Quote:
Originally Posted by JesseK
It's possible on a rudimentary level to figure out the torque requirements of a lead screw by figuring out the angle of the threads as if projected on a flat plane. I figured this out as we went along in 2011, so I don't know if it's the best way to do it:
e.g. There are 2" of travel in 1 turn, PI*0.625" of circumference rotation in 1 turn, thus Theta = Tan(2/(PI*0.625)) should give the angle of thread incline in radians. Sin(Theta) will then correlate to the amount of force due to gravity the motor must overcome just to move the load. You'll also want to add in a buffer for the losses within the threaded nut, thrust bearings and the gearbox. If I remember correctly, I did it as a ratio [sin(Theta)/(sin(Theta)+cos(Theta)) * weight of load] was the force at the tip of the sprocket on the threaded rod, and it seemed to be close to what we experimented with under heavy load.
Note that the '2" of travel in 1 turn' is the rating after the # of starts is taken into account.
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This is assuming that thread is square. The other thread angle(the incline directly out of the axis of the screw) is not flat on a standard acme screw. This adds to the amount of torque you will need and complicates the equations a little more.
But back to the OP, a quick and dirty calculation you can always do to figure out how fast your motor can lift your load(force) is take your motors' theoretical output power (Watts) at its max current draw for FIRST (eg 40A) and use the below fiziks equation. Of course this will only work on your 100% efficient system.
Power=Force*Distance/(Time)