interference fit problem

[limct]

Dear all,
To me, it should be more appropriate to post this question in the Mechanical section, but I think this section is so fat the most active one, and I believe I should be able to hear some valuable advice.
I have a aluminium die cast part in cylindrical shape, OD 37mm and ID 32N6 (-0.012, -0.028), made of ADC-12 grade.
This hole is meant to be pressed fit with a deep groove ball bearing of OD 32-0.011.
I assembled three pcs and I notice one out of three is problematic, the interference fit is not there, and the bearing can be easily pulled out, it is just like slide fit.
Is this because the wall is too thin and not strong enough to sustain the interference fit?

Best regards,
ct

 

[israelkk]

Check the actual dimensions of the parts it seems that either the hole is larger than it should be or the bearing is smaller. But I assume your part is larger because bearing manufacturing process is more accurate (grinding).

First, leaving only 0.001 mm interference at worst case is not a good idea (as you can see). I would use larger minimum interference and make the ID more accurate. Take into account that to accurately measure a dimension accurate to 0.001mm require a tool able to measure in accuracy of 0.0001mm. I assume that the manufacturer used a measuring tool of 0.001mm accuracy to measure the 32 mm ID and could not see that it may be actually 32-0.011 or even 32 -0.010.

Secondly it seems that the wall may be too thin but it depends or your requirements for the holding force/torque.

 

[swall]

Is the I.D. machined or as-cast? Also, did you look for cracks after you pressed the bearing in?

 

[limct]

Hi Swall,
the ID is precisely bored. There is no sign of crack or whatsoever.
Well, suppose I can add an extra information. During the mounting process, the bearing is actually pushed for the distance of some 20mm before it reaches a step and stop. I notice on that problematic unit the surface of the ID (which experience the rubbing of the bearing) looks very much glossy, kind of like being burnished.


Best regards,
ct

 

[swall]

Well, if the press fit didn't crack the housing, perhaps it expanded the housing. You didn't happen to measure the O.D. prior to the press fit, did you? If so, a re-measure of the O.D. would show if you expanded the housing. I don't think an I.D. measure would show anything at this point, since it sounds like you ironed the bore surface, as evidenced by the burnished appearance.

 

[codeeng]

For this application I would use the equavalent of a H7/t6 fit (Class FN3).
Max. interference = 0.066 mm
Min. interference = 0.025 mm