The combination of seals, wipers, and guides in hydraulic cylinders is determined by the application in which the machines are used, the arrangement of the cylinders, and the prevailing operating conditions. In order to meet the requirements of applications running in extreme conditions in an economical manner, Simrit developed the new high-performance HDR-2C U-packing. This new seal combines two polyurethane materials and their respective properties in a single seal in a way that makes optimum use of the specific advantages of each material.
The development of this innovation would not have been possible without the help of modern simulation and analysis methods, which provided an accurate insight into all functional areas of a seal¾even under load. These methods showed that because of their functions, the demands made on the sealing material by sealing edges, which are exposed to static and dynamic loads, are completely different to those made by the groove base or when bridging gaps. The press fit (Fig. 1, section 1) prevents axial movement of the seal in the housing. For this, a very stiff material with low permanent set and excellent adhesion is required. Static tightness (Fig. 1, section 2) prevents leakages via the groove base and ensures pressure buildup. To this end, the sealing material must boast good elasticity and a high contact force pressure. It is important that dynamic sealing is guaranteed at the sealing edge (Fig. 1, section 3) even at low temperatures and that leakages are prevented via the moving rod. At the same time, however, this area must also ensure pressure buildup when exposed to transverse loads on the rod even after a lengthy downtime in low temperatures. The ideal material for this function boasts excellent dynamic properties, low relaxation at low temperatures, low friction, and high tightness. The gap-bridging zone (Fig. 1, section 4) must be extrusion-resistant and must prevent the seal from being pressed into the gap under high pressure. This requires a sealing material with a high level of stiffness and hardness.
Simrit has succeeded in combining the optimum materials for these different functional areas in a single, one-part, high-performance seal (Fig. 2): the HDR-2C U-packing.
The low-temperature polyurethane 92 AU 21100, which is capable of withstanding highly dynamic loads, is used at the static and dynamic sealing edge. In order to prevent gap extrusion and to ensure an excellent press fit in the groove base, the extremely extrusion-resistant polyurethane 98 AU 928 is used. In this way, the outstanding low-temperature flexibility of the first PU material is combined with the excellent extrusion stability of the second PU material. This significantly increases extrusion stability¾e.g. in hydraulic cylinders with end-of-stroke damping¾and means that the seal can be used over a broader temperature range at extremely low temperatures without impairing its extrusion stability at high temperatures. This targeted optimization of extrusion stability also allows for wider production tolerances for the guide and greater permissible gap widths. It goes without saying that the HDR-2C U-packing renders anti-extrusion rings (back-up rings) superfluous in many cases, thereby simplifying assembly.
The high-performance HDR-2C U-packing increases serviceability and optimizes the service life of aggregates, e.g. of excavators at temperatures between minus 40°C and 100°C for pressures of over 400 bar (to pressure peaks of up to 500 bar). This innovative seal also significantly extends the functionality of systems such as the hydraulic systems in fork lift trucks used in cold stores at temperatures as low as minus 40°C.
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