by their very
classification, must live
up to some picky
expectations. Not just
any style will do,
however, as their
attributes must closely
match the system
Servo systems must be totally predictable where positioning is concerned, and this calls for limited play and high mechanical stiffness, from shafting and gears right down to the mountings. The servo-grade coupling, therefore, must have zero backlash and significant torsional rigidity while conforming to parameters such as misalignment, speed, and torque. Several distinct forms of servo couplings meet the many possible application needs.
Beam-type couplings are made from a single piece of material, usually aluminum. They’re cut spirally, forming platelike beams that bend to give lateral flexibility while remaining torsionally rigid and strong. This coupling style is a solid all-purpose choice, offering good performance at a low cost. The single-piece design, with no joints or mating parts, promotes zero backlash and requires no maintenance.
There are two basic variations of beam couplings – single-beam and multiplebeam arrangements. The single-beam style has one long continuous cut that usually encompasses several complete turns. This results in a coupling that’s very flexible and can minimize bearing loads. It accommodates all types of misalignment, but works best with angular misalignment or axial displacement. Parallel misalignment capabilities are smaller because the single beam has to bend in two different directions at the same time, undergoing high stress that could lead to premature failure. Although a longer beam will bend more easily, it is also more flexible in the torsional direction. The large amount of windup adversely affects the accuracy of the coupling and reduces its overall performance. Singlebeam couplings, while relatively inexpensive, are best for low-torque applications, especially in connections to encoders, tachometers, and other light instrumentation that might be installed away from the primary load path.
Multiple-beam couplings consist of two or three overlapped beams. Instead of a continuous helix, repeated helical cuts are overlapped, such as in multi-start threads on a screw, although each cut may be less than a full rotation. Overlapping allows shorter beams without losing much of the coupling’s misalignment capabilities. Shortening the beams and overlapping them to work in parallel increases the coupling’s torsional rigidity and capacity. Nevertheless, the loss of flexibility is enough to increase bearing loads (reactions from misalignment) a good deal over the single-beam variety, but these loads still tend to be low enough to keep the bearings safe. Multi-beam couplings are suitable for use in semilight applications, such as between a servomotor and a lead screw.
The beam concept can be further modified. Instead of one long cut or set of adjacent multiple cuts, two separate groupings of cuts can be made. This gives additional flexibility with better acceptance of parallel misalignment, as one set of beams bends in one direction while the second set bends in the other direction, with a rigid section between them. Such an arrangement is most often used in conjunction with multiple-beam construction rather than single-beam.
While beam couplings are most commonly aluminum, stainless steel is usually an option. In addition to corrosion protection, stainless steel raises the torsional strength and stiffness of the coupling; these can be close to double that of a comparable aluminum component. However, the higher rigidity and torque capacity is offset by a dramatic increase in mass and inertia. A smaller motor will spend a large percentage of its torque to overcome the coupling’s inertia.
This three-piece coupling is comprised of two hubs and a center disc. The disc is the torque-transmitting element and is made of plastic or, less commonly, metal. There are slots in the center disc, located on opposite faces of the disc and oriented 90° apart. Drive tenons (load-bearing projections) on the hubs are fitted to the disc slots with a slight press fit, and torque is thus transferred between disc and hubs. The press fit provides zero backlash, but over time the sliding of the disc over the tenons creates wear to the point that the coupling will have play. However, the discs are inexpensive and easily replaced, and inserting a new one 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 the hub and disc, oldham couplings allow bearing loads to remain the same regardless of the level of misalignment. Othertypes of servo couplings flex to fit the misalignment, with greater force required for larger deflections, and the load is ultimately taken up by the bearings.
Oldham couplings shift to accept significant parallel misalignment (from 0.025 in. to 0.1 in. or more depending on coupling size) but they allow less than 0.5° of angular misalignment and less than 0.005 in. of axial motion and are limited to speeds of 4,000 rpm. With larger degrees of angular misalignment the coupling loses its constant-velocity characteristic. As for axial movement, the three-piece design (with the center disc a floating member) prohibits use in pushpull applications. Furthermore, both shafts must be supported to keep the coupling from falling apart. Manufacturers generally provide smaller misalignment ratings in order to improve coupling life, but these ratings can be surpassed at the expense of endurance.
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One advantage of oldham couplings is the variety of disc materials available. For instance, one material is best used for zero backlash along with high torsional stiffness and torque. Another material absorbs vibration and noise in applications where positioning need not be as precise. Nonmetallic inserts are electrically isolating, and some can act as a mechanical fuse (similar to a shaft key made to shear if there’s an overload). Plastic inserts, for example, fail by breaking cleanly, completely stopping the transmission of power and preventing damage to more expensive machine components.
