how to calculate the friction force due to o-ring compression 1

[ForestGump]

I am trying to calculate the friction force due to a sliding o-ring. In this case, the o-ring has compression from the slider and also has contact with the side-wall of the groove. I can only find the calculation when there is no contact with sidewall. Please give me your 2 cents.

 

[btrueblood]

Do you mean sliding axially along the shaft, or sliding in rotation, or both? If axial, I can't see how contact with sidewall has any effect (unless you mean the groove is so narrow that the sidewalls increase the compression of the O-ring, i.e. both sidewalls are in contact, squeezing and "pooching" the ring?)

 

[ForestGump]

yes, I mean the sliding in axial diretion. Assuming the grrove is narrow, then the o-ring moves together with the slider until it has contact with the sidewall of the groove, i.e. its one side in contact with wall, the other side is driven by the fluid pressure.

 

[btrueblood]

Oh, I get you. Well, from statics, the force should be the same on both the i.d. and o.d. of the O-ring. The materials are the same, presumably, so the friction force should be approximately the same on both i.d. and o.d. Except, if the O-ring has been "stretched" slightly during installation, then there will be a preload on the i.d. that increases the friction force there. Note that the preload will "decay" over time with most rubbers, due to stress relaxation (aka creep).

 

[ForestGump]

Thanks. But how can I calculate the normal force or stress in this case? I mean when the o-ring has one side in contact with the sidewall of the groove, it is essentially deformed like a "D" ring.

 

[israelkk]

Enter this address

http://www.parker.com/o-ring/Literature/00-5700.pdf

and at chaper 5 you will find how to calculate O-ring friction.

 

[btrueblood]

Once the O-ring hits the sidewall, motion relative to the groove stops, and you only need consider friction on the other side (I think you know that already).

As far as any additional squeeze (and increased friction) occurring due to the deformation of the O-ring, you are now in the realm where you'll have to just make some assumptions, and go test. The amount of deformation of the ring will depend on the compound, pressure differential, temperature, fluids used, friction developed (note the recursion here), velocity of travel, etc. etc. etc. The Parker handbook reference shows a little bit of data, but it's pretty limited, and not much use if you aren't using the specific compounds they show data for. Good luck.