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FAQ  O-Rings

"O-Ring Seals in the Design of Hydraulic Mechanisms" A paper presented at the S.A.E. Annual Meeting, January, 1947 by Mr. D. R. Pearl of Hamilton Standard Prop. Div. of United Aircraft Corp.

A.  The seals can be made perfectly leak-proof for cases of static pistons and cylinders for fluid pressures up to 5000 p.s.i. (Limit of test pressure).  The pressure may be constant or variable.

B.  The seals can be made to seal satisfactorily between reciprocating pistons and cylinders at any fluid pressure up to 5000 p.s.i.  There may be slight running leakage (a few drops per hundred strokes) depending on the film-forming ability of the hydraulic medium.  O-rings can be used between rotating members with similar results but in all cases the surface rubbing speed must be kept low.

C.  A single O-ring will seal with pressure applied alternately on one side and then on the other, but in cases of severe loading or usage under necessarily unfavorable conditions, seal life can be extended by designing the mechanism so that each seal is subjected to pressure in one direction only.  Seals may be arranged in series as a safety measure but the first seal exposed to pressure will take the full load.

D.  O-ring seals must be radially compressed between the bottom of the seal groove and the cylinder wall for proper sealing action.  This compression may cause the seal to roll slightly in its groove under certain conditions of piston motion, but the rolling action is not necessary for normal operation of the seals.

E.  In either static or moving O-ring seals under high pressure the primary cause of seal failure is extrusion of the seal material into the piston-cylinder clearance.  The major factors affecting extrusion are fluid pressure, seal hardness and strength, and piston-cylinder clearance.

F.  Moving seals may fail by abrasion against the cylinder or piston walls.  Therefore, the contacting surfaces should be highly polished for long seal life.  Moving seals that pass over ports or surface irregularities while under hydraulic pressure are very quickly cut or worn to failure.

G.  The shape of the seal groove is unimportant as long as it results in proper compression of the seal between the bottom of the groove and the cylinder wall, and provides room for the compressed material to flow so that the seal is not solidly confined between metal surfaces.

H.  The seal may be housed in a groove in the cylinder wall instead of in the piston surface without any change in design limitations or seal performance.

I.  Friction of moving O-ring seals depends primarily on seal compression, fluid pressure, and projected seal area exposed to pressure.  The effects of materials, surfaces, fluids, and speeds of motion are normally of secondary importance, although these variables have not been completely investigated.  Friction of O-ring seals under low pressures may exceed the friction of properly designed lip type seals, but at higher pressure friction compares favorable with, and is often less than, the friction of equivalent lip type seals.

J.  The effect from temperature changes from -65F to 250F on the performance of O-ring seals depends upon the seal material used.  Synthetic rubber can be made for continual use at high or low temperatures or for occasional short exposure to wide variations in temperature.  At extremely low temperature the seals may become brittle but will still seal and will resume their normal flexibility without harm when warmed. Prolonged exposure to excessive heat causes permanent hardening and usually destroys the usefulness of the seal.  The coefficient of thermal expansion of synthetic rubber is usually low enough so that temperature changes present no design difficulties.

K.  Chemical action between the seal and the hydraulic medium may influence seal life favorably or unfavorably, depending upon the combination of seal material and fluid.  Excessive hardening, softening, swelling, and shrinkage must be avoided.

L.  O-ring seals are extremely dependable because of their simplicity and ruggedness.  Static seals will seal at high pressure in spite of slightly irregular sealing surfaces and slight cuts or chips in the seals.  Even when broken or worn out, seals may offer some measure of flow restriction for emergency operation and approaching failure becomes evident through gradual leakage.

M.  The cost of O-ring seals and the machining expense necessary to incorporate them into hydraulic mechanism designs are at least as low as for any other reliable type of seal.  O-ring seals may be stretched over large diameters for installation and no special assembly tools are necessary.



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