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The Mechanical Torque Limiter
April 21st, 2021
Damage caused to equipment by input shocks can have huge knock-on effects. Torque limiters protect equipment from mechanical overload, preventing harm to rotating components.
Here we will take a closer look at the essential role of torque limiters and what types are available, including tolerance rings.
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.
Why use a torque limiter?
If a critical component fails or breaks it will send the torque levels rocketing way beyond what a system can deal with. The results can be costly in terms of equipment repair, replacement and the downtime to production can be significant.
Torque limiters provide you with insurance. They introduce a fail-safe mechanism that will limit damage by limiting torque.
What is the difference between torque limitation and torque control?
Torque control is about maintaining the correct levels of torque for operation. Torque limitation is about preventing system failure due to the torque exceeding normal operating parameters. Control gives you optimum performance, while limitation prevents damage caused by excess torque.
How can I limit the torque in my system to prevent damage?
Torque limiting couplings act to disengage the drive and driven parts should the torque exceed a predetermined level. Adding a protective limiter allows each system to set a torque threshold, preventing costly damage from the outset.
Torque limiter applications
Torque limiters are found in applications with rotating parts. They are best-known for being a safety component in gearboxes and can prevent jams that cause damage on conveyor systems. Because power failures can cause high torque (even momentarily), generators need a limiter.
A good example is in robotic applications. If overload occurs, the robot can lose their ability to position accurately (if it can still function at all) as well as have a massive effect on production. A simple torque limiting device will protect the robot from damage and reduce downtime.
Torque Limiters are used extensively in cars and automotive vehicles as there are many rotating mechanisms. Powered tailgates need protection from mis-use when something or someone applies force to prevent the tailgate from completing the programmed open or close actions. A slip clutch or torque limiter will protect the motor, the spindle drive and the tailgate panels in the event of these mis-use cases, preventing mechanical damage.
Another automotive application is Electric Power Assisted Steering (EPAS). A torque limiter is needed to protect the plastic worm wheel gear in mis-use or shock loading events. Overload events such as potholes in the road surface, curb strikes or car mechanics forcing the wheels to turn during maintenance can all cause damage to the EPAS if not mechanically protected.
Types of mechanical torque limiters
There are two main types of torque limiter: slip clutches and disconnecting limiters. The difference between them is the way that they disengage the motor from the load or driven parts.
All mechanical torque limiters provide disengagement within milliseconds during torque overload conditions.
There are also electrical torque limiters, we have excluded them from here as they require sensors and devices for detection and monitoring. Mechanical limiters offer a physical disconnection from the motor. Because of this they are often more reliable and can be a simpler option. Mechanical and electrical torque limiters can be used in conjunction to support each other and offer dual torque limitation when used together.
The slip torque limiter
The slip torque limiter disconnects the components by slipping, hence its other name, the slip clutch or the overload clutch.
The slip torque limiter is set to a specific level of torque. During normal operation, the slip torque limiter transmits torque between the drive shaft and the driven part through friction discs. The discs are sprung-loaded creating a frictional force, the torque threshold is set by adjusting the spring force preload.
If the set torque level is exceeded, the torque limiter disengages the drive shaft from the components being driven, the friction is overcome and the drive shaft slips preventing damage. It then automatically resets and the job can continue with very little interruption. Resulting in no damage and very little downtime.
The disconnect torque limiter
This type of torque limiter works differently from the slip clutch by physically disconnecting the drive. There are various types of disconnect limiters:
Ball or roller release detent limiter
A ball detent limiter provides power transmission through metallic balls in a shaft. There are two rotating bodies, one with spring-loaded balls and one with mating sockets (or detents), both types are needed to mate the components together. Under normal operation the torque will be transmitted but if mechanical torque overload occurs, the balls break transmission by overcoming the spring forces and disengaging from the sockets or detents.
Shear Pin
This design has a metal pin linking two rotating parts. 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 of using shear pins is that they must be replaced after the torque overload conditions so maintenance downtime is inevitable. Another disadvantage is that it can be difficult to accurately control the level of torque at which the shear pin will break. Shear pins are found in applications such as propellers, tow bars and weapons.
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.
RENCOL® Tolerance Rings
The RENCOL® Tolerance Ring removes the need for a separate slip clutch and inline fixing. Like the friction-type slip clutch, the RENCOL® Tolerance Ring works by using spring force and friction. The ring, which is inserted coaxially into the drive, has wave features that provide a radial force that creates friction between two components. This transfers the torque until the set threshold is reached.
What are the benefits of using a tolerance ring for torque limitation?
A tolerance ring is one element of a three-part system, between two mating components. It has surface corrugations (“waves”), which compress to securely hold everything in place as well as acting as a limiting slip clutch. It works with the inner male and outer female components to join parts, transfer torque and enable axial force between mating components.
The tolerance ring is easy to assemble and low-cost, and because it’s multi-functional it reduces the weight as well as volume needed in a system. Depending on the system, the RENCOL® Tolerance Ring can be designed to slip onto the shaft or bore providing you with mechanical overload protection.
How is the torque set using a RENCOL® Tolerance Ring?
Each tolerance ring is designed to your unique specifications, giving you total control to set the torque threshold to your requirements.
The size, shape and location of the surface waves or corrugations determine the tolerance ring’s function. Adjusting the geometry of the waves as well as the size and material of the tolerance ring allows you to set the ring for the specific application or to reach the desired performance.
This works using spring theory. The greater the stiffness of the waves, the more force is needed to compress the ring. So, how an individual tolerance ring operates depends on the stiffness of its corrugations. This is why bespoke components are essential, as the corrugations should be adjusted for each application. The torque is determined by creating the right ring geometry for that application.
Does a RENCOL® Tolerance Ring make the coupling simple?
Yes, it does. Once the waves are engaged with the mating parts, the force level becomes stable for the rest of the assembly process. The spring element of the ring gives more consistent forces over a range of temperatures. This secure retention force helps prevent damage and misalignment.
There are two ways to assemble tolerance rings:
Housing Variable Ring (HV)
This is mounted onto the housing bore first and has inward-pointing waves. Before assembly, this type has a large gap between the ends of the ring. This is squeezed shut during assembly. The tolerance ring is inserted into the bore and then springs open, creating a force.
Shaft Variable Ring (SV)
This ring is mounted onto the shaft first and has outward-pointing waves. During assembly, the prised-open ring is slid over the shaft and then released. It closes then is retained on the shaft.
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
Sign up to our Torque Limiter Webinar
We’re holding a webinar on the topic of Torque Overload Protection were we will be sharing our knowledge on mechanical overload protection. We’re running the webinar twice, so you can attend at a time that suits your schedule. Both will be held on Tuesday 29th June, the first one will be at 9am UTC and a second at 2PM UTC (9am EDT). If you’re interested you’ll need to register as spaces are limited, the links are below:
