O-ring groove design - comments please

[octathorpe]

I am making a flat plate with some small (008 or 009 sized - about 7 to 9 mm in diameter) o-rings on the surface. I would like to have the largest o-ring size on a 9mm X 9mm grid. If I keep discrete o-ring grooves that do not overlap then I come up with a design as shown in plate cap2.jpg - a working prototype is in snap.jpg.

However, if I am willing to overlap the grooves as shown in plate 009 o-ring.jpg then i can get a larger diameter (a useful increase in size to me). This means that the o-rings will just touch each other and perhaps more importantly, there will be a sharp corner on the object (looks sort of like a diamond) in between the o-rings.

I am wondering if this will be OK if I have such a design milled out of stainless 304 or aluminum on a CNC machine. Can anyone tell me if I need to be sure and cut the corners down somehow or will it essentially be just fine as I've shown it?

Thanks for any comments.

[Gary55Ford]

What are the plates for and do they mate up with another plate?

[ckirchen]

Individual o-ring grooves are better, but I have used both configurations in the past (with a lot less o-rings, only two side-by-side).

You should definitely add a radius to the tips of the diamonds, perhaps .010" to .015". When the o-rings are compressed, they will expand sideways and fill up the groove. The sharp edges may cut the o-ring and cause a leak, which would be almost impossible to locate when it does.

What are they for?

[octathorpe]

Originally Posted by ckirchen Individual o-ring grooves are better

I agree, but I have to push the limits because I cannot change the spacing between holes and a larger hole helps in my application.

The situation, while not ideal, does seem like it will work. If anyone feels otherwise please speak up!

"You should definitely add a radius to the tips of the diamonds, perhaps .010" to .015". "

I am going to include that in the design spec. I'm trying to keep it very simple for the machinist - I think the part will be programmed directly into the CAM software manually rather than using a model to generate tool paths so I may ask to just knock the tips off with a flat end mill that could get two at once.

[octathorpe]

Originally Posted by Gary55Ford What are the plates for and do they mate up with another plate?

I suppose this part does look a little random without any context. It's a microtiter plate - about 3" X 5" with 96 wells. This part mates to a flat bottom plate to make a sealed 'well' that holds a chemical reaction. Each well holds a different reaction. The array is to facilitate automation of a process. In my case I need to be able to do a chemical reaction, evaporate the solvent, and take apart the plate. This leaves a solid on the flat bottom plate exposed for analysis.

[Geof]

Originally Posted by octathorpe ....It's a microtiter plate....

My goodness med-lab equipment or something; I hope you are not releasing intellectual property here.

Presumably the only pressure inside the O-ring enclosed cavities is that arising from the gravitational head? You should be fine.

I would be tempted to completely remove the sharp cornered remnant between the O-rings and just have them locate on the spigot. Part of the reason for saying this is that I am guessing a technician will fill the plate with O-rings and the sharp corner simply represents a hazard but has no real function.

[octathorpe]

Originally Posted by Geof I hope you are not releasing intellectual property here.

Oh, I probably am... Good thing I'm using an alias ...

Originally Posted by Geof Presumably the only pressure inside the O-ring enclosed cavities is that arising from the gravitational head?

Actually I will seal both sides and put the well under pressure by heating somewhat, which is why I thought I should keep the diamond shaped thing, otherwise i'd get rid of it too. Without the diamond the o-ring might expand too much.

[Geof]

Actually if they are going to expand far enough that the sharp pointed remnant is restraining them it may cut them.

Leave a perimeter all around the outside, you could even scallop it to fit the O-rings. Don't worry about restraining the individual O-rings; if you heat and expand them far enough they will simply assume a self-supporting hexagonal array.

[PoiToi]

if it's diamond pattern is just for positioning could you just replace the diamonds with raised circles that were tangent to all the O-rings?
no sharp edges, and it would still support each oring at 4 places.

.. =)