The trend of replacing mechanical systems with electrical systems continues. Even developers of hydraulic and pneumatic systems are following it. But, as is becoming evident through the latest unintended acceleration issues, electronic components can have a few drawbacks that should not be overlooked in a design.
When in comes to designing a system for safety, specifically when considering whether to choose a mechanical component such as a coupling, or to go electronic, remember this: Electronic safety components have two major disadvantages compared to mechanical safety components.
- Reaction time. Assume a machine crashes and causes an overload. According to engineers at R+W America, a signal from the monitoring circuit does not reach the motor controller until 5 to 7 ms following a sharp increase in torque. During this period of latency, the controller attempts to further increase torque to reach the setpoint value. Most likely, another 10 ms will pass before the motor is shut off. Depending on the drive train’s moments of inertia, more time can pass before the electronics brings the whole system to a stop.
- Multiple potential failure sources. Electronic monitoring systems need multiple sensors for data. Between the monitoring system and all of its sensors and other components, you have a system with multiple possible points of failure.
A mechanical safety coupling, on the other hand, completely disconnects the drive from the load within 3 to 5 ms; 1/3 of the time needed by an electronic cut-off. Noted engineers at R+W America, “electronic machine monitoring is not suitable for high speeds due to the large centrifugal mass of the rotating parts.”
Also with a mechanical safety coupling, you have one component per axis, reducing the number of possible points of failure.
Safety couplings must demonstrate two clear behaviors:
- Upon overload, separation of drive train and load should occur within a few milliseconds.
- After the coupling has disengaged, residual friction should not be excessive so as not to damage coupled components that continue to be driven due to mass moments of inertia.
According to R+W, safety couplings can be subdivided into five classes:
1. Rigid safety couplings used in indirect drive applications.
2. Torsionally rigid safety couplings for use between two shafts or flanges. These couplings resist twisting and can be subdivided into two groups.
A. Single-piece torsionally rigid safety couplings.
B. Press-fit couplings.
3. Vibration-damping safety couplings are fitted with an elastomer insert that damps incurred drive vibration.
4. Economy safety couplings suit applications requiring simple overload protection and functions as a variation of the ball-detent principle.
5. Torque-limiting line shafts, which span long distances between shafts.
(Some material, courtesy of R+W America.)