Coupling two shafts together sounds like a simple task, but the reality is somewhat different. Shafts are invariably misaligned to a greater or lesser extent, and the application requirement may be for accurate transmission of torque or, for example, some loss of accuracy here may be an acceptable downside to using a lower-cost coupling. The type of coupling that is best suited to a particular application will also depend on the shaft diameter, the space available, the power, torque and speed to be transmitted, the life expectancy required from the coupling, electrical conductivity (or insulation), the operating temperature range, and the nature of the shaft misalignment – which may be radial, angular or a combination of the two. Different coupling types are also characterised by different torsional, angular, radial and axial stiffnesses.
Unless a designer is always faced with having to specify the same type of shaft coupling, it can be difficult to ensure that the optimum coupling type is selected for each application. However, the following guidance should prove helpful.
Shaft couplings can be grouped into three types: high-precision, flexible and sliding/pivoting. High-precision couplings tend to have higher stiffness and are suited to applications where shaft alignment is relatively good. Flexible couplings are better able to compensate for radial or angular shaft misalignment, while greater misalignment calls for the use of sliding or pivoting couplings.
Rigid couplings, having a split cylindrical body that is clamped to the two shafts, are simple and robust, easy to install and can be readily adjusted. Of course, being solid they cannot tolerate any misalignment.
Membrane-type couplings (also known as disc-type) are available in several configurations: single-stage, double-stage or double-stage with spacer. The steel membranes transmit the torque between the shafts while being sufficiently thin and flexible that they can also take up the angular misalignment (they are not recommended for use where there is radial misalignment). Benefits of this type of coupling include a dynamically balanced construction, a long operating life, and tolerance to high and low operating temperatures. The choice of configurations enables different amounts of misalignment to be tolerated, though flexibility comes at the expense of torsional rigidity. If required, two single-stage membrane couplings can be used at either end of a cardan shaft.
Bellows couplings, using one or more stainless steel bellows to transmit torque, also offer high torsional stiffness and a wide operating temperature range but, unlike membrane couplings, are radially compact and can tolerate some radial misalignment (Fig.1). While standard products are typically available with three convolutions, some manufacturers are able to customise the number and size of the convolutions to enable the coupling’s flexibility to be fine-tuned to suit a particular application. For example, a larger number of convolutions can be used to create a longer coupling that is more tolerant of angular misalignment and axial displacement. Another alternative design uses a single convolution at either end of a tube to give high torsional accuracy. Because of their torsional accuracy, bellows couplings are often used in applications such as encoder drives and closed-loop servo systems.
A further variation of the bellows coupling uses stress-free electrodeposited nickel for the bellows instead of stainless steel. Because the thickness of the bellows can be tightly controlled, this type of coupling offers an excellent combination of radial and angular flexibility, as well as high torsional stiffness. Applications for nickel bellows couplings include high-precision drives and positioning systems, instrumentation and medical equipment.
A popular general-purpose coupling is the multi-beam type that is usually machined from solid stainless steel or aluminium with a multi-start helical cut (Fig.2). This type of coupling can accommodate any combination of axial motion, angular misalignment and radial misalignment, and is typically offered in three-beam or six-beam configurations, the latter being capable of accommodating greater misalignment but with reduced torsional accuracy. While stainless steel models have better torsional performance, aluminium versions are lighter and cost less. Something to be aware of with multi-beam couplings is that they can exert unwanted loads on bearings in some circumstances.
Single-beam couplings are similar in concept to multi-beam couplings but have a single-start helical design. Compared with the multi-beam alternative, the single-beam coupling provides lower torque capability and can suffer from ‘wind-up’ (Fig.3). Customers can often choose between short and long versions of single-beam couplings, with longer models offering greater tolerance of misalignment but reduced torque accuracy.
Traditionally high torsional stiffness and radial flexibility have been mutually exclusive, but Huco Dynatork has developed a novel type of beam coupling that gives the high torsional stiffness of a straight beam coupling and the radial flexibility of a helical beam coupling. Moulded from plastic, the result is a lightweight (and therefore low-inertia) Step-Beam coupling that can tolerate temperatures up to 150°C. In some applications, its resistance to corrosion and electrical insulation are also beneficial.
If greater angular misalignment needs to be accommodated, universal joint couplings are now available moulded from acetal, making them lower-cost than metal versions. Huco Dynatork, for example, produces Huco-Flex P couplings that feature controlled preload on the radial bearings to eliminate backlash. If both radial and axial misalignment is present, a double-jointed version of the coupling is available with two universal joints and a short cardan. Being lightweight and corrosion-resistant, these couplings are typically used in food, textile, paper handling and packaging machinery applications.
