Maximum efficiency under extreme operating conditions is critical for off-highway machinery. Downtime and malfunctions lead to reductions in profitability. Freudenberg Sealing Technologies developed the Cassette-S4 off-highway industrial seal to meet the demands of mobile machinery markets by enabling long-term reliability and power in harsh conditions with no speed, temperature or pressure limitations.
The Cassette-S4 features a three-component sealing configuration that protects all functional parts in a single component, reducing part complexity and accidental damage during installation. Robust to contamination and damage due to handling, the Cassette-S4 radial shaft seal is made up of multiple components.
“They're easier to install and to replace since the critical running surface and delicate oil lip is internal,” said Brett Randall, account manager for Freudenberg NOK-Sealing Solutions. With a traditional single element seal, which runs directly on the shaft, the responsibility has always been shared between the seal supplier and the shaft supplier. Since the finished shaft surface and the oil lip are inside the seal and self-contained, the whole sealing system is provided by the sealing supplier. When a cassette seal eventually needs replaced, the customer doesn’t have to consider resurfacing the shaft.
Freudenberg engineers developed the more robust seal to handle harsh environments but still allow for large equipment to reach the speeds required by end users. The company’s Cassette-S1, S2 and S3 for wheel hub and axle shaft sealing applications have been offered for decades. According to Randall, years of validation testing have shown that the S4 lasts up to four times longer than the S3 in head-to-head testing. “We've always had this balance between the competition of ‘I need more speed’ and ‘I need more robustness to contamination,’ so in certain applications this can have a direct impact on the service life of the seal,” he said.
Randall discussed the service life of a Cassette-S3 seal, an example being on the rear axle of an agricultural machine that was used to pull a belly scraper in an arid region of North America. The seal still functioned and was in great condition after more than 6,000 hours of use. “The only reason the seal had to be pulled was because the bearings needed to be reset. Meanwhile, the same S3 on a rear axle on a similar machine was used in a rice paddy. These seals rarely reached a thousand hours before needing to be replaced. Again, this highlights the real-world challenge: one solution that meets all the machine requirements such as high face speeds required by over-the-road transport, but at the same time seals that can also handle an extreme range of demanding environmental conditions,” he explained.
The S4 is not designed to be a drop-in replacement for mechanical face seals but rather an alternative. The metal face seal is still limited by the elastomer’s tolerance for heat, Randall noted. The metal faces generate heat-in to maintain a mechanical face seal’s robustness to contamination, so the faces need to be pushed together quite hard. “Conversely, when you need higher speed capability, that loading is backed off, which degrades their ability to protect against contamination from their environment,” he said.
A comparison test between the S4 and a comparably sized mechanical face seal replicated the real-world usage of a machine operating at low speeds (1 to 4 meters per second) and then traveling from one portion of the site to another or being used to transport to a whole other site entirely. The S4 begins to outperform mechanical face seals once the speed hits 1.5 mps.
Mechanical seals often require shims to reach the appropriate loading. Cassette seals can be installed with the seal being pressed into the housing, then the shaft inserted or pressed in with both the housing and the shaft already in place. According to Randall, the cross section of the mechanical face seal housing is a labyrinth that over time accumulates dirt and damages the seals. “For a cassette seal, the housing would be a simple rectangular cross section and the open layout would prevent dirt accumulation and allows for easier cleaning and washing,” he said.Continue reading »