Tolerancing Question: Steel in Nyloil FG 1

[DELurker]

(First time post. Apologies in advance)
I'm trying to design a steel insert to be pressed into a Nyloil FG part. The insert will be nominally 10mm in diameter and 18mm long and will be fabricated by a local machine shop. The Nyloil FG part will also be machined by a local shop. However, I'm kind of stuck on which tolerances to use.

We have a rule-of-tumb that says to use S7/h6 for plastic part fits (ie - 9.983/9.968 (S7) and 10.000/9.991 (h6) ), but there is no reference on the rule-of-thumb to show where it came from. Is this a reasonable fit or is it overkill? Does anyone have any references that would help to specifiy a better fit (or even support this fit choice)?

 

[patprimmer]

What is Nyloil FG. It sounds like some ones trade name for a lubricated nylon bar stock but I don't want to guess.

Regards
Pat
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[DELurker]

Nyloil FG is an oil-filled cast Nylon 6 plastic approved for food contact surfaces by the FDA (FG = Food Grade).

 

[patprimmer]

So it will expand as it absorbs moisture.

To have a precise fit, you need to condition to the average relative humidity in the environment of use. That can take a considerable amount of time at thick section.

The complexity is that the hole initially gets smaller as the surface only absorbs moisture, but as the moisture soaks or diffuses through to the core, the hole eventually gets bigger.

Equilibrium with 50% rh is about 3% moisture which is about 1% expansion.

This is from memory. There are published tables of moisture uptake rates vs section thickness and RH and temperature

Regards
Pat
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[DELurker]

Thank you, Pat. If I'm understanding you correctly, the hole can vary by up to 1%, which would be 0.1mm (0.0039"). That would make any press-fit pretty useless. I guess I'll just have to hold the insert in place by form rather than by fit.

 

[patprimmer]

It depends on the moisture content when it is machined vs in use. It is possible to compensate if you know.

You also have a problem with cold flow or creep when under constant load.

Nylon when conditioned is pretty tough and has quite high elongation so you can use a lot of interference

I am sure Ticona or DuPont or BASF will have datta on their web sites.

If you want it to stay tight for a long time you need undercuts and splines or whatever. Also beware of notches when designing retaining features.

Regards
Pat
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