Truck Parts and Service | Heavy Duty Trucking, Aftermarket, Service Info
Issue link: https://read.dmtmag.com/i/85632
By John G. Smith, Contributing Editor info@wordsmithmedia.ca Disc(uss) the benefi ts A ir disc brakes promise every- thing from shorter stopping distances to quicker mainte- nance procedures, but their benefi ts have been limited to a relatively small share of wheel ends on vehicles used in North America. Indeed, specialty applications like the fi re trucks and refuse vehicles that are us- ing these brakes are the exception rather than the trucking industry's rule. Most heavy-duty trucks use enlarged S-cam drum brakes to meet the National High- way Traffi c Safety Administration's recent cuts to allowable stopping distances. Th e next round of cuts scheduled to take eff ect in August 2013 — covering all 4x2 tractors and severe-service tractors with Gross Vehicle Weight Ratings above 59,600 lbs. — will largely be met the same way, says Dan Philpott, Meritor's engineering manager – air disc brakes. In spite of that, the number of fl eets experimenting with air disc brakes is growing, particularly as fl eet safety managers look for performance benefi ts that could be used to help improve CSA safety ratings. "Th ey [experimenting fl eets] very WWW.TRUCKPARTSANDSERVICE.COM Brake Shop Air disc brakes are gaining traction, and that can be good news for all concerned much liked what they found — sig- nifi cant improvements in lining life and signifi cant improvements in feel, " says Gary Ganaway, director – marketing and global customer solutions for Bendix Spicer Foundation Brake. Th e systems also perform well in the face of high temperatures, which come with harsh duty cycles. Today's allow- able stopping distances are measured at 250˚F, but temperatures in the real world climb much higher than that. Th e benefi ts hardly end there. Th e disc designs are less prone to fading because they deliver a consistent amount of friction as temperatures rise. And the brakes boast a low and consistent hysteresis, which is a measure of how well the components release aſt er being applied, Bendix Spicer notes in Th e Compelling Case for Air Disc Brakes in Heavy Truck Braking. "Where there are high duty cycles and you have to do regular lining chang- es, [fl eets] are looking at the disc brake as being more interesting because of the quicker pad change, brake can be replaced in about a quarter of the time that it takes to replace friction material on an S-cam drum brake. Today's systems certainly are more reliable than earlier generations that were fi rst introduced in the 1980s. Early disc designs, actuated with a single power screw or wedge, had their share of problems. Th e power screw designs applied well but had trouble releasing. Exposed chrome-plated slide pins were prone to fl aking, corrosion and seizing. Calipers were attached to the pads and subjected to unwanted clamp loads and brake torque. Single-piston system designs faced uneven pad wear, while undersized 15-in. rotors were prone to cracking, and exposed components had to be lubricated regularly. Reliability has improved thanks " says Steve Bell, Ben- dix Spicer's engineering manager – wheel end air disc brakes. Th e pads on a disc to twin-piston designs with internal adjustment mechanisms, as well as direct-mounted air chambers, Philpott says, noting how the twin-piston designs load the pads more evenly against their respective rotors. "Th e direct-mount air chambers take away any sliding seal surfaces we used September 2012 | TRUCK PARTS & SERVICE 23