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August 1997 - Page 51

moves. Although the son is not nearly as strong as his father, their combined effort gets the job done. Each is pulling as hard as he can, even though they are not equal. Furthermore, it makes no difference whether the son is pulling ahead of, or behind his father.

F reight Train Brakes
F ine, you say, but exactly h ow a re brakes controlled? That i s the job of the so-cal l ed triple valve (The name i s properly applied only to the earliest models. Newer designs are known as control valves.) on each car. B a s i c a l l y, t h i s v a l v e c o n s t a n t l y compares t h e brake pipe pressure with i t s car's reservoir tank pres sure. If the brake p i p e pressure i s h igher t h a n t h e reservoir pressure, the triple valve moves t o t h e re lease p osition. Any brake cylinder air is vented to the atmosphere, thus releasing the brakes. The valve will also open a passage between the brake pipe and the reservoir tank, re-charging the tank. This sequence happens when a train s i t s i n t h e yard, "pumping up its a i r " prior to making a brake test: The locomotive's diesel engine i s turning com pressor, pumping a i r t h rough the engineer's brake valve into the brake p i pe and, fin a l l y, t h rough the triple valves of each car into the reservoirs. This takes a lot of air. I t may take from 1 5 minutes to an hour to charge a train, depending on its length and how leaky the air hose couplings are. The standard brake pipe pressure i s 90 psi on my railroad. Once the cars' reservoirs are charged to the same pressure as the brake pipe (90 psi ) , the triple valve on each car moves to the neutral, or Lap, posi tion. The brakes are now ready for use, either on the road or for an air brake test. To set the brakes, I move the brake valve handle from the Release & C harge po sition to the Application position. This disconnects the locomotive's air compressor from the brake pipe and opens a small hole, allowing brake pipe air pressure to vent to the atmosphere. This venting causes the brake pipe pressure to drop slowly. On each car, the triple valve monitors both the brake pipe pressure and the reservoi r pressure and senses when pipe pressure is lower. This signals the triple valve on the car to move to the apply position, connecting the reservoir air pres sure to the brake cylinder, pushing a piston in the cylin der out, and applying the brake shoes. M e a n wh i l e , up i n t h e c a b , I ' m w a t c h i n g t h e gauge s . W h e n t h e brake p i p e p r e s s u re lowers t o where I want i t , I p u t the brake valve i n neutral or Lap . Lap simply seals the brake pipe, letting no a i r out n o r letting air from the compressor i n . Let's say I " made a 1 0 pound set . " This means I 've reduced the brake pipe air pressure from 90 psi to 80 psi then lapped the brake valve. The triple valve on the car was monitoring the brake pipe air pressure, and as soon as it dropped below reservoir pressure, it moved to the apply position and allowed reservoir air to flow into the brake cylinder. This flow of air will, of course, lower the pressure in the car reservoir tank. Remem ber, the triple valve always compares the pressure from the brake pipe to the pressure in the reservoir. I t allows air to flow from the reservoir into the brake cylinder until the reservoir pressure lowers to match that of the brake pipe. When the pressures match ( that's 80 psi in this example), the triple valve returns to Lap. But now all that air that flowed from the reservoir to the cylinder has applied the brakes on that car. The volume of the reservoir is about 2 . 5 times the volume

of the brake cylinder. So, to lower the reservoir 10 psi, from 90 to 80, enough air flowed from the reservoir that i t put 25 psi ( 2 . 5 multiplied by the 1 0 psi reduc tion equals 25 psi) in the brake cylinder. Simple, isn't it? As engineer, I now have the choice of leaving the brakes applied, making another reduction to get heav ier braking, or releasing the brakes. Let's say I ' m on a moving train and want to slow down quickly. I move the brake valve to the Application position and lower the brake pipe another 5 psi from 80 to 75 p s i . The triple valves on the cars sense, once aga i n , that the brake pipe (now 7 5 psi) i s lower than the reservoir (80 psi ) . Once again, the valve moves to a llow reservoir air to flow into the brake c y l i n d e r u n t i l the reservoir matches the 7 5 psi. The brake cylinder pressure goes up, and the braking effort correspondingly increases. Because of the 2 . 5 ratio of reservoir-to-cylinder vol ume, this 5 psi reduction results in 1 2. 5 psi more brak ing pressure, in addition to the 25 psi already there, for a total of 3 7 . 5 psi brake cylinder pressure. This air brake system, w hen fu lly charged, i s fail safe: Anytime the brake pipe air reduces, the brakes a p p ly. If a train comes u n c o u p l e d , or a n a i r hose bursts, the brakes apply fully and automatically. But t h e amount of braking force always relies on t h e amount o f charge present in t h e system . When I no longer need the brakes, I can release them by moving my brake valve t o the Release & C harge position. As before, this connects the locomo tive air compressors to the brake pipe, raising its pres sure back to 90 psi. The cars' triple valves sense that the brake pipe (now 90 psi) is higher than the reservoir (still at 75 psi) and moves to Release position, connect ing the brake cylinder to the atmosphere, releasing the pressure in the cylinder and thus releasing the brakes. It also connects the brake pipe to the reservoir to begin re-charging the reservoir from the brake pipe. Congratulation s ! You now know the basics of a i r brakes. But-as always in l i fe-there are complications. When the brakes released on the train's cars, the brake pipe was at 90 psi, the reservoirs were at 75 psi . U pon releasing, the reservoirs begin to recharge-a process that takes time. So for s everal minutes after r e l e a s i n g t h e b r a k e s , t h e r e s e rv o i rs a re n o t fu l l y charged, and an engineer does not have ful l braking power available. In the example, I had made a total reduction of 1 5 psi (reduced the brake pipe and reservoirs from 90 to 7 5 p si ) . Suppose, one minute later, I want to set the brakes again? The brake pipe may be at 90 psi , but the reservoirs may have only recharged from 75 psi to 79 psi . Now if I m ake a t o p si reduction of the brake pipe ( from 90 to 80) , what does a car's triple valve see? I t sees 80 psi in the brake pipe and 7 9 psi i n the reservoir. The brake pipe is higher than the reservoir, so the triple valve stays in the release position ! I get no brakes ! Nada! Zip! B ut, i f I reduce a further 5 psi, bringing the brake pipe down to 75 psi , the triple valve sees the brake pipe lower t han the reservoir ( 7 9 psi ), so it goes to ap ply position. The brake pipe is at 75 psi and the reser voir was at 79 psi, so the reservoir lowers 4 psi. The 2 . 5 volume ratio between the reservoir and brake cylin der means I will get (2.5 multiplied by 4 psi) 1 0 psi in the brake cylinder. That's very little brakes compared to a minute earliel when the same 15 psi reduction re sulted in 3 7 . 5 psi braking power! And this is how runaway trains happen. Imagine, while going down a long mountain grade, a dumb engineer makes several heavy sets and releases


RaiiNews 51

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