# Force applied to objects pressed together

1. Jul 30, 2014

### mkpn24

Hello all new to the forums but I have a question. I need to find out the force that holds two pressed objects together. For example I have a piece with a diameter of 1.000 in and another piece (the shaft) with has a diameter of 1.001. The shaft is inserted .25in into the 1.000in opening. The open diameter is heated up while the shaft is frozen and then the shaft is inserted into the opening. I need to pretty much figure out how to find how much force the shaft can take before it starts to spin in the other pieces open diameter.

Im assuming there is friction force and the force of the two objects pressed together I just can't seem to figure out how to go about finding these forces.

2. Jul 30, 2014

### 256bits

You would be able to find the friction force of a block on a surface, yes, no?

For a press fit the parts are pressing against each other causing a stress, pressure, force.
You need to find the force between the mating parts. Multiply that by the radius of the fit and you have the torque before failure.

For the material(s) have you the modulus of elasticity and Poison ratio?

How much knowledge do you have about all this?

3. Jul 31, 2014

### mkpn24

Yes I do have knowledge about this I'm actually in a strength of materials course right now but for some reason I just can not wrap my head around this problem. It's not for school this isn't a homework problem or anything it's for an internship I have right now.

The material is 52100 steel.

E=30,000,000psi
Poisson's ration = .30

I'm thinking maybe I could find the strain but I'm not sure about the deformation of the objects would it be .0001in, which the difference in the two diameters, Over the smaller open diameter 1in?

4. Jul 31, 2014

### 256bits

This site may help you out, from the point of view of a thick walled cylinder being pressurized.
http://www.faculty.fairfield.edu/wdornfeld/ME311/PressCylinderHam.pdf
which gives a good indication of first principles.
And from which, by the equations you can obtain the basic strength of the fit.
That should get you started.

Here are some more equations of interest, but which are more daunting, but which you may need.
http://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-equations.htm

And a brief explanation of the design criteria of the von Mises stress, from where else but,

5. Aug 6, 2014

### mkpn24

Can someone review this work and tell me if I'm correct please.

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