Controlling flatness of electronics enclosure covers for effective environmental and EMI seals

[Tunalover]

Can anyone provide guidance on how to control the flatness of a machined aluminum cover when flatness must apply when the cover is bolted to the enclosure? I'm finding that controlling the flatness in the free state is insufficient because of the bulging effects of the bolts. There are separate EMI (braided mesh) and environmental (o-ring) seals. The bolts must be tightened until they close the gap between enclosure and cover. Compression stops might work but I dunno.



Tunalover

 

[MikeHalloran]

Stiffen the cover between the bolts.

Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

By which I mean, flange the cover, weld on a thick doubler, add local stiffeners between the bolts, make the cover much thicker, use more bolts, etc.

For ad-hoc examples, look at all the aftermarket stuff that used to be sold for SBC rocker covers when they were made of thin sheet steel.

For analytical examples, search on bolted gaskets, and consult sheet gasket material catalogs. The problem of loading a stiff gasket with a thin flange and not plastically deforming the flange is not new, but has gotten more attention in the ChemE world and the automotive world than in the EMI world.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tunalover]

Thanks Mike,
Since I originally posted, I decided to stay with a simple, flat cover but 25% thicker, and much more compliant gaskets. The cost of using the larger, compliant gaskets was flange real estate. We had to widen the flange to accommodate the wider, deeper grooves necessary to retain both the new larger EMI and environmental gaskets. It worked!


Tunalover

 

[drawoh]

tunalover,

You have two choices with any sort of gasket.
[ol]
[li]Install the gasket into a recess such that the cover compressed it a known amount.[/li]
[li]Sandwich the gasket between the box and cover. Make the cover rigid. Engineer your fasteners to account for the non-rigid element in the joint.[/li]
[/ol]

--
JHG

 

[drawoh]

tunalover,

Hit the "Submit Post" button too early.

The advantage of the first concept is that you tighten all your fasteners down hard, as recommended by all current publications on vibration loosening. Can you thicken your covers? More than 25%?

--
JHG

 

[Tunalover]

drawoh-
I planned on putting compression stops (eg. captive pins) so the gaskets get compressed a known amount.


Tunalover