By Joyce Laird, contributing editor
Precision elastomer couplings are used for a variety of applications in which a customer may need backlash-free torque transmission, vibration damping, electrical isolation, and compensation of misalignment. These torsionally soft couplings have an adjustable stiffness and temperature range with the selection of the color coded elastomer insert or spider.
Alex Webb, Applications Engineer at Ringfeder Power Transmission USA Corporation explains that shore hardness, also known as the durometer rating, is the main characteristic behind fine tuning a precision coupling for specific applications.
“This is measured by using either a Shore A scale for ‘softer’ plastics or a Shore D scale for ‘harder’ plastics. Elastomer couplings are considered to be backlash-free couplings due to an oversized spider/exact production, size, and accuracy of tooth geometry/control of the mounting force.”
“Many drive systems endure some vibration but with a softer elastomer insert this vibration can be dampened between components. Each insert is injection molded with knobs on the insert face(Figure 1) which help separate the coupling, keeping it electrically isolated. (Fig 1)
These inserts are typically made from thermoplastic polyurethane for temperature ranges -76° to +248°F and nominal torque values from 0.22 ft-lb to 1416 ft-lb. An alternative thermoplastic polyester (hytrel) material is also offered for applications with temperatures exceeding +250°F and nominal torque values from 2 ft-lb to 299 ft-lb.
While companies may differ, Webb says that to keep selection easier, Ringfeder Gerwah servo coupling inserts are broken down into five color coded groups with different durometer ratings. (Fig 2) He goes on to explain the reasoning behind this and how it aids in proper selection.
“The 80 ShA(blue) elastomer can be selected for applications where vibration damping is a necessity although torque and temperature are lower than standard. A typical application for this insert would be for a drive in an electronic measuring system. The 98 ShA(red) and 92 ShA(yellow) are the most common inserts as they provide the coupling with a balance of damping, torque transmission, and operational temperature. Common applications for these are main spindle drives and positioning drives. Finally, the 64 ShD(green) inserts offer a high torsional rigidity but low damping capacity due to the hardness of the elastomer. The hytrel insert 64 ShD-H(green) will transmit the highest torque and have the greatest temperature range of five inserts per coupling size. These inserts will be found in machine tool spindles, control drives, lead units, and so forth.”
The application is the guide for perfect shore hardness. Application and elastomer coupling must be a perfect match for success. The following chart example sows a complete breakdown of a properly selected coupling for a servo motor to ball screw drive. (Fig 3)
Applications for elastomer inserts in couplings cover a huge area of industrial devices, machines and equipment. Some examples include encoders, precision drives, grinding/milling spindles, machine tools, packaging machines, linear motion drives, measurement and control equipment/technology and conveying equipment. (Fig 4)
In recap, the benefits of the characteristics in this technology, Include backlash-free torque transmission, pluggable coupling halves and vibration damping. They are electrically isolating, provide compensation of misalignment and are basically free from ware and maintenance free.
However, getting the correct selection for any specific precision coupling does take some expertise. “Getting input from experts in the field is always the right path to take. Be sure to talk with a specialist to ensure a perfect fit for your application,” Webb concludes.
Ringfeder
www.ringfeder.com
This is great engineering data; so many times the customer uses the “hit & miss” method. In today’s motion controlled world the electrical/motor characteristics can be very precisely defined and are repeatable. This article shows that the mechanical side can be just as clearly defined for a much better and predictable application results.
Ernie Judson
Applied Products Co.