U-ring
U-rings (also: rod seal or piston seal ) serve as an axial sealing element between the axis and the housing, which can perform a rotational or translational movement , such as. B. in hydraulic or pneumatic cylinders . They are therefore part of the dynamic seals . As a rule, they are made of polytetrafluoroethylene and should be greased before installation to improve their service life .
Classification
Depending on their function, U-rings are divided into internal and external seals. This refers to the side of the sealing lip that rests on the moving surface. In the case of symmetrical U-rings, the manufacturer's instructions must be observed. Outer sealing grooved rings, however, enable cheaper production , since both the recess in the axis and the smooth surface of the bore are not a major challenge. Manufacturers usually divide their products according to their design, as different properties can be achieved with different cross-sectional shapes, wall thicknesses and angles. Some properties are: friction , permissible pressure , installation space , operating temperature and chemical resistance . This is achieved, among other things, with different materials and additional spring elements that change the contact pressure.
function
The function of all U-rings is limited to the axial seal between two moving (cylindrical) components. They are placed in a rectangular groove so that only the sealing lip protrudes from them. This presses with a slight bias against the sealing surface of the counterpart. Due to the V-opening of the cross-section , the U-ring is expanded further with increasing pressure , which further improves the sealing properties. Therefore, less friction can be achieved with smaller pressure differences . Since the seal is significantly impaired by dirt particles, wipers are often installed. Some designs even contain their own wiper on the opposite side of the sealing lip.
Installation
The following should be observed during installation:
- correct alignment (sealing lip towards pressure side)
- Sufficiently greased
- Check sealing surfaces for damage
- Eliminate dirt particles
- If an adhesive is used, the compatibility should be checked (chemical resistance)
A thin hook or a rope can be used as a tool to put externally sealing groove rings over the shoulder of the shaft by hand. The dismantling is usually destructive.
literature
- Karl F. Berger and Sandra Kiefer: Seal Technology Yearbook 2018. ISBN 978-3-946260-01-1 .