July 15th, 2020
Using a Tolerance Ring can be a simple way to introduce a mechanical torque overload protection into a complex system. The torque limiter will increase life-time and reduce space and weight of the final product.
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. Such devices can be required for servos, stepper motors, or in gears driving the systems.
Torque overload protection devices are used by engineers of robotic companies, automobile manufacturers, or automatic equipment manufacturers. Accidental collisions of robots, collisions of production or assembly machines, or unintended driving conditions can cause overload situations. Often this happens during power transmission at high rpm and will damage the drive motors or speed reduction units. It can affect the ability of a robot to function or affect its ability to make its moves to positions accurately. In the case of robots or production/assembly machines, torque overload conditions can cause robot or machine downtime, which can have a big negative impact on productivity and maintenance costs. Torque limiting devices will protect the drive motors or gearboxes from damage.
Mechanical torque limiters provide disengagement within milliseconds during torque overload conditions. An advantage of mechanical torque limiters is that they are a cheap and simple design, while electrical torque limiters require sensors and devices for detection and monitoring. The main types of mechanical torque limiters are: Friction Type, Shear Pin Type, and Ball or Roller Release Detent Type.
Friction type torque limiters have 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.
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 is that the 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.
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.
A 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.
The RENCOL® Tolerance Ring is introduced between the drive gear and the output shaft, so if, for example, the output drives against an obstruction, or receives a shock load, the RENCOL® Tolerance Ring will permit momentary slip between the gear and the shaft. The output shaft position is monitored by the servo angle sensor or potentiometer, so when slip occurs, the system ‘knows’ what happened and can deal with it whilst not losing positional registration.
The ring can be engineered so slip occurs at a torque level suited to the rating of the servo, or to that of the driven system, depending on what should be protected. The graphs in the 2 charts below show 0.4Nm torque slip, but an even higher torque setting of 2.5Nm for this particular servo would be possible with a RENCOL® Tolerance Ring design.
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 Clutches on our product page.