Rust Never Sleeps

There are lots of reasons why you might see premature wear on your brake linings. Most have more to do with your spec or the way the truck is driven than with the linings themselves.

But there’s a condition some maintenance managers are seeing and are having a hard time explaining: cracked linings on their S-cam air brake shoes.

What we’re talking about is what brake engineers call “rust jacking.” Rust forms on the brake shoe table, almost always caused by moisture, and eats away at the shoe. As this progresses, usually with an irregular pattern, the brake linings no longer sit on the smooth level shoe table they once had. As the rust does its work, linings flex, start to work loose around the rivet holes, and, if not caught, develop cracks, breaks, and even come loose from the shoe. The first signs are often like heat cracks but without lining glazing.

Rust jacking is a serious concern. Visibly cracked linings are cause for “out of service” citations at roadside safety checks. Premature brake relines defeat the purpose and extra cost of the so-called long-life premium brake products which have thicker linings. Worse yet are the safety implications of reduced braking power as a result of cracked or broken linings. “Accidents waiting to happen” is what one fleet maintenance manager called rust and corrosion problems at a meeting of The Technology and Maintenance Council of the American Trucking Associations last year.

Cracks can be tough to spot, especially if vehicles are equipped with dust shields. Edge cracks in the lining more than a pencil-point wide are a good indication that rust jacking has set in. The only sure way to find out is to pull brake drums in order to see the entire brake linings. Some fleets are now doing this as part of their annual safety inspection.

Rust jacking seems to be most severe on aftermarket relined shoes, especially so-called “price” products. The re-liner may not coin the shoes to ensure a flat and true surface before installing new linings. How and with what the shoe is painted has a lot to do with the level of protection. Major brake suppliers, well aware of rust-jacking problems, are working to find paint and epoxy coatings that will have better resistance.

As it turns out, “rust jacking” is just one part of what appears to be a rapidly growing rust and corrosion problem affecting many areas of the vehicle whether it be a truck, tractor, or trailer. The primary suspect is the dramatic increase in the use of highway de-icing materials over the past 20 years. Many jurisdictions now use calcium chloride and magnesium chloride or blends in place of traditional rock salt. Both types of chlorides do a faster and more effective job of melting ice and snow than traditional rock salt. They are, however, likely a root cause of rust and corrosion. Other factors from the environment can affect your brake linings. Oxides of nitrogen (NOx) from engine exhaust and sulfuric acids (“acid rain”) from coal-fired power generating plants all contribute to the rust problem.

Trailers that sit around for long periods appear to be prone to rust jacking because once wetted with the bad chemicals, rust gets started sooner than on equipment in service, which is washed regularly and where heat from braking tends to evaporate off the chemical-laden moisture.

Some final words of advice: don’t be complacent and assume all is well underneath your vehicles and inside your brakes. It’s worth the time and money to ensure safety.

SIDEBAR:

The Performance Review Institute, an affiliate of the Society of Automotive Engineers, has released an updated list of aftermarket brake linings that meet test criteria established in TMC RP628A. The criteria includes dynamometer requirements specified in U.S. FMVSS 121; OE linings must meet these requirements, but they are optional for aftermarket linings. The list is intended to help fleets replace worn OEM linings with replacement linings of similar torque value to help ensure torque balance. A copy of the list is available in pdf format at truckline.com/cc/councils/tmc/PRI_brake_linings.pdf. Or phone TMC, 703/838-1763.