Dynamic sealing applications often call for pressure-energized seals. In the absence of system pressure, seals such as pure lip seals have only the light load of deflected lips to provide a sealing line. They have a definite tendency to seep at low pressures. As system pressure increases, the flexible lips are compressed against shaft and walls, increasing sealing effectiveness. The advantage is that the relatively small area in contact with the dynamic surface produces little friction, giving long life and good sealing at high speeds.
U-cups are suitable for applications to 5,000 psi. As pressure increases, harder elastomers and backup rings should be used. These seals are available in a number of shapes.
Asymmetrical U-cups are pressure-activated lip seals with different lip profiles for sealing dynamic and static surfaces. They are usually not adequate for low-pressure sealing. Symmetrical U-cups, while not as effective, are widely used because of their lower cost, and can be used as either a piston or rod seal.
For rubber U-cups, the metal support ring sometimes used in gland installations should not exert undue pressure on the packing since it retracts the lips and spreads the heel. The base of the supporting ring should be designed to properly center the ring. Internal threads should be offset to protect seal lips.
V-rings are installed in sets, each consisting of a number of rings, a male adapter, and a female adapter. The number of rings in a set depends on pressure and the ring material.
The function of the male and female adapters is to support the rings. The female adapter is the more critical and should have the same included angle as the rings (usually 90°). The male adapter should have the same angle as the female.
A small amount of leakage must be allowed to provide a lubrication film between the metal and rubber of the furthest sealing lips. Otherwise, the seal will "run dry" and be destroyed by heat buildup. For zero low-pressure leakage, a urethane vee should be inserted as the last (outer) member of the vee stack.
Cup seals are unbalanced, and seal only on the ID -- usually at the heel, or shoulder. Cups are used in large volume for hydraulic and pneumatic service and are available in leather, solid rubber, and fabric-reinforced rubber. Special formulations are available for difficult applications.
Cups used for low and variable pressures often need spring expanders that preload the cup walls. With reasonably high pressures, expanders are not needed.
One installation problem encountered with cups is excessive tightening of the inside follower. Excessive tightening crushes the bottom of the cup and causes poor sealing, high friction, and excessive wear. Some cups are made with extremely hard bottoms to combat excessive tightening.
Flange and collar seals are variations on the cup-seal design. They have developed a possibly unwarranted reputation for excessive wear and friction due to unbalanced operation. Flange and collar seals are widely used for sealing on shafts or pistons at moderate pressures. Because they are compact, they are typically used where there is insufficient space for a U-ring. They are generally considered to seal less efficiently than U-rings or V-rings.
Collar rings are very similar to flange rings, except for a slightly modified shape. Claimed advantage of the collar shape includes improved resistance to wear and scuffing.