TrainLife Staff updated April 14, 2011

Categories

TrainLife Staff's Tags

Archives

Browse Articles » Feature Text View Magazine View

  • Air Brakes Simplified

    By Al Krug

    The work of the air brakes is revealed in the smoke pouring from the wheels of this eastbound Conrail train, inching downgrade through the Berkshire mountains of western Massachusetts.
    RailNews - August 1997 - Page 48 RailNews - August 1997 - Page 49

    Here is a (very simplified) description of how North American freight train brakes work and some answers to other frequently asked questions.

    Basically, there is a reservoir (air tank) on each car which is charged with (nominally) 90 psi of compressed air, supplied by compressors on the locomotive and sent to every car through the train's brakeline. (That's the hose you see linking the cars-the one that goes "kapoosh!" when the train uncouples.) Once the reservoirs on all the cars are charged, the engineer can set the brakes on the entire train by bleeding air out of the brake pipe, using a valve in the locomotive cab. The reduction of air pressure in the brake pipe causes a valve on each car to connect that car's reservoir air to the brake cylinder on that car, applying the brakes.

    To release the brakes, the engineer moves the valve to the Release position, which once aga in sends compressed air back through the train. The increase in pressure in the brake pipe causes the valve on each car to vent the air in the brake cylinder to the atmosphere. A spring in the brake cylinder of each car causes the brakes to move away from the wheels. The brakes apply whenever the air pressure in the brake pipe drops. If the train accidentally uncouples, the brakes will automatically apply fully-since all of the brake pipe pressure will be vented to the atmosphere through the disconnected pipe. (This is why they are called automatic air brakes.)

    Because air pressure from the car's reservoir tank flows into the brake cylinders to force the brake shoes against the wheels, it is important to note that a train has no air brakes until each car's reservoir is initially charged from the locomotive. In other words, no air in the reservoir means no brakes. There is nothing new about this system. It was invented by George Westinghouse in the late 1800s (and first practically applied to passenger trains in 1868, to freight trains in 1887) and has been in use ever since with minor improvements.

    LEFT: On the traditional locomotive control stand, the engineer's brake valve is located alongside the engineer's seat. This handle (on a GP30) is painted the appropriate color, red. On these and the following pages, a typical sequence of air brake functions is illustrated using the diagram detailed on the previous page. To show each part's role in each action, air pressures are color coded; red denotes rising air pressure, green indicates steady or constant air pressure, and blue shows falling air pressure. For clarity, not all parts are shown. FAR LEFT: Once a train is assembled and before it departs, the locomotive "pumps the air" until the brake pipe and all reservoirs are charged to the amount of pressure specified by the engineer according to the feed valve. RIGHT: With full pressure established (in this case 90 psi), the control valve moves to Lap position and once tested with a trial brake application, the train is ready to depart.
    RailNews - August 1997 - Page 50

    Train Control

    The engineer can "graduate" the brakes on-that is, control the braking force in the "more brake" direction, but any release can only be a full release. He or she can't graduate a freight train's release as a car driver does by easing up on the brake pedal-one of the many characteristics that make running a train different from driving a car or truck. The engineer must not set too much brake, or he will stop short. Too little brake and he can set some more-if there is time. If there isn't time, he is fired-or dead!

    Passenger cars are a little different: some are equipped with graduated release brakes, and some even have anti-skid brakes. However, you won't find either feature on a freight car. (Some freight cars do have a load/empty sensor. Brake cylinder pressures can be higher when loaded and limited when empty to help prevent wheel sliding during braking.)

    Locomotives have two air brake systems-one operates in concert with the train brakes, and the other as an independent brake exclusively for the locomotives. The independent brake uses additional air brake hoses connected between units, is faster-acting, and offers graduate-on graduate-off control. This locomotive-only brake is usually used only during switching, light engine moves, or for holding a stopped train on level track.

    Most locomotives have a third braking system-dynamic brakes-which essentially rewire the traction motors to act as generators. The electricity generated is thrown away as heat in locomotive resistance grids. Generating this power creates rolling resistance, slowing the rolling train.

    Trailing locomotives are controlled by the engineer in the lead locomotive's cab and automatically do what ever the lead locomotive is doing, controlled via a multi wire jumper cable connected between each locomotive.

