Nitrile Static Friction Grip? 1

[ojc234gtnz]

I'm trying to use HNBR (Nitrile 70 Shore A) as a kind of stationary gland packing - at the same time holding and sealing a 304 SS tube (Bright 320 grit polish finish). I've made a test rig - see pictures below. But when I apply the load of 6000N, the tube slips. There doesn't seem to be a minimum load at which it doesn't slip (It was still slipping at 2000N when I stopped testing). I know static coefficients of friction are quite variable, but calculations tell me that it should easily hold the tube at 5-10 Nm of bolt torque.

So first of all - why is the tube slipping?
Secondly, is there a material I could use in place of nitrile that would ensure a slip-less hold and seal? (needs to be oil resistant and non-hygroscopic) - Polyurethane?

At this stage I'm not so interested in changing the design unless there are fantastical improvements to be made.

Thanks in advance

 

[btrueblood]

Try a taper on the end of the flanged piece, to better "wedge" the rings into the tube. Are those square packing rings or round o-rings? How much diametral clearance?

 

[ojc234gtnz]

They're square rubber rings, normally used for sanitary fittings.
I don't want to add a taper as I think this would jam the rubber spacers and inhibit further adjustment.
The rubber is tight on the tube with 0.05-0.1mm clearance on diameter in the housing.
I was thinking that at the force indicated, the pressure exerted by the rubber would be roughly the same in all directions.

 

[acspain]

I don't think changing the material is going to help much. Most elastomer materials are going to contain oils and/or waxes to aid mixing, processing, molding, and to reach desired properties. Sticky compounds do exist but will most likely prevent adjustment later on.

How did you arrive at the 47 kN clamp force?

I had to rebuild a frozen snow plow cylinder once and it was not easy getting it apart (the packings had swelled) which leads me to believe you can achieve your goal.

Have a good day!

 

[ojc234gtnz]

Yes, that does make sense of the observations. After one trial without cleaning, I cleaned the rings with a volatile solvent and they felt quite grippy afterwards. My guess is that there was still oils/waxes inside the nitrile that allowed it to slip.

Thanks.