O-ring depth

[StanFoster]

Recently I changed my main rotor transmission oil and filter, required every 100 hours. I went up to my local NAPA Truck and Helicopter supply store and bought a new filter as I have previously. I went to put the filter in and noticed the new O-ring was of a larger cross section than supplied on my other purchases before. I placed it in the groove....flush mount application where a simple plate has a groove and this is bolted up to the housing. I was skeptical as I had read that the diameter of the O-ring should project just a little above the groove it goes in....something like around 20-25% diameter above. This one was clearly a little too fat...but for experimenting sake...I used it anyway, the idea being just to monitor it for any slight seepage. My gut feeling said put the right size in, but I like to see for myself.

So....

I flew it about 5 hours, and was tired "thinking" about it...even though it was bone dry all the way around. I spent more time looking for oil seepage than it would take to change the O-ring to the correct diameter. I took it off yesterday and found ....maybe to no surprise....that the O-ring was deformed as you can see in this picture. I immediately put the correct diameter O-ring in and noticed it was projecting about 25% of its diameter above the groove like it should be. Sometimes...."more" is not better!

Kind of like seeing how your gut feelings should be followed, especially if backed up with information already meticulously researched.

Here is a link to a site showing where the depth, "L" shows that its roughly 20 to 25% greater than the O-ring thickness.

http://www.oringsusa.com/html/gland_design.html

[Jazzenjohn]

Good site Stan. I think the table you'd use is the one on the bottom,
INDUSTRIAL O-RING STATIC FLANGE APPLICATION.
You ran afoul of the warning on the right of it. Essentially the volume of the o-ring was bigger than the volume of the groove so it squeezed out and was pinched between the flanges. That's what the damage looks like to me.

[NoWingsAttached]

I'm going to pick up a 6mm thick (!!!) O ring for my project, tomorrow on my way home from Atlanta. Let me know if you want one while I am there.

[StanFoster]

Greg- 6 mm would be almost a quarter inch thick! Go ahead and get me one. I will order 4 longer bolts and s-q-u-e-e-z-e that baby down! Ha