Introduction
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 a sealing system with a configuration of seals is often required. For example, proven solutions to today's hydraulic applications include U-Cups with back pumping ability in combination with a double-acting scraper (single configuration) as well as Stepseals® in Tandem configuration.
One of the most important requirements for hydraulic seals is a long operating time without leakage. No leakage means reduced maintenance and repair costs and protection for the environment. Unfortunately, extended pressure, frequency and speed rates limit the reliable performance of standard sealing systems. Extreme temperatures and high demands on chemical resistance are also part of the challenging demands for sealing systems.
To meet these requirements, a new sealing system with a specific pressure between the primary and secondary seal was developed. For a tandem sealing system, a new primary PTFE seal was developed with a modified contact pressure distribution in the contact zone. This design causes a thicker oil film on outstroke and an interseal pressure between the primary and secondary seal. To relieve the interseal pressure, the primary seal is equipped with an integrated check valve. Under these circumstances, the differential pressure between the system and trapped pressure is loading the primary seal. The load of the primary seal is directly reduced, and the seal lubrication is optimized. Lower wear and friction are the results.
Characteristics
One of the most important factors for the operating time of a sealing system is the lubrication condition in the contact zone between the sealing element and counter surface. The possible deformation of a sealing element under pressure, time and temperature is also important. The oil film thickness at outstroke and return stroke is responsible for the lubrication of the contact zone and the leakage rate. The oil film thickness of hydraulic seals depends on the viscosity of the oil h, the rod speed n and the contact pressure distribution in the contact zone (Figure 1).
To prevent leakage at a standard seal, the oil film of the outstroke should be thinner than the film of the return stroke. Due to this distribution, the lubrication of the contact zone at outstroke is poorer than at return stroke. Improved lubrication conditions cause lower wear and friction as well as longer operating time. The new sealing system achieved these improved conditions. A primary PTFE seal with a modified contact pressure distribution effects a thicker oil film on outstroke and causes interseal pressure between the primary and secondary seal. To relieve the interseal pressure, the primary seal is equipped with an integrated check valve. The valve functionality is realized by a hole that is normally closed by the pretension O-Ring (see Figure 2).
The 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 (see Figure 3).
Friction
Due to the intermediate pressure between the primary and secondary seal, the friction and the wear of the primary seal is reduced. The friction tests were done with a test rig. The interseal pressure was varied in defined steps from 0- to 15-MPa. Figure 5 shows a significant friction decrease at higher interseal pressure.
To measure the friction at different system and interseal pressures, a hydraulic spring cylinder was used.
The tests were done with the following parameters:
With a sealing system of the new primary seal combined with a U-Cup, friction depends on the interseal pressure (see Figure 7).
Consequently, the improved lubrication and pressure balance of the primary seal will compensate for the increasing load on the secondary seal with a direct related increase of friction force. With an optimized choice of geometry and compound of the secondary seal, the increase of friction in a sealing system due to intermediate pressure is reduced. The secondary seal is designed for a low increase of friction depending on pressure load, so the benefit of friction and balance on the primary seal is greater than the increase of the specially designed secondary seal. Figure 8 shows that the friction decreases considerably at higher speed and at higher intermediate pressure.
Wear, Extrusion and Operating Time
An endurance test rig was used to investigate wear behavior.
The test parameters were as follows:
Without intermediate pressure, the seal profile made of filled PTFE material showed typical deformation and extrusion for these test conditions.
Unlike the direct comparable test, adding intermediate pressure showed significantly better results. Much lower wear and no extrusion at the primary seal leads to significantly longer operating time. Because the interseal pressure is limited to an allowed rate, the secondary seal also showed no wear.
Conclusion
A sealing system with a pre-lubricated primary seal and integrated check valve in combination with an optimized secondary seal can improve the service time and the reliable functionality of a sealing system. The reduction of the friction as an additional feature opens new possibilities for special demands. The choice of specialized elements in a sealing system enables sealing systems for much higher demands.