Torque Limiting Clutch and overload protection with zero backlash:

Backlash free ball-detent Torque Limiters and Torque Limiting overload Clutches offer the ultimate in machine drive protection as a fail-safe mechanical circuit breaker, disconnecting drive and driven elements accurately (+/-5%), and virtually instantaneously (<3 msecs), in the case of a machine jam or crash. While there are several keys to the high sensitivity of R+W torque limiters, one in particular stands out more than others. The digressive spring disk, which receives an axial load on its inner rim, and transmits it to the clutch mechanism on its outer edge, is highly sensitive to axial force. Isolation of the clutch mechanism from outside forces is therefore imperative to the proper functioning of the unit. Outlined herein are a description of the basic functioning of ball-detent torque limiters, the specialized use of the digressive spring, and an explanation as to how the axial loading of the torque limiter is controlled in normal application.

Basic Function

The free-wheeling element, or detent plate, is highlighted in blue in Figure 1. This component, with either a mounting flange machined into it, or a flexible shaft coupling assembled to it, rests on bearings over the base element of the torque limiter. In the event of an overload, the stainless steel clutch balls, which had been locked into the conical detents of the plate by the force of the digressive spring, roll out of their detents, allowing the detent plate to rotate freely, on its bearings, and relative to the base element. This disengagement is highlighted by the small explosion graphic in Figure 1. The orange arrows mark the axial movement of the actuation ring, which fits between the spring disc and the clutch balls. This ring, along with the outer edge of the digressive spring moves axially by a distance equal to that of the depth at which the balls had rested in their detents.

The Digressive Spring

In its disengaged state, the digressive spring does not apply a restoring force in direct linear proportion to its axial displacement, but rather changes shape slightly, placing only a very light residual force on the Torque Limiting Clutch mechanism. In its engaged state, the disk is already very near to being caused to disengage in terms of the amount of axial force required to affect this change. Only a very narrow range of axial force is permitted to be placed onto the digressive spring by the less than one allowable revolution of the finely pitched torque adjustment nut. It is partially the result of precise loading of the spring between the base element and the detent plate that such an accurate and immediate response to overload is possible. Further axial loading of the detent plate relative to the base element of ball-detent torque limiters would directly impede their ability to disengage properly at the pre-set disengagement torque.

Normal Application

The detent plate’s bearing is tolerant of radial loads up to, but not by far exceeding normal belt or chain tension in a pulley or sprocket mounting configuration. In direct drive applications, a stainless steel bellows or elastomer insert coupling is used to isolate the bearing from relatively high lateral and axial restoring forces caused by slight misalignment between the input and output shafts. These two types of flexible couplings in particular exhibit a very low resistance to axial compression. The accordion-like nature of the bellows makes compression very easy, and as proper installation of elastomer couplings calls for a 1-2 mm gap between the jaw sets, they too can collapse axially in operation while transmitting only a very light frictional load through to the clutch mechanism. While chain and belt drive configurations can cause an overhung load to be applied to the bearing, no direct axial force is normally applied.

While ball-detent Torque Limiters and Torque Limiting Clutches are perhaps the best way to effectively protect machinery from torque overload, as with anything they are not without their limitations. Precise control of the axial loading of ball-detent torque limiters is directly related to their performance in terms of the accuracy, reliability, and promptness of their response to torque overload. Any significant forces beyond those applied to the digressive spring by the placement of the torque adjustment nut relative to the detent plate will have a negative impact on performance.

With any questions or special application requirements, contact R+W America.

Torque limiting Clutch series SK: zero backlash, compact,
torque from 0.1 – 2,800 Nm.

Torque Limiting Clutch SK1: Made for belt or chain drives.

Torque Limiting Clutch ES2: The elastomer insert coupling is used to isolate the bearing from relatively high lateral and axial restoring forces caused by slight misalignment between the input and output shafts.

Torque Limiting Clutch SK3 with metal bellows: The accordion-like nature of the bellows makes compression very easy