If a lower-cost alternative to universal joint couplings is required, and low torsional stiffness is acceptable, double-loop polyurethane couplings are available with steel or stainless steel hubs swaged into each end (Fig.4). Exceptionally flexible, loop couplings are quiet and friction-free.
For applications with primarily angular misalignment, couplings based on a flexible rubber element are available. These typically use a pair of aluminium hubs, each with three arms that are attached to either side of the rubber element.
Sliding and pivoting couplings
If radial misalignment is relatively large but angular misalignment is small, a popular coupling is the Oldham type in which torque is transmitted from one hub to another via a central disc that is able to slide radially relative to the hubs (Fig.5). Backlash is eliminated though the preload between the disc and hubs, and different materials for the disc and hubs enable the same design to suit a multitude of applications.
For applications where both angular and radial misalignment must be accommodated, Huco Dynatork has developed the Uni-Lat coupling that combines the sliding action of the Oldham coupling with the pivotal action of a universal coupling. This type of design is only suitable for general-purpose, light-duty applications such as on stepper motors, encoders, resolvers and tachogenerator drives.
Another option for shafts exhibiting both radial and angular misalignment is the jaw coupling. This type has a cross- or star-shaped flexible element (typically polyurethane) that fits between hubs, each with lugs to mate with the insert. Relatively high torques can be transmitted by this type of coupling with little or no backlash. Different hardnesses can be specified for the flexible insert (sometimes referred to as the ‘spider’), giving a range of torsional stiffnesses, and polyurethane is capable of continuous use at temperatures up to 100°C. The hubs are typically available in aluminium or cast iron, depending on the size and power requirements of the application. A similar coupling design is available with a two-piece design that uses a pair of hubs moulded from plastic and no intermediate flexible element.
Where the need is for a compact coupling capable of accommodating angular misalignment and transmitting a relatively high torque for its size, nylon sleeve couplings can be appropriate. These feature a pair of steel hubs with barrelled external gear teeth that mate with a nylon sleeve that has internal gear teeth moulded into each end. These couplings are not suitable for high-speed operation, but they are electrically insulating and maintenance-free. All-steel versions of the same design are also available for transmitting higher torques.
While all of the above require physical contact, some applications benefit from non-contact torque transmission. For example, systems processing highly volatile liquids or requiring drive to be transmitted to equipment inside a vacuum chamber can use non-contact couplings to avoid the expense and risk associated with shaft seals. Huco Dynatork offers a range of magnetic disc couplings that deliver smooth motion across an air gap of almost 5mm thanks to the use of high-energy rare earth magnets. They can also accommodate up to 3 degrees of angular misalignment and a maximum radial misalignment in excess of 6mm. Four sizes are available, with the maximum running torque being 1.69Nm.
Heavier loads tend to require either larger versions of the above couplings or alternative designs. For example, pin-and-bush couplings use a pair of hubs with protruding pins that mate with bushes in the opposing hub. These have the advantage of being fail-safe in the event of the elastomeric bushes wearing, making them suitable for application such as elevators or fire pumps.
While many small-scale applications often require good torsional stiffness, larger applications can benefit from a degree of shock or vibration absorption in the coupling. Heavy-duty couplings for high shock load applications can transmit torques in the order of 6MNm via a set of rubber inserts. Once again, the design is fail-safe in the event of the inserts wearing.
Heavy-duty applications requiring torsional stiffness can benefit from chain-type couplings. Similar in concept to the nylon sleeve coupling described earlier, this type of coupling has a pair of hubs with sprockets and a duplex chain bridging between the two. Capable of transmitting torques in the region of 8MNm, the couplings offer torsional stiffness yet also accommodate some misalignment.
Capable of operating under the most arduous of conditions due to the simple, robust construction, pin/buffer couplings absorb shock loads and can tolerate misalignment. Torques up to around 250kNm can be transmitted.
Regardless of the application, cost will be a consideration. As a guide, the simpler the coupling design and the less accuracy required, the cheaper the coupling will be. Care should therefore be taken not to over-specify the coupling and spend more money than is absolutely necessary. As always, the advice is to contact the suppliers for advice if required, but the information above should give an indication of the types if coupling that are most likely to be correct for a given application.
With thanks to Huco Dynatork for assistance in preparing this article.