Written by Joel Johnson, VP Technology, Ben Westbrook, Sr. Application Engineer Simrit Division of Freudenberg-NOK, and Noriyuki Matsui, Mgr., GI Design Section NOK Corporation
Excessive temperature and fluid incompatibility often tax hydraulic cylinder seals. New materials and designs are necessary to meet the increasing requirements of smaller cylinders with higher pressures and hotter ambient temperatures. The needed features include:
• Pressures to 6000 psi @ 0.5mm diametrical extrusion gap
• Continuous 110º C or 120º C temperatures
• Compatible with biodegradable and standard hydraulic fluids
• Hydrolysis and glycolosis resistant
• Retrofit in existing standard grooves
The changes to the temperature range are especially demanding. Bench testing has shown that increasing the system temperature by 10º C can decrease seal life by a factor of 5. For a benchmark for comparison, we used our best-in-class sealing system in Disogrin 9250 urethane. This system is reliable and proven, but has a temperature limit of 100º C and is not hydrolysis or biofluid resistant.
The proprietary urethane, NOK U641 -110 C, withstands temperatures to 110º C. With it, a sealing system can provide similar service to that provided by Disogrin 9250, but at an elevated temperature. It is also hydrolysis and glycolosis resistant.
NOK UH05 – 120 C Urethane improves cold-temperature resistance while increasing the high-temperature capability by an extra 10º C. The tradeoff is that it is more difficult to process, and therefore only suitable for the thinner cross section of the buffer seal.
Baseline or benchmark system, using Disogrin 9250 urethane for low-temperature sealing.
A complement to urethanes, specially formulated elastomers are suitable for use in pressure applications. Elastomers take less of a compression set than urethanes, but they require backup support to prevent extrusion at pressures above about 1700 psi.
G928 HNBR (rod seal) is a resistant material for 120º C systems with standard and biohydraulic oils. This sealing solution provides long life at high temperature.
Residual interference provides a forecast of remaining seal life. The G928 material, left, likely has a much longer life remaining than the U641 material, right.
The JIS standard groove sizes allow for a backup ring independent of the rod-seal material, whereas the North American and DIN standard groove sizes do not. This creates a problem with retrofit of new designs into existing grooves. The IUY design is an alternative as it integrates the backup into the seal.
NOK U641 is a hydrolysis and glycolosis resistant option for 110º C systems with standard and biohydraulic oils, provided 100% sealing at extreme cold temperature is not needed. If 100% sealing at extreme cold temperature is needed, A505 NBR can be used in combination with a backup ring for the rod seal.
NOK UH05 (buffer) in combination with G928 HNBR (rod seal) is a hydrolysis and glycolosis resistant material for 120º C systems with standard and biohydraulic oils. The HNBR rod seal does require a back up ring to prevent extrusion. Our tests indicate that this is the best sealing solution for long life at any temperature.
Simrit Division of Freudenberg-NOK
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