When designing a machine, product or mechanism with rotating parts it is very easy to locate two components or shafts and simply note that a shaft coupling is required. Actually specifying the coupling is often left until late in the project, at which point the designer might discover that, for whatever reason, standard products are unsuitable.
It is also not uncommon for shaft couplings to fail in service, perhaps because they were incorrectly specified or because they are subjected to loads that were not foreseen during the design process, and sometimes it is necessary to introduce a compliant coupling to overcome a problem that is causing another component to fail. Fortunately, many of the manufacturers of shaft couplings are able to work with designers to create bespoke couplings that will function as required, and sometimes the 'coupling' component can be designed to incorporate functions that were to be performed by other components, thereby simplifying the design, as well as potentially cutting cost, reducing weight and improving reliability and serviceability.
Abssac manufactures and sells an extensive range of standard couplings, including bellows couplings (electrodeposited and hydro-formed), disc couplings (for standard and servo applications), gear shaft couplings, helical couplings, X-slot couplings, jaw couplings, Sure-Flex elastomeric couplings, rigid couplings and universal joints. In addition, the company designs and manufactures special products that are either variations of these standard shaft couplings or more complex components that integrate several functions.
For example, the basic dimensions of a helical coupling can be modified to suit the application's torque and compliance specifications, or alternative mounting arrangements can be incorporated to suit the customer's requirements. In many cases the final coupling-based component is an amalgamation of separate parts including items such as external gears, splines, threads and tangs (Fig. 1). Abssac is so confident in its ability to create successful customised products that it often offers a free sample for test and evaluation purposes.
The performance characteristics of a helical flexure are determined by six major factors: outside diameter, inside diameter, coil thickness, material, number of coils and number of starts. By altering these, it is possible to create a flexure that meets the application's specifications in terms of torque capacity, angular and parallel misalignment capabilities, and torsional and lateral bending rates.
By adapting the design of the helical beam shaft coupling concept, Abssac is also able to alter the coupling's resonant frequency and improve its resistance to fatigue. Other design options include shaft connections that enable the assembly to be dismantled easily and quickly and flexure configurations that permit more angular offset than would be normal for this type of coupling (Fig. 3).
With miniaturisation becoming commonplace, Abssac has also been asked to manufacture smaller couplings. For example, a fully functional 3.1mm diameter shaft coupling was supplied for a medical application. Using a left- and right-handed threaded shaft for attachment, the coupling could accept angular offsets while rotating at up to 10,000RPM. The same customer then requested a double-start flexure with a 1.5mm outside diameter produced in titanium.
Clearly the ability to manufacture shaft couplings from different materials is beneficial for some applications. Anodised aluminium is commonly used for helical beam couplings, but stainless steel is also a popular choice.
For some applications, such as those in aerospace equipment, titanium may be specified, and plastics can be used where the requirements for torque capacity, deflection, weight and cost make this option more suitable than other materials.
Note that plastic couplings are available both as standard products that have been injection moulded, and custom products that are machined from solid. In the case of machined plastic parts, of course, it is possible to incorporate other features as is the case with machined metallic couplings.
Huco Dynatork, another company with shaft coupling expertise, has recently selected Victrex Peek (polyetheretherketone) for a special variant of its Multi-Beam couplings. These are replacing metal components in a demanding, high-voltage electronics assembly application within the manufacturing process for plasma TV screens. Outlining the development process, Dave Lockett, the managing director of Huco Dynatork, explains: "One of our customers in Japan that specialises in the design and manufacture of high-voltage capacitors came to us with a request for a miniature coupling for an electronic assembly used in the manufacture of plasma screens. Serving to connect the drive from a small gearbox to a lead screw in a high-voltage capacitor, the coupling had to meet a number of stringent requirements such as insulation and flexibility properties."
This coupling had to prevent any current leakage or tracking across the drive. At the same time, it had to be flexible in order to compensate for any unintentional misalignment between the gearbox shaft and the lead screw.
Peek offers excellent insulation due to its high surface resistance and comparative tracking index. In addition, this material has a high dielectric constant and its mechanical properties meant it could be used for a coupling design that gave the required flexibility within an acceptable working envelope.
Lockett continues: "Other plastics were trialled, but Victrex Peek polymer proved ideal for this job. As a result, we have received a substantial ongoing order for a range of Huco Dynatork Multi-Beam couplings made from Victrex Peek polymer."
As well as being suitable for applications requiring electrical resistance, the Peek couplings can also be used where relatively high temperatures are encountered or chemical resistance is necessary, such are the properties of this high-performance polymer.
Expanding on the capabilities of his company's machined Multi-Beam couplings, Lockett says: "We are often asked to develop application-specific couplings and now have considerable expertise in recommending the correct material for the task. We can produce a Multi-Beam from any machinable material."
Aluminium and stainless steel are used most often, but Huco Dynatork regularly produces couplings in plastics such as nylon and acetal for use in the food and chemical industries.
For customers requiring lightweight components with high strength and good fatigue life, anodised titanium is available; titanium shaft couplings are typically specified by the aerospace, instrumentation and medical equipment industries (Fig. 4).
Depending on the application, a jaw-type coupling with an elastomeric 'spider' element may be more appropriate than a beam-type coupling. These are often available with the spider element moulded form different grades of polyurethane to give appropriate levels of dampening of impulse loads. One of the advantages of this coupling type is that it is 'fail-safe'; if the elastomeric element fails, the jaws on the hubs will continue to provide positive drive. As with helical beam couplings, jaw couplings can be customised to suit particular applications.
However, conventional polyurethane spiders are only suitable for use in temperatures ranging from around -30 to 90°C, which is insufficient for many applications, especially those where higher temperatures are encountered. To overcome this shortcoming, KTR has developed a 'Spider from Wire' for its Rotex shaft couplings that can be used in temperatures from -40 to 250°C.
As with the Rotex couplings incorporating elastomeric spiders, those with knitted stainless steel wire spiders provide vibration dampening and fail-safe characteristics. When operating at up to 180°C, the torque capacity, torsional stiffness and damping remain substantially unchanged; above this temperature, the operating characteristics of the knitted wire spider vary slightly.
Other notable features of the knitted wire spider are the non-corrosive and electrically conductive material properties, both of which can be advantageous in some applications.
As engineers seek to optimise designs and avoid over-specifying components - which can be wasteful in terms of cost, weight and space - there is a growing demand for customised shaft couplings.
The degree of customisation varies considerably, but many manufacturers today are in a better position to deliver custom couplings, which means that specifying non-standard components need not necessarily carry with it the burdens of higher cost and long delivery times as used to be the case.
Indeed, custom shaft couplings can now be very cost-effective, especially if those special component also perform other functions and help to achieve benefits beyond those normally associated with humble shaft couplings.