In hydraulic cylinders — used in everything from precise positioning equipment in machine tools to heavy earth moving machinery in the toughest working environments — the requirements for sealing are varied and difficult. The applications are dynamic, often operating at high speeds and significant pressures. A single seal is rarely a good enough solution and in most cases, a sealing system as a configuration of seals is required. For example, some solutions include U-cups with back pumping ability in combination with a double-acting scraper or step-seals in single or tandem configuration also in combination with a double-acting scraper.

One of the most important requirements for hydraulic seals is a long operating time without leakage. Seals with this property can reduce maintenance and repair costs. Without leakage they help to protect the environment. But performance requirements for hydraulic components increase continuously. Extended pressure, frequency, and speed rates lead standard sealing systems to a limit in terms of reliable performance. Additionally, more extreme temperatures and higher demands on chemical resistance complete the challenging demands for sealing systems.

In order to meet these requirements a new sealing system with a specific pressure between primary and secondary seal was developed.

Things to consider

One of the most important factors for the operating time of a sealing system is the lubrication condition in the contact zone between sealing element and counter surface.Figure 1 Furthermore, the possible deformation of a sealing element under pressure, time, and temperature is important. The oil film thickness at outstroke and return stroke are responsible for the lubrication of the contact zone and the leakage rate. The oil film thickness of hydraulic seals depend on the viscosity of the oil η, the rod speedv, and the contact pressure distribution in the contact zone a, Figure 1.

In order to prevent leakage on a standard seal, the oil film of the outstroke should be thinner than the film of the return stroke. The disadvantage then, is that the lubrication condition of the contact zone is poor. The idea is to develop a seal that creates a thicker oil film during outstroke that affects a limited pressure between primary and secondary seal and to use a primary seal with an integrated check valve that defines and limits the created pressure between the two seals.Figure 2

The improved lubrication conditions are realized by a modified contact pressure distribution that affects a thicker oil film on outstroke. The valve functionality is realized by a hole that is normally closed by the pretension O-ring, Figure 2.Figure 3

Interaction between the position of the pretension ring and the relief channel is essential for functionality. Different positions of the hole lead to different opening pressures, Figure 3.