How an O-Ring Functions

An O-Ring functions as a seal through the mechanical deformation of the elastomeric compound by mating metal surfaces. This creates a condition of "zero" clearance which blocks the liquid or gas being sealed. The pressure which causes the O-Ring to move is supplied by mechanical pressure or "Squeeze" generated by proper gland design, material selection, and by the system pressure transmitted by the fluid itself. 

Figure 1 shows an O-Ring IMPROPERLY used. As installed, it is not deformed but retains it s natural round shape. With the applicaiton of pressure as in Figure 2, the O-Ring is likely to deform (as part of its design characteristic) or open a leakage path.