how an o-ring functions \ physical properties of o-ring compounds \ types of seals \ general design and gland dimensions \ standard o-ring sizes and dimensions \ compound information

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 application of pressure as in Figure 2, the O-Ring is likely to deform (as part of its design characteristic) or open a leakage path

Figure 3 shows the proper installation of an O-Ring seal. Notice that the clearance for the O-Ring is less than its free outer diameter, that the O-Ring cross-section is squeezed diametrically out-of-round even before the application of pressure. This ensures contact with the inner and outer walls of the passage under static conditions.

Figure 4 now shows the action of this properly installed O-Ring when pressure is applied. Since both the inner and outer walls are in firm contact with the O-Ring material, the pressure tends to force it along its groove. Engineered to deform, the rubber compound flows up to the passage, completely sealing it against leakage. The higher the pressure trying to leak past, the tighter the seal that is thus formed. Upon release of the pressure, the resiliency of the rubber compound results in the O-Ring returning to its natural round form, undamaged and ready for similar cycles.

By this fundamental explanation, the criteria of design are clearly visible. The initial "diametral squeeze" is vitally important. An initial "diametral squeeze" of 10% results in a flat sealing surface of about 40 to 45% of the initial cross-section area of the O-Ring AT ZERO PRESSURE. Thus, at zero or very low pressures, the natural resiliency of the rubber compound provides the seal. It follows that very low pressure sealing may be improved by increased "diametral squeeze" (but note that such increased squeeze may adversely affect dynamic sealing at higher pressures).

The "diametral squeeze" induces a frictional force between the O-Ring and the walls of the sealed passage that tend to hold the O-Ring in "neutral" position. Until the forces applied are sufficient to either overcome the frictional force or deform the rubber compound, the O-Ring will retain its initially deformed shape and will seal purely by diametral pressure.

The diametral squeeze applied to the constant volume of the O-Ring material will produce an increase in length of rubber across the groove. Expansion or swell of the rubber compound in the fluid or from heat will further increase this length of squeezed rubber. The groove must always be made sufficiently long to allow for the maximum expansion of the rubber, otherwise very high stresses will be set up on the installation. Normally the groove length will permit the O-ring to slide or to roll a small amount within the groove.

When breakout force is applied, the O-Ring will slide or roll in the direction of force applied until it contacts the end of the groove. From this point on, further pressure or force can only result in deformation of the O-Ring into tighter contact with the inner wall, the groove end and the outer wall.

The O-Ring will initially deform into a "D" shape. This normal deformation will increase the surface contact area to 70 to 80 percent of the initial cross-section. Thus, the contact area of sealing under pressure is roughly twice the area of contact of the original zero-pressure seal resulting from diametral squeeze. It will be apparent from the foregoing that the O-Ring will seal in either pressure direction.

Figure 5 shows the extreme case of deformation just before failure. Note that a small portion of the rubber material has been forced into the small clearance beyond the groove. Assuming the rubber has reached its limit of flow under pressure, further increase of force will result in failure by shear or extrusion as shown in Figure 6. The clearance allowed will bear a direct relation to the force causing failure.

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