R+W is a leader in elastomer couplings and inserts. They shared with us the details of the different elastomer inserts which can help in any type of application. Read on for more.
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. On account of its elastic memory and damping characteristics, the 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.
Shore Hardness Testing
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.
Shore Hardness Types
Depending on their properties, R+W elastomer coupling inserts can be divided into five main groups. The most commonly used elastomer insert (type A) 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 (type C) 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 those 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.
Elastomer inserts which belong to the hardest group (types, B, D, and E) 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.
The chart below shows the angle of twist, and thus the torsional rigidity, of the three main hardness levels offered. Since the angle of twist depends on the torsional load and the hardness of the elastomer, it can be said that the more vibration damping the elastomer inserts are, the larger the angle of twist. The more rigid the elastomer inserts are, the smaller the angle of twist.
One specialty insert in this group (type D) is used for its electrical conductivity, which is achieved by the addition of graphite to the plastic granules. They have a Shore hardness of 65 Sh D and cover a temperature range of –10°C to +90°C. Due to their electrical conductivity, these elastomer inserts are suitable for explosion-proof applications (ATEX zones 1 and 2). The conductivity helps to prevent an arcing of electrostatic loads from one hub to the other, providing a path to ground for any current running through the shafting. The other specialty elastomer (type E) is made from HYTREL, and, as stated above, is used for temperature extremes.
In addition to their primary functions, to damp vibration and transmit torque, elastomer couplings are also designed to compensate for some limited amounts of all three types of axis misalignment. When misalignment levels are kept below the maximum recommended values for a given insert type, they are maintenance-free and have a very long service life. This is because when they are aligned within the allowable limits, the remaining misalignment is taken up in the flexing of the elastomer, rather than rubbing against the jaw. Because there are small spacers on the face of the elastomer insert, the hubs are always separated slightly, causing an electrically isolating effect, except in the case of the specialty insert (type D) mentioned above. Even though elastomer couplings exhibit a torsion angle under load (as do all drive components to a certain degree), precision elastomer couplings are noted for being backlash-free, a characteristic achieved by preloading the oversized elastomer insert between the jaws. This means that there is no lost angular motion resulting from mechanical clearances.
Being injection molded, the elastomer inserts start with large dimensional tolerances. Calibration is, therefore, necessary to improve dimensional consistency from one unit to the next. The elastomer inserts are pressed into a master hub and heated in an oven at a specific temperature for a specific amount of time. This leads to a larger contact surface between the insert and jaws, so the torque load on the insert is no longer concentrated on high points, but is distributed over a larger area, shown below. This improves the hysteresis characteristics as well as improving service life. It also allows the coupling to slide together more easily, reducing bearing loads during assembly and axial movement.
View the variety of R+W Couplings & Elastomer inserts that MISUMI carries here.