What is a safety coupling?
Safety coupling is a mechanical shaft component. It connects two pieces of the shaft. However, it has a special role: to protect the shaft line against excessive torque. That is why it is also referred to as a “mechanical fuse”.
Synonyms for safety coupling
The device is known under several names:
- safety coupling
- torque limiting coupling
- slip coupling
- overload protection coupling
Those names make the purpose of the coupling very clear.
Function of safety coupling
The function of safety coupling is to protect the shaft components against mechanical overload. As long as the transmitted torque is within the specified range, nothing happens and the safety coupling works as a normal joint. When the torque exceeds certain level, the safety coupling breaks and disconnects the protected part of the shaft. Therefore the names “torque limiting”, “overload protection” or “mechanical fuse”.
Breaking of the safety coupling is a controlled breaking. It does not necessarily mean that the safety coupling gets destroyed. In most cases the design is such that the coupling elements (halves) just slip when the torque exceeds certain threshold.
Use cases
When can safety coupling be used? Typical use cases are various test stands. The safety coupling protects the device under test (DUT), especially when the tests are very demanding and include high dynamics. Another case is to protect a torque meter. Modern torque meters offer possibility to accurately measure static and dynamic torque (usually combined with speed measurement as well). However, those device are costly. Especially in high power range and measured torque of tens or hundreds of kNm the torque meters are very expensive. At the same time, the torque meter does not have any overloadability. Dimensioning the torque meter for the highest possible transient torque is economically not an option (note that in case of a motor short circuit the transient peak torque may reach 5…8-times the rated torque!). When the torque meter is loaded with torque exceeding the specified measurement range, it can be permanently damaged. Thus, it is a good idea to protect such devices with a safety coupling.
References
[1] Safety and overload couplings, Jakob Antriebstechnik, https://www.jakobantriebstechnik.de/en/safety-couplings-overload-couplings/