    Ah, but then wouldn't a small locomotives be pulled or pushed by larger locomotives in the same consist? As it turns out, multiple unit locomotives of differing horsepower operate perfectly well together. Each will do its share of the work. Suppose a man is pulling on a rope tied to a heavy wagon. The wagon is too heavy for the man to move alone. His 12-year-old son comes along and begins pulling on the rope, and the wagon 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.

    RailNews - August 1997 - Page 51

    Freight Train Brakes

    Fine, you say, but exactly how are brakes controlled?

    That is the job of the so-called triple valve (The name is properly applied only to the earliest models. Newer designs are known as control valves.) on each car. Basically, this valve constantly compares the brake pipe pressure with its car's reservoir tank pressure. If the brake pipe pressure is higher than the reservoir pressure, the triple valve moves to the release position. 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 sits in the yard, "pumping up its air" prior to making a brake test: The locomotive's diesel engine is turning com pressor, pumping air through the engineer's brake valve into the brake pipe and, finally, through the triple valves of each car into the reservoirs. This takes a lot of air. It may take from 15 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 is 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, position. 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 & Charge position 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 reservoir 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 pressure to the brake cylinder, pushing a piston in the cylinder out, and applying the brake shoes.

    Meanwhile, up in the cab, I'm watching the gauges. When the brake pipe pressure lowers to where I want it, I put the brake valve in neutral or Lap. Lap simply seals the brake pipe, letting no air out nor letting air from the compressor in.

    Let's say I "made a 10 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. Remember, the triple valve always compares the pressure from the brake pipe to the pressure in the reservoir. It 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 it put 25 psi (2.5 multiplied by the 10 psi reduction 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 heavier 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 psi. The triple valves on the cars sense, once again, that the brake pipe (now 75 psi) is lower than the reservoir (80 psi). Once again, the valve moves to allow reservoir air to flow into the brake cylinder until the reservoir matches the 75 psi. The brake cylinder pressure goes up, and the braking effort correspondingly increases. Because of the 2.5 ratio of reservoir-to-cylinder volume, this 5 psi reduction results in 12.5 psi more braking pressure, in addition to the 25 psi already there, for a total of 37.5 psi brake cylinder pressure.

    This air brake system, when fully charged, is fail safe: Anytime the brake pipe air reduces, the brakes apply. If a train comes uncoupled, or an air hose bursts, the brakes apply fully and automatically. But the amount of braking force always relies on the amount of charge present in the system.

    When I no longer need the brakes, I can release them by moving my brake valve to the Release & Charge position. As before, this connects the locomotive air compressors to the brake pipe, raising its pressure 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, connecting 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.

    Congratulations! You now know the basics of air brakes. But-as always in life-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. Upon releasing, the reservoirs begin to recharge-a process that takes time. So for s everal minutes after releasing the brakes, the resevoirs are not fully charged, and an engineer does not have full braking power available.

    In the example, I had made a total reduction of 15 psi (reduced the brake pipe and reservoirs from 90 to 75 psi). 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 make a 10 psi reduction of the brake pipe (from 90 to 80), what does a car's triple valve see? It sees 80 psi in the brake pipe and 79 psi in 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! But, if I reduce a further 5 psi, bringing the brake pipe down to 75 psi, the triple valve sees the brake pipe lower than the reservoir (79 psi), so it goes to apply position. The brake pipe is at 75 psi and the reservoir was at 79 psi, so the reservoir lowers 4 psi. The 2.5 volume ratio between the reservoir and brake cylinder means I will get (2.5 multiplied by 4 psi) 10 psi in the brake cylinder. That's very little brakes compared to a minute earlier when the same 15 psi reduction resulted in 37.5 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 in a short time. He or she soon will have no brakes because there will be very little air left in the reservoirs. Railroaders call this "pissing away your air." (Now before you go and tell the press at the scene of a runaway train wreck that a dumb engineer must have been at the controls, please understand that runaways can also occur in ways that are not the engineer's fault.)

    ABOVE: Since air brakes are fail-safe with the reservoirs charged, this Canadian Pacific train came to a halt soon after these hoses separated and emergency valves were tripped in both directions, causing a full brake application through out the train. LEFT: When braking effort is needed, the engineer reduces the pressure in the brake pipe through his brake valve; the control valve, sensing the pressure difference, then admits air to the brake cylinder and the brakes apply. RIGHT: Once the reservoir pressure decreases to equal the brake pipe pressure, the control valve returns to Lap position, and the engineer's valve self-laps so that a constant pressure is maintained in the brake cylinder.
    RailNews - August 1997 - Page 52

    Another complication of this simple brake system is that a long train has a long brake pipe, containing a lot of air. When I want to make a brake application by reducing the brake pipe pressure, it takes time to vent enough air to do so. This is not a problem under normal braking conditions, but what happens in an emergency?

