Back-up rings help prevent extrusion in high pressure service or compensate for loose fitting parts. The use of loose fitting parts makes for cost reduction in the machining of unit components.

The 90 durometer curve may be used to find the maximum recommended clearance at a given pressure for temperatures up to 74oC. In using these curves, it must be remembered that cylinders tend to expand when pressurized. The extrusion curves can be used in 2 different ways to reap the benefits of backup ring, depending on the application. In low pressure seals, the curves will indicate wider permissible clearances than those given in the design charts. Tolerance on these can be opened up until they permit these larger clearances to occur, resulting in substantial machining economies. When tolerances are increased, however, one must check to be sure that squeeze values cannot become too high or too low.

In high pressure applications, the curves will indicate whether adding a backup ring will permit the use of standard catalog gland dimensions or whether the clearance will need to reduced further.

In double acting seal assemblies, a backup ring is required on both sides of the O-ring. It is cheap insurance to use 2 backup rings even in single acting installations.

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Quad-rings are four lipped seals with a specially developed sealing profile. A wide range of elastomer materials for both standard and special applications allows practically all liquid and gaseous media to be sealed. The choice of a suitable material is determined by the temperature, pressure and media. They are vulcanized as a continuous ring. Quad-rings are supplied to the American Standard AS 568 A.

Quad ring has a double sealing function as compared to O-ring. The advantages of using quad-ring are:

• Avoid twisting in the groove.
• The seal does not tend to roll in the groove during reciprocating movement.
• Low friction. It requires less radial squeeze than a comparable O-ring.
• Less squeeze mean less friction, less wear, and thus a longer service life
• Very good sealing efficiency. Due to it special profile, a high sealing effect is achieved.
• A lubricant reservoir formed between the sealing lips improves start up

Quad-rings are self energizing double-acting sealing elements. The forces acting in radial or axial direction due to the installation give its initial leak-tightness. These forces are imposed by the system pressure. An overall sealing force is created which increases with increasing system pressure. When under pressure, the quad-ring behaves in a similar way to fluid with high surface tension and the pressure is transmitted uniformly to all sides.

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