Stepper motor geared vs non-geared

[ZeroBacklash]

I am getting a 300kg-cm geared stepper motor NEMA34 at the same cost of a 300kgm-cm non geared stepper motor in NEMA42. I don't have the full spec of the 300kg-cm geared motor I mean the currrent rating but the NEMA42 one is an 8A motor.
My target rpm is around 10 to 20 rpm, the geared stepper has a gear ratio of 30:1.
Suprisingly there is not much difference in price between the 2 motors.

What would be the advantage of the geared motor vs the non-geared one?
Is the torque figures indicated are holding torque values. What is the % of holding torque that is generated by the motor when the motor is running? I believe it would be lower that the 300kg-cm value.

Thanks in advance

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[Bdrangdo]

The spec for the NEMA 42 motor is either wrong or you are misinterpreting it wrongly. It would be unlikely to be able to produce much more than 20n/m and that would be a holding torque, as soon as the motor starts turning the torque will drop rapidly and in general the larger the motor the faster the torque drops. NEMA 34 motors are largest practical size for stepper motors I have used NEMA 42 and they do not perform well. Motor torque for small motors is usually rated in Newton/Meters or CMs not KGs, A Newton is roughly 100 grammes. (0.981 Kg) You may need to redo your sums.

[awerby]

With a handle like yours, I'm surprised you missed the major difference. A regular stepper motor won't have any backlash, but if you add a gear train, you get it. Whether that's a problem or not depends on your application. As Bdrango points out, another problem is the torque curve. Since a gearmotor trades speed for torque, if the torque falls off with speed you don't gain much if anything. It makes more sense to use a servo with a gear train, since they start out with more speed than can generally be used.

[Jim Dawson]

I installed a NEMA 42 motor on a project, that was proabaly a mistake. The torque dropped off so fast that you have to bring up at accel very slowly or it would just decouple....running unloaded! At very low speed it worked pretty well, but needed to decel to zero RPM to reverse without decoupling.