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Interference fit between steel shaft & aluminium collar.

Interference fit between steel shaft & aluminium collar.

Interference fit between steel shaft & aluminium collar.

(OP)
Not sure if this is the right forum to post this in, so apologies if it's the incorrect place.

Its being 20 years+ since I've had to calculate an interference fit so I'm incredible rusty with the maths.

I have an Nickel Iron hollow shaft with a coefficient thermal expansion of 0,000008mC. The shaft has a 118,525mm/118,475mm Outer diameter with a 78,275/78,225mm internal diameter.

The shaft needs to go into a Aluminium collar, 7mm thick with a CTE of 0,0000247mC.

I'm trying to determine the size of the internal diameter of the collar to meet the following criteria:

The operating temperature of the assembly is -40 to 200C, so the aluminium collar will still need to 'grip' the shaft at those high temperatures.
During assembly, the shaft can be cooled to -15c & the collar heated to +250C.

I've did a quick calculation & I get the internal collar diameter to be approximately 0,53mm smaller than the shaft. This seems like a VERY big interference, but I know that the expansion the aluminium is 3 times that of the nickel-iron shaft.

If anyone has a spare 5 minutes & is a lot more clued up than me on thermal interferences, could you see if this figure seems about right?

Thanks.

Regards
Chis

RE: Interference fit between steel shaft & aluminium collar.

Hi

What's the od of the aluminium collar?
I'm not sure your fit will work because when you make the interference what are the stresses in the aluminium and steel a room temperature?
Interference fits are based on both materials remaining within their respective yield stresses, if you check the stress in the aluminium with a 0.53mm interference, will it be above the yield stress for the aluminium?

RE: Interference fit between steel shaft & aluminium collar.

(OP)
The outer diameter of the aluminium collar is 132.3mm.

I will try to calculate the stress, but will need to look up how to do this as its been a long time since I had to do any calculations like this.

RE: Interference fit between steel shaft & aluminium collar.

Hi

I did a rough calculation on just forcing the average diameter of the collar to grow by 0.5mm and the stress in the ring was around the 86.5 N/mm2 on average, before subjecting it to any additional stresses by varying the temperature.
I looked up a yield stress figure for aluminium and found it to be around 95N/mm2, which suggests to me that the joint doesn't have sufficient safety margin during its temperature cycle.

Desertfox

RE: Interference fit between steel shaft & aluminium collar.

i don't think the yield strength matters here a lot, unless we expect the part to be disassembled undamaged.
the question is, is there enough interference @200°C, since the parts have different expansion coefficients and is the aluminium part past the ultimate strength point @-40°C (although this doesn't matter for compression at this temperature, cracks are not good @200°C).

RE: Interference fit between steel shaft & aluminium collar.

I think the yield strength matters a lot in terms of keeping the mechanical integrity of the joint.
Read the paragraph just above the sub title "assumption" on this link.

http://www.fea-optimization.com/ETBX/interference_...

RE: Interference fit between steel shaft & aluminium collar.

Any concern about ultimate removal? Seizing is likely.

RE: Interference fit between steel shaft & aluminium collar.

(OP)
^ Theres no concern about removing the shaft from the body.

Once its in there, I want to stay there. Removal should only be possible with destuction of the body.

RE: Interference fit between steel shaft & aluminium collar.

Hi cmberry20

Are there any external loads imposed on the joint other than the temperature cycle?

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