Power Transmission
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.”
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).
Jaw couplings
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
Fig. 4. Titanium offers high strength and good fatigue life for shaft couplings used in the aerospace, instrumentation and medical equipment industries.
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 34
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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. l
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