LL631
Industrial
- Nov 30, 2018
- 24
I am looking into the design of interference fit splined hubs i.e. a plain bore hub with external male spline, in particular how to design for stresses in relatively small / thin walled designs. I posted something similar a while back but I haven't manage to get to the bottom of it. My question has two parts but both contribute to the main point which is, how to determine the maximum stress / minimum wall thickness before the hub will fail (split)?
First point: how to determine maximum allowable stress?
Using thick-walled cylinder theory I've calculated that the designs I'm working with are almost always going to be under elastic/plastic stress, it's unavoidable without going into tight manufacturing tolerances. This should be okay (shouldn't cause immediate failure) but how do I determine what my design limits should be?
[ul]
[li]Is it when the max stress at the ID is > UTS? I know this can effect fretting but can this cause the hub to split from the inside out? If true this would suggest a 100mm hub with 10mm bore at 0.001" interference would fail, not very likely. [/li]
[/ul]
[ul]
[li]Is it when the stress through the wall is fully plastic? I think this is a more likely design limit; if the material is brittle then yes it would fracture, if the material is ductile it would deform which would relive the interference and stress so for small strains probably wouldn't fracture but will be more prone to fatigue failure.
[/li]
[/ul]
Second point: how to account for the spline form?
In my calculations I've assumed the hub is a plain cylinder with OD = spline minor diameter, is this considered sufficiently conservative or should I also consider a stress concentration factor to account for the spline profile?
I've attached my spreadsheet to give some context to my question, I'm more interested in finding the right approach rather than a solution to the particular design but if you spot anything obviously wrong with my working then by all means point it out.
First point: how to determine maximum allowable stress?
Using thick-walled cylinder theory I've calculated that the designs I'm working with are almost always going to be under elastic/plastic stress, it's unavoidable without going into tight manufacturing tolerances. This should be okay (shouldn't cause immediate failure) but how do I determine what my design limits should be?
[ul]
[li]Is it when the max stress at the ID is > UTS? I know this can effect fretting but can this cause the hub to split from the inside out? If true this would suggest a 100mm hub with 10mm bore at 0.001" interference would fail, not very likely. [/li]
[/ul]
[ul]
[li]Is it when the stress through the wall is fully plastic? I think this is a more likely design limit; if the material is brittle then yes it would fracture, if the material is ductile it would deform which would relive the interference and stress so for small strains probably wouldn't fracture but will be more prone to fatigue failure.
[/li]
[/ul]
Second point: how to account for the spline form?
In my calculations I've assumed the hub is a plain cylinder with OD = spline minor diameter, is this considered sufficiently conservative or should I also consider a stress concentration factor to account for the spline profile?
I've attached my spreadsheet to give some context to my question, I'm more interested in finding the right approach rather than a solution to the particular design but if you spot anything obviously wrong with my working then by all means point it out.