Stack up analysis and Shim calculation

[lagpump]

hello all,

I've just done an axial stack up analysis for a loop in an assembly of 7 components. As a result,
The calculated shim thickness is .001+/- .009.
The questions is that do I need to modify some components' dimensions so that I can get a realistic shim thickness of at least .009 +/- .009 ?
Thanks for your help.

Sincerely,
lagump
HiPress Inc.

 

[rb1957]

i expect that your tolerance bands aren't equally spread over the nominal dimn.

and i don't think you've allowed a sufficient +ve gapping to create a +ve shim ... ie one element in your stack up should be "shim allowance.

 

[lagpump]

Hi rb1957,

Thanks for your response.

Actually the tolerance bands are equally spread over the nominal dimensions which are +/- .001. In the loop, there are requirements of one interference of .0005 and a gap of .030.

What do you meant by "sufficient gapping" to create a shim and "shim allowance" for one element in my stack up? Please help to explain.

Thank you very much.

lagpump

 

[rb1957]

it sounds odd to me to have both interference and gapping in a stack up ... either you can understand (physically) but both together ??. maybe you mean that the stack up can have 0.005" interference or 0.03" gapping ??

a shim allowance would be a layer in your stack designed to be a shim, so that with nominal dimns it might be 0.02" thick but it could be 0.03" or 0.01", depending on your tolerancing. so you'd define it as a 0.03" thick peelable shim or maybe as a "shim, thickness TBD"

 

[GannetAM]

Because you have seven components in your stackup and not all components will be at your maximum or minimum tolerance at the same time, I suggest you perform root sum square on your positive and minus tolerances. You may find the attached excel file helpful.

In addition, you may want to consider laminated shim stock

 

[ShaggyPE]

I agree with rb1957. Design in a nominal thickness shim pack. Take .010" off of one of your parts and replace it with a shim pack that can vary between 0 and .020". This could be a stack of .001" shims... or some .005"s and .010"s.

-Dustin
Professional Engineer
Pretty good with SolidWorks

 

[lagpump]

Thank you all for your suggestions!