Need help performing an RSS tolerance analysis

[sbaw88]

Hi guys!

I'm trying to perform an RSS tolerance analysis but I'm relatively new to it and having a hard time to understand the procedure.

I've attached a basic layout of the problem (sorry for the bad image). There are two pins, the distance between the two pin's axis is 1 inch with a tolerance of .002". Under the pin shoulder is a spring with .007" OD, .065" ID, and .003" tolerance on the OD. The pin diameter is .06" with .002" tolerance. I'm trying to find the minimum gap between the OD of the two springs. I've already calculated it using the worst case method but have no idea how to do it using RSS. Could anyone help me here or give a brief idea about how it could be done?

Thank you so much!

 

[3DDave]

Wellll... you take the tolerances and square them, then sum the squares of them, then take the square root of the sum. This is the 1 sigma value; about 60% of the items will fall within that tolerance.

It's really all there in the name RSS. Root. Sum. Square.

If you don't have a lot of tolerances, the central limit theorem doesn't work so well at making an accurate prediction of the variance, so the worst case may be the one to go with, at least until you get a lot of data from actual, produced parts. Of course then you might just measure the gap in the finished product to base statistics on.

 

[sbaw88]

Thanks!