There are four main factors that will determine the torque required for the rotational axis, inertia, acceleration, friction and efficiency.
The inertia should be calculated on a worst case with the arm a full horz. reach with max load, convert to in-lb-sec^2.
The acceleration will be determined by your cycle time requirements and should be in rad-sec^2.
Once you have these two the calculation is (inertia times acceleration) this will give you torque in in-lbs.
Friction is the force to rotate the robot with the drive disconnected. This should not be much if you are using roller or ball bearings at the pivot. This can be calculated but the bearing and seal type needs to be known.
Efficiency will basically be the rated efficiency of the gearbox if you are using one. I am guessing that your reduction is going to be over 100:1 unless you are going to use a very large stepper. Some robot mfgs are going to using very large pancake servos on the rotational axis instead of a reduction.
There are several websites that go thru this in detail. Sanyo-Denki and Baldor both have downloadable pdfs that go thru this step by step for sizing servos which will work just as good for steppers. Sanyo-Denki and I think Baldor also have free sizing software you can download so you can just punch in the numbers.
I also have a little spreadsheet I made for doing this that you can have if you email me at barry1961@aol.com.
Barry
