O ring groove dimension?

[blwilliams]

Getting ready to cut an O ring groove in my blower intake to seal blower to intake.
I have a .250 diameter Buna O ring.
I contacted good vibrations who refered me to the blower shop. Blower shop only had a dimension for smaller diameter O ring.
I have found several differet sights with dimensions but they seem way off (IE .215 deep X .300 wide)
Has anyone had any expearance in this process?

[turbo camino]

What 'seems way off' about .215 x .300? The area of the groove ( = .0645in2) has to be larger than the area of the o-ring cross section ( = .0491in2).

[blwilliams]

Turbo Camino thank you for your reply.
In my foggy mind I was thinking that the O ring area should be slightly greater than the area of the groove
so as it would be pushing against its sealing surface to inhance the seal.

[ijames]

No direct experience on a blower but a decent amount designing vacuum flanges and low pressure piping. The biggest thing that gets forgotten is that Buna and other rubber orings are deformable but not compressible. That means that the cross-sectional area of the groove has to be at least as big as the cross-sectional area of the oring or the oring won't fit and will be extruded and mangled. A little extra room is good. The Parker oring manual, available online, has lots of design formulas and tables but here's the simple version. First, design for a crush of 20-30%. For low pressure seals 20% will work. For high pressures use 30%, and in between, well, use in between . The more you crush and deform the oring the better it will seal and the fewer cycles of tightening and releasing you will get before it cracks or otherwise fails. Seriously, orings are very forgiving so it's not that critical and assuming you will be in the 10-50 psi range, 25% crush will be fine. If your oring is 0.250" diameter you need to make the groove 0.188" deep. To provide for the cross-sectional area of pi*0.25^2/4=0.0491 sq. in., the groove needs to be 0.0491/0.188=0.261" wide at minimum, plus 0.025-0.05" extra. So you need a groove 0.188" deep and 0.286 to 0.311" wide. If you are doing a vacuum seal, make the inside diameter of the groove the same as the oring so when vacuum is applied the oring is already in position pressed against the "inside" wall, and reverse that for a pressure seal like yours. If you put the oring against the inside wall of the groove and then apply pressure the oring will "squirm" to the outside of the groove which can cause seal failure. Vertical walls for the groove are fine but since the oring will be loose in the groove installation can be annoying. Ideally you will have a tiny radius on the bottom corners of the groove and a very light chamfer on the top corners to avoid stress raisers. You can get fancy and cut a dovetail shaped groove for better oring retention to make installation easy, but that's a bit trickier to design. Oh, if you are doing a rectangular flange you will need to radius the groove in the corners of the flange. For a 0.250" oring off the top of my head I'd start with a 1" radius to the inside wall. Bigger would be gentler on the oring, and tighter would probably work but the oring will be harder to keep in place for installation.

Finally, for bonus points cut a channel the depth and width of the oring groove from the oring groove to the inside of you chamber. One is ok, two on a round flange is common, and two or four on a big rectangular flange would be nice. This allows ready access to the oring by the vacuum or pressurized gas in your chamber which makes sealing faster and more reliable, and also makes leak detection easier on vacuum flanges.

Hope this was helpful and not toooo longwinded .

[blwilliams]

Wow, Mr. ijames that is very impressive.
Thank you very much.