Press Fit vs. Shrink Fit 1

[qwicker]

Hello everyone. This is my first post, so bear with me.
My project involves fitting a solid shaft of Beryllium Copper (BeCu) into a BeCu sleeve. I am trying to figure you the best method to do so. My calculations are a bit rusty so I am not sure if I have done this correctly. I suppose the best way to compare the 2 methods is to see which one has lesser stress and less radial expansion.

I have computed the stress in the press fit by using the following formula (from engineeringtoolbox.com)
sr = abs((((a)^2*p)/(b^2 - a^2))*(1 - (b^2 / r^2)));
Sorry for the raw Matlab code. If it is easier, please view the formula @

http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html

I computed the radial stress (Sr) for the shrinkage fit by using the formula found @

http://www.ameritherm.com/PDFs/shrinkfitcalcs.pdf

With both stresses, I simply used Hooke's Formula (sigma=E*epsilon) to find the radial deformation.

It seems that the shrink fit causes higher stress and deformation. Thoughts?

Thank you!

 

[israelkk]

The stresses are function of the part dimensions therefore, the way you assemble them should not matter. The first link is for the stresses in a pipe under pressure. In your case the pressure is the missing value that you need calculate. Both parts are contributing to the pressure between them. The second link either is not what you are looking for because it doesn't let you find Delta_D i.e. the change of the inner diameter of the sleeve.

You need to get a hold of a mechanical design book such as from Shigley or Norton or Machinery's Handbook.

Do a search in this site, the issue is already posted many times.

 

[qwicker]

Ok, well i did some digging on this site (there is alot to offer!) and i think i can successfully calculate the press load required. I suppose I am wondering how to do the next step. I need to make sure that my BeCu sleeve will not crack/split/break open during or after the press. Therefore I believe I need to find the hoop stress, is this correct?

After that, I need to find the total deformation AKA the new outside diameter of the Hub. Also, I have Machinery's handbook, but is a bit vague in this area.

Thank you

 

[Heckler]

I would also suggest looking at Roark's Formulas for Stress & Strain...secion 15.6 Temperature Stresses.

It's hard to give direct answers when we do not know the design details. But I would break this down into parts:

1. How this tube functions in the assembly.
2. The interfaces sleeve, shaft and Bore.
3. Static stresses
4. Dynamic stresses
5. finish the list because I am going home

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 3.0 & Pro/E 2001
XP Pro SP2.0 P4 3.6 GHz, 1GB RAM
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he's all right." -- George Best

 

[qwicker]

Ok, thank you. Thats a good place to start. However, please let me give more specs to help. I have a solid BeCu pin to be press fitted in a BeCu sleeve. The pin is .995-.997inches. The sleeve is OD 1.295-1.297inches, ID .9937-.9957inches. Max interference is then .0033inches, BeCu modulus (E) = 18500ksi. poisson=.27 UTS 150ksi. Shear modulus (G) = 72800ksi. Shear Strength = 75ksi.

Ok, so I found the load required to fit this to be 2.03 tons. I also found the contact pressure to be 12550 psi. Now I need to check to make sure the BeCu is not yielding (i'm sure it isnt, but I need to show proof). I think I will use a pressure vessel setup to find the radial and circumfrential stresses then check to make sure they are below the Yield strength for BeCu.

Also, this pressed assembly will not be rotating.

 

[israelkk]

Assuming your calculations are correct s far the circumfrential stresses are under 50 ksi. Yes, use the pressure vessel/pipe under internal pressure to calculate the stresses. It is not quite thin wall pipe but very close.