Basics of Design Engineering: Hydraulics New Sealing Systems Take the Heat

Joel Johnson Vice President,
Technology Simrit Div.
Freudenberg-NOK
Plymouth, Mich.

Edited by Kenneth Korane

Excessive pressures and temperatures, as well as fluid incompatibility, have always spelled trouble for hydraulic cylinder seals. And the problems promise to continue. Tighter emissions regulations for mobile equipment, for instance, are leading to a new generation of diesel engines that run hotter and will likely need high-temperature catalytic oxidizers. That will subject hydraulics to more heat.

The growing green movement has more machine operators turning to so-called environmentally friendly hydraulic fluids, but bio oils often aren’t so friendly to traditional seals. And to get more work out of smaller, lighter actuators, hydraulic-equipment manufacturers continue to push pressures ever higher.

Overall, new seal materials and designs are necessary to meet these increasingly tough requirements. Recent advancements should help fluid-power engineers design more robust cylinders with longer life and lower warranty costs.

Raising the bar
The good news is the next generation of mobile equipment should run cleaner, and probably use less fuel than today’s machines. The bad news is that design changes will, in turn, demand performance that exceeds the capabilities of most off-the-shelf sealing systems. These include:

Handle pressures to 42 MPa (6,000 psi) with short 55 MPa (8,000 psi) spikes.
Handle continuous 110 or 120°C system temperatures, as well as cold-weather extremes to –40°C.
Work with biodegradable and standard hydraulic fluids.
Resist hydrolysis and glycolosis.
Fit in existing standard grooves.

Higher temperatures are especially concerning as they affect a broad range of applications. Bench tests show that increasing system temperature by 10°C can decrease seal life by more than 75%. Most commercially available materials are only capable of 90°C continuous system temperatures (CST), with select blends reaching 100°C

For instance, top-of-the-line sealing systems often include a buffer seal, asymmetrical rod seal, and vented rod wiper. Our baseline systems use 100°C CST urethane seals (Material A in the accompanying charts) and have decades of proven field experience, but undesirable hydrolysis and biofluid resistance.

Here’s a look at several new urethane and elastomeric blends, how they stack up to current seals, and design trade-offs involved in real-life hydraulic systems.

New materials
Several years ago, we developed a proprietary urethane blend (Material B) that can withstand 110°C CST. As part of the development, the sealing system (buffer, rod seal, and wiper) was lab tested to 500 km (0.5 million cycles) at 32 MPa, 0.4m/sec, and 110°C without leakage. Actual field results correlate well with the test data. In this case the enhanced material provided similar life to our baseline urethane, but at higher temperatures. The urethane also resists hydrolysis and glycolosis.

However, urethane B does not work well at extremely high and low temperatures. This led to developments in newer urethanes (Material C) with better cold and high-temperature resistance. The trade-off is that Material C is more difficult to process and, therefore, only suitable for seals with thin cross sections.

To compensate for urethane’s processing limitations, developers turned to specially formulated elastomers for high-pressure applications. One advantage elastomers hold is they exhibit less compression set than do urethanes. Lower compression set improves residual interference and typically means longer life.

(Residual interference measures compression of the seal against the bore and shaft remaining after a test. In other words, it is the ability to fully rebound after being compressed for a period of time, and takes into account both wear and the physical state of the material. Because leak-free designs depend on material resiliency to ensure that they seal at low (or no) pressure, as well as having sufficient strength to prevent extrusion under high loads, residual interference is a strong indicator of remaining life.)

But elastomers require backup support to prevent extrusion. Using filled and reinforced-PTFE backup rings lets elastomers resist extrusion at pressures above 40 MPa.

Combinations of urethane and elastomer seals have been used in Asia for a decade. When applications demand low-temperature performance, a urethane B buffer with an NBR (Material E) rod seal — with a backup ring — meets all design goals up to 110°C.

To meet the 120°C requirement, substitute urethane C for the buffer seal and a special HNBR (Material D) for the rod seal. This is a hydrolysis and glycolosis- resistant option for standard and biohydraulic oils. The HNBR rod seal does require a backup ring to prevent extrusion. Based on our testing, this is the best sealing solution for long life at any temperature.

Field tests show that NBR-Material E systems can run at least 8,000 hr in excavators. And based on higher seal-residual interference, HBNR-Material D systems could last five times longer even at elevated temperatures. However, be aware that factors such as contamination, rod damage, and oil degradation greatly affect seal life in actual applications.

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