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


For fleets to match all their units, they would have had to go through every trailer in the fleet, and for some companies that just wasn't an option. "Say, FedEx, they've got 85,000 trailers. How would you do that?" Szapacs asks. "It would just be impossible. We have 1,800 or 2,000 trailers, roughly, but we always looked at that and we spec'ed a certain valve, and that afforded us the opportunity to make that work.

"I'm not saying we didn't have problems," he goes on. "With the first systems we had slide pin bushing problems. They kept wearing out and getting real floppy." Complaints to the supplier resulted in better bushings and O-rings to keep the dust out, but Szapacs found that when the systems were tighter, the calipers would hang up on the slide pins.

"We laughed at ourselves for that," Szapacs recalls. "We should have kept our mouths shut!"

The only other problem Szapacs can recall is the time a new tractor cracked both rotors on the same drive axle in its first three weeks of service. Turned out the truck was mistakenly equipped with a seven and a half pound crack pressure valve on one of the drive axles, but a one and a half pound valve on the drive axle that lost both rotors.

"We put the seven and a half pound valve on it, and the truck went through its life without an issue," Szapacs says. "So you can see how fleets that had a vast variety of equipment would really be plagued with problems."

"GENERATION TWO"
Amazingly, Air Products' fleet rode on those early "Generation One" Meritor 1560 air disc brakes for nearly 15 years. "We used them on drive and steer axles up until 1998, and they didn't have the new model out yet, so the '98 tractors that we purchased all had S-cam brakes on them," Szapacs recalls. "We actually still could have ordered the Meritor 1560 brake, but I knew it was being discontinued. We were having problems getting rotors and so on for the units we had, so at that point I thought, ‘Why buy another order of trucks with the disc brake that's going away? We're having troubles getting it now.'

In the 1999 model year, Meritor's new DX 195 air disc brake for steer axles became available, and from that year on Szapacs has spec'ed the DX 195 on the steer axles for all his new Mack and Freightliner tractors.

The biggest difference between the old brakes and the new DX models, according to Szapacs, is the way the brake actually operates on the slide pins that allow the caliper to slide freely on the rotor.

"The pins are totally sealed and lubricated, and in the past there was nothing on them; the pins were exposed, kind of like they are on a passenger car," he says. "That worked okay for 100,000 miles, but after that is when you hit the issues.

"The new systems have no external moving parts," he continues. "The diaphragm housings are all bolted directly to the caliper, so the brake is pretty much more pervious to water."

When Meritor introduced the DX 225 for drive axles, Szapac considered returning to air disc brakes front and rear. But he found that he couldn't beat the brake performance and cost-per-mile he was getting with S-cams on the rear and the DX 195 on the steer axle, so that's what the fleet has stuck with.

Air disc brakes were once considered for Air Products' trailers as well, but the cost/benefit equation never added up for Szapacs. "We tested early on with the disc brakes on the trailer, and we had some rotor issues and pad life issues," he reports. "We didn't see any improvements that we felt would really counteract those negatives. So we haven't gone with them on the trailers at this point, but that's not to suggest that we wouldn't test some of the new systems."

ORIGINAL PADS & ROTORS
Is Air Products' configuration the ideal braking combination?

Szapacs pays more up-front for the air disc brakes, but his trucks have more stopping power and maintenance costs on the DX 195s are, he says, "virtually nonexistent." Most of the units easily go 500,000 miles with no pad replacements or relines, and no rotor issues. Air Products trades its tractors out at 600,000 miles, and it's not uncommon for those trucks to be sold with their original pads and rotors up front.

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