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Question about air brakes
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   Author  Topic: Question about air brakes  (Read 556 times)
dtiman
Historian
Posts: 578
Question about air brakes
 
« on: Mar 5th, 2006, 4:18pm »
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Doesn't air in the brake system hold the brakes off, meaning that if air is lost the brakes are applied?
 
When an air hose is broken and all train air is lost, a train automatically makes an emergency stop, whether the engineer likes it or not right?
 
So, if a train loses it air, it should not become a "runaway" right?
 
Please correct me on this if I am wrong.
 
Thanks for any response.


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JhnZ33
Former Member
Re: Question about air brakes
 
« Reply #1 on: Mar 5th, 2006, 4:28pm »
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on Mar 5th, 2006, 4:18pm, dtiman wrote:       (Click here for original message)
Doesn't air in the brake system hold the brakes off, meaning that if air is lost the brakes are applied?

 
Yes, correct.
 
Quote:
When an air hose is broken and all train air is lost, a train automatically makes an emergency stop, whether the engineer likes it or not right?

 
If the air pressure were to go low enough to allow the brakes to engage, yes. But guages and warning indicators/sounders would alert the crew who operated the train well before the brakes "locking up", depending on the severity of the leak.
 
Quote:
So, if a train loses it air, it should not become a "runaway" right?
 
Please correct me on this if I am wrong.
 
Thanks for any response.

 
That sounds about right, as long as the brakes themselves are in satifactory operating condition.
 
 
John


« Last Edit: Mar 5th, 2006, 4:30pm by JhnZ33 » Logged
razor
Chaser
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Posts: 51
Re: Question about air brakes
 
« Reply #2 on: Mar 5th, 2006, 5:44pm »
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Each car has its own independent brake system. The main brake pipe works as a switch. If the main pipe has a sudden loss of pressure, it will trigger a release of the air stored in each car, causing an application of the brakes.
 
The engineer or conductor can "dump" the train by simply opening an air valve on the main brake pipe. This causes an emergency application of the brakes on the entire train. The EOTD (End of Train Device) can also be activated to dump the train from the rear. The train will stop faster if dumped from both ends. Also, if you have a heavy train, it's preferable to dump it from the rear, so the brakes apply on the rear cars first.
 
At some point, the air will leak off the cars and the brakes may release.
 
-r


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dtiman
Historian
Posts: 578
Re: Question about air brakes
 
« Reply #3 on: Mar 5th, 2006, 7:05pm »
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on Mar 5th, 2006, 5:44pm, razor wrote:       (Click here for original message)

At some point, the air will leak off the cars and the brakes may release.
 
-r

 
Thanks, both of you, for the informative responses!
 
But I am confused a little by the quote above. Releasing air applies the brakes, so how does leakage cause them to release?


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George_Harris
Historian
Posts: 3829
Re: Question about air brakes
 
« Reply #4 on: Mar 5th, 2006, 10:28pm »
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If you search this subject you can find some information that has diagrams., Maybe even within railfan.net.
 
The reason total loss of air equals loss of brakes is because there is a reservoir air tank on each car and it is the pressure in this tank that holds the brakes on when the air pressure in the brake line is reduced.  When the brakes are applied, some air from the reservoir tank is used resulting in some reduction of the air pressure in the reservoir tank.  When the brakes are released, the air pumps on the engine keep pumping air into the train line until the air tanks are fully recharged.  This recharging takes some time.  Therefore, if the brakes are released and reapplied several times without allowing time for the reservoir tanks to be recharged, it can reach the point where there is not enough air left in the reservoirs to hold the brakes on.  At this point you can have a runaway.  
 
That is why total loss of air results in loss of brakes.  It is not loss of air in the trainline, but loss of air in the reservoirs on the cars.
 
Razor, any clarification or corrections?
 
George


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kyblue
Enthusiast
Posts: 43
Re: Question about air brakes
 
« Reply #5 on: Mar 7th, 2006, 2:44am »
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Everyone is correct so far but i think what u are asking is how the air brake system works.  Everyone pretty much nailed the emergency question...  However i dont think it answered your question.  Razor is correct the cars will eventually bleed off, sometimes rather quickly, every car is diffrent.  i found this out one time on a temp transfer to South Carolina!  The first switch i ever ran through was caused by a car that bled off much quicker than i thought it would.  We cut a car off on a side track that wasnt ready to be spotted yet so we broke for beans came back and needless to say that the car was not where we left it!!!  We were lucky and could fix it without having to call in the TM.  Well i am rambling now, anyways, brake pipe pressure for frieght trains is 90 psi every car has 2 resevoirs a main and an emergency inside of the main.  both resovoirs on the car are also charged to 90 psi the main resovoir on the car does the normal brake functions while the emergency, well just as the name says is there for emergencys in case the air from the main is pissed away.  now the brakes are applied with air pressure but to get the brakes to come on u have to take air away from the brake pipe.  confusing yet  think of it as a cup under water if it is completly full of water the pressure is equal inside and out of the cup however if u take water from the cup the pressure is greater on the outside now. when u take air from the brake pipe, air from the main tank on the car moves to the brake clyinder compresses the spring and the brakes apply.  when u knock the air off u add air back to the brake pipe the resovoirs become equal and the air that applied the brakes is exausted out through the triple valve.  
 
now i did not use technical terms because being a conductor i dont know all the ins and outs i just know how to get my train over the road as safe as possible.  like was said if u want to know excactly how they work then look at the graphs.  hope this helped somewhat. and everyone feel free to correct me if there is anything wrong with this.


