Calculating a press fit 4

[nick00]

Hello all,

Well, what a great place this is.

I did a level 3 City & Guilds course in Mechanical Production Engineering years and years ago and I'm trying to remember how to work out a press fit dimension for a plug.

The plug will be made out of PTFE and fit a hole of 41.5mm.

I remember bits and bobs of limits and fits but not enough to get started, I'm pretty sure that if one of you guys can get me started I'll do OK.

Any help would be greatly appreciated.

Cheers,
Nick.

 

[kingnero]

to get started:

1) google press fit or Lamé equations
2) eng-tips search
3) roymech.co.uk
4)

http://www.tribology-abc.com/calculators/e3_8.htm

 

[desertfox]

http://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator.htm

 

[Tmoose]

Are you creating a seal, or retaining the plug against a part bumping against it? or ???

Generically Teflon has the reputation for "cold flowing" so the press fit will decrease somewhat over time.
Might need some mechanical retention as could be provided by a "snap fit"

When one component is pressed into another it's pretty easy to get some broaching or skiving if one is metal and the entry geometry does not have a gentle entry profile and is not carefully deburred. Sometime polishing is necessary

 

[nick00]

Hi guys, thanks for the links - haven't looked at them in great detail yet but I will do, they look good to me.

Tmoose, it's a plug for the back of a magnetically coupled impeller drive that's being used with slurry. Solids build up in the magnet and I want to stop that by plugging it. The bit the plug goes into is 316 Stainless Steel and the operating temperature is 160 Celcius at around 100 barg pressure.

The first attempt at making the plug was just achieved by machining the plug down little by little until I got a gentle push-fit as I had calculated that the PTFE would expand greater than the Stainless Steel when at operating temperature.

Over time it has loosened, I thought this might be due to impurities in the plastic that had degraded at operating temperature but maybe your cold flowing theory is the real reason there.

Anyways, I decided it might be a good idea to calculate the press fit 'properly' to get the tightest fit I could so that's why I asked the original question. never heard of a snap fit so maybe you can enlighten me?

Thanks for your comments,
Nick.

 

[Pud]

Try something glass filled - e.g. PEEK or Torlon.


www.tynevalleyplastics.co.uk

Beauty is in the eye of the beer holder.

 

[desertfox]

hi Nick

You need to look at the interface stress between the stainless steel and PTFE.
What type of PTFE are you using because the plug can creep particularly at temperature and thats why PUD's suggested a glass filled plastic.

desertfox

 

[desertfox]

I should of added why can't the plug be stainless steel

 

[dgallup]

Why not an elastomer like fluorocarbon?

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[KENAT]

As others mention, ptfe is probably not a great material for this kind of application. Why did you choose it out of interest?

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[nick00]

Thank you all - what a great response.

Sorry I've not been around, busy family life at the moment and I see to get more time to myself when I'm at work (and that ain't much)!

desertfox, KENAT, I'm not sure what type of PTFE it is, we just used a spare bit we had lying around but I'll be able to find a datasheet for it if that helps. Thanks, PUD, dgallup for the glass filled plastic / elastomer suggestions - I'll keep those in mind for future projects!

It didn't really matter what the plug was made from as it was only used to test a thoery on one pump so we could put it in place on all three pumps if it worked (an it did).

So now you guys have taught me a good bit about PTFE and why I shouldn't be using it - what material would be best? I suppose Stainless is the first option as desertfox suggested but if you have any better ideas the nere's what I need:

Something cheap.
With a low coefficient (or one matching SS).
Easy to machine.

Thank you all.
Nick.

 

[desertfox]

I would use the stainless steel that the impeller drive is made from by using the same material there is no issue with galvanic corrosion and assuming uniform heating no problems with differential expansion.
Whatever the material you use the stress will be limited to the weaker one so the impeller is already stainless its seems a natural choice.
One important question is does the plug need to be removed for servicing as this might change things?

desertfox

 

[nick00]

Hi DF,

All good points, the heating will definately be uniform, so no problems there.

Yes, I'd need to get the plug out but I suppose a looseish press fit would do to keep it in. Then I was thinking I could tap a thread in the plug and use some kind of puller to take it out.

What do you reckon?

All the best,
Nick.

 

[desertfox]

how will you seal the thread?
Also if its a press fit you will need to rotate the interference fit portion in order to get it in and out.

 

[nick00]

I could seal it with a stainless steel bolt. I didn't know I'd have to rotate the magnet to get the plug out, why's that? Can't I just make the plug so that it will pull out?

Thanks,
Nick.

 

[Tmoose]

http://www.pineappleracing.com/ProductImages/FreezePlugKit1a.jpg

 

[KENAT]

OK, so if removal is a requirement, and it has to be a simple plug to suit existing hole then how about making it a transition fit and having an o-ring or something in there?

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[desertfox]

Hi nick

Sorry I misread your post, I thought you were having a plain portion for the interference fit at one end and a thread at the other.
It depends, you could use a puller I suppose it depends how often it needs to be removed, if its frequent then a plug with a thread and some kind of seal might be a better option as stated by others