Shaft-hole fits, metal and plastic 2

[wtlebo]

I am looking for some tables on shaft-hole fits for a stainless dowel pin into a plastic part (ABS and Polypropylene). Does any one have this information, or a link? The machinery's handbook only talks about metal-metal fits.
specifically i an looking to press an M5 pin into an ABS disk, that will need to be removed periodically for maintenance.

 

[israelkk]

Best is to check the dowel pin manufacturer literature or Dupont "General Design Principles for DuPont Engineering Polymers". However, plastic material can not tolerate sustained tensile loads without creep. Therefore, if the press fit cause too high stresses the hole will grow and you will loose the fit.

 

[mitchm]

Actually, because of creep properties typical in plastics, you don't even need high stress to get significant relaxation over time. It's no a good application for plastics to design for press fits.

 

[patprimmer]

As stated by others, creep will be a problem.

To overcome this in plastic inlet manifolds on cars, they insert metal ferules or crush tubes, which bear the creep load.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[wtlebo]

Thanks to all for the help. I understand that the plastic will tend to creep over time and the press will become less and eventually disappear. Does that mean there are no standards (as there are for metals) for a recommended interference for a material like ABS? Or maybe even a recommended limit, for example the press should not be greater than 15% interference.

 

[patprimmer]

Another problem with most amorphous plastics and interference fits is it can lead to environmental stress cracking.

I don't have data, and can only advise that you enquire with all the major long established, reputable raw material suppliers like Bayer, Dupont, Ticona, BASF, GE, DSM, Basel, Atofina etc

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

wtlebo:

I again refers you to Dupont "General Design Principles for DuPont Engineering Polymers" which can be found on the web. They have a whole chapter on the issue. I do not believe the press fit will disappear but it will decrease. Using the info on Dupont document you can calculate the residual press fit.

You might consider a splined pin and a splined hole, the teeth will take the torque.