Concentric Housing to Shaft design question
Concentric Housing to Shaft design question
(OP)
I am building a piece of test equipment that has five housings (approx 6" OD x 3" thick) bolted together. On each end of the bolted together housing assembly I have ball bearings which guide a shaft through the housings. I am trying to maintain the housing to shaft concentricity as tight as possible. For example, we originally had the housings with an ID of 2.002" and a shaft of 1.999". Upon assembly there is contact between the shaft and housing. This is predictable based on our tolerance stackup as each fit can be up to ~.0007" out of alignment. The answer is not just to try and tighten up the tolerances because:
a) that will be very expensive if it can be achieved. Each housing needs to be machined on each face so it is done in two operations in our NC. The fact that it is clamped/unclamped/reclamped to do second operation makes me think we can't ask for much more than the locational clearance fit we have now. Measurements of the existing parts seem to confirm this.
b) It will be very difficult to assemble if every fit is .0001" clearance on a 4" dia fit.
c) I'd like to do something that is practical on a scale larger than our test equipment. ie: production if we get there.
So, I am looking for your suggestions on how to best design and machine such an assembly. Press fitting the pieces together is an option we tossed around but I still don't know whether it will get us close enough (doesn't address the actual machining of the parts but will eliminate the error with a clearance fit). Assembling them and then boring the housing concentric would help but it is unlikely to be assembled the exact same way twice (clearance fits and stackup) so this likely will only work some times. Dowel pins are nice but sure a pain based on my experience.
I'd greatly appreciate you sharing your input and expertise.
Thank You
a) that will be very expensive if it can be achieved. Each housing needs to be machined on each face so it is done in two operations in our NC. The fact that it is clamped/unclamped/reclamped to do second operation makes me think we can't ask for much more than the locational clearance fit we have now. Measurements of the existing parts seem to confirm this.
b) It will be very difficult to assemble if every fit is .0001" clearance on a 4" dia fit.
c) I'd like to do something that is practical on a scale larger than our test equipment. ie: production if we get there.
So, I am looking for your suggestions on how to best design and machine such an assembly. Press fitting the pieces together is an option we tossed around but I still don't know whether it will get us close enough (doesn't address the actual machining of the parts but will eliminate the error with a clearance fit). Assembling them and then boring the housing concentric would help but it is unlikely to be assembled the exact same way twice (clearance fits and stackup) so this likely will only work some times. Dowel pins are nice but sure a pain based on my experience.
I'd greatly appreciate you sharing your input and expertise.
Thank You





RE: Concentric Housing to Shaft design question
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RE: Concentric Housing to Shaft design question
If it's a one-up machine, this isn't really that big a task and it ensures that the parts can be disassembled and reassembled accurately in the future. But it's probably not suitable for mass production.
Don
Kansas City
RE: Concentric Housing to Shaft design question
Don
Kansas City
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
I have made something similar to your description where the parts registered off each other. After these parts were bolted to gather we used an Engis Diamond hone to finish the bore.
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
Thanks
RE: Concentric Housing to Shaft design question
I think your suggestion of a dowel hole is right on.
A second dowel hole with a slot in the mating part
might be realistic. I do not think it is that hard
to control two dowel holes.
You might want to consider fixturing the parts but
you need at least a dowel hole, pilot or whatever.
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
Low clearance is not necessaary for concentricity, but yes, concentricity is required to maintain a consistent low clearance.
So we will assume that the design goal is low clearance.
You can throw whatever tolerances you want on the hole diameters, it will get you nowhere because the machining of the individual holes does not control the assembly's concentricty. The assembly process controls the concentricity.
My first thought was to line-bore the assembly, as previously mentioned by eromlignod. Mango's suggestion of tolerance rings might be even simpler.
But an appropriate ring in each bore, install shaft, secure the individual sections to each other, remove shaft, remove rings, install bearings and shaft.
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
If I were smart enough I could attach a sketch of the system but my ASCII art will hopefully do.
-BRG-|____|-- --seal-|___|-- --seal-|____|-seal-- --|___|-seal-- --|____|--BRG-
Housing 1 Housing 2 Housing 3 Housing 4 Housing 5
I'll do some testing with dowel pins and have to read up on face couplings because I'm not sure what you mean unclesyd.
Thanks Again.
RE: Concentric Housing to Shaft design question
http://www.amcprecision.com/v-tooth.htm
RE: Concentric Housing to Shaft design question
RE: Concentric Housing to Shaft design question
What needs to be sealed 5 times?
RE: Concentric Housing to Shaft design question
Cheers
Greg Locock
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RE: Concentric Housing to Shaft design question
Everyone agrees that a shaft that is running out is much harder on a shaft because it exerts a higher peak load and the loading is cyclic. The four seals are really two redundant sealing systems back-to-back in order to simplify testing. Redundant sealing systems are everywhere and that's essentially what I'm testing.
The issue I started asking about is concentricity (yes I really mean concentricity) because as designed, that is the biggest problem I have with the test fixture. I am interested in the previous suggestions like: tolerance rings, dowel pins, etc which help in solving the concentricity problem. I'm interested in talking about seal runout, STBM, etc but that's not what I meant to ask about.
Thanks to all for sharing your collective knowledge.
RE: Concentric Housing to Shaft design question
Greg Locock wrote "The latter would put a lot more energy into the seal."
Hi Greg, were you referring to hysteretic losses from stretching the seal material?
I imagine the available energies would be heat from rubbing, and heat from hysteresis from seal lip bending and stretching.
In the limit the surface speed of the ever-so-slightly smaller diameter shaft would have ever-so-slightly lower surface speed, so I'd guess >less< heat from rubbing would be generated there.
In my static eccentricity example, As the lip bending/stretching happened during assembly, there is no real motion during operation, thus, no hysteretic energy loss.
RE: Concentric Housing to Shaft design question
"Art without engineering is dreaming; Engineering without art is calculating."
Have you read FAQ731-376: Eng-Tips.com Forum Policies to make the best use of Eng-Tips Forums?
RE: Concentric Housing to Shaft design question
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Concentric Housing to Shaft design question
What is the shaft diameter? It would seem it must be mighty small to need support every 3 inches. Then I'd expect some issues of assembling the bearings with (depending on operational loading type and direction) a slight interference fit on the shaft. If the shaft or bearing ID develops a polished or worn appearance it implies the shaft-to-bearing fit is incorrect. The design has to allow assembly AND proper bearing fits.
The housing faces' perpendicularity (re: the bearing bores) must be mighty accurate or their assembled centerlines will look look this ^^^ or maybe this ^-^- or this ~-^
RE: Concentric Housing to Shaft design question
Thanks again to all