In mobile and stationary hydraulic applications, rod seals are exposed to high pressure and friction for extended periods. To achieve a long service life seals must be wear and extrusion-resistant, compatible to aggressive media, have a wide operating temperature range, and demonstrate low friction capabilities. In addition, seals need compact installation dimensions and should ease assembly.
A new sealing design from Trelleborg Sealing Solutions, the Zurcon U-Cup RU9, meets all these criteria, and has proven in tests to out-perform standard U-cups. This seal is able to give longer service life in rod seal applications. It was tested alongside standard U-cups against all their main operating criteria, including leakage, friction forces, and level of lubricating oil and extrusion, with the last three measured over a range of pressures. In all of these tests and all pressures, the Zurcon U-cup RU9 performed significantly better than standard U-cups.
Zurcon U-cup RU9’s strong performance is partly due to one of its unique features. When the system pressure is higher, the contact surface between the seal and the piston rod becomes larger. The seal deforms to such an extent that its entire friction generating inside surface is in contact with the piston rod, improving the pressure distribution on the rod. The resulting tribological benefits (the effect of friction, lubrication, and wear of interacting surfaces) strengthen the friction characteristics of the seal.
Another contributing factor is the microstructure of the Zurcon U-cup RU9, where sliding surfaces in the seal are combines with back-pumping channels. On the forward stroke of the shaft an oil lubrication film is distributed under the seal. On the return stroke the oil is back-pumped into the system, preventing leakage. This extends seal life by reducing dynamic friction and breakaway force, even after prolonged periods of rest.
The Zurcon U-cup RU9 in manufactured from Trelleborg Sealing Solutions’ proprietary polyurethane material specially engineered for production of sealing elements. This material offers high wear resistance, low compression set, has a wide operating temperature range, and almost universal media compatibility.
- Article courtesy of Trelleborg Sealing Solutions America
Hercules Sealing Products’ own Sonny Goldsmith was a presenter at this year’s AIST (Association of Iron and Steel Technology) training seminar “Building the Future of Excellence Through Maintenance, Reliability and Lubrication” held in Charleston, SC, September 24-26, 2012. Sonny brought to the seminar over 30 years of experience in the fluid power industry. During the seminar, Sonny demonstrated the many Seal Compounds for use in fire-resistant fluids.
The AIST workshop-based training seminar provided over 150 attendees with hands-on instruction, tools and the best available technologies for mechanical, electrical, lubrication, hydraulics, management system maintenance and reliability solutions. Manufacturing reliability is an integral part of sustainability in the metals industry. Improvement in reliability is essential to assuring manufacturing results at the lowest cost. Both operations and maintenance personnel must understand the direction their organizations need to take with respect to improving and managing their equipment maintenance programs.
The conference was intended for maintenance, operations and engineering personnel. It was useful for individuals who are in middle management or frontline supervisor positions, as well as maintenance, operational and reliability personnel responsible for equipment reliability processes and techniques. Each attendee qualified for up to 12 Professional Development Hour (PDH) Credits during the course of the seminar.
To view other training seminars presented by Sonny Goldsmith, please visit the Hercules website at www.HerculesUS.com and click on the YouTube Channel icon found on the left-hand web panel.
Recently, the new snap-in type of wiper ring has been introduced to the industry. This new design is generally called a “slotted” heel wiper. Features include several axial slots arranged around the circumference of the wiper at the ID of the heel and is now offered in some form by all seal manufactures.
These slots were designed into the snap in wipers to help avoid “wiper extrusion” caused by a build-up of an excess of hydraulic fluid between the rod seal and wiper ring. The fluid build-up is created due to the heel of the rod wiper fitting too closely to the rod, essentially creating a partial seal condition. When an excessive amount of fluid builds up in this “cavity” (between the rod seal and wiper), it creates pressure and eventually causes the wiper ring to extrude out of the wiper ring groove.
This is most likely to occur during “cold start up” conditions when the hydraulic fluid may be thicker, the rod seal material is less flexible / less responsive to pressure changes and allows relatively large amounts of fluid to “leak” past the rod seal. This fluid then becomes “trapped” in the cavity between the wiper and rod seal and causes the wiper to extrude.
As the hydraulic component is operated and heat is generated in the hydraulic system, the heat softens the rod seal material which becomes more reactive to pressure changes and no longer allows the fluid to by-pass in the larger amounts which caused the wiper to extrude.
However, by the time this occurs the wiper may have already extruded causing the cylinder to be removed from the equipment for repair causing down-time.
By designing the wiper ring with the slots, excess fluid which may by-pass the rod seal on initial start-up is allowed to leak past the wiper ring temporarily preventing a build-up of pressure which may cause the wiper ring to extrude.
Any amount of fluid leakage should be avoided if possible. But the amount of leakage allowed by the slotted heel wipers prior to the rod seal regaining effective sealing is very minimal and may be a preferred option to having to immediately shut down the machine to have the cylinder repaired and loosing productive time.
Hercules Sealing Products is proud to announce the release of the Hercules Facebook, Twitter and LinkedIn Pages, along with the Hercules YouTube Channel and Blog.
