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The Mechanical Torque Limiter
July 15th, 2020
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 Limiter
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. At robots, it can affect the ability to function or affect the repeatability 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.
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.
Torque limiter applications
Torque limiters are found in applications such as motors where rotating parts are used. They are best-known for being a safety component in gearboxes, and can prevent jams causing damage on conveyor systems. Because power failures can cause high torque (even momentarily), generators need a limiter.
There are two basic designs of torque limiter, depending on how they disengage the driver from the driven parts or load: slip clutches and disconnecting limiters.
Another option are electrical torque limiters. But we will exclude them as they require sensors and devices for detection and monitoring. And mechanical limiters are a cheaper and simpler option.
How slip torque limiters work
The torque limiter is set to a specific level of torque. If this torque setting is exceeded, the torque limiter disengages the driving shaft from the components being driven, preventing damage from high torque levels. It disconnects the components by slipping, hence its other name, the slip clutch or the overload clutch.
The useful component doesn’t hang around waiting for overload to happen. When it’s not providing overload protection, the torque limiter transmits torque between the driving shaft and the driven part through friction discs. The discs create a frictional force that enables torque transmission, and if overload occurs and the set torque level is exceeded, the friction ceases and the driven part slips.
Crises averted, the limiter automatically resets, and the job can continue. No damage and very little downtime: you can see why a torque limiting clutch is essential. However, it’s worth noting that slippage will continue to happen unless the overarching problem is fixed.
The disconnect torque limiter
This type works differently to the slip clutch, by physically disconnecting the drive. A ball detent limiter provides power transmission through metallic balls in a shaft. If mechanical overload occurs, the balls leave their spaces, breaking their transmission.
The shear pin has a pin that breaks if high torque is reached. Obviously, the downside is that they are single use; however, they provide a fast and decisive break if the torque is exceeded, and are found in applications as diverse as propellers, tow bars and weapons.
The magnetic torque limiter combines the disconnecting and slip torque approaches. Its magnets break off if the set torque is exceeded, causing the clutch to slip.
The problem is with this type of torque limiter is that they need manual re-engagement after uncoupling, and response times for doing this can vary. Due to the downtime, these are not low-cost options in the long term.
Types of mechanical Torque Limiter
We still focus on mechanical torque limiters; they provide disengagement within milliseconds during torque overload conditions. The main types of mechanical torque limiters are: Friction Type, Shear Pin Type, and Ball or Roller Release Detent Type.
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 Type
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.
RENCOL® Tolerance Ring Type
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.
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 Clutches on our product page.