Face Seal Fastener Placement and Spacing 3

[innominata]

How much does it matter where the fasteners are placed on an o-ring face seal?

Park O Ring handbook doesn't say anything about it. I'm currently using a racetrack groove. I've made the radius of the groove curvature 3x the diameter of the o-ring but I worry about where I've placed the screw holes.

The enclosure material will be injection moulded polycarbonate with the groove using either an NBR or FKM o-ring.

I can understand that the fasteners should be placed symmetrically but beyond that I'm not sure what the placement and spacing requirements should be or why it's not often mentioned in guides. My question is from your experience, apart from symmetry does it happen often that the placement of fasteners influences the seal in this type of application (plastic electronic enclosure)?

 

[TugboatEng]

This depends on how much force it takes to compress the o-rings. Your configuration would not likely compress an off the shelf 70 durometer o-rings. There are form in place and foam options that would work with your current configuration. Softer o-rings are also available. NBR is just about to the worst choice of material for your application. FKM will be cost prohibitive. I suggest silicone. EPDM would be better but risks damage from exposure to oil.

 

[innominata]

Thanks Tugboat,

So fundamentally I need to find the force requirement for the material and find some fastener configuration that distributes the forces over the length of the o-ring correctly.

Ty

 

[TugboatEng]

I mean to say the 4 circled screws will likely be insufficient. The 8 screw co figuration would likely be acceptable.

 

[3DDave]

If you have access to FEA make an analysis where the desired contact pressure is distributed along the groove and the force from that is resisted by constraints placed where the heads of the screws would bear. Then look at the deflection along the groove and see if the deflection is larger than the deflection required to deform the o-ring to produce the desired contact force. It might also be valuable to check the stress the plastic will be exposed to and compare to what the plastic can take long term.

 

[hendersdc]

Side note, you have a lot of very sharp corners in the o-ring groove and you may have a hard time getting the o-ring to stay in place. You say that 3X o-ring diameter was used as the min but the corners to either side of each screw hole look much sharper than that.

 

[DogPAnt]

I'm far from an expert but I've used quite a few enclosures similar to this.

Can you find a quality enclosure similar to what you are trying to design and base your spacings / screw sizes and the seal material on what they have done.

Having said this I recently had issues with warp-age on some enclosure lids that allowed weather ingress.
From memory they were a Rittal enclosure, good quality stuff. 5 years in the Australian sun had taken its toll.
The lids warped and the seals had hardened.