How an O-Ring Works

All robust seals are characterized by the absence of any pathway by which fluid or gas might escape. Detail differences exist in the manner by which zero clearance is obtained - welding, brazing, soldering, ground fits or lapped finishes - or the yielding of a softer material wholly or partially confined between two harder and stiffer members of the assembly. The O-ring falls in the latter class.

The rubber seal should be considered as essentially an incompressible, viscous fluid having a very high surface tension. Whether by mechanical pressure from the surrounding structure or by pressure transmitted through hydraulic fluid, this extremely viscous fluid is forced to flow within the gland to produce “zero clearance” or block to the flow of the less viscous fluid being sealed. The rubber absorbs the stack-up of tolerances of the unit and its internal memory maintains the sealed condition. 

O-ring seated on piston.

Figure 1-4 illustrates the O-ring as installed, before the application of pressure. Note that the O-ring is mechanically squeezed out of round between the outer and inner members to close the fluid passage. The seal material under mechanical pressure extrudes into the micro fine grooves of the gland. 

Piston and O-ring installed 
in bore.

Figure 1-5 illustrates the application of fluid pressure on the O-ring. Note that the O-ring has been forced to flow up to, but not into, the narrow gap between the mating surfaces and in so doing, has gained greater area and force of sealing contact. 

O-ring sealing fluid toward 
the pressure source.