Are they LEADscrews or BALLscrews? With ballscrews, the finer pitch screw should indeed provide a much better mechanical advantage.
With leadscrews, the mechanical advantage almost doesn't matter, because friction is by far the dominant force. Somewhere between 60% and 95% of the torque required to turn a leadscrew under load is due to the friction between the screw and nut rather than the force actually needed to raise the load. I'm not sure why the 10tpi screw would actually be worse - I would expect it to be roughly the same. However, many factors come into play: 1) materials: are both screws steel? both nuts bronze? (pr plastic, or whatever) The materials and lubrication can make a huge difference in the amount of friction. 2) surface quality: a rough screw or nut will have more friction than a polished one 3) thread form: 60 degree threads have a lot more friction than Acme due to the wedging action of the 60 degree flanks. Square threads are best because the flanks are perpendicular to the load, but Acme is almost as good. 4) diameter: if all of the above are equal, the friction FORCE will be the same. But the TORQUE required depends on the radius of the screw, so a larger diameter screw will require more torque. On Thu, Aug 18, 2016, at 10:14 AM, Todd Zuercher wrote: > I have a machine that I converted from step-motors to servos, and I'm having > a little trouble with the Z axis. It has an anti backlash lead screw with a > 5tpi screw. This sort of worked, but the servo was working hard to move the > head, and I wanted a little higher encoder resolution for better tuning. So I > swapped in a nearly identical 10tpi lead screw set I happened to have on hand > (removed from another stepper machine to get better speed and perfomance). I > thought that the 10tpi screw should be easier for the servo to turn, but I'm > finding that the opposite is true and the servo can't raise the motor without > counter balance assistance when it could with the 5tpi. Does that make sense, > or is the problem more likely that the 10tpi screw and nut are worn out and > binding? > > -- > > ======================================== > > Todd Zuercher > mailto:[email protected] > > ======================================== > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users -- John Kasunich [email protected] ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
