Shrink fit

[climbingtech]

I've run several calculations for a shrink fit and for likely user error reasons I'm getting very high numbers.

Looking for a 30-40 kn axial force requirement to break a shrink fit on 7075-T6. I ran my calculations with a 0.007" interference not accounting for surface roughness at a frictional coefficient of 1.1 (contact is naked 7075-7075).

The shaft is ID 0.625", OD 0.75"
The collar is ID 0.743", OD 1.125"

Length of engagement is 0.5"

I understand wall thickness and length of engagement are not optimal, just trying to get a feel for where it sits as far as part design is concerned. I just need the press fit to be able to handle >15 kn applied dynamic force to exceed the weakest failure point which is the shaft itself.

My last calculation using Lames equations and conversion from standard to metric yielded 225 kn static frictional force. lol I think I'm making an error somewhere or I personally underestimated the power of the interference.

Any help is appreciated, stay safe!

 

[BrianE22]

Without running any calculations that seems like a lot of interference for a 3/4" shaft. What material is the collar made from? Did you look at the hoop stress in the collar with that much interference?

 

[climbingtech]

Thanks for the reply. They're both 7075-T6. I didn't bother since that pressure would be ridiculous for the given material. I thought I'd shoot on the high end of getting them together since I was interested in maximizing the force to prevent it from slipping. I just didn't think that much force would be generated in aluminum on a shrink fit.

In short, it's a shrink fit collar containing a shrink fit pin perpendicular to the shaft with an attachment point . General load is roughly around 30 degrees off perpendicular to the shaft but looking to optimize the axial force so slip won't occur should the force be parallel to the shaft. Working on graphing out multiple tolerance scenarios now, and also working in reverse.

I also ignored hoop stress for now since the wall thickness of the barrel is less (understandably different scenario) because yielding at the collar/shaft will be the failure point in this application in all max- over-load scenarios.

What would you off the hip recommend for a level of interference with this in mind?

 

[climbingtech]

looks like 0.003 gives a fit of 32 kn at the specified dimensions. Anyone able to corroborate this?

Taking fit pressure into the equation Pa = Friction*pi*L*D*(Pc from lames equations).

 

[drawoh]

climbingtech,

Can you assemble two pieces of aluminium like that without it galling?

--
JHG

 

[climbingtech]

The pressure would be below the modulus of elasticity at roughly 66% its rated value. I don't think what ever little bit that did occur would impact it's overall performance for it's application. If you disagree I'd love to hear your perspective on it. This would be a shrink fit not press.

 

[mfgenggear]

climbingtech
so what you want is ever one here to do your work
.015 press will most likely be propagate cracking the collars. but I could be wrong.
I suggest go to the shop and run some test.
.003-.005 interference is more or less ball park
I too believe that's to much for 3/4 diameter, why was aluminum chosen

 

[hydtools]

Hot aluminum can be sticky.

Ted

 

[climbingtech]

mfgenggear, certainly not the case, and you didn’t need to reply to the post or offer any help. I figured it would be a simple question to offer general advice on for most advanced members here, and was looking forward to learning. It’s a starting ball park and will have to undergone specific testing conditions anyway, so I’ll proceed to empiric testing.

Thank you hydtools. Planning on just cooling one component and not using any heat so we’ll see how it goes. Thank you.

 

[climbingtech]

Mods please delete this thread. Thank you.

 

[3DDave]

Aluminum coefficient of thermal expansion is 13 ppm per degree Fahrenheit.

1 million * 0.007/0.750 = 9333 parts -> 720 degrees Fahrenheit difference to make for a line-to-line fit.

The thin wall of the shaft will not allow the collar to be damaged.

I would consider shaft locking compound and a close, non-interference fit.

 

[climbingtech]

Thank you 3DDave. With the .003 I can still maintain temper with heat or use nitrogen for the shrink. Going to just run some pulls and be done with it. Appreciate the reply. Going to look into some compounds and see what I can find to compare the two in pull tests.