By Holger Jordan and Dr. Nils Kohlhase,
Trelleborg Sealing Solutions
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. 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.
Figure 1. Oil film thickness and contact pressure distribution.
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. Check valve functionality of the primary seal, showing it as depressurised (left), pressurized (middle), and pressure relieve (right).
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. Different positions of the check valve hole.
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.
Sealing reduces friction
Due to the intermediate pressure between the primary and secondary seal, friction and wear of the primary seal is reduced. Tests show that despite the higher friction of the pressure-loaded secondary seal, the friction of the sealing system in total is decreased. This leads to lower wear and longer operating time. The friction tests were done with the test rig shown in Figure 4. The intermediate pressure were varied in defined steps from 0 to 15 MPa while the friction was measured at different stroke speeds, as seen in Figure 5.
Figure 4. Friction tests were completed on a test rig.
Figure 5. Friction at different rates of pressure and speed.
A hydraulic spring cylinder was used to measure the friction at different system and intermediate pressures, Figure 6. The tests were done with the following parameters:
- System pressure: 7 / 12 / 15 MPa
- Intemediate pressure: 0 – 15 MPa
- Velocity: 2 mm/s
Figure 6. Friction measurement in a hydraulic spring cylinder.
With a sealing system of two identical PTFE seals, no significant impact of the friction at different intermediate pressures occurred. The friction only depends on the system pressure, Figure 7.
Figure 7. Friction of two similar seals depending on intermediate pressure and system pressure.
This means that the increasing load on the secondary seal with a direct related increase of friction force will be compensated by the improved lubrication and pressure balance of the primary seal. With an optimized choice of seal geometry and compound, the increase of friction in a sealing system due to intermediate pressure can not only be kept neutral but reduced. The secondary seal is designed for a low increase of friction depending on pressure load so that the benefit of friction and balance on the primary seal is bigger than the increase of the special designed secondary seal. Figure 8 shows that the friction decreases considerably at higher speed and at higher intermediate pressure.
Figure 8. Friction of optimized profiles depending on intermediate pressure and system pressure.
Lower wear means increased operating time
For the investigation of the wear behavior an endurance test rig was used, Figure 9. The following test parameters were used in the investigation:
|• Temperature||T|| |
|• Rod diameter||50||[mm]|
|• Surface coating||chrome|
|• Medium||HLP 46|
Figure 9. Endurance test rig.
Without intermediate pressure the seal profile made of filled PTFE material shows typical deformation and extrusion for these test conditions, Figure 10, unlike the direct comparable test with intermediate pressure that shows significant better results, Figure 11. Much lower wear and no extrusion at the primary seal leads to a significant increase of operating time. Because the intermediate pressure is limited to an allowed rate, the secondary seal also shows no wear.
Figure 10. Rod Seal as Single Element (no intermediate pressure).
Figure 11. Sealing system with defined intermediate pressure.
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 reliable functionality of a sealing system by far. The reduction of the friction as an additional feature also opens new possibilities for special demands. The choice of specialized elements in a sealing system enables sealing systems for much higher demands.