One advantage to jaw couplings is the ability to mix and match spiders based on the application. Manufacturers of zero-backlash jaw couplings offer multiple materials with different values of hardness and temperature capabilities, so users can specify inserts that meet the application's exact performance criteria.

Delicate disks
At the very least, a disk coupling includes two hubs and a thin metallic (or composite) disk that transmits torque. However, some manufacturers offer disk couplings with two disks.

Disk couplings

At the very least, a disk coupling includes two hubs and a thin metallic (or composite) disk that transmits torque. (Usually this disk is fastened to the hubs with a tight-fitting pin that prevents play or backlash between the parts.) However, some manufacturers offer disk couplings with two disks that are

• Separated by a rigid center member and
• Attached to a hub at each end.

This rigid center member is usually metallic, but plastic versions offer electrical isolation. (Insulating property gains do come at a loss of torque capacity and torsional stiffness.) Interestingly, the difference between one and two-disk variations is similar to the difference between single and multiple-beam couplings. Single-disk couplings don't accommodate parallel misalignment due to the complex bending of the disk that is required. On the other hand, in two-disk styles, the disks bend in opposite directions to harness any parallel offset.

Other disk-coupling properties are comparable to those of bellows couplings, another type we'll explore shortly. In particular, the way they transmit torque is very similar. Torsionally, the disks are very stiff, with stiffness ratings slightly lower than those of bellows varieties. The disks are also very thin, so they bend easily under misalignment loading. This means the coupling handles large amounts of misalignment (up to 5°) with some of the lowest bearing loads available in a servo coupling. One drawback is that these couplings are very delicate and prone to damage if misused or installed improperly. So, for proper operation, special care must be taken to ensure that misalignment is within coupling ratings.

Bellows for bending
In most cases the hubs/bellows assembly is either welded or glued together. Thinner bellows walls allow bending under loads caused by angular, parallel, and axial misalignment between shafts.

Bellows couplings

A bellows coupling is a conclave of two hubs and a thin-walled metallic bellows. In most cases the hubs/bellows assembly is either welded or glued together. Although other materials are used, the two most common for the bellows are stainless steel and nickel. Nickel bellows are manufactured using electrodeposition. This involves machining a solid mandrel in the shape of the finished bellows. Nickel is electrodeposited onto this mandrel; the mandrel is then chemically dissolved to leave behind the finished bellows. This method allows precise control over wall thickness and produces thinner walls than other forming methods. Thinner walls give these couplings greater sensitivity and responsiveness, making them suitable for extremely precise, small instrumentation applications. However, the thinner walls also reduce torque capacity, putting a limit on useful applications. Stainless steel bellows are stronger than nickel versions and are usually manufactured with a process called hydroforming. (In this process, a thinwalled tube is placed into a machine and hydraulic pressure is used to form the convolutions of the bellows around specialized tooling.)

The characteristics of bellows make them ideal for transmitting torque in motion control applications. Thin, uniform bellows walls allow bending under loads caused by the three basic types of misalignment between shafts — angular, parallel, and axial. Generally, bellows allow for up to 1° to 2° of angular misalignment and 0.010 to 0.020 in. of parallel misalignment and axial motion. The thin walls result in low bearing loads that remain constant at all points of rotation, without the damaging cyclical high and low loading points found in other coupling types.

Perhaps most importantly, bellows couplings are the one of the most rigid styles under torsional loads. The stiffer the coupling, the more accurately motion is translated from the motor to the driven component — very beneficial to applications that require higher accuracy and repeatability.

The use of aluminum hubs with a bellows results in a coupling with very low inertia, a useful feature in highly responsive systems. Some bellows manufacturers also balance their couplings so they can be used in faster applications (10,000-plus rpm) as well. In addition, bellows couplings are available with stainless steel hubs, which can be useful in applications where corrosion resistance is important. However, the extra mass of stainless steel does detract from the main benefit of this otherwise lightweight component.