Sandwiched for safety
An oldham coupling is a three-piece coupling comprised of two hubs and a center member. Unlike other couplings, there are no bending members to increase bearing loads as shafts become increasingly misaligned. If the center insert fails, it breaks cleanly and prevents damage to more expensive components.

Oldham couplings

An oldham coupling is a three-piece coupling comprised of two hubs and a center member. The center disk, which is usually made of plastic (or metal) is the torque-transmitting element. Torque transmission is accomplished by mating slots in the center disk, located on opposite sides of the disk and oriented 90° apart, with drive tenons on the hubs. The disk's slots are slightly press fit on the tenons of the hub. This press fit allows the coupling to operate with zero backlash, though with time, sliding of the disk over the tenons can create wear — until the coupling exhibits backlash. However, the disks are inexpensive items that are easily replaced, and a new insert quickly restores the coupling's original performance.

In operation, the center element slides on the tenon of the hub to accommodate misalignment. Because the only resistance to misalignment is the frictional force between hub and disk, oldham-coupling bearing loads do not increase as misalignment increases. Unlike other couplings, there are no bending members to act as detrimental springs that increase bearing loads as shafts become increasingly misaligned.

This type of coupling allows a small amount of angular misalignment (less than 0.5°) and axial motion (less than 0.005 in.) and is limited to speeds of 4,000 rpm. Larger amounts of angular misalignment cause the coupling to lose its constant velocity characteristic. Also, axial motion is limited by the three-piece design of the coupling, which does not allow for push-pull motion. Further, because the center disk is a floating member, both shafts must be supported to keep the coupling from falling apart.

As far as parallel misalignment goes, this design is particularly well suited for handling relatively large amounts — from 0.025 to 0.100 in. or more depending on coupling size. Coupling manufacturers generally provide conservative misalignment ratings so users can obtain maximum life. However, these ratings may be surpassed at the expense of coupling longevity. The ability to choose different disk materials is an advantage of this type of coupling. Several manufacturers offer material choices to meet application needs. Generally, using one material is best where zero backlash with high torsional stiffness and torque are required. Other materials are useful in applications that have less precise positioning requirements, can tolerate some backlash, and also benefit from a quieter coupling that absorbs vibration. When nonmetallic, inserts are electrically isolating and can act as a mechanical fuse. If the plastic insert fails, it breaks cleanly and does not allow any transmission of power, preventing damage from occurring to other more expensive machinery components.

A quick bite
Curved-jaw couplings — a variation on the jaw design — are well suited to servo applications. Curved jaws help reduce coupling deformation and limit the effects of centrifugal forces during high-speed operation.

Jaw couplings

Conventional straight-jaw couplings are not appropriate for servo applications where accuracy of torque transmission is required. However, curved-jaw couplings — a variation on the jaw design — are well suited to servo applications. Curved jaws help reduce coupling deformation and limit the effects of centrifugal forces during high-speed operation.

These zero backlash curvedjaw couplings consist of two metallic hubs and a star-shaped elastomer insert, commonly called a spider. This multi-lobed insert fits between the drive jaws on the coupling hubs. (In other words, jaws from each hub alternate with the lobes of the spider.) As in the oldham coupling, there is a press fit between the jaws and the spider so there's no backlash. However, in contrast to the oldham coupling (in which torque disks are in shear under torsional loads) a jaw coupling's spider operates in compression.

When using a zero-backlash jaw coupling, users must be careful not to exceed the manufacturer's rating for maximum torque with zero-backlash, which can be significantly below the physical limitations of the spider. If torque does go higher, the spider can be compressed until there's no longer a preload; then backlash occurs, sometimes going unnoticed until a larger problem results. Jaw couplings are well balanced and can easily handle high-rpm applications (to speeds of 40,000-plus rpm) but are not able to handle big misalignments — especially axial motion. Large parallel and angular misalignments cause bearing loads higher than those produced by most other servo coupling types.

End users must understand how jaw couplings behave upon failure. If its spider fails, this kind of coupling doesn't disengage. Instead, the jaws from the two hubs pair (mating like teeth on two gears) and continue to transmit torque with metal-to-metal contact. Depending on the application this may be desirable; if it isn't, mating can cause problems in the larger, overall system.