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August 1997 - Page 57


ABOVE: Much as it has done for more than 1 00 years, the railroad air brake continues to slow and stop thousands of trains every day in North America. LEFT: If another application isn't made first, the pressure in the brake pipe and reservoirs returns to 90 pSi, and the control valve returns to Lap. RIGHT: In an emergency application, the engineer lets a lot of air out of his brake valve, which triggers each car's emergency valve. This sharp reduction opens both reser voirs to the brake cylinder, resulting in maximum braking pressure, and vents brake pipe pressure to the atmosphere at each car. End of train devices are now designed to release the brake pipe pressure from the rear end so emergen cy applications happen quicker and without the risk of an obstruction in the line allowing only some brakes to apply.
s hould go i n to emergency, the higher braking effort may be enough to lock up and slide the wheels, caus ing wheel dam age at the very l e a s t . I t a l s o t a k e s longer t o charge a train initially to 1 00 psi instead o f 8 0 or 9 0 , a n d higher pressures cause more leaks. So why the 80 psi system? Long ago, l i ke in the 1 920s, the brake pipe was 70 psi. That was fine for the 40-ton cars of the day. H owever, by the 1 940s, coal cars had grown to 55 tons and brake pipe pres sure was pushed to 80 psi. A decade later, cars were 70 tons, then grew to 1 00 tons in the 1 960s. Still, 80 psi handled the braking adequately. By the 1 970s, coal and grain cars had climbed to 1 3 5 tons, and 80 psi had little margin for error. Emergency stop distances for heavy trains were growing longer and longer. During the 1 970s, my railroad's rule book dictated an 80 psi brake pipe for all trains except l oaded unit coal and grain trains, which were to use 90 psi. This shortened emergen cy stopping d i stances for t h e s e heavy trains, b u t it also created other problems. F o r in stance, when the cars were unloaded, the pressure had to be reduced. If a coal train using a 90 psi pressure handed off cars to a freight using an 80 psi brake pipe, the "over charge" condition had to be reduced. This wasn't always done properly, resulting in stuck brakes on some cars and overheated wheels. As the weight of l u m ber, t a n k , a n d o t h e r cars caught up to the coal and grain cars, and load/empty sensors were installed, my railroad mandated 90 psi for all trains. However, rai l roads that don 't operate unit coal trains, or don't have steep grades, still find 80 psi adequate. Some yard and transfer operations working at low speeds still use 70 psi, taking advan tage of the shorter charging times. As you can see, air brakes can get complicated but then life is like that, isn't i t ? Raj) lews

R aiiNews 57

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