Data Store Buffer seal

The test-rig consisted of a pressurized test cell, a piston rod, an electrical drive and a hydraulic pressurizer. The hydraulic oil had a viscosity class of 46. An additional pressure sensor was installed between buffer and primary rod seals to investigate possible intermediate pressures.

Test Protocols

During verification testing, five buffer seals were examined:

1. Lip seal with a round sealing edge and no back-up ring.
2. Lip seal with friction adjusted for heavy duty applications made of polyurethane with a thermoplastic back-up ring.
3. Lip seal made of polyurethane with a thermoplastic back-up ring with high sealability for medium to heavy duty applications.
4. Glide ring seal with an O-ring energizer for light- to medium duty applications, made of PTFE 40% bronze with an NBR O-ring.
5. Glide ring seal with an O-ring energizer for light- to medium-duty applications, made of hard grade polyurethane and an NBR O-ring.

The following performance criteria were considered and analyzed during or after testing:

The seals’ friction force levels and bands

The friction level and band should be minimized to improve long-term performance. High friction levels increase temperatures, which shortens seal life. The height of the friction band indicates how well the primary rod seal is lubricated.

This graph above shows the ratio of intermediate pressure to system pressure over distance and time. If the ratio is above 60%, the buffer seal will not absorb pressure spikes

Intermediate pressure between buffer and primary rod seal

Buffer seals must prevent intermediate pressure build-up to protect the primary rod seal. In heavy-duty applications, pressure spikes can exceed 600 bar and should be handled by the buffer seal. The primary rod seal is often a U-cup made of polyurethane; it handles pressures up to 400 bar. Therefore, the overall distance where intermediate pressure is higher than 60% of the system pressure and the time to build up intermediate pressure were analyzed.

Visual analysis of buffer and primary rod seal before and after testing

Our team looked at wear and extrusion on the seal, as well as discoloration. The before-and-after comparison provides direct evidence of seal performance. For example, if the buffer seal is not working properly and there’s evidence of leaks, the primary rod seal will show high extrusion due to intermediate pressure. And if there’s discoloration at the sealing lip of the primary rod seal, the buffer seal was too tight and did not allow enough lubrication to get to the primary rod seal.

Leaks

Generally, oil that escapes from the hydraulic cylinder into the environment must be kept on a minimum level. Recall that the chances for leaks increases as the ratio of outstroke to instroke piston-rod velocities increases. In addition, the primary rod seal needs good back-pumping ability. This is important for the buffer and primary rod seals.
The lubrication level determines the friction level. But the lubrication level also determines the number of leaks. The lubrication level is influenced by instroke and outstroke piston rod velocity, as well as seal design and hydraulic oil used.

The tests were done in four phases:

• Running-in phase: to set up constant starting conditions for each seal to guarantee correct comparisons.
• Pre-performance phase: to evaluate characteristics such as absorbing pressure spikes and time to build up intermediate pressure.
• Long-term phase: to evaluate characteristics such as wear and extrusion resistance.
• Post-performance phase: to evaluate characteristics as the seals age.

The testing parameters include several different pressure and piston rod velocity cycles, with pressures and piston rod velocities set so that pressure was low when speed was high and vice-versa. This mimics an operating excavator; the high loads are mostly at low operating speeds, and the higher speeds go with low loading.

Test Results

Lip seal without back-up ring

The graph above shows the system and intermediate pressure curve of seal #11 at the pre-performance test. It is clearly visible that the intermediate pressure increases immediately. Over 96% of the entire testing distance, intermediate pressure equaled the system pressure which means no buffering capability is available.
After the running-in phase, the first day of testing started with constant in- and outstroke piston rod velocity. After about a kilometer of motion, intermediate pressure equals system pressure. After a full system shutdown overnight, the second day started with twice the outstroke piston rod velocity. This caused a quick build-up of intermediate pressure to system pressure, which system pressure and pressure spikes must be handled by the primary rod seal. In this case, the pre-performance test was extended but it was not possible to decrease the intermediate pressure.

The above images show damage to the seal’s profiles. The buffer seal didn’t show any major damage. But the primary rod seal already shows some wear and extrusion after a testing distance of 90 km. The seal did no work as expected.

4 and 5. O-ring energized glide ring seals

The above figure graphs chart friction level of glide ring seal 4 and 5 at pre- and post-performance test as well as profile cut analysis after the whole verification test.
Seal 4 demonstrates little friction. The PTFE element shows extrusion in both directions because it sealed in both directions and did not vent intermediate pressure as desired. As a consequence, the primary rod seal showed extrusion, which confirms that glide ring seal 4 was ineffective in reducing pressure spikes.
Glide ring seal 5 has slightly more friction than 4, but still an acceptable good level. The decrease in friction after the test is lower compared to glide ring seal 4 because of increased wear and extrusion resistance of the hard grade polyurethane glide ring.

The graphs above show intermediate pressure at 50-bar of backstroke system pressure during the long-term test for both 4 and 5 glide ring seals. Seal 4 immediately built-up intermediate pressure to a multiple system pressure level, the result of no venting capability. Seal 5 shows no to low intermediate pressure built-up at several pressure stages between 50 and 400 bar.

Here’s a comparison of leaks with Seals 4 and 5. The good performance of Seal 5 improves the performance of the entire rod seal package; it had far fewer problems with leaks than Seal 4.
The results clearly demonstrate that it is very important to consider the operating parameters when selecting the buffer seal. Lip-type buffers with thermoplastic back-up rings can handle high pressures, while O-ring energized glide ring seals have limitations in extrusion resistance, especially in Bronze filled PTFE (which showed unacceptable extrusion). Consequently, these seals should only be used in medium-duty applications. Lip type buffers without back-up rings and round sealing edge didn’t work as expected and show no improvement compared to systems without an additional buffer seal.
Future evaluations of rod seal packages, including buffer, primary rod seal and wiper, are already underway. The goal is to find rod seal packages for applications from light- to heavy-duty that guarantee longest service life of the entire sealing system.

This document was taken from:

Thomas Schwarz is manager of testing, materials technologies & research; Wolfgang Swete is manager of product technology and development; Silvio Schreymayer, is manager of testing; Martin Wallner is the manager or product development; Emmanuel Pichlmaier is a product development engineer; and Michael Liebminger is a testing engineer for SKF Seals.