determining needed screw pressure for compression plate

[becma27]

I have a aluminum plate that is 2mm thick, diam 1.5 inches. I am screwing it down with four 2-56 screws into another aluminum plate underneath. Between the two plates is a polystyrene plate, same diameter, 1mm thick. this compresses against several small lenses to keep them in place. I want to determine the pressure that the polystyrene will apply on the lenses. I guess this goes to how tight I tighten the screws, but how do I determine the force the screws apply and then the force the plate applies? Below is a quick diagram of the sandwhich.
||(|
||(|
|--- screw
||(| these lines are the top plate, polysty plate,
||(| lens, and bot. plate from left to
right


Thanks for the input

 

[CoryPad]

This is a nontrivial problem due to the geometry (nonuniform force distribution due to point loads from fasteners) and the different materials in cross-section (which results in different compliances).

Do you know the axial compliance of the lenses?

What is the property class of the screw? With this, you can get an upper bound of the force that can be applied by the screw.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Sparweb]

Can you make an analogy with a durometer test? Using a durometer with a spherical impactor, you can apply a calibrated force and penetration into the polystyrene. Using a few different impactors will give you a curve - extrapolate to the size of your lenses.

Just an idea. If I'm way off it's because I don't work with plastics much.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout