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Concentric Housing to Shaft design question

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

RE: Concentric Housing to Shaft design question

Research "tolerance rings" on Google.

"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

If your bearings are close enough that you can assemble and line-bore the holes together (as you mentioned), then you could "field dowel" it.  That's where you simply drill and ream the mating parts by hand while they are assembled and insert dowel pins.  The holes can basically be placed arbitrarily, as long as you have two of them per mating surface.  

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

...I might add that these field holes should be bored with a magnetic drill, or in a machine tool, if possible.  The holes must be very perpendicular or you won't be able to dissasemble the parts!

Don
Kansas City

RE: Concentric Housing to Shaft design question

You seem to confusing CLEARANCE and CONCETRICITY.

RE: Concentric Housing to Shaft design question

Can you machine your parts where they register off each other?

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

(OP)
Thanks for the replies guys.  I am looking into tolerance rings, possibility for line boring and pinning.  I don't think I'm confusing concentricity and clearance.  If everything is concentric I need little or no clearance.  Because things aren't concentric I'm forced to increase the clearance.  No confusion.

RE: Concentric Housing to Shaft design question

Which do you need, concentricity or low clearance?

RE: Concentric Housing to Shaft design question

For low clearance, don't you need concentricity?

RE: Concentric Housing to Shaft design question

(OP)
ewh:  Thank you for re-iterating what I said earlier.  I think the two go hand in hand.  I need to maximize concentricity in order to minimize the clearance required.  I hope we can get back to discussing real design ideas to solve the problem.

Thanks

RE: Concentric Housing to Shaft design question

cnuk,
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

I've just got to ask; why is such a tight clearance required between the shaft and bore?

RE: Concentric Housing to Shaft design question

The thread's subject line is "concentric".

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

But = Put

RE: Concentric Housing to Shaft design question

Could you use a "face coupling" between the parts?

RE: Concentric Housing to Shaft design question

(OP)
The tight tolerance are at the request of a supplier.  Each of the housings carries a lip seal and the seal manufacturer wants that tolerance.  Seals don't like shaft-to-bore misalignment so I need to minimize it...keep concentricity as close as possible.

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

Lip seal are far more sensitive to the runout of the shaft than they are to concentricity of shaft to housing. It sounds to me like the two isues are being confused with each other.

RE: Concentric Housing to Shaft design question

When I see a fairly large tolerance for acceptable range in shaft sizes, but a very tight tolerance for allowable bore concentricity, I get very confused wondering how a section of seal lip can tell whether it is in the presence of a fat shaft, or a slightly smaller eccentric shaft.

What needs to be sealed 5 times?

RE: Concentric Housing to Shaft design question

The latter would put a lot more energy into the seal.

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

(OP)
CompositePro:  I know that seals really don't like runout and that it's a much bigger problem that STBM (shaft to bore misalignment).  The problem with my test fixture is not runout.  The shaft bearing and sealing surfaces are ground in one operation.  The shaft runs on bearings so runout is not an issue.  

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

I said "the presence of a fat shaft, or a slightly smaller eccentric shaft", meaning a small statically eccentric shaft.

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

Can you just mount the bearings to the shaft, then shim the bearings to ensure concentricity of the entire thing?

"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

I meant that a dynamically eccentric shaft (a cam) would flex the seal continuously. I see what you mean by a statically off-centre shaft, ie one where the shaft is concentric about its rotation axis, but that rotation axis is not correctly aligned with the seal.

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

I think 5 ball bearings in a row on an straight, round shaft would object more to housing centerlines roving concentricity misalignment than the seals would.  The shaft may well be your alignment tool especially if the housings can be left loose during assembly, and finally secured with through bolts.

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

(OP)
I think my ASCII art is confusing.  There are only two bearings in the assembly, not five.  The |__| I was drawing represent the contours of the housing and not the bearings.  Only where the word BRG is written are there actually bearings.  Certainly five bearings on one shaft would cause significant problems if there was eccentricity between the housings and shaft.  Because I only support the shaft at two points that's not my problem.  I think we all agree on the relative importance of STBM vs. runout.  Tmoose is describing the STBM scenario with what he referred to as a statically eccentric case.  The shaft diameter is 2".  The idea of using the housings for alignment has some potential however it is will complicate getting a proper seal installation.

Thanks again to all

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