Subject Matter Expert (Press Fit install of a steel tube into an aluminum housing)

[BigPrizz]

I'm looking for a subject matter expert in press fits (specifically steel tubes into aluminum housings).

We have a pump that requires we press in a steel tube into an aluminum housing and we are having issues with the parts leaking on our production line but no leaking in our lab environment. We've looked at what's different/same between the setups and cannot come up with anything viable. The interference fit values are set by standards by our customer.

Anyone out there able to help or provide some insight?

 

[3DDave]

Almost always such differences between development and factory production are because of a difference in procedure. Someone in one of the locations is doing something that makes it work or makes it fail.

My guess would be someone is using an abrasive to clean the tubing or the hole and leaving axial scratches. They likely believe there is a burr that needs to be removed and they are doing that.

My favorite was a rotary coupling with minor leakage. Someone told a tech they thought it was from runout, so he chucked the subassembly up and made a fine pass on a lathe. Turned out it was from roughness and his fine pass was about 10 times rougher, so the seal gave up about 50 times faster. The good news was he was honest about what and why and it delivered an experiment none of management would have signed off on, allowing the genuine fix to be developed (2-5 microinch instead of the 10-20 originally spec'd vs the 100 to 200 the lathe made.)

 

[mfgenggear]

is there a seal leaking if so 3DDave nailed it., it's about concentricity and surface finish.

 

[GregLocock]

We used to have problems with press fits of steel hub units into aluminium spindles. I got asked to investigate, and I suppose the quick outcome was that you have to account for the flexibility of both parts and use Lame's equation to actually assess the interference of the joint. The coefficient of friction is the big unknown in deriving a push-in force.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mfgenggear]

Since it is an aluminum housing and steel liners, it should be a easy to heat the housing and freeze the liners.
Should just drop in.
Now I said concentricity sould of said
Cylindricity. Of holes and liners should
Be close. If either of the liners or the holes in the housing are out of round.
Will cause failure.

 

[drawoh]

BigPrizz,

What temperatures are you assembling, testing and then running this thing at? CTE for aluminium is around 23e-6K[°]^-1, and carbon steel is around 12e-6K[°]^-1.

I am crunching numbers here and I am getting that [Δ]D for thermal expansion is way in excess of an acceptable interference fit.

[edit]
I just realized that I did my calculation in millimetres, and I interpreted the results in inches. My bad. The differences due to CTE still are significant, and must be accounted for.
[/edit]

--
JHG

 

[BrianE22]

Is the leak axial between the aluminum and steel? Are you press fitting in the lab, testing it and then sending it to production? Or is the press fit done by different people (lab and production personnel)? I'd watch the assembly and testing in both locations - you don't always get the whole story from the people doing the work. If separate groups are doing the assembly I'd guess maybe a nick or burr on the steel is scratching the aluminum ID during assembly on the production line.

 

[GregLocock]

drawoh- my specific issue was that heat from the brake disc would bleed down through the hub unit into the spindle, which then expanded, loosening the fit, resulting in a clicking noise as the hub unit settled itself into the new fit. No actual harm done but it was annoying.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[drawoh]

GregLocock,

Isn't that the sort of thing that loosens screws?

--
JHG

 

[mfgenggear]

The point of CTE is a very good point.
Controlling temperature is critical.
Aluminum does in fact expands very rapidly.
I was in the assumption there was no running temperature issue.
But that may be an actual contributing facture.
The OP has never responded to any and all suggestions.
Waiting patiently.

 

[EdStainless]

When stronger stiffer materials are inserted into softer more flexible ones great care must be taken to not exceed the yield strength of the Al (in this case). Once it yields you loose the clamping force for the joint. A tighter fit does not help, you need just enough.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

May in fact require material change to cast iron.

 

[racookpe1978]

But Aluminum-steel and aluminum cast iron would still face metal-metal corrosion issues over time.

 

[rb1957]

that was my knee-jerk reaction too. Maybe if you can keep fluid (water, air, etc) out of the contact surfaces ?
(or maybe it's an Al/Fe thing that just needs contact ?)

another day in paradise, or is paradise one day closer ?

 

[kjoiner]

Would one of the Loctite shaft retaining compounds be of use in this situation?

Kyle

 

[FACS]

OP only logs in once every 10 months so we have a bit of a wait