By Joyce Laird, Contributing Editor
Torque Limiters are a particular category of ball-detent clutches, which use a series of hardened balls, spring loaded into a detent plate, to clutch the load, and disengage once the drive line torque reaches a value sufficient to actuate the spring set, and release the balls from their detents. They are used in applications requiring a high level of accuracy and repeatability, with a rapid response to torque overload. These mechanical devices are designed to protect machinery and equipment from torque overload, resulting from malfunctions, jams, and crashes.
“They come in a variety of designs, including shear pins, slip clutches, magnetic clutches, and ball-detent clutches, and more,” Andy Lechner, Product Manager R+W Coupling Technology says.
“Low inertia and zero backlash versions have become increasingly popular in recent years for servo driven systems. The term, ‘multi-position’ refers to the pattern of balls and detents in the clutch, which are in this case arranged in such a way as to allow the balls to re-engage with the detents at multiple intervals within a single rotation after overload (30, 60, 90 or 120°).”
Applications and Variety
While “single position” types are more popular, “multi-position” torque limiters have key applications. The most obvious reason to use one would be on a very slow speed system, where there is no need to wait for the shaft to rotate one full turn before re-engagement. This can arise when the slow speed results from a significant mechanical speed reduction, which is often accompanied by sufficient mechanical advantage to cause excessive torque to be generated.
Another reason is when a machine is required to start-up with the torque limiter disengaged. In this scenario the drive line stops after an overload with the balls oriented away from the detents. If there is a rapid acceleration at start-up, it becomes possible for the balls to jump over their detents, and fail to re-engage in time. Use of a multi-position torque limiter ensures that the balls are never far from their detents, reducing the amount of time before an opportunity to re-engage occurs.
Manual indexing is another possibility. In this situation a dial plate, or round indexing table, can be made to disengage from a back stop, and freely rotate until the next set of detents is reached.
“The two most common commercially available multi-position torque limiters include simple mechanical designs, in which the disengagement torque is set manually prior to use, and pneumatically controlled designs, wherein the spring pressure on the ball detent clutch can be controlled remotely during operation, but many other possible uses arise any time there is a need to disengage and re-engage at a set angular interval less than 360°. We’ve supported many specialty applications,” Lechner adds.
Specifying for an Application
“When selecting a torque limiter for a new or existing application, it is important to allow sufficient headroom for upward adjustment,” Lechner says. “Since their intended purpose is to protect from uncontrolled spikes in torque, their application often comes in situations where the real required peak torque for operation is unknown. Under-sizing a torque limiter can exacerbate problems with machine downtime by preventing the machine from running at all, in case there is no margin left to increase the disengagement torque as needed. ”
“Multi-position” torque limiters come from a family of high performance ball-detent clutches, which are often compact, backlash free, and capable of releasing drive line inertia much more rapidly than an electronic control system would otherwise be able to react. While not all machines need them, they can be a very useful tool in preventing machine damage as a result of unintended interference or malfunction. It is important to always make sure to contact supplier application engineering prior to selection in order to ensure the optimal torque limiter solution will be reached.”
Filed Under: Coupling Tips