Simplicity offers advantages
Rigid couplings are stiff under loads caused by misalignment. However, in situations where misalignment can be tightly controlled, rigid couplings offer excellent performance characteristics.

Rigid couplings

Rigid couplings are known for being imprecise, inexpensive, and often homemade components for simple shaft-to-shaft connections — so many designers wouldn't consider using this coupling type in motion control applications. But now, smallersized rigid couplings (especially in aluminum) are increasingly common due to their high torque capacity, stiffness, and zero backlash.

As the name implies, rigid couplings are torsionally rigid with virtually zero windup under torque loads. The drawback is that they are also rigid under loads caused by misalignment. If any misalignment is present in the system, the forces cause shafts, bearings, or the coupling itself to fail prematurely. This means that these couplings cannot be run at extremely high rpms, because they don't compensate for any thermal changes in shafting. However, in situations where misalignment can be tightly controlled, rigid couplings offer excellent performance characteristics.

Most important in motion control applications is that the rigid coupling itself does not introduce misalignment into a system where it cannot be absorbed without damage to bearings, seals, or system performance. Frequently, the coupling itself is used to establish the needed alignment. The motor and other component mounts are loosened so that there is free play. Then, the shafts are connected to the rigid coupling that, if precisely made, will align the shafts. Finally, the components are centered on any remaining free play and the mounts tightened.

Shaft alignment is best when its 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 assures a large percentage of shaft contact and greater torque transmission ability.

Rigid couplings lack a mechanism to absorb the vibration inherent in many mechanical systems. Consequently, vibration can loosen hardware and degrade torque transmission during normal use. Placing a nylon treatment on the screw threads can reduce these effects on the hardware for increased reliability. Also, as a dissimilar material nylon reduces galling of the screw threads in stainless steel couplings.

Setscrews fix the simplest rigid couplings to shafts through impingement. A superior alternative is clamp-style rigid couplings, because they wrap around the shaft to provide high torsional holding power without shaft damage and the fretting that is unavoidable with setscrews. Two-piece styles also allow for disassembly and maintenance without removal of other components. When the hardware on a two-piece rigid coupling is opposing, it dynamically balances and the coupling can operate at higher speeds. (As a general guideline, one-piece rigid couplings are evaluated for applications up to 3,000 rpm. This can increase to 4,000 rpm when a two-piece style with opposing hardware is used.)

Most clamp-style rigid couplings have cap screws close toby-gether and arranged in pairs. This design, especially in combination with a crosscut, facilitates greater holding power and accommodates slight deviations in the shaft sizes being connected. One warning: It is recommended that this coupling style be installed by tightening the paired screws alternately in several steps. 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.

Acknowledgements to Fred F. Ruland and thanks to Robert G. Ruland. For more information, call (800)225-4234 or visit www.ruland.com.