U-seals (also u-cups or u-packings) are used in hydraulic systems where they fulfil a number of functions. The u-seal must hold hydraulic fluid and must not allow any leaks; u-cups must prevent penetration of solid particles and fluids and it must also maintain the correct pressure in the hydraulic system. To allow the u-seal to fulfil all the aforementioned functions, it is necessary to select the right type of hydraulic sealing ring.
When sealing a hydraulic piston using a u-seal (u-cup), the sealing ring must keep its maximum efficiency for the longest possible period of functioning of the hydraulic system. In addition, the u-seal must withstand the operating conditions that occur in the seals of the hydraulic system. When selecting the right hydraulic ring one must consider whether the piston is single- or double-acting. For a single-acting piston a sealing ring is required that provides optimal sealing capacity with the thinnest possible lubricating film remaining between the u-seal and the cylinder wall. For a double-acting piston a two-side hydraulic seal is required. Using two single-acting u-seals may result in a build-up of excessive pressure in the space between hydraulic seals.