# Torque to spin motors?

This may seem a bit random, but does anyone know how much torque it takes to spin an unpowered 393 motor?

The stall torque of a 393 Motor set in torque mode is 14.76 in-lbs
The stall torque of a 393 Motor set in speed mode is 9.2 in-lbs
The stall torque of a 269 Motor is 8.6 in-lbs
(These are the specifications from the vex store and you can view them here and here)

I don’t think he was asking how much torque motors deliver. I can’t give you an answer, but i think i might be able to fermi it. If you get a ratio of 25:1 or more, your gears will begin to break before it turns. this happens slightly before your axle. Axles handle about 118 inch pounds of torsion, so im going to estimate that gears handle about 100. since gears dont transfer power at full efficiency, i would lower the value slighly more, to maybe about 90. Remove the mass of the gears and you can lower it even more to like 80. Divide it by 25 for the ratio and

you get about 3.2 inch pounds. if you have it in the speed ratio, its about 2 pounds

Edit, then again i might be overestimating the strength of gears, cause i remember doubling them up and they still wore down. That means it could be as low as 1.6 to 1 inch pounds

Thanks, but that is not what I am looking for. The stall torque is the torque at which the motor will stall, meaning that at that torque, a powered motor will no longer spin. I am looking for the torque needed to make an unpowered motor spin.

Stall torque can mean the motor is at no speed or when the motor physically stalls.

Torque does not make a motor spin (unless you are trying to back drive it). The current flowing in the windings due to the potential difference (voltage) across them does. To start the motor spinning the current will be high, it is the same as the current that will flow when the motor is stalled. As the motor starts to move it will act as a generator and create “back emf”, a voltage of the opposite polarity to that which is being supplied, the faster the motor spins the higher the voltage produced, eventually the motor speed will reach an equilibrium depending on the load driven where the difference between supply voltage and back emf (an over simplification, see some of vamfun’s posts and my smartmotor library for the math) determines how much current is needed and therefore how much torque is produced at that speed.

That is exactly what I am trying to do

I measured this a while back for a 3 wire motor, found it to be 33 oz-inches of torque to overcome static friction. Don’t have the number handy for a 393 though.