Only Kaco U-rings of the PNN …V series may be used for sealing hydraulic equipment. A description of the sealing lip of these U-rings was already given on pages 4 and 5.

The characteristic projection of material over the sealing edge displays considerable advantages for the design of the installation location and functioning of the U-ring. As the ring body projecting over the support shoulder lies free during the expansion of the internal sealing lip by the rod, the resulting radial forces can only find support in the area of the sealing edge and this results in the fact that the tension force is at its maximum at the sealing edge. As a result of this, especially in the case of highly viscous hydraulic oils or in the case of low starting temperatures, permeation of

the sealing lip contact surface by the highly viscous medium is avoided and good sealing is obtained. But also in normal applications, the necessary pressing of the seal is guaranteed by the sealing lip projection, even if in the course of time the characteristics of the elastomer material alter due to material fatigue and aging.

PNN …V type U-rings are installed without support ring (figure 27). As a result of this, the back can be kept narrower and axial space requirements are reduced. As a result of the reverse displaced sealing edge, the relatively wide front side of the material projection can find sufficient support when approaching the groove wall (e.g. in the case of excessive stick-friction).

This type of U-ring can be used not only for piston but also for a rod sealing with operating pressures up to 160 bar. Figure 28 shows the installation conditions of a piston and a rod seal. A comparison of figure 26 and figure 28 shows the differences found in the installation of U-rings with and without support ring.

The width of the recesses (groove width) in the machi-ned path for mounting of the U-ring must be1 mm larger than the U-ring height H.