Tolerances for Bushings on Parallel Shafts
Tolerances for Bushings on Parallel Shafts
(OP)
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.eng ineering.c om/downloa d.aspx?fol der=33b9f9 55-3d5a-4e 3d-94ab-73 16fda91316 &file= DRW-016-A- 4100-A.PDF
http: //files.en gineering. com/getfil e.aspx?fol der=0019b3 2f-b3b0-46 c2-a103-cb a0ce070d9c &file= DRW-016-A- 4120-A.PDF
http: //files.en gineering. com/getfil e.aspx?fol der=48cbb7 02-9299-47 1e-943d-bf 77358063b4 &file= DRW-016-A- 4110-A.PDF
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:/
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RE: Tolerances for Bushings on Parallel Shafts
- 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
RE: Tolerances for Bushings on Parallel Shafts
RE: Tolerances for Bushings on Parallel Shafts
http:
RE: Tolerances for Bushings on Parallel Shafts
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
RE: Tolerances for Bushings on Parallel Shafts
RE: Tolerances for Bushings on Parallel Shafts
RE: Tolerances for Bushings on Parallel Shafts
RE: Tolerances for Bushings on Parallel Shafts
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