Rust is a powerful force, eating through the toughest steel plate and turning the stoutest bolts into welded rivets. But if you're not familiar with the term “rust jacking” it might come as a surprise that it can also split apart a heavy-duty brake block even though that block contains no steel or other rust-prone material.
“Any ferrous material open to the environment is liable to corrode,” says Dave Poleykett, R&D manager for Carlisle Motion Control Industries, a supplier of heavy-duty brake blocks and a remanufacturer of brake shoes.
While the blocks have no exposed ferrous material, the shoe table or platform in the company's remanufactured line is steel. “There can be a buildup of corrosion between the shoe block and platform because water sits between them,” says Poleykett. As it develops, iron oxide, the chemical name for rust, expands, causing what the brake industry calls rust jacking. “High forces are created by that expansion,” he explains. “The blocks are riveted in place, and the rivets don't fail, the block does. It cracks, forming what we call rough scale, and you can even have the [brake block] material coming off the shoe.”
While rust jacking has always been a concern, in recent years it's become a much more widespread problem for all types of heavy-duty vehicles.
“There are a couple of factors involved,” says Poleykett. In an effort to cut costs, many areas are replacing sodium chloride — ordinary rock salt — with less expensive magnesium chloride or calcium chloride as the primary chemical for treating icy roads.
“These chemicals are cheaper than they were in the past and have become more cost-effective than common salt for removing ice,” says Poleykett. Unfortunately, both are also more corrosive than the salt they replace.
But the increase in rust jacking isn't limited to the Snow Belt states. Warm coastal areas like Southern Florida that combine high humidity and saltwater have also been experiencing the problem.
“I believe another contributing factor [to the increase in rust jacking] is the paint used on the shoe table,” says Poleykett. “In the past, solvent-based paints were used, but due to environmental pressures, there's been a switch to water-based paints. I think that is also a factor.”
Not only has the problem become more common, but it can also affect even recently replaced brake shoes. “It's amazing how quickly they can corrode,” Poleykett says. “Fleets see a cracked lining where there's little wear, so they think it's a defective block and return it. You have to remove the lining to see the rust buildup. When we do we often find warranty returns for cracked brake blocks are really a rust problem.”
With a degree in chemistry from the University of Wales, Poleykett turned to the material sciences to develop a solution to rust jacking for Carlisle's Altec brand of remanufactured brake shoes. “We've come up with a trademarked ultra violet (UV) coating called RustGard,” he says.
Under an exclusive agreement with the coating supplier, Carlisle has added another step to its remanufacturing process. “We apply a thin film of the coating as a liquid spray and then cure it with UV light before it goes to the paint dip and the rest of the remanufacturing process,” he explains.
It took Carlisle's R&D team about four years to identify the right anti-corrosion material and develop the manufacturing process.
“After testing it side-by-side [with non-coated shoes] in a salt-spray booth, it was pretty clear that this would be an effective means of countering the corrosion problem,” Poleykett says. “We moved into field tests on trucks, including some running along the Florida coast, and the results were good, so we moved into production just about a year ago.”
Although the RustGard coated shoes have only been in real-world use for one winter season, monitoring warranty returns indicates that the new process has addressed the rust jacking problem. “It goes on every shoe we make now,” says Poleykett.