Using rigid couplings for precision applications

Rigid couplings historically have been imprecise, inexpensive, and often home-made components for simple shaft-to-shaft connections. It is not surprising that in the past, many people would not consider using rigid couplings in a servo application.

However, smaller-sized rigid couplings, especially in aluminum, are increasingly being used in motion control applications due to their high torque capacity, stiffness and zero backlash. It is important though, that unlike their common counterparts, the rigid couplings used have the precision and features necessary for low maintenance and accurate performance.

As the name implies, rigid couplings, which are sometimes called sleeve or muff couplings, are torsionally stiff couplings with virtually zero windup under torque loads, but they are also rigid under loads caused by misalignment. If any misalignment is present in the system, the forces will cause the shafts, bearings or coupling to fail prematurely.

This also means that the couplings cannot be run at extremely high rpm, since they cannot compensate for any thermal changes in the shafts that can be caused by heat build-up from high-speed use. However, in situations where misalignment can be tightly controlled, rigid couplings offer excellent performance characteristics in servo applications.

Shaft alignment

A sometimes overlooked advantage of rigid couplings is that they can be used to establish shaft alignment in some systems. First, loosen motor and other component mounts so that there is free play. Then connect the shafts with the rigid coupling, which, if precisely made, will align the shafts. Centre the components on any remaining free play and tighten the mounts.

There are several features required of rigid couplings to ensure proper performance in motion control applications. Most important is that the coupling itself does not introduce misalignment into a system where it cannot be absorbed without damage to bearings and seals or by causing poor system performance.

The strictest control of shaft alignment will result when the bores are honed, since honing assures that both bores are collinear. Honing also corrects any residual distortions caused by stresses introduced during the manufacturing process, resulting in a round, precisely sized bore. Proper sizing and geometry assure a large percentage of shaft contact and greater torque transmission ability.

The simplest style of rigid couplings has set screws to fix the coupling to the shaft through impingement. A superior alternative is clamp-style rigid couplings, since they wrap around the shaft to provide high torsional holding power without the shaft damage and fretting inherent in set-screw types.

Two-piece styles have the additional benefits of allowing for disassembly and maintenance without removal of other components. When the hardware on a two-piece rigid coupling is opposing, the coupling can be operated at higher rpm, since it is dynamically balanced.

As a guideline, one-piece rigid couplings can be evaluated for applications up to 3,000 rpm. The guideline can be increased to 4,000 rpm when a two-piece style with opposing hardware is used.

Rigid couplings lack a mechanism to absorb the vibration inherent in many mechanical systems. Vibration can cause hardware to loosen and torque transmission ability to diminish during normal use. Placing a nylon treatment on the screw threads can reduce the effect of vibration on the hardware for increased coupling reliability. The nylon also provides the necessary dissimilar material to reduce galling of the screw threads in stainless steel couplings.

Proper installation

Most clamp-style rigid couplings have cap screws close to one another and arranged in pairs. This design, especially when combined with a cross-cut, facilitates greater holding power and also accommodates slight deviations in the size of the two shafts being connected.

It is recommended that this style of coupling be installed by tightening the paired screws alternately in several steps. This is because the close proximity of the screws results in a mutuality of the hoop stress developed in the coupling by each screw in a pair. As each screw is tightened, it tends to relax any tension developed by its companion. Alternately tightening the screws in several steps distributes the tension more evenly, assuring a tighter fit and the desired better holding power.

A less common but sometimes useful type of rigid coupling is the three-piece clamp-style. This design allows for more convenient exchange or adjusting of coupled shafts, particularly where the shaft cannot be axially detached. Either side of the coupling can be detached and the shaft removed without disturbing the other shaft connection. The three-piece design also accommodates a slightly larger variation between the sizes of the shafts being connected by clamping independently on the two shafts.

Summary

The benefits of rigid couplings include their economy, high torque capacity, torsional stiffness and zero backlash.

Selecting a coupling for a servo application involves many different performance factors, including torque, shaft misalignment, stiffness, rpm, space requirements and others that all must be satisfied for the coupling to work properly.

When they are precisely manufactured with honed bores and other features to assure performance, rigid couplings are increasingly used in motion control applications where components are properly aligned. Frequently, the coupling itself is used to establish the needed alignment. In addition to motion control applications, rigid couplings are often used to connect line shafting or other components such as a motor to a gearbox. MRO

Bill Hewitson is director of manufacturing and engineering at Ruland Manufacturing Co. Inc.

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