By Joyce Laird, Contributing Editor
Andy Lechner, Product Manager for R+W America, explains that it is important to look at how to break down the classifications of elastomer insert couplings to better understand how everything fits into industry use today.
He says that elastomer insert couplings generally fall under one of two main classifications: “standard” versions, which have been serving industry well for more than a century, and “precision” versions, which were developed later in the 20th century to address an increasing need for speed and precision, mostly in electric powered machinery. “Standard versions are generally made from low cost metals, with cast or sintered hubs, and have some clearance between the jaws and the elastomer insert, keeping things nice and loose between the driving and driven shaft. ‘Precision’ versions typically have a fully machined hub and a slightly harder elastomer insert, which is preloaded between the hubs, eliminating backlash,” he says. 
Because precision versions are in wide use today and expanding, and also more complicated than standard insert versions it pays to focus on a brief discussion of the properties of the elastomer inserts of the this variety.
“Precision coupling inserts are normally made of thermoplastic polyurethane (abbreviation: TPU). This thermally stable material can be used over a temperature range of –30°C to +120°C. They can also be made of HYTREL material, which has a special structure and is mainly used for applications with temperature extremes. Because of its elastic memory and damping characteristics, this type of elastomer insert reduces and compensates for torque impacts. It also damps vibration between the driving and driven shafts, and can serve to filter it out. This allows the entire drive system to run more smoothly. To compensate for the various torque impacts and vibration in specific applications, elastomer materials are available in various hardness levels, measured on the Shore hardness scale,” Lechner says.
He states that it is important to understand that there are two different testing procedures to determine the Shore hardness of the types of elastomers used in precision elastomer couplings. In both procedures, the penetration depth of a foreign object into the test specimen is measured. In the Shore A test, the object is a ball, and in the Shore D test, it is a sharp-edged pyramid. “Soft” elastomer is measured with the Shore A test, and “hard” elastomer is measured with the Shore D test.
“The most commonly used precision elastomer insert has a medium hardness level at 98 Sh A. It provides a good combination of vibration damping and load carrying capacity. They are generally suitable for torques from 2 to 20,000 Nm and over a temperature range of –30°C to +100°C.”
“The softest elastomers offer the best damping characteristics, with a hardness of 80 Sh A. They are primarily used for applications with a high level of vibration. Due to the composition of the TPU material, the temperature and torque range covered by the softest elastomer is smaller than that covered by the other types. They can be used over a temperature range of –20°C to +80°C and over a torque range of 2 to 400 Nm. They also exhibit a much larger twisting angle under load than do the harder inserts, as a result of having a lower torsional stiffness in coupling applications,” he adds.
“Elastomer inserts which belong to the hardest group have a Shore hardness of 64-65 Sh D, a relatively high torsional rigidity, and a rather low damping capacity. Compared with the other elastomer inserts, they cover the widest temperature range, and transmit the highest torques (up to 25,000 Nm). On account of the high rigidity of the elastomer insert, the angle of twist under load is the lowest. Torque impacts and vibration can therefore only be damped to a limited degree.”
Lechner notes that other specialty versions of TPU are also available, including special electrically conductive versions, as well as HYTREL versions, which are made for temperature extremes. Going beyond the “precision” variety of elastomer couplings, a very wide range of different materials are available for elastomer coupling inserts, and this paper should only be considered an introduction to one specific type, generally for use in electric motor applications requiring high speed or high precision.
“As always, make sure to contact your coupling suppliers for more details about whether a ‘precision’ coupling with TPU insert is appropriate for your application,” Lechner concludes.