Concentric Housing to Shaft design question

[cnuk]

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

 

[MadMango]

Research "tolerance rings" on Google.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of Eng-Tips Forums?

 

[eromlignod]

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

 

[eromlignod]

...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

 

[MintJulep]

You seem to confusing CLEARANCE and CONCETRICITY.

 

[unclesyd]

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.

 

[cnuk]

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.

 

[MintJulep]

Which do you need, concentricity or low clearance?

 

[ewh]

For low clearance, don't you need concentricity?

 

[cnuk]

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

 

[dimjim]

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.

 

[sreid]

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

 

[MintJulep]

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.

 

[MintJulep]

But = Put

 

[unclesyd]

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

 

[cnuk]

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.

 

[unclesyd]

The case is thus or somewhat similar. There all types of variations on this design.

http://www.amcprecision.com/v-tooth.htm

 

[Compositepro]

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.

 

[Tmoose]

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?

 

[GregLocock]

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

Cheers

Greg Locock

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