Sitction in bushed ways

You might also try plastic bushings. Something like teflon impregnated bushings.

Dan

You're speaking of turning the rod while a small adjustment is being made? Should work well.. What about vibrating it? Like when you tap a piece with a hammer as you're moving it? AC coil set at the end of the rod?

John

This sounds like a good idea! Be aware that you are probably trading in for a new disadvantage: if the rod has lengthwise backlash.

Nick

I like JohnM's suggestion. A piezo crystal mounted at every station near the end of a rod could provide just the right amount, about a tenth of a degree, of *rotation* (perpendicular to way travel), to break stiction once per travel clockwise and counterclockwise. Not quite as perfect as a rotating rod, but sooo much simpler. I think to move the rod axially would just introduce random errors into way travel.

Whadda say, is this one patentable? You could sell premade shafting with piezo ends, in modular lengths. You would lose the infinite flexibility in design of having yard long shafting that can be cut to any length, but gain in the fight for smoothness. A simple controller could be patched to every rod. A controller with capacity for sixteen rods would be cover pretty much any machine tool situation. Various other drives are possible, such as an electromagnetic rod end that merely needs to be hooked to 110 VAC.

--Doug

Interesting! When I was using servo hydraulics on an experimental rig, it was normal to use "dither" to prevent stiction of the spool valve with its o-rings. I drove it without the usual driver board, and had to add dither back into the signal. Of course, the servo spool dither wasn't directly related to the final position of the cylinder, so you probably couldn't tolerate the magnitude of dither I used, ~5%. On the other hand, your bushings shouldn't be as sticky as o-rings.

Pete Keillor

SNIP

Hey Doug,

Geez, I'd have to give that idea a big "Hummmmmm". Maybe some female operators would like it though?!?

Take care.

Brian Lawson, Bothwell, Ontario.

I Dunno if it's still done this way but two of my old CNC lathes have dither built in to the servo drives. And you can hear it in one machine when all else is quiet. The repair guy who adjusted the dither on one machine said it was left over from the hydraulic driven days and when used in the servo driven machines helped prevent stiction in the ballscrews. ERS

A piezo rod end wouldn't have backlash, it'd be of unitary construction. An electromagnetic rod end might have some. I'll check. There's a magnetostrictive alloy that could be used.

Can anyone here name any machine tools that use rod ways. I'll start: Unimat SL.

Doug

I understand. But do machines exist that use piezos for that purpose? And is the travel big enough? And if your way is stiff, the piezo will have a lot of work. I really don't think that this is an elegant solution. But you can try to convince me.

Many wabeco's

What? That doesn't count? It's too exotic? Man, the Unimat is from Austria, so Germany counts! :-P

Greetings, Nick

I have an email out to Channel Industries asking if piezos can be supplied with OD and ID electrodes, poled or patterned to create rotary motion when energized. It sounds like Nick is thinking of axial motion. That would certainly be simpler, to have piezos coordinated at both ends driving axial motion rather than rotary, but it seemed to me that since the rotary motion would be across the way's motion, that this would provide more continuous relief from stiction. Ideally, the rod way would rotate completely continuously, eliminating stiction.

I didn't see any rod ways on the wabeco website. Note doubled http:// in Nick's post.

How about a rod way composed of two lead screws chain driven together, or two rods as ways with a central lead screw, chain driven so the rods would break stiction whenever the lead screw turned?

This does seem unnecessarily complicated, with lots of bearings and chains.

I think I need to come up with something a little less kinematically redundant.

Doug

Interesting idea.

You wouldn't need to lose the flexibility of yard long shafting cut to length. Just make rotational dither mounts that accept stock shafting.

Giant magnetrostrictive devices (as Terfenol D) might work better than piezo here because they exhibit greater strain, hence more motion per unit length. However, there are more sources for piezoceramic materials and structures than for giant magnetostrictive materials.

Google searches on "magnetostrictive" and "piezoceramic" will find lots of material for you.

Piezo and/or magnetostriction aren't the only possible approaches here. One might also devise a structure that is constrained against displacement while compliant rotationally, actuated by an alternating magnetic field. It could be a variable reluctance structure or one with a permanent magnet. Resonance could be used to magnify rotation. The SoniCare toothbrush uses these principles: two supermagnets, a coil actuated by AC, and a resonant beam.

Yes, I did.

Rotary motion would be better in my eyes too. But still I think that a piezo would not make enough travel to break the bushing loose. Or the hole thing is vibrating like a ****** (censored). A continous (that is not back and forth) rotary movement is the best you can have (at least thats my _oppinion_).

That's what you get when I think & type & copy & paste. :-)) Sorry.

But here they are:

Don't blame me for the long URLs

Greetings, Nick

This is interesting, Doug.

No, I'm not going to steal your idea. I'm (mostly) retired!!!

It might be important to learn some about stiction: specifically, how much relative motion is necessary to overcome it in a given situation of materials, transverse load, surface finishes and lubrication. The 0.1 degree you mention seems like a reasonable guess, but I'll bet it is only a guess. The "answer" may be more one of displacement rather than degrees; e.g., a 1/2" dia shaft might need more angular rotation than a 1" dia shaft to achieve the same effect.

You might have to do that by instrumented experimentation. Doing that might give you proprietary knowledge and a "leg up" toward selecting the best method of achieving sufficient motion to do the job.

It might be an error to fixate on any given method (piezo, magnetostriction, etc) this early. Better to understand what needs doing before selecting a way to do it, even if your instrumentation uses one of these methods.

Dither has been used for decades in gyroscopes, particularly laser gyros, to overcome stiction -- or "lockup" in the case of a laser gyro. Some of them use random noise rather than a fixed frequency. Random noise (sort of a hiss, like escaping air) might be less onerous in a shop than a whine or single tone.