    Solution: an emergency vent valve has been added to each car. This valve monitors the brake pipe air pressure. If the pressure drops slowly, the emergency valve does not react, no matter how low the pressure goes. But if it drops quickly, the emergency valve opens the car's brake pipe to the atmosphere. This quickly dumps the brake pipe air to the atmosphere at the car. In other words, all the air does not have to go through the entire brake pipe, up to the engineer's valve, and out to the atmosphere.

    All I have to do is start the emergency application by quickly venting brake pipe air at the head end. The first car's emergency valve senses the fast drop and vents all brake pipe air at that car quickly; the next car senses a fast drop and also goes to emergency, then the next, and so on. Within seconds, the entire train is in emergency, dumping all the brake pipe air at each car. I get a fast and full application of the brakes throughout the train. If you are standing near a train when the locomotive uncouples, you can hear these emergency valves vent the brake pipe pressure locally as the car you are next to makes the tell-tale "psssssht." If you are standing some distance off to the side, you can hear each car trigger in succession, rapidly down the train. These emergency vent valves stay open for about two minutes, ensuring that the train will be stopped before the engineer can release the brakes.

    Anything-the train coming uncoupled, a bursting hose, the engineer, the conductor-that causes a quick drop in brake pipe pressure at any car will trigger that car, which in turn triggers adjoining cars and puts the whole train in emergency.

    All well and good-in theory. But what about that doofus engineer who used up all his brakes, leaving little pressure in the reservoirs?

    If he puts his train into emergency, he will still get very little braking-in effect, he will just get what is available more quickly. To ensure that there is always air pressure on each car for an emergency application, the basic system was modified by adding a second-or emergency-reservoir to each car.

    The emergency reservoir is charged with compressed air from the brake pipe, just like the service reservoir. After the initial charging time in the yard, it contains 90 psi. This air is never used during normal braking. However, if the engineer initiates an emergency application by making a quick reduction, each car's emergency valve triggers, just as described above, but now it also connects the emergency reservoir air to the brake cylinder in combination with the service reservoir air.

    When I make a service application, the brake pipe air vents through a small hole in my brake valve, lowering brake pipe air pressure slowly. When I want an emergency application, I move my brake valve over to the emergency position-a big hole in the valve that allows air to escape quickly, hence the terms for an emergency application, "Big hole 'em."

    Just like freight cars, locomotives have air brakes that apply when brake pipe air pressure is reduced. This is not always desirable, especially when "stretch braking" with the throttle open and car brakes set to control slack action. At these times, an engineer can prevent the locomotive brakes from applying by depressing the independent brake handle and holding it down-sometimes called "bailing the air."

    "An Electronic Future" (posted at the bottom of this article)
    RailNews - August 1997 - Page 53

    As I mentioned, locomotives also have an independent "straight air brake," so called because air pressure goes directly from the locomotives' compressor reservoirs to the locomotives' brake cylinders. This brake is controlled by the independent brake handle and is generally used to apply the locomotive brakes during switching or for holding a stopped train on level track.

    You are now an expert on train brakes. There will be a quiz on Wednesday.

    Quiz. (Is it Wednesday already?)

    Question: You have made a 10 psi brake pipe reduction on a fully charged 11O-car train. The brakes have applied, but one car has a leak in its service reservoir. What happens to the brakes on that car? What happens to the brakes on the entire train?

    Answer: (no peeking)

    The key is to remember how the triple valve works. It senses the difference between the brake pipe pressure and the service reservoir pressure. If you have made a 10 psi reduction, from 90 to 80 psi, and the brakes have set, the reservoir and brake pipe are both now at 80 psi. As the reservoir slowly leaks, the pressure drops, from 80 to 79 to 78 to 77 etc. As soon as the reservoir pressure leaks from 80 to 79 psi the triple valve "sees" that the brake pipe (80 psi) is higher than the reservoir (79 psi) and will release the brakes on that car. Whoa! That is not good! But you still have brakes on the other 109 cars of your train.

    Hopefully.

