Tolerances for Bushings on Parallel Shafts

[erk1313]

I'd like advice on using fixed-alignment bushings in a parallel shaft application.

McMaster says that fixed-alignment bushings are used in parallel shaft applications, but I don't see how that can be done. I end up with a positional tolerance of .0006" for a 3/16" shaft.

http://www.mcmaster.com/#catalog/117/1076

Bushings (MMC# 5986K65))
ID: 3/16" + .0015 clearance

Shafts (Precision Shoulder Bolt)
3/16"OD; Tolerance +0 -.001

This leaves a minimum clearance of .0015" for a perfectly aligned shaft. For figuring out the positional tolerance for locating two shafts, I divide by two, yielding .00075". Then subtract clearance for an RC2 sliding fit (.00015"), yielding .0006".

Am I thinking about this clearly? I'd like to save cost by actually going to a cheap bronze bushing (5986K651) with similar clearance, but I'm wondering if I need use a self-aligning bearing instead.

Below are PDFs using bronze bushings (also attached as a zip file). Please offer advice on my use of geometric tolerances as well. Thank you for your help. I am a novice, so be easy on me

Links to Drawings

http://files.engineering.com/downlo...d-94ab-7316fda91316&file=DRW-016-A-4100-A.PDF

http://files.engineering.com/getfil...2-a103-cba0ce070d9c&file=DRW-016-A-4120-A.PDF

http://files.engineering.com/getfil...e-943d-bf77358063b4&file=DRW-016-A-4110-A.PDF

 

[MikeHalloran]

To use parallel shafts effectively, you need either:
- extremely tight tolerances on everything, which is rarely cost effective.
OR
- some flexibility, or intentional slop, somewhere in the mechanism. It does not need to be the same in every direction, and in most cases should not be the same in every direction. In particular, you can use shaft mounts that allow one of the shafts to move easily so that the center distance need not stay exactly constant, or you can use a carriage that is particularly flexible in the between-shafts direction, so that it will not bind even if the shafts are not perfectly parallel.

I.e., the solution to your problem is probably not within the realm of GD&T, but within the realm of controlled flexibility >by design< in parts that are nominally but not actually rigid, or in adding yet more parts, with yet more tolerances.


Mike Halloran
Pembroke Pines, FL, USA

 

[erk1313]

Good advice. So perhaps replacing one of the bushings with a slotted hole, and making sure the other bushing is long enough to support the entire load.

 

[erk1313]

How about my GD&T, am I going in the right direction here?

http://files.engineering.com/getfil...2-a103-cba0ce070d9c&file=DRW-016-A-4120-A.PDF

 

[MikeHalloran]

I think you need to make the tight location tolerance apply only to the counterbore with the tightly controlled diameter, and allow a looser location tolerance for the other nominally coaxial features. Or refer them to the tight c'bore with a looser location tolerance.

Also, I don't think you've got a lot of thread engagement on the 8-32 hole.

Or rethink the whole thing; it's a damn complicated hole to make.




Mike Halloran
Pembroke Pines, FL, USA

 

[erk1313]

Good point. Thank you for the helpful advice, Mike.

 

[erk1313]

To designate only the counterbore having positional tolerance, would I simply move the GD&T frame to the right of the counterbore instruction? (I will likely substitute looser tolerance bushings, but this will be helpful in the future.

 

[erk1313]

Also, the shoulder bolt has a 8-32 thread length of only .188", hence the shallow depth.

 

[MikeHalloran]

I would associate the feature control frame with the appropriate counterbore, probably by putting it on the same line as suggested, and designate the cbore surface as a datum for the other associated features.

You may need to control the location of the 8-32 closely in order to allow it and the shoulder to engage properly.

... which is yet another reason to rethink the complete assembly.



Mike Halloran
Pembroke Pines, FL, USA