Press-fit diametrical interference recommendations 2

[Indicator1]

I have a 1095 Steel shaft and a ULTEM 1000 polyetherimide part press fit. Diametrical interference is about 0.04.
I got an on-line press-fit calculator, recommended by this website, and it states Diametrical interference is 0.001 typically.
We have a problem of the plastic part cracking, even before the thermal shock. Could it be because of the wrong Diametrical interference value, please?
Thank you.

 

[DesignerGuy16]

Even without looking at your situation, .04" press-fit seems pretty excessive.

.001 to .005 would probably be reasonable if you have a nice chamfer lead in on the Ultem piece. Any more than that and I'd worry about cracking. Ultem's some excellent material. Very strong, very good dimensional stability at very low and high temps.

James Spisich
Design Engineer, CSWP

 

[ShaggyPE]

What are the units we are talking about? Interference of .04mm is reasonable, but .04 in is excessive. .04mm is .0016 inches.

I agree with James' statement, .001-.005 inches is reasonable.

-Dustin
Professional Engineer
Certified SolidWorks Professional
Certified COSMOSWorks Designer Specialist
Certified SolidWorks Advanced Sheet Metal Specialist

 

[RebelBrill]

The correct amount of diametral interferrence also depends on the diameters of the parts, the thickness of the walls, length of engagement, and elastic properties of the materials involved, as well the functional requirments of the joint.

 

[Indicator1]

James,

This is a RELATIVE value: interference divided by the shaft OD. No inches.
Sorry, I thought it is clear without any units.
Thanks.

 

[Wrenchbender]

As already stated the amount of interference depends on the nominal diameter, the moduli and thickness of each component, and also on how tight you want the fit to be. Suggested dimensions and tolerances for a given ‘class’ of press fit can also be found in Machinery’s Hdbk (FN fits); however polymers are prone to stress relaxation (function of time and temperature). PEI may be better that others, but your part still may eventually loosen. I would recommend avoiding plastics if at all possible.