The effort to reduce maintenance continues
The demand for components that last longer and are easier to maintain is never-ending. And wheel seals are no exception. But are fleets willing to pay for them?
One example is the unitized hub assembly, perhaps the most significant change in wheel seals. As Meritor points out, this unit helps eliminate the problem of improperly adjusted wheel ends, the biggest enemy of wheel seals. In addition, it has the advantage of being virtually maintenance-free. In spite of these advantages, however, only about 10% of new vehicle purchases are spec'd with this option, according to Federal-Mogul, with expense cited as the primary stumbling block.
The industry is still struggling with the tradeoff between the pressure for low-maintenance components and the drawbacks that can accompany them. Wheel-end lubrication is a case in point. Although 90-weight gear oil is still the industry standard, there's been a trend toward the use of semi-fluid grease by some fleets - particularly on trailer wheel ends - because it's less likely to leak than oil.
Meritor contends, however, that grease is not as good a lubricant. Oil does a much better job of dissipating heat, and works better than grease in extremely cold weather. Fleets have to weigh the maintenance advantages of grease against its performance disadvantages.
According to Freudenberg-NOK, some manufacturers tried using a No. 2 (solid) grease for wheel-end lubrication, but didn't have the best results. Semi-fluid grease is a reasonable alternative because the warning signals that are so crucial when oil is leaking will still be there.
The trend toward outboard brake drums has added to wheel-seal life since brake work can be done without removing the entire wheel end. But as Meritor points out, there is a downside: wheel ends don't get serviced as regularly as they should.
In terms of the seals themselves, considerable strides have been made in developing premium products - for those willing to make the extra investment.
Chicago Rawhide's Scotseal Longlife is a premium performance seal based on the one-piece, unitized design of the original Scotseal. To address the issue of improper installation, which Chicago Rawhide says is responsible for as many as 90% of premature seal failures, the company developed the Scotseal Plus, which can be installed by hand without the use of any special tools. The seal's four-lip design includes Waveseal, which uses a pumping action to run cool and keep the bearing properly lubed.
Stemco's high-performance, long-life Discover seal is one of the company's two unitized, hub-installed products. While the Discover was designed primarily for longhaul carriers, the Voyager for regional-route and urban P&D applications. Both products have Stemco's "smart" anti-rotation feature to ensure that the ID surface is positioned accurately. Double steel case construction provides strength and helps avoid damage during installation, which requires only a single tool.
To meet performance demands of low-maintenance wheel end systems, Freudenberg-NOK developed the Outrunner, a premium oil bath seal. The lip of the seal can withstand the higher temperatures generated by longer hauls and heavier loads; bi-directional helixes are used to continuously pump oil away from the primary sealing lip; andprelube grease protects against dry start wear. Although the unitized seal's outer and inner diameters of are made of rubber to minimize installation problems, a special adapter plate is available to hold the seal securely during this process.
Federal-Mogul developed the National Gold oil bath seal in response to research indicating that levels of wheel end movement were higher than expected, and in excess of what most seals were designed to handle. A "lay down" bi-directional lip absorbs wheel end movement in multiple directions. The seal can withstand both extreme low and high temperatures, and can be used with mineral and synthetic oils. In addition, four exclusion lips provide protection against outside contamination.
Meritor Automotive has also developed a hub-installed, unitized version of its premium MVP oil bath wheel seal. Leaks caused by faulty installation are eliminated because the one-piece seal is s part of a unitized wheel-end system and doesn't have to be installed separately. The wear and tear on seals that results from high brake temperatures is minimized through use of a graphite-loaded rubber compound. Large vent holes also help protect the seal radial lip from brake heat by enabling lube to flow freely to and from the seal.