I have an application in which speed of the shaft is 0.54 rpm. The input speed of the motor (induction 4 pole AC motor) is 1400 rpm. So the gear reduction is enormous. I had two helical redactors coupled together and I read that in this situation when you have such big reduction (2 stage reduction) and super low speed you have to limit the input torque. I don’t understand this – why is this necessary? The start up torque of AC motor is 2-4 times higher than the nominal torque, how to start this application? Can someone tell me why in big reduction systems it is necessary to limit input torque?
Because the torque is multiplied by the same factor that you reduce the speed with. For large reductions this can create a lot of torque. If the output is seized up then this results in large forces which will start damaging things like bending or shearing shafts, stripping gears or cracking the case.
A common way to limit torque is to include a sacrificial plastic gear that gets stripped when torque becomes too large. This is then the designated weak point just like the fuse in an electrical system.