Air Disc Brakes – Can't Live With Them, Can't Live Without Them

How one fleet has made air disc brakes and S-cam brakes live together in peace and harmony


It's not exactly accurate to say that fleets can't live without air disc brakes. Many, if not most, fleet maintenance managers are getting along just fine without them. But that could all change by the end of the decade. The U.S. Department of Transportation (DOT) and the National Highway Traffic Safety Administration (NHTSA) have, for several years, been working on new standards that would require heavy truck stopping distances to be reduced by 30 percent. It's all part of the agencies' goal of reducing heavy truck-related fatalities by 50 percent by 2010, and although there is no requirement in the proposed standard for any specific new braking technology, odds are that air disc brakes will play a big part in the years to come. A very big part.

None of this would be an issue if the heavy truck industry could shake the bad memories of the air disc brakes that were brought to market in the 1980s. Back then air disc brakes were seen as the next big thing, but reliability and wear issues took their toll, and fleets retreated en masse to the less effective but proven technology of the S-cam air brake.

20 YEAR HISTORY
But it seems nobody told Ron Szapacs. Here he is, 20 years later, still running his 650 Class-8 tractors with Meritor air disc brakes on the steer axles, without any complaints. His fleet may already be close to meeting the proposed DOT/NHTSA stopping distance requirements, without changing a single spec'.

Szapacs is fleet equipment maintenance specialist, power vehicles, for Allentown, PA-based Air Products, Inc., one of the world's largest suppliers of gaseous materials for industry and transportation. When your fleet regularly hauls hazardous materials, and one of your regular runs is supplying fuel to the Space Shuttle, you'd better know a thing or two about what works and what doesn't, and Szapacs has a pretty good idea why air disc brakes fizzled in the ‘80s.

"Being a private fleet, we were able to control our tractors and trailers a little bit more precisely than some other fleets, especially the huge fleets that have all kinds of van trailers, thousands of them being sent all over the country," Szapacs says. "We paid specific attention to brake timing, and brake application pressure. So we made the tractors and trailers work together, and that is a very important function that needs to be addressed with air disc brakes."

Szapacs points out that an air disc brake doesn't have a return spring like an S-cam brake does. And because it takes between nine and 11 pounds of air pressure just to overcome the S-cam brake return spring, a driver who puts five pounds of pressure on the brakes will engage the air disc brakes while the S-cams are still being held back by the return spring.

To back up his case, Szapacs cites industry statistics showing that 60 to 70 percent of the brake applications a truck driver makes are 20 pounds or less. On every one of those brake applications, the disc brakes are doing much more work to. And so, "the disc brakes would just overheat and wear out, and create a lot of problems," Szapacs says.

MATCHING UNITS
To address this issue, Szapacs paired every air disc brake system with a seven and a half pound crack pressure valve on the trailer's brakes. Figuring in air line length and air loss, Szapacs estimates that the system easily reached the eight to 10 pound criteria for overcoming the S-cam return springs before engaging the air disc brakes, and so the steer, drive and trailer brakes all worked together in unison.

"I think other fleets understood that," Szapacs says, "but, you know, you can get a trailer with a zero pound crack pressure valve, or one and a half pounds or four and a half, or six and a half, and I think a lot of fleets never spec' that valve, so they weren't sure what they got."

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