How to maximise the benefits of custom couplings
Custom shaft couplings can be useful for applications where standard products cannot be accommodated. Paul Stevens investigates.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. 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.
Miniaturisation
With miniaturisation becoming commonplace, Abssac has also been asked to manufacture smaller couplings. For example, a fully functional 3.1 mm 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 10,000 RPM. The same customer then requested a double-start flexure with a 1.5 mm outside diameter produced in titanium.
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.
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.
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.
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.
