Press Fit Bushing Knurl Design for Nylon

[jmarkus]

I am designing a press fit bushing to fit into a nylon part. The bushing is actually a compression limiter so that a bolt can be torqued down over the part (and not compress the nylon). I have specified a medium diamond knurl pattern, based on seeing similar press-in inserts, but I don't really know what is the appropriate knurling detail (size, TPI, spec?) to use. We are going to make a prototype and try it, but I want to start with something reasonable.

Thanks,
Jeff

 

[Pud]

Hi jmarkus,

imho I would suggest the best route would be to talk to suppliers of inserts. You will find that they will be the cheapest and have the capability to design exactly what you need, and indeed may have an "off the shelf" product for you!

Rgds

Harry

 

[jmarkus]

Unfortunately, that is the problem. The insert suppliers don't have an off the shelf part that fits, and their costs to provide a custom solution are more expensive than us providing our own custom solution. My problem is, I do not possess the knowledge to design a knurled press fit.

Jeff

 

[Pud]

Hi Jmarkus,

In that case, I would suggest just a medium diamond knurl - if it's a compression limiter then I assume the insert will not fall out in use as it is retained and there is no force pushing/pulling it out. Keep the interference fit as small as possible to prevent stress raising around the insert (although unfilled nylon is pretty tough). If you can mould the insert in - then all you need is hex bar with a groove in the OD down to the across flats size.

btw, avoid zinc plating of inserts with nylon - especially if zinc can corrode - nylon does not like zinc salts, which can cause environmental stress cracking.

Rgds


Harry

 

[jmarkus]

Harry,

I've been asked "which medium diamond knurl - give me a TPI" I don't know how many TPI to specify for a fit.

As for the zinc plating and nylon - we were planning on using a zinc/nickel plating to prevent corrosion. Is that a bad thing too?

Thanks,
Jeff

 

[patprimmer]

Do not put zinc anywhere near the nylon. On its own it's no problem, but as soon as any water containing any chlorine (like normal tap water) also hits the scene, you have a serious stress crack situation.

Under normal circumstances (ie no zinc or chlorine nor acids) nylon has exceptional stress crack resistance.

Unless there is something we don't know about your application, it sounds to easy and the knurling will not be at all critical.

Why not get a sample of an off the shelf insert that is the wrong size, but knurled, and copy the knurling.

Regards

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[MikeHalloran]

The diamond knurl will abrade the nylon unless it's pretty fine.

I'd use a straight knurl, or push the inserts in with a big 'ol soldering iron.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[jmarkus]

Pat,

Do you know of any corrosion resistant coatings that can exist in contact with nylon parts? Everything I am aware of contains Zinc!

Thanks,
Jeff

 

[patprimmer]

I'm not a coatings expert, but what's wrong with copper, or epoxy or whatever bolt manufacturers use apart from zinc.

Regards

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.