I found this geared stepper motor on robotshop and I am confused as to what maximum torque it can give: http://www.robotshop.com/en/geared-bipolar-stepper-motor-3v-555-oz-in.html
It says 48 kg-cm of torque in the specs, but in the description is says a maximum of 250 kg-cm of geared torque and 48 kg-cm or continuous rotation torque and 100 kg-cm for brief overloads.
It is a geared stepper motor so I'm guessing it can do 250 kg-cm of holding toque and not massively damage the motors, I'm not sure though. If it's just 48 kg-cm of holding torque then it's no use to me!
The description is quite clear, the motor+gears can in theory generate 250 kg-cm
(24.5 Nm) but that is more than the gears can stand, so its quite capable of destroying
itself if you overload the output. The continuous torque rating of 48 kg-cm (4.7 Nm) is
the value to use.
Put another way the 250 kg-cm value just means the motor itself is rated at 2.5 kg-cm
(in other words they have put an inadequate gearbox on the motor!)
The description is quite clear...in other words they have put an inadequate gearbox on the motor!
After reading the description myself, you are quite correct! I wonder how many people break those?
I had a DC gear-motor (purchased surplus for $10.00) that easily generated enough torque to break it's output gear when stalled (steel gears, too); I found this out the hard way. I contacted the sales rep for the company that currently manufactures the motors, and they could sell me replacement gears, but they had a minimum order quantity of 50 pieces, at $7.00 USD each! I passed on that, and purchased another motor instead.
With a 99.5:1 reduction gear as a stack of epicyclic stages the last stage will be
taking much more torque than the others, the design is rather a compromise.