Good way to measure o-ring (tiny donut shaped rubber with a radius of 1-2mm) deflection

[magomago]

I'm taking this thing and trying to measure force vs defelection of the o-ring. I have a setup where I have a tool that measures displacement attached to a base...and then I have a force transducer. put this o ring on this base, and cover it with a thing peice of metal because if I did not the probe of the force transducer is not big enough to cover the entire o ring. Then using my adjustable base I slowwly turn it and cause defelections on the order of .0005 inches and measure the corresspond gram force on the o ring.

To give a better idea: imagine a microscope with the top part of the microscope ripped off. The base that can adjust has a displacement measurement tool secured to a point that doesn't move, and the over point is secured to the base. Thus when I turn the microscope wheel, I cause the base to shift up and down and this is measured by the displacement measurement tool. Then on top of that I have a force transducer that is suspended above any object I would want to measure the applied force on. So as I riase the base up, it connects with the force transducer. This way I can measure both the force and displacement of said object.
Now that object I'm using is a rubber o ring that is laid down on the surface of the base. But since the probe is not big to cover the entire o ring, I put a very thin sheet of sturdy metal on top of it.

The only problem is that the results are not repeatable to any degree. I can spend forever making sure that it is all callibrated and zeroed properly, and that the force transducer is just touching the said object without applying force (the force i'm dealing with ranges from 0 to about 50 grams, so as long as initial contact force is <1 g I'm comfortable with the apply...the displacement as I stated earlier, varies from .0005 to .0035 in .0005 increments) to ensure accurate results. However despite this it is varying too much between the same points.

So now I'm looking for a DIFFEERENT way to measure this. Anyone have any ideas? I was thinking of maybe using a more mechanical approach of tossing on weights (something like weight about 1/10th of a gram to 5 grams in size) but I can't seem to find the bloody weight set .

I figured I can try to call the manufactorer of these o rings and see if they know of a good way to measure this, but just before I wanted to see what high tech could come up with

 

[Born2bwire]

Maybe you could use a strain gauge. The measured force from the gauge is dependent upon the deformation of the gauge. Theoretically, you could calculate the deformation given the original dimensions and force. I do not know if you could accurately do this, but I'm sure if you asked a manufacturer they could tell you.

 

[Armitage]

I wonder if the o-ring is rolling or collapsing on one side? If your setup isn't perfecty centered, I would wager something like that.

How about a setup closer to how an o-ring is used. Put the o-ring over a very short shaft and make a collar that just fits over the shaft. Maybe with a rabbet on the inside of the collar. I'll sketch something up...

Not sure how to deal with the issue of off-center loading. I remember having fits aligning the test frame for reproducible compression tests on small samples.

 

[Armitage]

http://pics.bbzzdd.com/users/Armitage/oring.png

 

[CycloWizard]

What exactly are you trying to measure? There's no reason I can think of that you would really want to know the force-displacement relationship for such a convoluted setup. There are well-developed testing methods for compression testing of O-rings, and I can suggest many much better ways to determine mechanical properties if that's what you're after. Just give us an idea of what you're after here and maybe we can help more.

 

[herm0016]

armitage's setup is the way to go, but you must be sure to lubracate the collar so you dont get too much noise in your data from the friction of the ring on the collar or the pin on the collar.

 

[jagec]

Originally posted by: Armitage
http://pics.bbzzdd.com/users/Armitage/oring.png

That's a pretty good idea; I'd probably omit the flange on the top section in order to give the rubber somewhere to expand, though.

And you might want to use a more precise method to cause the deflection than a microscope base. Use two or three bolts with fairly fine threads, which will allow you to make very small, precise adjustments, and have the added benefit of holding that adjustment fairly well if you accidentally bump it.

Also, you could just look up the modulus of rubber, and run a FEMLAB simulation

 

[CycloWizard]

Originally posted by: jagec

Originally posted by: Armitage
http://pics.bbzzdd.com/users/Armitage/oring.png

That's a pretty good idea; I'd probably omit the flange on the top section in order to give the rubber somewhere to expand, though.

And you might want to use a more precise method to cause the deflection than a microscope base. Use two or three bolts with fairly fine threads, which will allow you to make very small, precise adjustments, and have the added benefit of holding that adjustment fairly well if you accidentally bump it.

Also, you could just look up the modulus of rubber, and run a FEMLAB simulation

But the modulus of rubber depends on many things, chiefly chemistry and processing techniques. There are more kinds of rubber than you can shake a stick at, and they can have vastly different properties.