Lubrication management for hydraulic applications
by Holger Jordan

The performance requirements of multiple sealing systems are continuously increasing. Friction, wear and service time are all key performance criteria. Lubrication management is one method of improving performance. Lubrication management is about adjusting the lubrication conditions of all sealing elements within a system so that the load on each element can be reduced and the performance can be optimized.

When engineering a sealing system, all elements—the seals and lubrication, or pressure fluid, in particular—must work together as a team. The pressure fluid can be seen as both a challenging element and a supporting team member. The pressure is the load and challenge while the fluid in the contact area between seal and counter surface is the supporting partner in reducing friction and wear.

The best practice in demanding fluid power sealing applications is a primary and a secondary seal. The primary seal does the sealing job for as long as possible, and the secondary seal is the redundancy. The challenge for this secondary sealing element is length of life under dry running conditions. The more effective the primary seal is at sealing in lubricant to the hydraulic system, the drier the running conditions of the secondary seal. Starvation of lubrication can lead to wear issues that come into play when the secondary seal is needed.

Lubrication Management

This difficulty of optimizing the performance of the primary and secondary seals brings us to the new discipline of lubrication management, where the risk of lubricant leakage is balanced with the need to ensure performance of the primary seal and the extended life of the secondary seal. This balance is achieved by allowing a better, thicker oil film under the first sealing element, which has a higher pressure load, to reduce the load on that seal. The fluid film volume transferring past the primary seal is then controlled—normally with an integrated check valve in the primary seal—to the contact area of the secondary seal to the counter surface.

The lubrication management principles have been tested to determine the optimum method of control in terms of wear inhibitors and lubrication type. For instance, in a test using a polyurethane buffer seal, compression set behavior was reduced by almost 50 percent simply with the addition of a wear inhibitor and partner lubrication. Overall, reduction ratios in friction of about 50 percent were achieved. Taking the entire sealing system into consideration, the reduction reached a maximum of 70 percent.

Because friction and wear are significantly influenced by lubrication management, the added value for the user is in improved wear resistance and performance of the secondary seal. This satisfies many of the demand-driven trends in the world of fluid power. These trends include reduced size and weight in hydraulic systems, leading to an elevation in pressure and increases in speeds of hydraulic applications so polyurethane seals reach material limits. In these situations, sealing systems can be improved through lubrication management.

Coatings

Another issue is the coating of counter surfaces in hydraulic applications. These are often decided for technical reasons without any consideration of the sealing system but can be a limiting factor for the service life of seals. Spray coatings, such as high velocity oxygen fuel (HVOF) coating, differ greatly from standard galvanic processes, such as hard chrome plating. The big difference in seal performance is related to the different surface topography of the coating technologies.

Roughness characteristics like embedded holes in the counter surface are a general challenge for sealing systems, as hole edges on the contact area can act like a knife, cutting the seal as it passes the holes, causing abrasive wear on the seals which rapidly wears them out.

The cost of modifying coatings or post processing to prevent wear can play a significant role in competitiveness on the international market. Implementing lubrication management on the sealing system can give more freedom in terms of seal performance as it relates to roughness specifications, potentially lowering total cost of hydraulic systems.

Conclusion

Teamwork is critical for optimizing the performance of sealing systems in fluid power applications. It is important to ensure the development of the most effective system in terms of operation, wear resistance and limitation of leakage. Users should work with a seal supplier that can bring a wealth of knowledge, test results, seal profiles and materials to each individual hydraulic system design, including an understanding of lubrication management.