New Design in Torque Limiters Delivers High Power Density

By Tobias Wolf, Product Engineer, R+W GmbH and Andrew Lechner, Product Manager, R+W America

The concept of weight reduction through the use of high tech materials is not a new one. But for those involved in the design of motion control and automation systems, the elimination of excess mass and inertia is often the difference between success and failure. Energy savings, higher throughput rates, and reduced downtime, all without compromise to quality or accuracy, are the new requirements. To address them, R+W has introduced a new torque limiter, SL Series, with half the inertia and less than half the mass, allowing for a rapid and automatic recovery from torque overload even in the most advanced drive technology.

Some designers who prefer to control torque overload through electronic current limitation often view the use of mechanical torque limiters as outdated. While this approach is effective in many cases, as machinery becomes more dynamic, the inertia of moving parts becomes more critical. It is indeed possible to abruptly decelerate a rotating mass through unintentional blockage or application of a dynamic braking system at a faster rate than the drive would normally accelerate. This deceleration creates torque overload through reflected inertia, which is completely independent of the electronic system, and can easily exceed the peak torque rating of the motor. Modern mechanical torque limiters offer a high level of sensitivity and accuracy, with a smaller impact on the size, mass, balance, and power consumption of the drive system.

The SL Series uses the proven spring loaded ball detent system, along with a previously patented preload for zero backlash operation. But to achieve its target of 50% weight reduction, we embarked on a two-year collaborative effort with local universities, designing the product from the ground up. The result is a torque limiter constructed from state of the art materials with unique surface treatments and innovative assembly technology–surpassing weight reduction targets and simultaneously reducing its footprint. One example of this size reduction is a torque limiter rated to disengage at 160 Nm, which in the past would have had at best a mass of 1.3 kg and a moment of inertia of 1.6 x 10-3 kgm2. It now weighs 370 grams with a moment of inertia of 0.8 x 10-3 kgm2. What that amounts to is an automatic torque limiter with unparalleled power density.

In addition to custom material specifications, specially designed spring systems, and some improvements to the ball detent configuration (resulting in a 40% increase in torque capacity for a given size) the weight reduction was also achieved through the compression of individual components. This, of course, is without negative impact on the precision or service life of the torque limiter. The SL Series can handle in excess of 10,000 disengagement events, depending on rotational speed.

Custom material specifications, specially designed spring systems, and improvements to the ball detent configuration increased torque capacity by 40% for a given size torque limiter.

The four sizes (Series 30/60/150/300) cover disengagement torque values from 5 to 700 Nm, and involve various mounting options, including direct and indirect drive versions. Models SLN (clamping) and SLP (keyway) attach by flange to sprockets, sheaves, pulleys, and gears, and include an integral dual bearing system to support belt and chain tension when properly located over the shaft. Models SL2 (bellows coupling) and SLE (servo insert coupling) mount inline between two independently supported shafts, such as motor to ball screw connections, and compensate for the small but inevitable misalignment which exists in this type of machine layout. All four types are field adjustable, and come with both English and metric bores.

R+W America
www.rw-america.com

::Design World::