Updated May 2015 by Mike Santora || Couplings are devices that connect two shafts for transmission of torque from one to another, usually on rotating equipment such as motors. They should be designed to allow for some end movement. Two types are available — rigid and flexible. Flexible couplings compensate for misalignment, while rigid couplings work where shafts are already in alignment.
Within these two broad categories exist a variety of coupling styles. Rigid couplings include sleeve-style and clamped or compression-style couplings. Flexible couplings include bellows, jaw, Oldham, disc, and beam couplings. Some of these are precision (motion-control) couplings that allow for precise transmission of velocity, angular positioning, and torque.
Rigid couplings are torsionally stiff and are most suitable when shafts are already in proper alignment. One drawback is that they are susceptible to vibration and cannot be run at high speeds. Most engineers agree that rigid couplings require a parallel shaft misalignment well below a thousandth of an inch.
Sleeve-style rigid couplings are suitable for light to medium-duty applications. The one-piece sleeve — which is essentially a tube with an inner diameter that is the same as the shafts it is joining together — has two set screws to fasten it to the shaft. They are very easy to use and offer high torque capacity, stiffness, and zero backlash.
Clamped or compression style couplings come in two parts that completely wrap around the shaft. Like most coupling designs, this protects the shaft from damage while providing high torsional holding power. Their advantage is their two-piece design, which allows easy removal for maintenance.
Flexible couplings go where there is a slight amount of misalignment between shafts. They accommodate misalignment while still transmitting torque.
Of course, misalignment — whether lateral, axial, angular, or skewed — makes for less efficient use of motor speed and torque. Misalignment also contributes to premature wear including broken shafts, failed bearings and excessive vibration.
Flexible couplings are typically the most compliant components in mechanical motion systems, so their torsional stiffness is a critical factor in maintaining positional control over a load. Many users of servomotors require the shaft to start and stop multiple times per second, which requires a torsionally stiff coupling to help diminish the settling time between cycles. However, torsionally flexible couplings provide maximum torque capacity for a given body size. Torsionally flexible couplings are naturally better for vibration damping, which is needed in continuous motion applications and cyclic-duty applications alike.
Recall that on machine axes, motion can be continuous or start-and-stop. With the latter, couplings can help damp vibration and diminish the system settling time for higher throughput. In contrast, continuous-motion applications benefit more from torsional strength than damping capabilities. Motion applications that require precise motion control (such as packaging and scanning and inspection) call for zero-backlash couplings.
Bellows couplings excel in motion-control applications that require precision control despite shaft misalignment. If an application requires precision, understand the performance factors critical to selecting a bellows coupling suitable for the task.
They are flexible with zero-backlash.
Note that there is a difference between backlash (which is a true mechanical clearance, such as that which is found between gear teeth) and torsional deflection, or wind-up, which all components exhibit to some degree. Most couplings are preloaded to eliminate backlash or are inherently backlash free, like the bellows coupling. But they all have different levels of torsional stiffness, which is often traded off for lateral flexibility during the coupling selection process. Bellows couplings have very high torsional stiffness, do not handle quite as much misalignment as others, but also do not impose heavy reaction loads onto the shafts and bearings as they flex.
Key benefits of bellows couplings include misalignment compensation and precise transmission of velocity, positioning and torque. Bellows couplings are known for their exceptional torsional rigidity, and flexibility in dealing with axial, angular and parallel shaft misalignment. Bellows couplings are typically made from a stainless-steel tube hydroformed to create deep corrugations that make them flexible across axial, angular, and parallel shaft misalignments. The bellows absorbs any slight misalignments between the mounting surfaces of two components.
Bellows couplings also absorb slight misalignments from perpendicularity and concentricity tolerances between the mounting surfaces of the two connected components. Plus bellows couplings tend to have the highest torsional stiffness of any coupling paired with servomotors, and are common in applications requiring highly precise positioning.
Jaw couplings feature two metal hubs and a spider insert, usually made of elastomer, which are fitted together to absorb vibration and shock. The elastomer is available in a variety of hardness and temperature ratings, so the spiders can be chosen for specific applications. Because they are not as torsionally stiff as other couplings, they are better suited to constant motion applications.
They are available in two types: straight jaw and curved jaw with zero backlash. Because accuracy of torque transmission can be an issue, straight jaw couplings are not used in most servo applications. Curved jaw couplings, on the other hand, reduce deformation on the spider and the effects of centrifugal forces during high-speed (up to 40,000+ rpm) operation. Both types can easily handle axial motion.
