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| Deflection/Stress from Compressed O-Ring |  | ||
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| Posted by: kevinh673 ® 05/02/2008, 09:11:15 Author Profile eMail author Edit |
This may be a trivial problem, but I'm a new Engineer and the stress I am calculating from an O-Ring compressed roughly 30% seems a bit high, and am looking to see if I did this correct. I have included the .jpg of my hand calculations My stresses calculated are 16 ksi and 28 ksi, which is a significant difference... Deflection is only 6/1000s this seems fair. The O-Ring goes into the 0.070" gap. The O-Ring is 0.070" wide, and is compressed to 0.050"
Modified by kevinh673 at Fri, May 02, 2008, 09:12:24 |
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Posted by: randykimball ®  05/03/2008, 00:38:40 Author Profile eMail author Edit |
Compression of o-rings actually depends on the PSI of the fluid acting upon it. Now that I have your attention: I'm not sure where the teaching goes wrong, but the way an o-ring works is that it is placed into a grove that it "touches" both the inside and outside diameter OR it touches the top face and the bottom face (some light contact compression NOT cross sectional compression). The o-ring groove should gernerally not be small enough to compress the o-ring, in other words the groove cross sectional area must be larger than the o-ring cross sectional area. The o-ring seals because the fluid "tries to push it out the leak" and the o-ring profile compresses against the two surfaces (usually a face and a diameter) forming a seal. The working fluid side has a higher pressure than the non-working or ambent side so the o-ring displaces and "compresses towards an escape". I have seen several cases to where engineers were taught to design an o-ring groove to be of less cross sectional area than the cross sectional area of the o-ring. What then often then happens is as the cover or head of the device is torqued down one side is tightened more at some point and the o-ring being compressed is forced to escape out the less tight side. This results in a pinched o-ring because some of the material WILL extrud into the gap before the two surfaces are completely pulled together. Also, it is often nearly impossible to get the two surfaces to actually make physical contact because the compressed o-rings require considerable forces to be compressed into a smaller space. This squeezing of the o-ring most frequently damages the o-ring. So... please listen to an old fart and please take heed, this will keep you out of additional troubles and we all have enought of those. The short version.... guys and gals... an o-ring is NOT a gasket and indeed makes a very lousy gasklet. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Sat, May 03, 2008, 00:54:34 |
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| Posted by: kevinh673 ® 05/08/2008, 07:36:10 Author Profile eMail author Edit |
Thanks for the input, Randy, I appreciate the reply. This actually isn't a fluid problem, however; the o-ring is actually up against a mirror. The piece I have shown is a cover to a mirror holder that is screwed in, and the o-ring touches the mirror to keep it in place (as well as keeping it from touching the cover directly). As such, I am not looking for the o-ring to act as a gasket, as it's not trying to seal any fluid per-se. If the cover was clamped down directly on the mirror, it would scratch it, so the rubber o-ring is used. What I am actually after is if I correctly calculated the deflection and pressure on the section of the cover that the o-ring is incontact with. |
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| Posted by: Marky ® 05/08/2008, 07:44:07 Author Profile eMail author Edit |
I think that you could go to around a 5% compression....but I would consider changing to a Neoprene sponge gasket. |
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| Posted by: randykimball ® 05/08/2008, 13:45:25 Author Profile eMail author Edit |
If you do not want the frame to touch the mirror simply make sure the groove does not have enough area to accept the cross section of the o-ring. It doesn't take much less area, I see it as simple as that. Sometimes it is easy to make things more complex than they need to be. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. |
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| Posted by: kevinh673 ® 05/08/2008, 14:57:28 Author Profile eMail author Edit |
Well what I am after is whether the calculations I did for pressure and deflection are correct for the O-Ring compressing 30%. I am concerned about the strength of the cover piece's thin groove that the O-Ring fits in is strong enough to not deflect (bow out). |
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| Posted by: randykimball ® 05/08/2008, 23:49:15 Author Profile eMail author Edit |
You say your aren't sealing and are only interested in preventing the frame from contacting the mirror.... I doubt you need any where near 30%.. It will take enormous power to compress an o-ring into a cavity 30% too small to fit its cross section area. I agree with the other posts, 5% too small an area in the groove is plenty. Here is a real life example of o-ring compression forces: We had a situation in which some o-rings were about 6% larger than the groove cross section. We were damaging o-rings because they were extruding out of the grooves during torque down. I realize you are not torquing them but understand we were clamping down with four M8 socket head cap screws and were deforming 3/8" thick steel caps only 2.5 inches square and still not compressing the o-rings enough to get the plates flush with the mate surface. If we pulled the corners down where the bolts were the caps folded warped leaving gaps between the the plates, between the cap screws. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. |
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| Posted by: kevinh673 ® 05/09/2008, 07:59:49 Author Profile eMail author Edit |
I suppose I am looking at worst case scenarios. The O-Ring is only sticking out about .02", however if it was compressed the full .02" that would be a poor design as it would result in the mirror being scratched from touching the cover. I'm assuming it will only comress very little, probably the 5% you mentioned is correct and is good advice; however say I changed the 30% compression to 5%, would you say my calculations would be correct. I'm curious as to whether or not I analyzed the problem correctly. I also sketched a bit more on there to clear up any confusion. I should have noted that the depth of the groove is .07".
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| Posted by: Kelly Bramble ® 05/09/2008, 08:30:37 Author Profile eMail author Edit |
I think there are other factors that should be considered and understood regarding your o-ring design and analysis. First, o-rings have cross sectional tolerances, typically on the order of +/- 10% but never less than +/- .005. Please double check my claim as manufacturing processes and material stability continue to improve. If you use a .070, then .07x (+/-) 10% = .007 or up to .014 of size variability. O-rings are not dimensionally precision components. With a fixed o-ring gland or groove and only 5% of compression and an o-ring that shows up on the small side, you could see contact with the cover. Now there are ways to design this interface to minimize the chances of scratching the mirror assuming this is a real issue. I would look at alternative o-ring or elastomer shapes. There is "T" and "L" shaped elastomer extrusions that may protect my mirror better. Typically these shapes are bonded on the ends to achieve the correct continuous length. There are other options to prevent abrasion on your mirror as well. You could use felt, Teflon and other materials with a controlled or limited compression applied to the mirror to prevent breakage. |
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| Posted by: kevinh673 ® 05/09/2008, 10:06:29 Author Profile eMail author Edit |
Interestingly enough, I haven't heard of any problems where the mirror has been scratched from touching the metal of the cover. There really isn't a problem with the design at hand from what I've gathered, I just don't know that anyone's double checked the calculations. I'm in the process of redesigning an assembly involving this part, and was curious as to the calculations involved. Thanks for the insights so far I've learned quite a bit about o-rings from this post. |
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| Posted by: randykimball ® 05/09/2008, 23:54:39 Author Profile eMail author Edit |
Ya know, Kevin......
We don't need to prove EVERYTHING with calculations... wink.. ..The old addage "if it ain't broke don't fix it" often has merrit, as long as we do not let it put us into a box. During this whole thread I've been thinking... WHY?.. all you need is something insulating the breakers from the breakable.... end of thought process... go work on the next problem... big smile. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. |
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