    Controlling Slack

    Becaue all the brake pipe air has to vent through the locomotive's control valve during a normal service application, it takes a long time for the brakes set throughout the train: The pressure first drops near the front of the train and then drops further and further towards the rear. This causes the free-rolling rear end to run into the slowing front end-slack action.

    So some smarty came up with the idea of modifying the triple valve on the cars. Now when a car first senses a drop of pressure, it opens a passage from the brake pipe to a small reservo ir (a third reservoir, called a "quick service reservoir"). This reservoir is sized in such a way that filling it with air from the brake pipe reduces the brake pipe by 7 psi. When I make a 10 psi reduction, 7 psi worth of it is reduced at each car, resulting in a faster (but not fast enough to trigger emergency) and more even application of the brakes through the train. It only works the first time, since, once filled, that reservoir will remain filled until the brakes are released.

    Because of the long brake pipe of a train and all those cut-off valves at the ends of each car and other restrictions, it takes time to pump air back through the train to release the brakes. As a matter of fact, as each car goes into release, it begins re-charging its reservoir from the brake pipe, consuming air and further slowing the build-up of pressure toward the rear. This results in more slack action problems if the head end releases first and moves away from the still-anchored rear portion.

    ABOVE: The prominant position of the air brake pressure gauges is shown in this view of the control stand on an F45, as well as the position of the brake valve relative to the throttle and dynamic brake handle. LEFT: When braking is no longer needed, the engineer moves the valve handle back to Release & Charge. This starts to increase the pressure in the brake pipe, releasing the brake cylinder pressure to the atmosphere via the retainer valve. The retainer valve can be manually set to hold some pressure in the cylinder until manually released. To help release the brakes quickly, pressure from the emergency reservoir is vented into the brake pipe. RIGHT: With the pressure gone from the brake cylinder, the compressor continues pumping the air into the brake pipe and reservoirs in order to restore the system to 90 pSi. BELOW RIGHT: Brake components are easily seen on a tank car where they cling to the framing. Visible on this car is the control valve (on the len side), the brake pipe running the length of the car towards the bottom, underneath the car are the rods connecting the brake cylinder to the brake rigging on the trucks.
    RailNews - August 1997 - Page 54

    In the early days, this problem was solved by putting chokes in the pipes carrying air from the brake pipe to the reservoir on each car. This allowed a more rapid build-up of air pressure in the brake pipe all the way to the rear of the train, since each car reservoir was consuming brake pipe air at a slower rate because of the choke restriction as it recharged following a release. But as trains got longer and heavier, this was not enough, and the chokes slowed down the initial charging of the trains in the yard and the recharge on the road. Here comes smarty again, and like a Congressman eyeing the Social Security trust fund, he can't stand to see a surplus go unused.

    Remember that emergency reservoir on each car? It was initially charged to 90 psi and never used if the engineer did not need an emergency application. Hmmmm. All that air there.... The triple valve was modified again so that when a car goes into release, it vents the cylinder air to atmosphere, releasing the brake as before; connects the brake pipe to the reservoir to begin re-charging, as before; and connects the 90 psi emergency reservoir to the brake pipe to boost the pipe pressure quickly at each car. This results in fast releases throughout the train, but it depletes part of the emergency air if it is needed before the system can recharge.

    And, basically, that's how train brakes work today.

    Remember the leaky reservoir question? With an entire train of this type of equipment, what are the answers to the two questions posed in quiz No. 1 under the same conditions? Answer: same as before-the brake will release on the leaky car. However, that one leaky car will dump its emergency reservoir air into the brake pipe when it moves to the release position, raising the brake pipe pressure on that car and on the cars next to it! When the cars nearest the leaky one see the brake pipe rise slightly above their service reservoir pressure, their valves interpret this as a release signal and they also move to release! Now they also dump their emergency reservoir air into their brake pipes, triggering the adjacent cars to release as well. Because of that single leaky service reservoir, the entire train will release. It is for this reason that it is against the rules to "bottle the air" (close the brake line angle cock on the train) when uncoupling the engines.

    Retainers

    On long steep grades it may be necessary to set and release the brakes several times because of grade changes, etc. But if the brakes are released on a steep hill, the train immediately accelerates. If the engineer quickly resets them, he gets less braking than before because the car reservoirs have not yet had time to recharge. Because of the long recharge time on lengthy freights, a way was needed to keep the brakes applied on the cars yet allow them to recharge.

    RailNews - August 1997 - Page 55

    Enter the retainer valve.

