O-ring groove specifications and questions

  • Thread starter Jynx18
  • Start date
  • #1
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I am looking for some info. I have two hollow tubes that need to slide into each other and have the inside one not fall out due to friction from the oring INSIDE the larger one. It is not doing any sealing, the oring is just to cause friction. The inside tube will be moved to remove and replace it but it is not a constantly moving part. The tube diameters and oring size are set. I just need to size the groove for the oring to sit in.

Oring
Size: 018
ID: 0.739"
OD: 0.879"
Cross section: 0.07"

Tube 1:
0.75" ID

Tube 2:
0.74" OD

Tube 2 goes inside Tube 1.

From the parker handbook it looks like the oring groove should be slightly wider than the oring. Also to keep some friction the OD of the groove should be slightly smaller than the OD of the oring.

Here is what I am thinking from the handbook:

It says Gland Depth sould be 0.050" to 0.052".

So 0.75" + 0.050" + 0.050" = 0.85" to 0.854"

Groove Diameter: 0.85" to 0.854"

And right from the handbook:
Groove Width: 0.093" to 0.098" (sounds a little wide to me though)

Any input on design of this would be great. Thanks
 

Answers and Replies

  • #4
Baluncore
Science Advisor
2019 Award
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You want a frictional grip rather than a fluid seal so hydraulic data will be misleading since fluid must move around the O-ring to pressure seal.

The O-ring must be deformed sufficiently to apply the friction you require.

The volume of the O-ring is fixed. If not under tension the O-ring will have a fixed cross section. Deformation into a flat sided ellipse will require a groove width that does not pinch the O-ring.

1. Investigate the flattening needed to grip OK between cylinders. O-ring grip thickness = t.
2. Measure the O-ring width in that situation. Width = w.
3. Machine a grove of width w and depth t.

Tolerance will be taken up by filling the unused corners of the groove.
 
  • #5
2,017
85
If all you are interested in is friction, why use an Oring at all?

Use something spring energized, that way you can control the level of friction.
 
  • #6
13
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I also need that the oring kind of centers the tube but am open to other methods. What would you recommend?
 
  • #7
Baluncore
Science Advisor
2019 Award
8,118
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The great thing about deformation of a partially constrained rubber cross section is that it is elastic, just like a spring.

Unfortunately, water is a natural lubricant of rubber. The coefficient of friction will depend on the (water / oil / dry) state of the contact. If not lubricated, a dry friction surface may gall the rubber seal. Abrasive wear can be reduced by the use of a polished stainless steel or a chrome plated sliding surface. If grit is present it will become embedded in the rubber which will wear the metal sliding surface faster than the softer rubber.

One problem with O-ring seals is that they get pushed into, or pinched in, the small gap between the piston and cylinder. O-ring hoop tension can be used in the absence of fluid pressure by cutting the groove for the O-ring with a deeper dovetail section, axial pressure will then tend to lift the O-ring away from the friction surface. That can significantly reduce galling of the rubber and sensitivity to lubrication.
 

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