While browsing the Hercules Facebook page, visitors can “Like Us” to take advantage of Fan Only Specials, view new products as they are released, take product and service surveys, and sign up to win Hercules 50th anniversary merchandise. Followers of the Hercules Twitter account will also have the benefits of Followers Only Specials, along with receiving up-to-the-minute product and event announcements. Guests to the LinkedIn page can connect and network with Hercules staff members from all departments. The Hercules YouTube Channel gives visitors access to the latest Hercules seal training webinars. The Hercules Blog gives visitors an opportunity to read the latest industry technical tips and product review articles. All pages can be accessed through the Hercules website at www.Herculesus.com.
Before we discuss the two styles of u-seal lips, let’s briefly look at the evolution of the u-seal and how different lip styles came about. The u-seal shape evolved from the vee ring. Initially the u-seal was a vee ring with a flat back or heel and all u-seals were symmetrical in shape. Gradually the shape evolved and instead of a sharp pointed lip design the “D” lip style was used on all u-seals to improve sealing effectiveness.
In the late 1950’s a major seal manufacture introduced the bevel lip or “B” lip design. The characteristics of the “B” lip and the new geometry of the u-seal created a more effective seal.
Loaded u-seals were introduced as a cost effective way to improve low pressure fluid leakage. When the “B” lip design is utilized in conjunction with an energizer (o-ring / quad shaped ring, etc.) in the u-seal, a very high radial loading is created at the seal lips which provides very effective sealing and a very dry surface. In rod sealing applications this is a very desirable situation. However, in double action piston sealing applications the effectiveness of the loaded “B” lip seal may actually create problems.
In piston sealing applications the primary purpose of the piston u-seal is to keep significant amounts of fluid from passing through the gap between the piston OD and bore of the cylinder. A slight amount of leakage past the seal lips is not a problem like in rod sealing. Due to the cycling of the piston and alternating high / low pressure cycles of the fluid, some leakage past the seal lip is normal and not a problem. However when loaded “B” lip seals are used in piston sealing applications, their sealing effectiveness may actually be too good. As fluid bypasses the sealing lip on the high pressure side of the piston it will accumulate between the two seals and should be able to “leak” past the seal on the low pressure side of the piston relieving the pressure build up between the two seals. But, if the “B” lip seal on the low pressure side of the piston seals too effectively, the pressure between the two seals begins to build up. As the pressure builds it causes each of the two seals to seal more effectively and trap more fluid increasing the pressure even more. As the pressure trapped between the two seals increases, it causes the seals to twist in the groove towards the outside edge of the piston (away from the center of the piston) until one of the seals eventually fails – pressure trap failure. This type of failure is typically indicated by one or both of the piston u-seals showing evidence of extrusion of the seal lip towards the edge of the piston (away from the center of the piston) as well as evidence of a twisting of the seal cross section.
Typically pressure trap is more likely to occur in applications where there is a very rapid cycling of the cylinders.
There are several ways to avoid this situation. One way would be to use non-loaded u-seals. Two unloaded seals could be used or just one unloaded seal and one loaded seal. If using two loaded seals, using a “D” lip seal will lessen the sealing effectiveness of the seal and reduce the possibility of pressure trap between the two seals. Since the “D” lip seals less effectively it is less likely to trap pressure. “D” lip seals were specifically designed by seal manufactures to be used in double action piston applications to avoid the problem of pressure trap. However, if only “B” lip seals are available, they may be used by removing the energizer from one of the two seals. This reduces the low pressure sealing effectiveness of the seal which has the energizer removed (thus allowing the pressure between the two seals to by-pass the seal to the low pressure side of the piston) but has no effect on seal performance at high pressure.
Using unloaded u-seals, “D” lip seals, or “B” lip seals with the energizer removed offers no guarantee of avoiding pressure trap. But it is less likely to occur if these steps are taken. The only way to avoid pressure trap is to use a piston designed for a single piston seal.
Sonny Goldsmith, Territory Manager
Hercules Sealing Products takes its role in the Seal and Cylinder industry extremely seriously. To this end, Hercules periodically prepares online seminars, or Webinars, for training. Until now it was only possible to get copies of past webinars by ordering a cd, but in the interest of broader distribution Hercules is making the series available on YouTube. Currently there is an informative ‘Hercules Webinar Series’ of four video clips on hydraulic cylinder seal theory, application, performance and failure. To search for the series on YouTube, simply type the words “Hercules Webinar” in the search box.
The educational series has been compiled with the aid of special instructors and make for a comprehensive guide for those who wish to train their staff in hydraulic design and seal theory.
Hercules Sealing Products is an ISO 9001 registered distributor of hydraulic and pneumatic seals, seal kits and cylinders for mobile and industrial applications. The company employs more than 150 people. Their distribution warehouses are spread over 100,000 square feet, and usually house some 35,000 parts.
Apart from being one of the top distributors of cylinders, seals and seal kits, it is also a leading distributor of a wide range of assorted industrial and mobile applications, such as mobile cylinder heads, repair accessories and kits, leak detection products, dismantling and removal tools, motor and valve repair kits, and similar products. The company boasts an impressive stock of seals and cylinders for well known brands of industrial and construction vehicles such as Caterpillar, J.I. Case, John Deere, Komatsu, Nissan, and Toyota, to name a few.
The Hercules Webinars are still available on CD. Send your request to HelpDesk@herculesus.com to receive your free copy.