Single-acting piston seals
A single-acting piston seal is designed for cylinders where pressure is applied from one side only. The piston in single-acting cylinders may have oil on the pressure side only, while the opposite side is open to atmosphere. Therefore, the piston seal must leave a minimum of oil film when passing along the cylinder bore since the transportation of oil otherwise would result in a leakage to the exterior.
In single-acting cylinders, the open end may push air out and draw air in as the piston reciprocates. This air may carry moisture and contaminants into the cylinder, which can lead to seal damage. Vent filters may be fitted to the open side of the cylinder to reduce contaminants entering the inside of the cylinder. The cylinder bore may be hard chromium plated to prevent corrosion.
In addition, to prevent damage to the cylinder bore or piston seals, SKF can supply special piston wiper seals on request. For additional information, contact SKF.
UNP profiles (fig. 1) are single-acting U-cup seals made of thermoplastic polyurethane (TPU). They are suitable for pressures up to 350 bar (5 075 psi) and are available in metric (→ Size list [pdf]) and inch sizes (→ Size list [pdf]). For additional information, refer to UNP profile data.
In case of higher pressures, SKF provides full-face anti-extrusion rings on request. For additional information, contact SKF.
Single-acting piston seals in double-acting cylinders
Two single-acting U-cup profile seals, facing in opposite directions, can be used in a doubleacting cylinder. It is important to select seal designs which can relieve reverse pressure for such arrangements to prevent build-up of pressure between the two seals. UNP profiles are suitable for such double-acting arrangements because the dynamic seal lip can flex to relieve reverse pressure.
Rod seals used as single-acting piston seals
Some rod seal profiles are designed with similar inside and outside sealing geometry and, therefore, can also be used as single-acting piston seals in single- or double-acting cylinders (fig. 2). PTB, STD and DZ rod seal profiles (→ Rod and buffer seals) can be used for those applications.
Rod seals with loaded-lip U-cup profiles may not relieve reverse pressure, but it is possible to remove their energizer (X-ring) from one of the seals to allow reverse pressure relief (fig. 2).