By John Smith
Magnesium chloride looked like a miracle cure for highway crews when it emerged as a de-icing option in the 1990s. It could be sprayed onto the surface of a road before freezing rain began to fall, ensuring that ice never had the opportunity to form. By reducing the amount of conventional road salt, transportation departments were even helping to protect nearby fields and greenery in the process.
Brakes were hardly that lucky.
This “miracle” de-icing compound also will seep into the smallest crevices and coat every piece of exposed metal such as the brake shoe tables found under a lining block. Unlike road salt, it will continue to suck moisture out of the air, even after a truck leaves the coated roads of the Northeast for the supposed shelter of a dry Texas highway.
Then the corrosion begins.
“Rust jacking does not instantaneously appear. It comes in stages,” Bendix Commercial Vehicle Systems engineer John Hawker explained this year during a presentation to the Technology & Maintenance Council (TMC).
Initially the problem will emerge as hairline cracks in the friction material – usually small enough to escape the attention of a roadside inspector. These eventually evolve into cracks that would require a lining replacement. As the oxidation builds up, the lining will begin to separate at the rivets before it completely comes off the brake shoe table, ultimately buckling around the rivets and causing cracks that run diagonally between the rivet holes.
It is the type of technical challenge that grabs TMC’s attention. In fact, rust jacking is expected to be a prominent issue during the February 2009 meetings. The Society of Automotive Engineers and federal government are studying the issue as well.
ArvinMeritor’s existing research into lined brake shoe cores already suggests that some wheel positions may be more prone to rust jacking than others.
“Steer axles tend to be hit the worst, and their incidence can be up around four to five percent,” says Joe Kay, ArvinMeritor Commercial Vehicle System’s engineering manager-foundation brakes, North America. In contrast, about two percent of drive axle brakes and three to four percent of trailer brakes were affected.
In each case, there was still 1⁄8 inch of usable brake lining above the rivet heads. But the lining had lifted off the shoe, causing a gap between the brake shoe table and the bottom of the lining block, and telltale cracks were found at the edge of the lining blocks and near the rivets.
“Our leading theory is the fact the steer axles in North America typically run the coolest. They’re not braked really heavily,” Kay suggests. Aggressive braking may generate heat that can evaporate most of the moisture around the brakes, but brakes on the steer axle will seldom reach 150° F. Then there’s the fact that brakes at the front of the vehicle are on the wheels that plow through the slop on the road.
But, not everyone agrees. Bendix aftermarket brake product manager Carl Swanson suggests that trailer brakes are more prone to the problem, largely because they are in service for extended periods of time.
Regardless, Hawker offered the TMC crowd some disappointing news. “We are not in a position to eliminate rust jacking,” he said. “There is no magic wand that we can [use to] get rid of it.”
Some anti-corrosion strategies may even cause more problems than they solve. One fleet thought it was taking every possible step to control the issue. It was buying premium shoes with brake shoe tables that were protected by an epoxy coating. Every chassis was regularly power washed.
The problem came in the power washing. The jets of water were also collecting magnesium chloride from the surface of the parking lot, and driving the rust-promoting material into crevices and under the lining material, Hawker said.
But as challenging as the issue can be, manufacturers have been responding with products to help lessen the impact of rust jacking.
“The place we’re focusing on is the metal shoe itself, and getting the corrosion protection on that,” Kay says, referring to electrostatic painting methods. ArvinMeritor has been applying an e-coat paint since 1992, ensuring that hard-to-reach areas are evenly coated in the process.
The importance of a protective coating is reflected in some remanufactured products as well.
Swanson suggests that the remanufacturing process is particularly important for any buyers who want to combat rust jacking. Bendix, for example, uses an abrasive blast of steel shot to remove rust and paint from any used brake shoe tables. Then it uses a hydraulic press to re-shape the metal, flattening any peaks and valleys that may remain in the surface.
Without the latter step, components that are protected by a 2 mm coating will still show signs of surface rust within a matter of weeks, he insists.
“There are several people out there who have tried over the last 20 years to put an intermediate piece between the shoe and the lining,” adds Bendix marketing manager Doug King. While these pieces filled the voids, they eventually settled to the point that the rivets lacked the required clamping power.
The quality of the coating is equally important. Some remanufacturing shops use little more than an off-the-shelf, rust-resisting paint, but Bendix bakes on an enamel coating. Carlisle/Altec applies its RustGard UV coating.
Carlisle/Altec tests even suggest that its newly coated metal components demonstrate less than 10 percent surface rusting after 250 hours of exposure in an ASTM B-117 salt spray test. In contrast, the company says that brake shoe tables covered in traditional coatings were showing 50 percent surface rust after a mere 24 hours in the testing tank.
Other products and research initiatives are in the works. An ArvinMeritor spokesman, for example, alludes to the fact that the company will soon be announcing a product to address rust jacking issues. And this winter, Bendix will be testing a number of different washers in a bid to identify the best ways to remove the troublesome de-icing compounds.
In the meantime, customers that are struggling with excessive levels of rust jacking may want to reconsider the use of remanufactured shoes, Kay suggests. Despite the preparation process, some corrosion can remain.
Other operations, meanwhile, are experimenting with dust shields to control the road splash, especially on the steer axle where they might even help to increase operating temperatures. But that is still a theory. And while there are ways to ensure a tighter seal between the linings and their respective tables, these methods would require remanufacturers to burn off any lining that had to be removed, Kay adds.
“It’s a work in progress,” Hawker said, noting how friction material also is being evaluated to determine if a higher density would help to repel the chemicals and moisture. Ceramic coatings and plating might offer extra protection. Even the adhesives might change.
“We’re also looking at sealants, laminates, maybe different shoe materials,” he added. “We’re not stopping.”
