Round into Hex Interferance fit.

[lobos8403]

Hello,

I have a question regarding a hex receptacle accepting a round peg for a friction fit retainer. This is essentially the opposite of a crush rib and i'm curious what the advantages are of this particular type of press fit. I've observed this particular mate in the automotive industry. I have found formula's for regular round round press fits but nothing on this. I would really appreciate if someone could shed some light on this.


Backround info:
I'm looking something that can be attached and dissembled up to 3-4 times in its life cycle and that will allow a long term fitment solution. I understand creep will be an issue but id like to get some info on the concept first. The purpose is essentially a lens retainer that has multiple pegs around its perimeter that holds a lens onto a bezel. Each peg has approx .125 engagement.


Any help would be greatly appreciated!

 

[loughnane]

I've used the 'bullet pin into hex hole' multiple times, and my general rule of thumb is that the diameter of the pin (~.100 - ~.200) should nominally be .005 larger than the width of the hex. I generally do this with some kind of PC blend. Practically I'll start @ a L2L fit, and encourage the molder to walk it in from there until we get a really good fit.

Also, in an autoclave environment the pull strength seems to fall of initially (can't remember the percentage...im going to say ~40%), but then flattens out. It doesn't sound like that's too relevant to your application, but something to keep in mind.

Also, test whatever you do, these can be finicky in special environments.


All that being said, it is a ONE TIME ASSEMBLY. I have not seen anyone doing really strong press fits that can be assembled/disassembled (not that it can't be done, but it seems like opposing constraints). Sometimes if you try to pull them apart you can break the housing before you pull out the pin.

Also, if you are concerned about the tolerance on the position of the pins (depending on your design), give the base of the pin a little bit of wiggle room.

Chris Loughnane - Product Design

http://www.pdnotebook.com

http://www.twitter.com/_chrisloughnane