The Mechanical Torque Limiter

Robot toys often fall off a table top, distracted people can stumble over robot vacuums, and electrified hinges on toilets seats or cars get abruptly stopped. Damage caused by such misuses or input shocks can be avoided by using a torque limiting device. Torque limiters protect equipment from mechanical overload, preventing harm to rotating components. Such devices can be required for servos, stepper motors, or in gears driving the systems.

Torque Limiters

Torque overload protection devices are used by engineers to protect systems from misuse or input shocks. Accidental collisions, collisions of production or assembly machines, or unintended driving conditions can cause overload situations. Often this happens during power transmission at high rpm and can damage the drive motors or gear boxes.

When torque thresholds are exceeded in robots, it can affect the ability of the robot to function or affect their repeatability. When robots are used for production and assembly, torque overload conditions can cause robot or machine downtime, which can have a considerable negative impact on productivity and maintenance costs. Introducing torque limiting devices will protect the drive motors or gearboxes from damage.

What is a Torque Limiter? 

The torque limiter exists to protect equipment from mechanical overload. Critical components can fail or break at a moment’s notice, sending the torque levels rocketing way beyond what the system can deal with. As you can imagine, the results aren’t good.

By preventing torque overload, these clever-yet-simple parts are essential to protect rotating components. We take a closer look at the torque limiter and the different types available. There are two basic designs of torque limiter, depending on how they disengage the driver from the driven parts or load:

1. The slip clutch is set to a specific level of torque. If this torque setting is exceeded, the slip clutch disengages the driving shaft from the components being driven, preventing damage from high torque levels. It disconnects the components by slipping. 
2. The disconnect torque limiter works differently to the slip clutch, by physically disconnecting the drive.

Another option are electrical torque limiters. But we will exclude them as they require sensors and devices for detection and monitoring. Further, mechanical limiters are a cheaper and simpler option.

Friction Type 

 

At friction type torque limiters, there are spring loaded friction disks that interface with each other, similar to an automobile clutch. The torque slip threshold is created by adjusting the amount of spring force preload on the friction disks. Under normal torque load conditions, this torque limiter will allow the entire torque to be transmitted. When the torque exceeds the threshold, the friction disks will slip against each other. An advantage of this type of torque limiter is the immediate re-engagement after the torque overload condition.

 

 

Shear Pin

 

For this type of torque limiter, metal pins are used to link two rotating bodies together. A constant shear force is applied to the shear pins when torque is being transmitted. During usual power-transmission, this torque limiter will allow the torque to be transmitted. When the set torque is exceeded, the shear pins will break. The disadvantage, shear pins must be replaced after the torque overload conditions. And it can be difficult to accurately control the level of torque at which the shear pin will break.

 

 

 

Ball or Roller Release Detent Type

A series of balls or rollers in one rotating body are matched with mating sockets or detents in the other rotating body. The balls or rollers are spring loaded so they remain engaged with their mating sockets or detents. The entire torque will be transmitted under normal operation. When the torque exceeds the threshold, the balls or rollers overcome the spring forces and disengage with their mating sockets or detents.

 

Magnetic torque limiter

The magnetic torque limiter uses a combination of the disconnect and slip approaches. If a set torque level is reached, the magnets disconnect and the clutch slips.

 

 

Tolerance Ring Type

The RENCOL^® Tolerance Ring, which uses spring force and friction as its operating principle, is not like an actual Friction Type torque limiter. The Tolerance Ring is installed coaxially into a drive assembly, instead of being installed or clamped in series with an assembly. The RENCOL^® Tolerance Ring has wave features which provide a radial force. This radial spring force provides friction between two components and transfers torque until a threshold value is exceeded. The Tolerance Ring can be assembled between a gear or sprocket and a drive or output shaft.

 

Slip Clutch

Most of these systems are complex, bulky and expansive. A RENCOL^® Tolerance Ring uses one component in line with the mating existing components. It reduces space and weight at low cost. Find further information on RENCOL^® Tolerance Ring as Torque Limiter or Slip Clutch on our product page.

For a toy robot we developed the world's smallest RENCOL^® Tolerance Ring. In this case study we share the story of how we shaped our capabilities across design, testing and manufacturing that ultimately led to the development of the smallest slip clutch in our history.

 

CASE STUDY

 

 

Torque to us 

To find out more about RENCOL^® Tolerance Rings, torque limiters and their role in overload protection, talk to us at Saint-Gobain, engineer to engineer. We have a wealth of experience to share. You can Contact Us or email: makingabigdifference@saint-gobain.com