There are two general types of jaw couplings, the conventional straight-jaw and the zero-backlash curved-jaw coupling. Jaw couplings consist of two metallic hubs and an elastomer insert, commonly referred to as a “spider.” Straight-jaw couplings usually aren’t suitable for servo applications where accuracy is essential. On the other hand, the curved jaws of the zero-backlash variation help reduce deformation of the spider and limit the effects of centrifugal forces during highspeed operation.
The spider is a multiple-lobed insert located between the jaws on the coupling hubs, with a jaw from each hub alternately fitted between the lobes of the spider. Similar to oldham couplings, a press fit between the jaws and insert lets the coupling maintain zero backlash. Also, multiple spider materials are available, with different temperature capabilities and hardnesses, so there is the opportunity to mix and match spiders to suit the application. Counter to the oldham coupling, where the torque disc undergoes shear during torque transmission, the jaw coupling’s spider is compressed.
Jaw couplings are well balanced and able to handle high speeds – ratings can be up to and beyond 40,000 rpm. But, they cannot handle much misalignment and are especially intolerant of axial motion. With this coupling type, large parallel and angular misalignments lead to higher bearing loads than with most other servo couplings.
With a zero-backlash jaw coupling, the spider is usually capable of withstanding loads well in excess of the maximum torque rating for zero-backlash operation. However, if the coupling is pushed past this rating, the spider can be compressed so that the preload is eliminated and backlash will occur, possibly without the user noticing until a related problem arises.
Failure is something to be aware of. If a spider fails, the coupling will not disengage. The jaws on the two hubs will mate much like gear teeth and continue to transmit torque with metal-to-metal contact, a situation that may be desirable or problematic, depending on the application.
This coupling is assembled from a thin-walled metal bellows joined, usually with a weld or an adhesive, to a hub on each end. There are numerous bellows materials, but stainless steel and nickel are the most common.
Stainless steel bellows are stronger than nickel versions and are usually hydroformed. In this manufacturing process, a thin-walled tube is placed into a machine where hydraulic pressure forms the convolutions of the bellows around specialized tooling.
Nickel bellows are manufactured with an electrodeposition method. This involves machining a solid mandrel into the shape of the finished bellows. The nickel is electrodeposited onto the mandrel and the mandrel is then chemically dissolved, leaving the finished bellows. Electrodeposition allows for precise control over the bellows’ wall thickness and makes for thinner walls than other methods of bellows forming. Thin walls promote flexibility and lower inertia and give the coupling greater sensitivity and response for extremely precise small instrumentation applications. However, the thinner walls reduce the torque capacity, putting a limit on potential uses.
Bellows couplings in general are superior for a wide range of motion control applications. The uniform thin walls of the bellows let it flex easily under axial motion as well as angular and parallel misalignment. These couplings generally accommodate maximum angular misalignment from 1 to 2°, and parallel misalignment and axial motion from 0.01 to 0.02 in. The flexibility and uniformity of the bellows results in low bearing loads that stay constant throughout the rotation; there are none of the damaging, cyclical high and low load points found in some coupling styles. This is accomplished while maintaining high torsional rigidity. In fact, bellows-type couplings are some of the stiffest available and therefore ideal for applications requiring a high degree of accuracy and repeatability.
The hubs of bellows couplings are usually stainless steel or aluminum. While stainless hubs fend off corrosion, their mass and inertia detract from performance.
Bellows couplings can be balanced (sometimes this is taken care of before purchase) to withstand speeds exceeding 10,000 rpm.
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This style of coupling, at its most basic, is comprised of two hubs and a thin metallic or composite disc as the torquetransmitting element. The disc is ordinarily fastened to the hubs with a tightfitting pin that prohibits play and backlash between them.
A variation on the disc coupling has two discs separated by a rigid center member and attached to a hub at each end. The difference between this and the more basic single-disc form is quite similar to the difference between beam couplings with one set and those with two separated sets of cuts. While the singledisc coupling is not very adept at accommodating parallel misalignment due to the complex bending that would be required of the disc, the two-disc style allows each disc to bend in opposite directions to harness this parallel offset.
The thin discs bend easily, and these couplings accept large amounts of misalignment (up to 5° with multiple discs) with some of the lowest bearing loads. Torsionally they are very rigid, with stiffness ratings only slightly lower than those of bellows couplings. A downside to these couplings is their delicate nature. They are prone to damage if misused or improperly installed. Therefore, it’s best to make certain that misalignment is within the coupling’s ratings.
Smaller-sized rigid couplings are increasingly being used in motion control applications. They have high torque capacity and provide zero backlash, and windup under torsional loading is virtually nonexistent.
However, they are completely intolerant of misalignment. If any is present, forces will cause the shafts, bearings, or coupling to fail prematurely. This also means rigid couplings should not be run at extremely high speeds because they will not compensate for the subsequent heat buildup and thermal expansion in the shafting. Nonetheless, in situations where misalignment can be tightly controlled, rigid couplings offer excellent servo-grade performance.
William Hewitson is Product Manager with Ruland Manufacturing Co. Inc., Watertown, Mass.