On Thursday 18 August 2016 11:16:02 Todd Zuercher wrote: > They are both almost identical screws and nuts except for their leads. > They are both Kirk Motion VHD series LEAD screws. > http://www.haydonkerk.com/LinearActuatorProducts/LeadScrewsAndNuts/Nut >s/VHDSeriesNuts/tabid/148/Default.aspx > > So you are saying that all things being equal the turning force should > be about the same for the 2 screws. Which must leave me with the > conclusion that the older lower lead screw I pulled off the shelf is > probably mostly worn out. I'll go disassemble it and inspect the > threads in the nut. > > I'm considering swapping to ball screws, but I'm a little concerned > about it back-driving when off, and I don't think I can find a 0.1" > lead ball screw (screws are currently direct coupled to the motor). > So far, I have not found that to be a problem as the steppers I have used have a sufficient cogging action when powered off that the only time I have had one unwind and drop the head was when I took the 1600oz/in off the G0704 to replace it with the 940oz/in.
It unwound and stopped on some relatively 'cushionable' wood stuffs laying on the mills table, no damage to anything other than my surprise when I realized and understood why it was happening. I won't say its not happened, like the knee on a bigger bridgeport, which weighs about 15x what the head on a G0704 weighs, but I've not seen it happen, just read about it. So I'd go a decent way out on a persimmon limb and say thats a servo problem 99% of the time. If anyone here has had a problem like that while using steppers, please speak up, we would like to hear about it. > ----- Original Message ----- > From: "John Kasunich" <[email protected]> > To: [email protected] > Sent: Thursday, August 18, 2016 10:50:59 AM > Subject: Re: [Emc-users] Leadscrew Efficiency? > > 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 Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
