In hydraulic and pneumatic applications standardized sealing technology generally includes the use of oil-resistant rubbers mostly on a nitrile-butadiene rubber (NBR) basis. In mechanically stressed seals rubbers on a polyurethane (PU) basis are preferred. For brake glycol-based fluids rubbers made of styrene butadiene (SBR) are used.
Before the installation of hydraulic or pneumatic seals in spaces under construction, it is necessary to remove all undesirable residues and impurities from the whole system. To improve friction force of the u-cup, we recommend a thin layer of lubricant, which is not harmful to the sealing ring rubber and the working media, be applied onto the u-seal. U-seals and other rubber seals must not be pulled over sharp edges, threads etc. In cases where there is a risk of damage to hydraulic seals, use the respective fixtures for the u-seal installation.
Installation of parts containing u-seals must be performed without shocks. We recommend that at least three travels be completed to check the sealing ring installation. The movement of mutually sealed parts in machines should be fluent. The working media should not leak through the u-seal (u-cup) at any time.
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. Based on the method of application, the main types of u-seals include:
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.
If a rod is sealed using a hydraulic seal (or a pneumatic seal), the rod surface may become worn. Wearing will apply higher stress to the u-seal, which will result in the reduced service life of the hydraulic seal. Moreover, the sealing ring must withstand changing temperatures and maintain its sealing function when high or low pressure is applied to the u-seal. For this reason it is necessary to select the u-seal material that fulfils the conditions required for the operation of the sealing ring.
Hydraulic wiper rings prevent penetration of impurities into the hydraulic cylinder. This type of u-seal is called a wiper ring because it wipes off impurities from the outer side of the piston rod. Application of the wiper ring should not be underestimated as the correct function of this u-seal protects the hydraulic cylinder as a whole as well as other u-seals and sealing elements (seals) contained in the cylinder.
• Carrier rings (u-seals)
This type of u-seal is made of nitrile-butadiene (NBR) and is used for pneumatic applications (pneumatic seal). The u-seal withstands temperatures from -25 °C to +80 °C. The sealing ring may be exposed to pressures of up to 1.2 MPa.