Gears, belts, and motors may be the workhorses of industry, but without locking devices, they wouldn’t accomplish a thing. Locking devices make multi-component solutions possible, holding almost any power transmission element on any shaft and letting it go when necessary.
Today, because of intense pressure to maximize uptime, precision, and efficiency while minimizing material, machining, and operating costs, keyless frictional locking devices are the hot trend. Although conventional keyless devices — now decades old — remain viable, the new breed of frictional types are clearly the future.
The old way
The goal of any motion control system is to repeat a specific move or move sequence many times without deviation. Timing belts and pulleys are frequently used to achieve such predictable precision because they are highly accurate as well as efficient.
Standard methods of mounting timing pulleys on shafts, such as keys, setscrews, taper-lock and QD bushings, reduce efficiency by introducing unwanted backlash into the system. And, if the pulley or other drive element is subjected to reversing or intermittent loads, backlash will worsen with time. Ultimately, you can count on a steady loss of accuracy and eventual connection failure.
As accuracy deteriorates, it may be necessary to adjust the system to restore the original timing of all indexed components. This process can be quite time consuming and is fraught with errors.
Another drawback to traditional technology is that it requires expensive machining. Keyways may need to be broached or milled, holes drilled and tapped, and tapers machined into drive components. Though partially offset by the efficiencies of mass production, the resources required to complete these manufacturing steps represent a hidden cost with standard solutions.
In light of the shortcomings associated with standard locking technology, keyless locking devices offer an attractive solution — a true zero-backlash, permanent yet fully adjustable connection that requires no complicated machining of the drive element.
Keyless locking devices connect components by means of a mechanical interference fit. Torque, thrust, and other loads are transmitted through frictional resistance generated by pressure applied simultaneously to the shaft and mounted component. Contact pressures easily exceed those achieved through traditional interference fits.
The resulting connection is entirely backlash-free, eliminating the problems inherent in keyed connections; micro-movement, fretting corrosion, impact loads, and loss of accuracy are of no concern here. In fact, assuming proper selection and installation, a keyless locking device connection should never fail.
Keyless locking devices also can be made quite small; they’ll even fit on thin-walled drive elements using a single taper to maximize concentricity. Any shaft you might find in a newer motion control system is fair game.
Because a key is not required to transmit torque, system components such as shafts and bearings don’t need to be as large — smaller parts, lower costs. Likewise, timing pulleys and other drive elements need only a straight-through bore, saving more time and money.
As for advantages of ownership, keyless locking devices can dramatically shorten maintenance cycles because they can be relaxed quickly and easily to original fit clearances. This can add up to a huge savings in terms of downtime when it comes to mounted components requiring periodic timing adjustments or removal.
How they work
Keyless locking devices work according to the wedge principle. Integrated high-strength steel rings with one or more tapered interfaces convert standard screw-clamp loads into radial contact pressure. In the case of internal devices, the pressure is applied between the shaft and component bore. With external devices, the pressure is applied around the drive component’s hub.
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Locking assemblies, which work internally, are like a bushing, inserting between the shaft and component bore. The concentric steel rings are pulled together by means of metric Grade 12.9 socket-head cap screws. The tapered interface converts the known screw clamp load R into a radial force N equaling.
where a = taper angle, p = friction angle (the angle at which gravity overcomes friction), and μ = coefficient of friction (μ = 0.12 for oiled steel-on-steel).
Contact pressures on the shaft and hub are easily determined.
where d = shaft diameter, D = locking assembly OD/bore diameter, and L = locking assembly contact length, all in inches.
Load capacities for locking assembly connections are determined as follows.
where N = radial force (lb), d = shaft diameter (in.), and T = peak drive torque (lb-ft).
Because locking assemblies exert internal pressure, components must be sufficiently sized to hold up. A classic example is open gearing.
Shrink discs, by contrast, work by “shrinking” or squeezing a portion of the mounted component down onto the shaft. Any thin-walled drive element is fair game for a shrink disc.
Fit clearances between the shaft and hub bore and between the hub OD and shrink disc are controlled to ensure the hub contracts within its elastic limits. The fact that there isn’t any plastic deformation means all components return to original fit clearances when the connection is relaxed.
As with locking assemblies, the radial forces required to contract the component hub are generated by pulling together tapered steel rings. For this purpose, shrink discs are typically supplied with metric Grade 10.9 hexhead cap screws. With the screws generating a clamp load R, the radial force N produced by a shrink disc is.
where a = taper angle and p = friction angle.
This force puts pressure on the outside of the component hub, a portion of which is required to contract the hub down around the shaft. The remaining pressure is applied directly onto the shaft, generating the interference required to transmit torque, thrust, and other loads as follows.
where P = pressure on hub OD (psi), PCL = pressure required to bridge the clearance between shaft and hub (psi), L = shrink disc contact length (in.), d = shaft diameter (in.), and T= peak drive torque (lb-ft).
Other factors that come into play when selecting and sizing locking devices include concentricity, overall component dimensions, space limitations, access to locking screws, and load requirements (torque, thrust, bending).
Keyless locking devices get into motion control
Traditional component mounting technologies were designed primarily for simple shaft-turning applications. In contrast, keyless locking devices were created with high-performance motion control in mind. As the limitations of older technology become more obvious, designers and users of precision industrial automation equipment are more frequently taking the modern approach.
A large consumer products manufacturer, for example, recently retrofitted all of its timing pulleys on a production line with single-taper, flange-type locking assemblies. Standard QD and taper-lock bushings, used previously, were repeatedly losing accuracy.
The switchover required no new components as the manufacturer was able to re-bore its existing pulleys and mount them back onto the original keyed shafts. The net savings in both downtime and maintenance costs have been dramatic now that there’s no need to regularly re-time indexed components.
In another case, a leading OEM of custom industrial automation equipment switched from standard keyways and setscrews to shrink disc connections for a zero-backlash coupling used on a line of packaging machinery. The elimination of backlash from the coupling connections optimized the accuracy of the entire motion control system.
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To ensure the long-term viability of a keyless locking assembly connection, you need to know several things about your application.
Shaft diameter — Locking assemblies are specified by the shaft diameter they’re to be mounted on, and are available in both inch and metric sizes. Shrink disc types are typically metric only, with each size accommodating a range of different inch or metric sized shafts.
Peak torque — Like all interference fits, keyless locking devices have a known load-carrying capacity. If its capacity is exceeded, the connection (theoretically) is expected to slip for as long as the excess load is applied. To avoid this kind of failure, know your application’s peak torque and make sure it doesn’t exceed that of the locking device. Watch, however, because some keyless locking device manufacturers report torque capacities with a safety factor built-in, while others do not.
Yield point (psi) — Keyless locking devices generate a mechanical interference fit by exerting pressure on shafts and components. Because shrink discs subject components to external pressure, material strength is rarely a critical factor. In contrast, locking assemblies exert internal pressure on mounted components. Naturally, components must be sufficiently sized or they will yield from the stresses generated by the locking assembly.