If a spider breaks, the driving jaws can still contact the driven jaws directly, maintaining operation, making jaw couplings failsafe designs.
Oldham couplings can be preloaded to eliminate backlash and can handle misalignment of all types depending on the disc material. They are being used more often as an alternative to straight jaw couplings on general industrial equipment such as pumps, valves, gearboxes, and conveyor systems. They are versatile, easy to use and install, and offer long lives when misalignment is an issue. Their three-piece design—two hubs and a torque-transmitting center—makes them easy to install and disassemble.
Oldham couplings are very versatile, and can be specified in a variety of materials to meet the needs of different applications; for example, if zero backlash is required vs. vibration reduction.
They are best suited when parallel misalignment may be high. And because of their three-piece design, axial motion is limited.
Disc couplings are a logical choice for servomotor and other demanding applications because of their ability to transmit high torque, operate at high/changing speeds, and to handle misalignment and system loads.
While a coupling’s torque, misalignment, and speed capacities need to be evaluated against a system’s requirements, the disc-pack usually is the most important aspect of the coupling’s construction because it will affect all critical performance aspects of the coupling and the system in which it is used.
The most common type of disc-pack is made of metal and can be found in different shapes (straight-sided, scalloped edges, square, etc.). In the case of metal disc couplings, double flex designs need to be used if there is to be any parallel shaft misalignment. The single flex variety of metal disc coupling is good for angular misalignment but not parallel. This can be quite advantageous in case a user needs to suspend a load between two single flex couplings, because their lateral stiffness can support the weight of the intermediate component.
For details on a different design from Zero-Max Inc., see the sidebar on its CD couplings.
Beam, or helical couplings are almost always manufactured of aluminum but stainless-steel versions are also available for use in corrosive environments and increased torque and stiffness. Their one-piece design makes them easy to maintain. Offering zero backlash, they feature spiral cuts that transmit torque and can handle all types of misalignment, angular, parallel, or axial motion. Parallel motion is more of a challenge for the single beam design because it must bend in two directions, which causes stress and possibly even failure.
Two designs exist under this style—single and multiple beams. Single beams are best suited to low-torque applications where no parallel misalignment is present, while multiple-beam designs are stiffer, for higher maximum torque capabilities.
Selection tips for couplings
When selecting a coupling, determine the most important design objective: Is it to provide torsional stiffness, offer vibration damping, or accommodate radial misalignment? Also determine shaft size and torque needs, as these dictate coupling size. In addition, identify the coupling materials will withstand the application environment — steel, stainless steel, elastomers, or plastics. These specifications will help identify suitable couplings for the application at hand.
Composite disc-packs: Rugged disc couplings
Most disc couplings feature a metal disc-pack. However, Zero-Max’s CD coupling features a uniquely shaped disc-pack that is constructed of a special composite material rather than metal. This composite material provides an alternative over metal disc couplings. The advantages include its ability to absorb shock and vibration, its misalignment capacity, electrical isolation and elimination of fatigue and fretting, common failures of metal disc couplings. Whereas metal disc couplings may be less expensive initially, overall cost of composite disc couplings usually will be lower because they are maintenance free and are rated for long life.
The ability to accommodate misalignment is a critical aspect of a flexible disc coupling. Misalignment between coupled shafts often exists due to manufacturing tolerances, improper installation, or from loads on the system.
Parallel, angular, and axial misalignment between coupled shafts should all be examined to see if the coupling selected is up to the task. It is important to know a coupling’s misalignment rating as well as the stiffness rating. The stiffer a coupling, the higher the reaction load misalignment will transmit to the coupled items. These reaction loads will have a negative effect on the life of the system. To limit these reaction loads, the Zero-Max composite disc couplings are designed less radially stiff than metal disc couplings. Therefore, they transmit lower reaction loads on the coupled equipment and thereby increase the life of connected (and often expensive) components.
The amount of misalignment that a system can experience will typically determine the selection between a single-flex (one flexible disc-pack) and a double-flex (two flexible disc-pack) coupling. While more compact in size than the double-flex variety, a single-flex coupling will have lower misalignment capacity and higher reaction loads.
A common misconception is that single-flex disc couplings cannot accommodate parallel misalignment. Although this is true for metal disc couplings, the design of the disc-pack used in the Zero-Max CD coupling allows the single-flex CD coupling to accommodate limited parallel misalignment. This permits designers to implement a single-flex disc coupling into designs that may not have space for a double-flex coupling.