Do mechanical torque limiters still make sense?
This article was originally written in the period 1995-2000
Mayr Transmissions argues that electronic overload protection still can’t react quickly enough.
Electronic devices for overload protection used to be extremely slow, compared to mechanical torque limiting systems, such as the Mayr EAS-NC. With substantial improvements in servomotors and intelligent controls, this gap has been reduced and servomotors today can react much faster than ever before. Lighter and more powerful motors with responsive electronic controls make motors extremely dynamic.
Some manufacturers argue therefore that, with lower inertia of the servomotor, it is no longer necessary to use mechanical torque limiters to protect a machine against the costly damage and downtime caused by jams or collisions. In order to evaluate this situation, Mayr Transmissions has undertaken research, including all the ranges of low inertia motors from the leading manufacturers. Technical data, including inertia and acceleration, has been considered.
The minimum stopping times of motors have been calculated at different inertias on the driven side of the motor. In order to avoid any controversy about the electronic reaction times in the system, zero delay was assumed, which in practice never happens Ð a more realistic value would be 5ms.
The stopping times of the fastest motors on the market have been applied to an actual collision test of a machine.. The maximum torque would occur in this machine after 15-20ms, whereas the fastest motor would take approximately 30ms to come to a complete stop. A mechanical torque limiter reacts in this realistic example in 3ms. This means that when using electronic protection, the maximum collision force that occurs in 10-20ms still applies before the feed drive can be stopped. The damage still occurs.
This of course is only example using a specific machine, but, considering the fact that the more commonly used motors would take 50-60ms to stop the drive, it is not possible for an electronic system to beat an instantaneous mechanical system. Only if there is very slowly increasing torque will an overcurrent relay be adequate.
Any future limitation to reduce motor inertia to a value that would allow faster stopping times relies on the ratio of motor inertia to ballscrew and turret inertia not exceeding 1:2.5.
- Mayr Transmissions
- Tel: 01535 663900
- Fax: 01535 663261
- Contact: G A Harrison (Technical Director)