Expressed as both a decimal measurement (in inches and/or millimeters) and as a percentage of the original O-ring cross-section (width), squeeze is compression of the O-ring’s cross-section between mating surfaces. For example, an O-ring with an original cross-section of .040" (1.02 mm) that is squeezed .007" (.18 mm) has been compressed approximately 16%. Likewise, a .275" (7 mm) O-ring that is squeezed .035" (.89 mm) has been compressed approximately 13%.

There are two main types of squeeze for static O-ring seals: radial and axial. Radial compression occurs on an O-ring’s O.D. and I.D., as with cap and plug type configurations (see Figure 73). Axial compression occurs on the top and bottom surfaces of the O-ring, as with face (flange) type designs (see Figure 74).

Because of the nature of their installation and movement, dynamic seals (either reciprocating, rotary, or oscillating) almost always employ only radial squeeze, though there might be rare instances (as with a face seal involving rotary motion) in which axial squeeze is used.

Squeeze depends on three variables: the amount of compressive force applied to a seal (as measured in pounds per linear inch, or pli), the hardness of the seal (its resistance to compression, as typified by a durometer reading), and the cross-section of the seal. As previously noted, the cross-section is reduced and flattened when the O-ring is stretched. A seal under a high degree of stretch will typically require a greater amount of squeeze (or, alternatively, a gland with less depth) in order to maintain the proper amount of contact between the seal and the mating surface.

As might be guessed, the proper amount of O-ring squeeze differs from application to application. Most static seals should never be squeezed more than 30%. Because of friction and wear considerations, the maximum recommended squeeze for most dynamic seals is only 16%. These percentages may vary, however, depending on factors such as the size of the O-ring and the temperatures to which the seal will be exposed. For example, a smaller O-ring or an O-ring that won’t have to withstand higher temperatures can function effectively under greater squeeze. The necessary amount of O-ring squeeze can also fluctuate within a given application if O-rings made of differing compounds (with varying hardnesses) or having different inside diameters are used interchangeably. No matter what the size or amount of stretch, all O-rings must be squeezed at least 0.007" before the adequacy of the seal can be accurately determined.

Keep in mind that the amount of squeeze being employed affects a seal’s susceptibility to gas permeation. As squeeze increases, permeability decreases. This is true for a couple of reasons. First, more squeeze translates to less groove depth, meaning less area available for gas to enter initially. Second, an O-ring under increased squeeze is wider, meaning gas must travel further (i.e. through a greater length of material) before reaching the low pressure side to escape.

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