As a primary or redundant safety device, backlash-free ball-detent torque limiters serve as a mechanical circuit breaker for machine drive protection, disconnecting drive and driven elements accurately (±5 percent torque) and virtually instantaneously (&3 msec) in the case of a machine jam or crash. A common misconception is that limiting current supplied to the drive will inherently protect the mechanical system from overload, though when placed on the output of servo worm or planetary gearboxes for example, precision torque limiters protect the mechanical system from reflected load inertia, where sufficient energy to do harm has already been supplied well in advance of the impact. Backlash, repeatability and response time are key to the successful application of mechanical torque limiters in high-speed servo applications.
“Traditional torque limiters would not respond fast enough to an overload situation in higher speed applications, where over travel would occur too rapidly,” says Ben Cucci, design engineer, Schumacher Automation, Belmont, NH.
“A high level of repeatability is desirable to ensure a consistent breakaway torque across the line,” says Duncan Quinn, design engineer, NBS Card Tech, Paramus, NH.
Servo-rated torque limiters normally possess internal preloads between mating components to eliminate backlash. But it is the digressive spring characteristic which stands out as the most significant adaptation in the development of precision mechanical torque limiters for servo-driven systems. Their enhanced sensitivity to axial movement heightens accuracy and response, but also requires that greater care be taken in layout and assembly.
The output flange, or detent plate, (highlighted in blue, above) presents a series of conical detents, into which ball bearings are spring loaded. This component, with a bolt circle or flexible coupling output, rests on bearings over the base of the torque limiter. In the event of an overload, the engaging ball bearings ramp out of their detents against the spring force, allowing the detent plate to rotate freely relative to the base element, and transmit zero torque. Upon disengagement, an actuation plate, located between the disc spring and the engaging ball bearings, moves along its axis by the depth to which the balls had been loaded into their detents, signaling overload to an adjacent proximity sensor.
The Digressive Spring
Servo torque limiters use a digressive disc spring in lieu of the traditionally employed coil spring to load the input and output. The digressive spring, which receives an axial load on its inner rim from the torque adjustment nut and transmits it to the ball-detent mechanism on its outer edge, is highly sensitive to axial force. 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 clutch mechanism. In its engaged state, the disc 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 primarily 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 preset disengagement torque. Isolation of the clutch mechanism from outside forces is therefore imperative to the proper functioning of the unit.
In belt drive applications, overhung load resulting from belt tension and transmission of the torque itself must be controlled. While no direct axial force is normally applied to the torque limiter in these cases, it is important to avoid tipping moments symptomatic of excessive overhung load. The detent plate's bearing is tolerant of radial loads up to, but not by far exceeding, those found in more aggressive servo applications. Designers need to be aware of a torque limiter's overhung load rating, but more importantly ensure that the load will be centered axially within a manufacturer's recommended range of positions.
In the case of directly coupled loads, axial flexibility is an absolute requirement. Torque limiters mounted in-line with a coupling unable to absorb axial endplay will experience a high level of compression of the output flange against the ball-detent system, directly impacting the torque limiter's ability to disengage with any repeatability. Stainless-steel bellows or servo insert couplings are recommended to isolate the torque limiter 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 servo insert 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 torque limiter.
While ball-detent torque limiters are perhaps the best way to ensure protection 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 an impact on performance.
Taking simple measures to ensure proper torque limiter installation serves to ensure safety and reliability. Many standard products are available to ensure proper loading, though special applications require special solutions.
Andrew Lechner has been torque limiter product manager at R+W America since 2002, responsible for special applications and technical support.