Misalignment of high speed shaft due to locking nut
Misalignment of high speed shaft due to locking nut
(OP)
I haven't got as far as vibration yet so I'm not sure if this is the right place?
I have a shaft which clamps a series of rotating discs. All are balanced and true until I tighten the clamping nut (M10 x 1 LH) then the shaft shows some runout. We've improved the situation with a sleeve nut but if anybody has any better ideas I'd be interested. I hear some machine spindles have a complex grub screw arrangement but I'm not sure it's possible on a 10mm shaft.
I have a shaft which clamps a series of rotating discs. All are balanced and true until I tighten the clamping nut (M10 x 1 LH) then the shaft shows some runout. We've improved the situation with a sleeve nut but if anybody has any better ideas I'd be interested. I hear some machine spindles have a complex grub screw arrangement but I'm not sure it's possible on a 10mm shaft.





RE: Misalignment of high speed shaft due to locking nut
fwiw I would think the solutions involve improving the geometry of those disks.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Misalignment of high speed shaft due to locking nut
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Misalignment of high speed shaft due to locking nut
Of course, we know very little about your machine. I'm sure with more details others might chime in more specific and useful comments.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Misalignment of high speed shaft due to locking nut
Machine tool spindles typically have tolerances < 0.0001 inch ( " a tenth ") on shoulder runout, spacer face parallelism, etc. That is necessary for the running accuracy and preload control. And locknuts with faces ground presumably perpendicular and true to thread pitch features.
Every now and then things will still "kick" a little when the nut is tightened. Some manuals describe a process of "easing down" the nut face by selective scraping or controlled grinding to keep the assembly true.
If I was really in a bind washers with Spherical mating faces are available, and offer the least influence when tightened.
Extreme example -
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Some of the spindles we made for GM had 4 screws positioned radially to allow tweaking the centering of the piloted flange mount tooling a little better than 2 tenths runout and a few tenths clearance could.
RE: Misalignment of high speed shaft due to locking nut
• Individually locate each part with its own shaft step, split ring or snap ring. Design each part for an interference fit to the shaft. This can be difficult to assemble and disassemble.
• Drive each part with a polygon fit to the shaft to eliminate the need for a drive key. This can be very expensive to machine.
• Clock the parts using a trial and error process of rotating individual parts one at a time and finding the orientation of that part that produces the lowest run-out result. This can take a long time and a lot of work.
• Change from a standard nut to a "Super-Nut" configuration with axial set-screws against a hardened washer to provide the clamping force. This allows for some adjustment capability to minimize the run-out.
None of these are probably a very good choice because of the small size of your application. But without more details, it is hard to be sure.
Johnny Pellin
RE: Misalignment of high speed shaft due to locking nut
The device is a axial flow compressor with 5 stages. We've got it much better by using a sleeve nut and hardened washers so we can live with it for now but it's not really a production solution. We have everything clocked up to less than 2/10 thou and balanced.
I'd be interested if you can point me to any pump info Johnny as this is most like our application. We haven't tried spherical washers yet but it's a possibility.
RE: Misalignment of high speed shaft due to locking nut
The nut all by itself can be the problem. The improvement with the sleeve nut suggests it is a big part.
Are the "hardened" washers faces also ground ?
Depending on how snug the spacers fit on the shaft, parallel faces on impellers and spacers may not be enough.
I'd be inclined to do things like probe for gaps between faces with the nut just barely tight, or the shaft stacked vertical with no nut. Also to remove any drive keys, and paint the mating faces with dykem or magic marker, then wring the pieces together by hand to test for even contact.
Also mark axially "high" spots to be sure they are not all in a line when assembled
RE: Misalignment of high speed shaft due to locking nut
How do you know your discs are balanced before you apply the nut?
I see two possibilities.
RE: Misalignment of high speed shaft due to locking nut
Sulzer pumps Model MSD uses individual shaft steps with interference fits.
Conhagen (Houston) made a multistage pump for us that uses polygon fits and split rings to locate and drive each impeller.
Ruhrpumpen Pumps has a shop in Tulsa (the old Byron Jackson shop) that makes vertical multistage pumps using snap rings to locate the individual impellers.
Multistage compressors made by Elliott or Dresser Rand often have individually located impellers with shaft steps. It is common to leave thermal gaps between the stages but no axial contact between stages.
I like to retain impellers from movement in the direction of normal thrust using shaft steps or split rings. Depending on the design, the impellers can be retained from moving opposite of normal thrust using snap rings, thermal gaps or nothing. Any major API pump manufacturer would routinely use these techniques.
Johnny Pellin
RE: Misalignment of high speed shaft due to locking nut
We torque the nut to 10ft.lbs. It's a very fine LH thread so it isn't going to come off but it needs to provide enough friction force as the discs are not keyed.
I'll investigate the stepped shaft etc. but we're hoping to be making this in high volume so need a robust, low cost solution.