    When a triple valve moves to release, it connects the reservoir to the brake pipe to begin recharging the reservoir. It 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 close off that exhaust. 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 allows 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 hold, and high-pressure hold). Usually only a percentage of the cars are "retainered," just enough to keep the train from running away down hill when the brakes are released and re-charging during the trip down the mountain.

    Once the crew member is back aboard, the train may proceed down the mountain. The air brakes work normally until they are released. Then the cars with the closed retainers will hold their brakes applied, slowing acceleration while the reservoirs recharge for the next brake application. 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 retainers are regularly used, since 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 emergency (for example, because of a burst air hose) and stop on a long, steep grade, the engineer would not 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 reservoirs to reapply the brakes. If the train doesn't have dynamic brakes, or it is not sufficient to slow the train, the train will run away. To avoid this, 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.

    Empty/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 just 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 must be designed so that when in emergency (when the highest brake cylinder pressure is obtained), the empty car will not slide its wheels. Unfortunately, this means a heavily loaded car would be under-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 cylinder 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, allowing 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 attached to the close-off valve, and the other end rests on the truck frame. If the car is empty, the car body rides high on the springs and the arm moves the close-off valve to the open position. A loaded car rides low and the arm is pushed up, moving the close off valve to the closed position.

    ABOVE: Much as it has done for more than 100 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 reservoirs 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 emergency applications happen quicker and without the risk of an obstruction in the line allowing only some brakes to apply.
    RailNews - August 1997 - Page 56 RailNews - August 1997 - Page 57

    Equalizing Reservoir

    I stated earlier that the engineer makes a service application of the brakes by moving his brake valve handle to the application position, opening a small hole, which reduces brake pipe pressure slowly. He watches the brake pipe pressure fall on the air brake gauge. When he gets the amount of reduction he desires, he moves the brake handle to the Lap (blocked-off) position.

    Actually, this is only true with the very early air brake systems. As trains got longer, and had more brake pipe volume, it took too much time for the air to travel through all the cars to vent at the engineer's brake valve. Compromising safety, his attention was fixed on the air brake gauge for an inordinate time. So another small reservoir, known as an equalizing reservoir was installed; its size allows pressure to be reduced almost instantaneously. The engineer's brake valve now reduces the air in the equalizing reservoir instead of the brake pipe. He can get the desired reduction (say 10 psi) very quickly and then can take his eyes off the equalizing reservoir gauge to look out ahead. An equalizing valve is connected between the equalizing reservoir and the brake pipe; it is this valve that vents the brake pipe air to the atmosphere until it matches the equalizing reservoir pressure.

    Also, since the late 1950s or early 1960s, the engineer's brake valve has been self-lapping. He or she no longer has to move the brake valve back to the Lap position after making a reduction, because the position of the brake valve handle determines the amount of reduction.

    One more consideration:

    The engineer can change the maximum pressure of the brake pipe by adjusting the Feed Valve at his control stand. I have used 90 psi as the standard pressure to which the brake pipe is initially charged and subsequently recharged. On the railroad I work for; 90 psi is the standard. Some railroads use 80. Some mountain grade lines use 100 psi on loaded coal and grain trains.

    What is the significance of the different pressures? During normal service braking operations, there is none. A 10 psi reduction from a 100 psi brake pipe, a 90 psi brake pipe, or an 80 psi brake pipe all result in 25 psi in the brake cylinder and thus equal braking effort. But what happens if you make a 26 psi reduction from a 90 psi brake pipe, since 90 minus 26 equals 64 psi in the brake pipe? As service reservoir pressure flows into the brake cylinder; the brake cylinder pressure rises. Because of the 2,5 to 1 ratio of volumes, when enough air has flowed into the brake cylinder to lower the service reservoir 26 psi, the brake cylinder pressure is 64 psi (2.5 times 26=64). This air came from the service reservoir which is also now at 64 psi. Since the reservoir pressure and the brake cylinder pressure are equal, no more air will flow into the brake cylinder. This condition is called a Full Service brake application, because reducing the brake pipe further will not increase the amount of brake cylinder pressure. (Even if you reduce the brake pipe pressure to zero psi, the reservoir and brake cylinder pressure will stiLl be 64 psi-the same as with only a 26 psi reduction.)

    This Full Service, or equalization of pressures, oc curs at 64 psi for a 90 psi charged system and 71 psi for a 100 psi charged system (resulting in higher full service brake effort).

