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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?

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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.

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  • $\begingroup$ Oks, but how is suppose to limit the torque. The gear is driven my AC motor i think the only one way to is ti limit AC motor current, but it''s interesting how the motor is going to start - the start torque is about 4 time nominal $\endgroup$ – next_for Aug 29 '16 at 18:19
  • $\begingroup$ @next_for it means that if torque gets too high the plastic gear will simply fail. If the output shaft is not blocked then the torque just won't get that high. $\endgroup$ – ratchet freak Aug 29 '16 at 18:22

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