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| SMEE (Re: Questions from the eager & curious) (860289) | |||
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SMEE (Re: Questions from the eager & curious) |
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Posted by straphanger9 on Wed Nov 18 02:45:26 2009, in response to Questions from the eager & curious, posted by Mike W. on Wed Nov 18 00:49:34 2009. Eh, I'll take the first one ("what is SMEE"). So here is probably "more than you ever wanted to know" :DSMEE = How it works: SMEE is pretty straightforward. You have two trainline air pipes, all that means is they run the length of the train. The two pipes are: A straight air pipe, and a brake pipe. Think of the straight air pipe as the service brake (the brake used in regular operations, such as stopping the train at a station, or slowing down before a curve). Think of the brake pipe as the emergency brake (the brake used when the emergency brake cord is pulled, a train goes through a red signal, or at the end of the line when the train is about to change ends and go back the other way - a loud "chow" can be heard when the emergency brake applies and the train stops cold). So here's how it works below. I will try to keep this simple so you can (try to) follow it. If others want to chime in after me with details of the various magnet valves, slack adjusters, feed valves, variable load valves, spotting wire, the old INSHOT, etc. have at it, but this will be the simple form. 1-The brake pipe As mentioned above, the brake pipe is related to the emergency brake. OK, so assume there is no air in the brake system, and this is the "default condition" (which would be the case when taking a train out of the yard). In order to get the train ready, one of the things that must be done is to charge the train's air brakes. Once compressor air is added to the car, but before the system is charged, the brake shoes will automatically apply. This is because in order to take his brake handle out, the last guy had to put the train into emergency, and before the brakes are charged, the train is still in emergency. So with compressor air added in emergency, the brake cylinders will have air in them keeping the brake shoes pressed against the wheels and preventing the car from moving. However, the brake pipe has no air in it. It is not charged. So when there is enough air from the compressor, the train operator charges the brakes. This brings them out of emergency. As a result, the brake pipe will charge to 110 pounds (130 pounds on R44 equipment). Now the brake pipe acts as a fail safe. If the pressure gets too low in the brake pipe, or drops too rapidly, the train will go right back into emergency, with no brake pipe air, and lots of brake cylinder pressure to apply the brake shoes to the wheel, stop the train, and keep it from moving further. Some ways this can happen: (1)Someone pulls the emergency brake. (2)The train operator, who has to keep a Deadman's feature of his controller satisfied, has something happen to him where he does not do this, so the train stops for safety assuming he is no longer at the controls or coherent at the controls. (3)The train runs through a red signal and is tripped. (4)The Brake pipe ruptures. (5)The train operator puts the train into emergency using the brake handle (something he'd want to do when changing ends at a terminal, for example, so he can take his brake handle out) So as mentioned, the brake pipe is meant to hold that pressure (110 or 130 lbs) until the train is intended to go into emergency again. Any rapid decrease in brake pipe pressure, such as would be caused by the 5 ways above, will cause the train to go into emergency and stop immediately, by venting all the air in the brake pipe and adding air to the brake cylinder and holding it there to stop the train immediately. 2-The straight air pipe If the brake pipe is the emergency brake, then the straight air pipe is the "service brake" aka the brake you feel when a station stop is made. When the train operator goes to make a stop, this pipe ultimately feeds air into the brake cylinders. Same as above, when the air pressure increases, the brake cylinders cause the brake shoes to be applied to the wheels and stop the train. So when running a train, obviously you don't want the brakes applied when moving, so "normal" straight air pipe pressure is zero (release) when moving between stations. Full service aka "maximum" straight air pipe pressure is 80 pounds, which is the most air a T/O can pull using the brake handle without going into emergency. During operation, air from the main reservoir (the big tank of air that the compressor fills), will move into the straight air pipe, and on down into the cylinders to stop the train. The main reservoir air is constantly refilled by the compressor, which shuts on and off like a home heating system when the pressure in the main res gets too low or high. The brake system is pretty responsive, so a train operator can pull as much or as little brake as he wants to make a stop, up to the full service amount. So that's all well and good for a brake system, but a lot of stopping and starting in the subway means those brake shoes would take a lot of wear and tear. But then of course there's... 3-Dynamic Braking A DC subway motor like on a SMEE car is basically a big electromagnet, which means that, like a bisexual, "it goes both ways." Which means when you put power to it, it spins (acts as a motor). However, when you spin it, it generates electricity (acts as a generator). So the car is accelerating, electricity goes to the motor, motor starts spinning and we're on the move. All dynamics are is the opposite. The car wants to slow down now, and it's going fast enough that dynamics are active. The T/O moving the brake handle to service brake energizes a dynamic brake wire, which connects the already spinning motor to resistance grids, and therefore places an "electrical load" on them. Now the motor acts as a generator and the spinning of the wheels creates electricity through the motor...that electricity needs somewhere to go, so it goes to the grids, which heat up, and the energy is used up and goes away. And of course, that slows the train down because there is now "drag" on the motors. But meanwhile a lot of brake shoe wear and tear is saved through the dynamics. Electrically, the train is capable of detecting the amount of electrical current created by dynamic braking (since the faster the train is going the higher this will be), and if that is not enough (ie the train is going slowly), the air brakes are added into the equation to help stop the train. (Note: this is also why a car with dead motors will apply air brakes at higher speeds - because dynamics are doing nothing). Electrically, the train is also capable of detecting how much brake was requested, so it knows how much of the resistor "load" to place on the motors to get the desired stopping rate. And if it doesn't get that (detected by the current created by the motor acting as a generator), then in come the air brakes to help slow the train down some more. The dynamic brake works seamlessly with the service brake to make stops. So a train operator pulls a train into a station at 30MPH, grabs brake. The dynamics apply and start slowing the train down. Then as he gets under 10MPH and further down, the air brakes will start to apply to make the final stop while the dynamics naturally fade. Dynamics do not work with the emergency brake however. Emergency stops are all air, no dynamics. And that's the basics of SMEE. What it means SMEE was a big upgrade in terms of being easier to use over older systems, because like a car brake, "the more brake you took, the faster you stopped." Older systems had an "add, subtract, or do nothing" system (with a different pipe layout) where the motorman controlled the flow of air into the brake cylinders by increasing the amount (applying brake), decreasing the amount (releasing brake), or leaving the air amount the same (lapping brake)...alternating between these positions as needed to stop the train. SMEE improved on that in making brake operations simpler. This is known as the "Self Lapping" feature of SMEE. Also, unlike older AMUE type systems (that's for another day), SMEE applied the brakes electrically throughout the whole train at the same time. AMUE systems could do something similar, but it required a break plug to be inserted to do it. SMEE, it was basically standard assuming the train had been set up for service properly. So hopefully that's enough to answer your first question :D |
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