    An engineer who makes a reduction greater than these values is just wasting time-no higher braking effort results.

    All of this is academic, however, since normal train operations seldom require a brake application greater than 15 psi. Indeed, any reduction greater than 12 psi is considered heavy braking. So why would mountain grade railroads use 100 psi in the brake pipe? Two reasons. As we just saw, the Full Service braking effort is higher if it is needed. Suppose a 10 psi reduction is made from a 100 psi system, resulting in 25 psi (10 psi times 2.5) in the brake cylinders. Part way down the mountain, the grade lessens and train speed drops. The engineer releases the brakes, and the brake pipe returns to 100 psi. The train immediately begins to accelerate down the grade. The engineer immediately resets the air brakes by making another reduction. But the car reservoirs have only just begun to recharge, so they contain only 90 psi. If he makes a 10 psi reduction of the brake pipe, he will get no brakes, because the brake pipe will be at 90, as are the reservoirs. But if he makes an additional 10 psi reduction (a total of 20 psi), the engineer will get the same braking effort as the original set, 25 psi in the cylinders.

    This means the 100 psi system gives the engineer one additional 10 psi set and releae before he begins to run out of air.

    So why not always use 100 psi? There are penalties that go along with that extra pressure. Weak hoses or valve gaskets may fail. And, if the train should go into emergency, the higher braking effort may be enough to lock up and slide the wheels, causing wheel damage at the very least. It also takes longer to charge a train initially to 100 psi instead of 80 or 90, and higher pressures cause more leaks.

    So why the 80 psi system? Long ago, like in the 1920s, the brake pipe was 70 psi. That was fine for the 40-ton cars of the day. However, by the 1940s, coal cars had grown to 55 tons and brake pipe pressure was pushed to 80 psi. A decade later, cars were 70 tons, then grew to 100 tons in the 1960s. Still, 80 psi handled the braking adequately. By the 1970s, coal and grain cars had climbed to 135 tons, and 80 psi had little margin for error. Emergency stop distances for heavy trains were growing longer and longer.

    During the 1970s, my railroad's rule book dictated an 80 psi brake pipe for all trains except loaded unit coal and grain trains, which were to use 90 psi. This shortened emergency stopping distances for these heavy trains, but it also created other problems. For instance, 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 lumber, tank, and other cars caught up to the coal and grain cars, and load/empty sensors were installed, my railroad mandated 90 psi for all trains. However, railroads 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 it?

    An Electronic Future

    Electronic control is the latest development in air brakes and the systems being tested now may well hold the key to a giant leap forward in brake response time and reservoir depletions.

    With conventional air brakes, the delay from when the engineer starts a brake pipe reduction to when the brakes actually apply can range from five seconds to nearly a minute, depending on the temperature, length of the train, state of charge, and any restrictions in the brake pipe. With an electronically controlled system, a wire extends the length of the train, and a signal to apply or release the brakes travels at the speed of light, rather than at the speed of changing air pressures.

    To set the brakes on this system, the engineer leaves his brake valve in the release position and presses a button on the electronic control box. An electrical command tells circuits on each car how much braking to apply. Air pressure from the car's reservoir is connected to the car's brake cylinder until the proper cylinder pressure is obtained. Although the air is taken from the reservoir, keep in mind that the reservoir is constantly being recharged because the engineer left his valve in release. which supplies air pressure from the locomotives to the brake pipe. Since the engineer never reduced the brake pipe pressure, the cars' triple valves remain in the release position. So, as air is used from the car reservoir to apply the brakes, it is simultaneously replaced from the brake pipe.

    Since the reservoirs remain charged, this system eliminates the period of time just after release when the reservoirs a repartially empty. In that respect, the safety of the braking system is greatly improved.

    When the engineer is ready to release the brakes, he again pushes a button on the control box, and the box sends a release command to all the cars simultaneously. The instant apply and release signals eliminate the slack run ins and run outs caused by the slower reacting all-air system.

    Electronic control also allows brakes to be graduated off-partially released to reduce braking effort if the grade lessens or to refine the stopping point. (With conventional air brakes you can grad uate the brake in crease but the release is all or nothing.)

    Several railroads are currently testing electronic air brakes. The conventional air brake components remain on the cars as a safety backup and to permit the cars to be used in conventional trains.

    Al Krug

    Article Details

    • Original Author Al Krug
    • Source RailNews
    • Publication Date August 1997

    Article Album (1 photo)

    Share - Report
0 comments