Mehdi,

Try using a sizing utility that works out the math for you and sizes the motor based on your system description...

Here's one for example:

http://www.parkermotion.com/sizing/in...#home=true

Just remember that you get linear increases in torque by increasing the gear ration, linear decreases in speed, and exponential decreases in inertia (that the motor will see)...

Also, remember that with a stepper, the torque curves aren't linear, so you can tailor your move profile to take advantage of higher torque at lower speed, and lower torque at higher speed.

I have used a parabolic velocity curve in the past for rapid acceleration, and this allowed me to use a smaller motor for the same load...

-db