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Hi all do-it-your-selfers ! Since my recent post about using O-rings I have been receiving many enquiries for design help and it seems that this kind of information is badly needed among us. So, I made some notes on the very basic rules to design a static assembly (that is, the parts are not moving, rotating, etc.) with an O-ring. This can be used for lamps, electrical wire feedthroughs, video housings, etc. It would be nice to send some drawings for this, but I will try to explain it in words. Dynamic (moving, rotating,... parts) sealing is more demanding and designer must be aware several issues not presented here. If you follow all these rules you will not have problems with sealing... Happy moments in your workshop ! Terho --------------------------------------------------------------- MATERIALS - usual body/chasis materials are metal or plastic - both can be used but.. - plastic is difficult to machine to accurate dimensions - plastic is not suitable for high power lamps (it melts) - plastic gets easily damaged (sealing surfaces) - plastic bens under pressure - metals are much easier, I recommend metal - I have used aluminium, stainless steel and titanium - if you have housing that will be opened often, use metal - threads are strong in metal, in plastic they are easily damaged - use glass for windows, not transparent plastic SEALING PRINCIPLE - O-ring is placed in a groove or slot - pressure forces the ring to reshape against the slot walls - if there's no wall near, the ring will pull out/in, damage or move until something will stop it - pressure must be allowed to act on high pressure side freely - with high pressures (tanks) the ring can penetrate to very small slots (small tolerances for slots needed) - long penetration damages the ring (cutting effect) - after the pressure is reliefed the ring will reshape to the original shape - if you cut the ring, the cut area is constant, when the ring reshapes, the area remains constant - the ring is moving and reshaping with the pressure, therefore the lubrication is needed SLOT SHAPE, SIZE AND TOLERANCES - for most uses the square shape is okay (and easy to make) - the outer wall of the groove is not obligatory, easier to do but the ring may slide aside while tightening, the both walls are recommended - select the ring size and the design the slot so that the inner circumfere is the same or sligthly bigger that ring's - slot depth must be selected so that it is 70-75 % of the thickness of the ring (for normal hardness, for harder rings use something like 80-85%) - slot width must be selected so that the cut area of the slot is bigger than the cut area of the o-ring (otherwise the ring can not fit inside the slot and will be damaged) - when the opening is closed, there should not be any small slots or grooves (from big tolerances) in which the ring could penetrate, so the slot surfaces should close very thightly - surface quality in slot must be the best you can make, mirror like, no visible scratches or grooves, if you feel it with the finger you shall not feel anything (finger is very sensitive to surface quality, you can feel a 0.01 mm 0.0004") scracth) - all slot walls are not sealing surfaces - usually there are two sealing surfaces - make sure that the pressure (water) can penetrate to the pressure side of the ring and press the ring against the walls (you may need to make small holes to let the water/gas to go in freely) - I have used tolerances for closing sealing surfaces something like 0.1-0.05 mm (0.0039-0.0019") for small rings and metal housing (only for sealing grooves), for small pressures (lamps, video housings) bigger tolerances can be used (0.3-0.2 mm) - finally, shape the body so that the ring is not visible outside when closed (visible ring will be damaged some day by scratches, mud can go in and damage surfaces, etc.) BODY / CHASSIS - by sealing pressure I mean the force applied to the housing body by thightening the sealing mechanism, no water or gas pressure applied - when the opening is closed and the ring is in place the sealing pressure should not act against the o-ring - sealing pressure must be received by the housing (flange, etc.) - you must be able to close it without the ring in place and fully thighten the bolts or whatever you are using O-RING - normal rings are useable in water and small depths (hardness is around 70 shore) - higher pressure needs harder material (90 shore) - high pressure (tanks) may need a pressure ring placed beside the ring in the low pressure side in valves or other places when bigger tolerances are needed for functionality - rings must be replaced time after time - for static assemblies, the rubber gets old and will be broken after some years (it get hard and splits) - bigger thickness is better, but of course no reason to use huge diameter, thin ring assemblies are difficult OTHER - keep the things clean - lubricate the ring before assembly (carefull with O2) - not scratching allowed, no hairs or anything between the surfaces - no sharp edges in the body over whitch you slide the ring in place - if possible, do not rotate or move the ring when you are putting it in place and thighten COMMON MISTAKES SEEN - if the sealing pressure is received (and only) by the o-ring, the assembly will not work at all and many problems are invited - sealing surface quality is poor, there is no way the sealing principle could work - wrong sized slots used (remember the cut area and proper slot depth) - too big sealing tolerances, the ring will penerate to grooves and be damaged (you can see pressed traces on the ring when removed, any visible marks show that you have something wrong in the design) - on lamp flange, too few bolts used for mounting, will bend the flange if it's not thick enough and results big slot tolerances, use at least 5-6 bolts, more if you are using plastic, I prefer one big thread aroung the glass for mounting it for a lamp (no bolts at all) - no lubracation when closed (ring lifetime will be reduced) - when somethign is wrong, the design may work for a while and then more and more problems, leaks, etc. will occur as the damaging ring will get worse and worse - with electrical feedthrough, if the wire is pulled the pulling force is applied to the o-ring (will damage the ring), use some kind of pull relief system, take the force but not by the ring (if nothing takes the pull, then the force acts on the ring) - plastic used in wrong places (melts by the high power bulb, bending by the pressure, soft material with high sealing forces will not work -> calculate the force applied the your lamp glass in deep dive and get supriced !) - too thin ring is used, use at least 3-5 mm diameter
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