beginning of the recessed-socket screw industry.

Setscrew collars derive all their holding power from the screw tightened onto the shaft. Holding power depends greatly on the shaft material and condition. For a setscrew collar to achieve maximum grip, the shaft must be a softer material than that of the setscrew. This allows impingement of the screw point into the shaft, which helps keep the collar in its installed position under torque and axial loads, and prevents it from sliding along the shaft.

Setscrews damage shafts, however, which is undesirable for functional and cosmetic reasons. Screw impingement causes an eruption of material around the screw, resulting in a raised burr on the shaft surface. This raised material makes it difficult to remove the collar for replacement or adjustment. What's worse, small angular and lateral adjustments are almost impossible because the screw point is always drawn back to the center of the original impingement.

No one knows who actually invented the clamp-style collar. They've been around at least since World War II when they were used in bombsights and guiding systems. These essentially mechanical military instruments consisted of precision gearing, differentials, couplings, and collars in combination with electrical selsyn motors, resolvers, precision potentiometers, and a smattering of electronics. They were considered very high tech — and top secret — and were the forerunner of the analog computer. Clamp-style collars, though common today, were, at that time, cutting-edge precision components used only in advanced equipment.

Tightening a screw is a deceptively simple task. When it's done to attach collars to shafts, any number of difficulties can arise. One common problem is stick-slip. Stick-slip can create a false impression that a screw has been tightened to its appropriate stress level.

During tightening, a screw rotates uniformly as it's torqued down, then it reaches a point (before achieving design torque) where its rotation gets sticky. Here, the screw begins to turn in a choppy manner, stopping and starting even though tightening torque is constantly applied. The lost torsioning effort during stick-slip is typically absorbed as excess friction between the threads (or underside of the head) and the mating parts of the clamp body, instead of contributing to the stress in the joint elements. If these stresses (in the joint) are too low, the collar won't hold well.

The best way to ensure stick-slip isn't making screws appear tighter than they actually are? Using specially coated elements. For example, black oxide smoothes the torquing of screws without diminishing the frictional characteristics (and holding power) of the bore. Clamping screws that operate smoothly during torquing are the best assurance that stick-slip is not present.

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