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August 1997 - Page 55


q uickly resets them, he gets less braking than before because the car reservoirs have not yet had time to recha rge . B e c a u s e of t h e l o n g r e c h a rge t i m e on lengthy freights, a way was needed to keep the brakes applied on the cars yet allow them to recharge. Enter the retainer valve. When a triple valve moves to release, it connects the reservoir to the brake pipe to begin recharging the reservoir. I t also vents (exhausts) the brake cylinder air to atmosphere to release the brakes. The retainer valve is mounted on the exhaust pipe of the brake cylinder and can restrict or c lose off t h a t exha u s t . This restriction holds some of the air in the brake cylinder, keeping the brake applied even though the triple valve is in release (where it a llows recharging of the reservoirs). Retainer valves are completely manually operated, i.e., the train must be stopped (usually at the top of a long grade ) ; the brakes released; and a crew member must walk back along the train , turning the retainer valve on each car (retainer valves have four positions: direct release, slow release, low-pressure h o l d , and high-pressure hol d ) . Usually only a percentage of the cars are "retainered, " just enough to keep the train from running away down h i l l when the brakes are released and re-charging during the trip down the mountain. Once the crew member i s back aboard, the train may proceed down the m o u n t a i n . The air brakes work normally until they are released. Then the cars with the closed retainers will hold their brakes ap p l i e d , s l ow i n g a c c e l e r a t i o n w h i l e t h e r e s e rv o i rs recharge for the next brake a p p l i c a t i o n . The train must stop at the bottom of the grade, where a crew member walks back and returns the retainers to their open (direct release) position. Today there are very few places in the United States where ret a i n ers are regu l a r l y u s e d , s i n c e dynamic brakes serve much the same purpose. Because dynamic brakes slow the rate of acceleration, air brakes have longer to recharge before they are needed again . Also, the retarding effort of the dynamic brakes allows the engineer to use lighter air brake applications to control train speed in the first place, taking less air from the reservoir and requiring less time to recharge. However, if a train should happen to go into emer gency ( for example, because of a burst air hose) and stop on a long, steep grade, the engineer would n ot
Dean Sauvola

want to release the train brakes after fixing the hose. The brakes would release completely, and he or she would have almost no air remaining in the car reser voirs to reapply the brakes. I f the train doesn't have dynamic brakes, or it is not sufficient to slow the trai n , t h e train will run away. To avoid t h i s , before releasing the brakes, a crew member turns on the retainers to hold the brakes on some cars. Then when the brakes are released, the train can roll down the hill with these "retained" cars controlling acceleration.

R ELEASE & CHARGE

E mpty/Load Sensors
Traction between the wheels and the rail is directly proportional to weight on the wheels. The amount of traction determines the amount of braking that can be applied without sliding the wheels. Even sliding j ust a few feet can cause wheels to develop flat spots. Train cars have a large weight difference when loaded and empty, especially with modern coal hoppers and grain cars. Consequently, the maximum braking effort of a car m u s t be designed so t h a t w h e n i n e m e rgency (when the h i gh e s t b r a ke c y l i n d e r p res s u re i s o b tained ) , t h e empty c a r wil l n o t slide its wheels. Unfor tunately, this means a heavily loaded car would be un der-braked, even in emergency. A way to correct this dangerous disparity was needed. The first step was to put larger reservoirs on the cars so that the traditional 2 . 5 to 1 ratio of reservoir volume to brake cylinder volume was greater, allowing higher brake cyl inder pressures for any given brake pipe reduction. But because higher pressure will slide the wheels of an empty car, a pressure-limiting valve was attached to the brake cylinder. On an empty Cat this valve's exhaust is open, a llowing it to vent excess pressure. On a loaded car, it is closed so the higher pressure can be used for higher braking effort. The exhaust close-off valve is mounted on the car frame just above the truck frame and is controlled by a load/empty sensing ann. One end of the arm is at tached to the close-off valve, and the other end rests on the truck frame. I f the car is empty, the car body rides high on t h e springs a n d t h e arm moves t h e close-off v a l v e to t h e o p e n p o s i t i o n . A l oaded car rides low and the arm is pushed u p , moving the close off valve to the closed position.

RailNews -55

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