Figure 4Due 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 5A 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 6With 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

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.

Figure 8

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.

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:Figure 9

• Pressure p 30 [MPa]
• Temperature T

60

[°C]
• Stroke s 300 [mm]
• Endurance t 250,000 [cycle]
• Rod diameter   50 [mm]
• Surface coating   chrome  
• Medium   HLP 46

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.Figure 10

Because the intermediate pressure is limited to an allowed rate, the secondary seal also shows 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 reliable functionality of a sealing system by far. Figure 11bThe 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.

For more information, contact Holger Jordan, Manager Innovation Management, R&D Europe, Trelleborg Sealing Solutins, at holger.jordan@trelleborg.com or visit www.tss.trelleborg.com.