From: null@void.com   
      
   "Jim Wilkins"  writes:   
      
   > "Richard Smith"  wrote in message news:m1wmbd29er.fsf@void.com...   
   >   
   > Hi everyone   
   > I have been shown how to do this with power-electronics and   
   > "off-the-shelf" gearbox, etc.  With all speed-control needed.   
   > A friend had made a small welding positioner.  Hence adapted it:   
   > * VFD - 1ph-to-3ph   
   >   v   
   > * 3ph induction motor   
   >   v   
   > * worm-drive gearbox   
   > You turn a knob setting the output Hz.  Adequate speed range.   
   >   
   > Having seen this - that it works and does everything wanted - will   
   > simply do it this way.   
   > Have friend's equipment on "unspecified loan' in my car to take home and   
   > play with more.   
   >   
   > -----------------------------------------   
   >   
   > That is an excellent way to learn what works. The potential problems   
   > are cost and custom machining. In that instance I'd look up and save   
   > the components' data sheets and see how their max power ratings   
   > compare to what was needed and what you intend to do. I've been burned   
   > by guessing that something was good enough, my sawmill left a trail of   
   > broken ball bearing innards until I switched to a more reputable and   
   > expensive brand for the blade guides.   
   >   
   > I hardly ever find the same device twice as second hand and must   
   > decide on the spot if what I found is suitable, which may depend on   
   > its power rating. People dispose of what they can't use, typically   
   > because they are either inadequate or broken.   
   > I tested that contactor for pull-in voltage and contact resistance.   
      
   I will go with "off the shelf".  Now seen what's needed.   
   Time si too valuable.   
      
   * have the thing working (hopefully!) and making inroads here in   
     Cornwall   
      
   * free to get on with other things I have to do - would cost me more in   
     what I have not done than I could save not "placing orders" for what   
     is spec'd for the job.   
      
   One "blessing".   
   Gearbox must for sure have a torque rating.   
   Derive torque.   
   Found it's gloriously simple - relation of power, torque and revs.   
   P=tau.omega   
      
   P=power (Watts)   
   tau = torque (Newton.metres)   
   omega = rotation-rate (radians/s)   
   Latter makes total sense - well it does for me :-)  Radian is where a   
   radius is wrapped around the circumference.  Very often gives vast   
   simplifications (compared to working in angular Degrees or Revs Per   
   Minute, etc.).   
   Prompted that way, I can easily see from first principles how   
   "P=tau.omega" can be derived, giving total satisfaction in applying it.   
      
   So I can look through spec.sheets for "off-the-shelf" gearboxes.   
      
   I have a 30:1 gearbox on loan.   
   However, reckon 7:1 would make the drive "synchronous" (if ran motor at   
   mains frequency, would give the right drive speed).   
   Of course you need speed adjustment to make the mill(s) work exactly   
   right - but what you are asking is very achievable - say +-25% - which   
   an induction motor can do no problem.   
   [welding positioner - is so low power draw for small parts that eg. 10% of   
   the mains synchronous speed it is designed for is not a problem]   
   So going to have to get a 7:1 ish gearbox for mill.   
      
   Speed control should be wide enough to run rod-mills and ball-mills on   
   the same rollers-on-a-frame, no mechanical changes needed.   
      
   I calculated for the "metallurgical" part of the rod-mill - everything   
   which is and is within the "shell" ("drum"):   
   * 65RPM of shell rotation rate   
   * 127W of power draw   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)