On Friday 15 February 2013 08:44:14 Erik Christiansen did opine: > On 14.02.13 09:37, dave wrote: > > Most machines I'm familiar with use either timing belts for relatively > > low power and gears for high power. > > The thing is that even a hefty little NEMA23 stepper can come with a > radial thrust limit of 15 N, and an axial limit even lower. (I've just > bought one from a local CNC goodies supplier.) There's no way the > toothed belt pulley could be directly mounted on the motor shaft, and > the belt then tensioned to the "twang level" recommended on this list, > for acceptable backlash on the belt drive portion, is there?
I was concerned with that when I built my new z drive, where I have a 17 tooth pulley driving a 42 tooth pulley which in turn drives the nut on a non-rotating bolt, which is suspended by a pair of rather large, axial thrust ball bearings. I was surprised by the amount of tension I had to use on a 1/2" wide belt to eliminate the backlash. That was about 5 years ago and there have been no detectable complaints from that triple stack 425 oz nema 23 motor. The 17 tooth and the 42 are well aligned, so the weight of the damper on the upper shaft end is about the only addition to the axial thrust. That combination can put 155 lbs on a bathroom scale to be used to push a drill bit should it be needed. That is 150 lbs more than the OEM lashup, with the z screw behind the post could do even with the hand crank installed on the OEM screw. > I can see that I'll have to drive the toothed pulley, mounted on a shaft > with bearings either end, through a flexible coupling, to decouple the > radial thrust. The supplier has helical beam couplings with torque > ratings modestly exceeding the motor torque, but like Pete, I'm also > trying to find out what will actually reliably in the long term. I haven't found it to be a problem, and apparently the folks selling cnc kits like for the GO704 etc, where a 2/1 or 3/1 belt drive runs the table screws haven't found it to require any aux shafts. > The bellows couplings seem impossibly expensive. The disk couplings > sound a bit like an X-shaped coupling I've seen - a bit like half a > universal joint. (CV coupling, in fancy language, IIUC.) > > Maybe an Oldham coupling with a disc/X adaptor added will do the job at > an acceptable cost? (Or does a small angular misalignment merely cause > the Oldham coupling to wear faster, as the tenon slides in the mortise > of what is then a nearly zeroed swash-plate?) There does seem to be a concern for that, and I could see it being valid where the center disk is some sort of plastic as the commercially made and torque limiting versions seem to be. My disks are made from the same mine shafting as the hubs, and wear has absolutely not been a problem in the years since I made them. Machining marks were still present on the tenons the last time I had one apart. > Erik > > > - Not all systems support hard links > > Rolls eyes. Sure, but all _my_ systems do! UNIX dammit! > - Cameron Simpson, on mutt > ML. > > > ------------------------------------------------------------------------ > ------ Free Next-Gen Firewall Hardware Offer > Buy your Sophos next-gen firewall before the end March 2013 > and get the hardware for free! Learn more. > http://p.sf.net/sfu/sophos-d2d-feb > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! My views <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> Try the Moo Shu Pork. It is especially good today. I was taught to respect my elders, but its getting harder and harder to find any... ------------------------------------------------------------------------------ Free Next-Gen Firewall Hardware Offer Buy your Sophos next-gen firewall before the end March 2013 and get the hardware for free! Learn more. http://p.sf.net/sfu/sophos-d2d-feb _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
