Interference Fit Plastic Stress Help?

[LL631]

I have a small interference fit hub that I need to find the acceptable range of internal diameters for.

I've calculated that for loosest case (+0.007mm) a hub ID range of 8.5mm - 13mm should provide enough torque. However, due to machining tolerances on the shaft and hub the minimum tightest interference case possible is going to be approx. +0.025mm (.001"), at this interference I've found that for almost all diameters the equivalent stress due to interference pressure is beyond the tensile strength of the material. But this assumes that the hub is elastic. If the material is assumed to be perfectly plastic then increasing the interference (strain) beyond the point to cause yielding will not lead to an increased interface pressure, only further deformation of the hub? So the interface pressure can only increase to the point where it will cause yielding? Maybe..? I don't know..

Can anyone advise where I go with this? How I'd go about determining the integrity of the hub under such conditions?

I know changing the material / geometry could solve this problem but I am trying to analyse an existing design (which works) which was based off "+0.001" should be alright." I'm also reasonably confident in my calculations as I've got the same data from a Kisssoft simulation.

 

[SwinnyGG]

Check your calculations. Calculating by hand, I get similar interface pressures to your chart but much lower stress values.

Sanity check 1: 1 thousandth of interference is very, very low. I would not expect .001" interference fit to cause parts to fail due to hoop stress unless they were very small.

Sanity check 2: where did your material properties come from? Judging by density and young's modulus, this is a low grade of steel you're calculating for. Your yield stress is a little low for even A36; your UTS is very low even for very low quality alloys.

 

[LL631]

Thanks for the reply, I've redone the calculation by hand but can't see where I've gone wrong; for +0.007mm interference & 8mm ID I've calculated:

- Pressure = 65.916N/mm²
- Radial Stress = -65.916N/mm²
- Hoop Stress = 95.958N/mm²
- Von Mises Stress = 140.99N/mm²

These results match the results in the first graph, perhaps you can point out where I've gone wrong?

I agree 0.001" seems low and is never going to rupture the hub - that's why I am questioning the stress calculation.

Material properties are for EN1A (230M07) low carbon steel Link. Yes this is a low grade mild steel (not the best hub material) but as I said I'm trying to prove an existing design. When analyzing hubs even with a stronger material I still often find yielding will occur at tightest fit, so I'd like to try and understand the effects of this.

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