By Zak Khan
Selecting a bellows coupling involves considering a few important factors. These include torque, degree and type of misalignment and the level of extension and compression the bellows coupling experiences. We also must consider the operating temperature range in which the bellows coupling is expected to be used.
Torque is the primary concern when selecting a bellows coupling. Start by determining the maximum instantaneous torque the bellows coupling experiences in the system. If there are any breaks or clutches, make sure to measure their maximum instantaneous torque values as well. Torque also goes in hand with misalignment. A higher misalignment means a lower capacity for the bearing to transmit torque. A well-aligned coupling can transmit more torque.
Because torque and alignment exist on a range, manufacturers can rate their bellows couplings in various ways. Some are more conservative than others for their ratings relating peak torque versus maximum misalignment. Usually, the conservative estimate of misalignment tolerance is 0.1 to 0.2 mm, which is the range most motion systems can handle. The peak torques for varying bellows couplings given this misalignment range are usually similar to one another.
Misalignment has its concerns as well. This involves finding the maximum parallel misalignment. The misalignment of the shafts, parallel but offset from each other, is the most severe loading condition the bearing can undergo. In addition to this, we have to find the maximum angular displacement between the shafts as well, if present.
The extension and compression forces a bellows coupling undergoes is the degree to which it becomes longer or shorter during given a load condition. Often the values of torque, misalignment and extension-compression are specified as mutually exclusive. That way, an engineer can determine a value for each, and a percentage value assigned so that they can be used on their own and in combination.
Manufacturers usually list temperature ratings. The type of bellows coupling and its materials affect this rating. Be aware of the heat produced by the system in addition to ambient temperatures, as this can have a significant effect on the overall thermal load. Be aware of the units used. Sometimes metric and imperial units are used in combination.
Comparing these calculated values to values provided in documentation allows engineers to select a proper bellows coupling. Also, remember that varying materials, metallic and composite, exist to make bellows couplings. When in doubt, ask the manufacturer for clarification.
For more information check out:
How to size and select bellows couplings for motion applications
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