designing a snap fit

Your best bet would be to do some research online. Look for guides published by industry insiders or search industry blogs. There are also books out there that cover this topic quite well, it's just a matter of finding them.

contact Jordon Rothheiser
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He's written books on the subject. Tell him I said "hi"

Bill tobin

I wanted some guidance about designing and snap well to when I came across this post and found the suggestions here quite useful!! I bought a couple of books recommended here and it was truly helpful for me!! I am sure that the books suggested here would have come in useful for many like me and hope that we get more of such useful advice from the forum in the future too – great work being done here – truly appreciated!!
www.burco-molding.com

I am a retired electrical engineer. I have a provisional patent on a product with significant market application. I have identified three (3) markets. I require a mold maker,experienced in snap fit annular mold making. The final product will be natural PP. Please contact me e-mail mcwbenyon@hotmail.com

Maurice Benyon

"Has anyone seen this and are their any oddities about it in terms of stresses."

Yup. One problem is that the radial loads during use go directly into the cantilever arms, causing them to deflect. Even the axial loads tend to put enough moment on the fingers to deflect them inward. It may work for disposable toys but it's not the most reliable way. If you can plug the gap between fingers with something after insertion then it's much better.

:) Yes, I do that all the time, just to think about other ways of doing things. I'm really just looking for a protocol for making them. I have to good hanser books and the web has a lot. But like I said, I have never seen it express the way I put it, and so wanted to check myself in general

By the way, one particular design I have not seen on the web or in books (and I cant imagine why not) is a two part annular type. The kind use for attaching small wheels to an axial. The axial being a cylinder with a slot in it forming the two cantilevers with a ridge at their ends that must be over come to get the wheel over. Has anyone seen this and are their any oddities about it in terms of stresses.

Large material suppliers like Dupont have excellent design handbooks you can take typical designs out of for their mateials.

This may sound like cheating, but a lot of times i will cruise wal-mart or home depot etc. and find similar product, check it out, buy one just to check design features, fits etc..

A good book is also is Desigining for Plastic Part Assembly ,I believe you can get this at Hansen Gardner through IMM.

GOOD LUCK

We use Finite Element Analysis to design a snap feature. If your snaps are complex and have a requirement of meeting a certain insert/pull loads, then I would recommend using FEA. Hand calculation does not take account of the actual sliding contacts between the snaps and mating parts.

Thanks

I have a few books and one just for snap-fits, but non really say what I did in so many words.

This is a late response, but for what it's worth you are correct. If you don't care how much force it requires to insert/withdraw the snap then you can simply find minimum length that ensures a snap load (or deflection) below the material limit.

In practice, there are a few more factors. Wall deflections can have a significant effect on the snap behavior. Also, some materials can take a while to recover from severe deflections even if the yield is not exceeded. Of course, tolerances and the freedom of the mating parts to shift with respect to each other can throw off a typical text book snap design calculation by a factor of 2 or even more. Then there are stress concentrations...

I'll bet that few people design snaps that work as anticipated on the first shot. I've had my share of failures. It's not rocket science, but snaps give the appearance of being overly simple.