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NSWDriver
TRAINing
Posts: 7
Re: Question about air brakes
 
« Reply #6 on: Apr 27th, 2007, 12:32pm »
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Im guessing you are asking about the Westinghouse style breaking system.
It all starts with the triple valve.
It does 3 things, Applies the brakes, Releases the brakes and Recharges the tanks on each car.
Each car on a train with Westinghouse style breaks has large tanks holding Pressurized air.  Connected to these are the triple valves. Connected to the triple valves is the break pipe which is hopefully continuous throughout the train.  When air is release from the break pipe, generally through the drivers brake stand (but any way will do) the triple valve reacts to this by pushing the tank air into the brake cylinders to equalize pressure with the break pipe.  Thus the brakes come on.  
When the driver releases the brakes he does it by recharging the brake pipe.  When the triple valve senses more pressure in the brake pipe than in the tanks it releases a portion of the cylinder pressure and takes air out of the brake pipe using it to recharge the tanks.
Runaways can occur for a few reasons.  Firstly im yet to find the brake cylinder that doesn't leak at least a small bit so over time they run out of air pressure and the train starts rolling away.  As there is no longer air left to recharge the brake cylinders the train is now a runaway with nothing more than time (if someone is aboard they can release the now useless brakes and recharge the brake pipe and tanks then reapply the brakes) or some sort of park brake (often the SPB or spring park brake which is applied when main res pressure is removed (this main res pressure hold the spring off the cylinder and when removed the heavy spring takes over)) or a wound/ratchet type (applied by brute force).
A second less common occurrence is the Driver error. When you apply the brakes you use a certain amount of air from the cars tanks that needs time to recharge with the brakes released.  If you apply and release in a couple of times in a short period you can/will find yourself with empty tanks.  You hit the brakes and the cylinders don't move.... oh *#&%89.  This leaves you with two options again, Option one apply some sort of park break or release them and wait for everything to recharge then apply them.  On a fright train 2km's long this is going to take a long long time.  One one of the suburban MU trains i drive which have multiple compressors this takes around 12-14 seconds for full recharge (it feels like weeks when it happens).
On many trains the Westinghouse system is more of a backup system than anything.  Usually only used for emergency applications or when the main system of breaking fails.
I for example mainly driver using Regenerative brakes and EP (Electro Pneumatic) braking. Basically a wire sends a signal down the train tell the EP unit to apply Xkpa of pressure to each cylinder out of the trains main res. When i back things off it says release Xkpa.  So i can almost apply and release these all day long without running out of air as the main res charges constantly off the trains compressors not just when the brakes are released like in Westinghouse.
I so wish i could post diagrams on how things work but i don't think my employer would appreciate me posting training material. So i will go for the best way i can explain it.


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prostock19
Historian
Posts: 486
Re: Question about air brakes
 
« Reply #7 on: May 17th, 2007, 9:47pm »
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Here are some things that I know (or think I know) about air brakes.
 
Service reservoir is 2.5x larger than the piston.  Emergency reservoir is 3.5 times larger.  When you drop 1lb off the brake pipe, 1lb of pressure is released into the brake cylinder, and there is 2.5 pounds in the brake cylinder.
 
Minimal or first service is a 6 to 8 pound reduction.  That means that there is 15 to 20 pounds in each brake cylinder.
 
Full service braking occurs when the brake cylinder and reservoirs balance out.  That occurs about 64psi.  (26 X 2.5 = 65)  Emergency braking causes about 70PSI in the cylinders.
 
Some of the newer engines automatically dump the EOT when there is either 1) a trainline emergency or 2) engineer initiated emergency.  I found that out when I was cutting in the air on a cut of cars that just came off yard air.  The rear part with EOT was bottled because I just removed the yard air.  When I opened up the sticky angle cock on the head cars, the head end dumped, followed a few seconds later by the rear.  Sorry, rambling.
 
Airbrakes release after an approximately 2 pound increase in brake pipe pressure (but not always).  If the cars are off air, they usually start to release after being charged to somewhere between 60 and 65 pounds.
 
When releasing the air brakes, the emergency reservoir assists in the release by taking air in there and adding it to the brake pipe to get a quicker release.
 
If anything I said is not 100% right, please correct me.  Any questions, feel free to ask.


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Birdman
TRAINing
Posts: 6
Re: Question about air brakes
 
« Reply #8 on: Nov 11th, 2008, 1:27am »
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The lead unit is the only one to charge air on the whole train, and it does it through a 19/64 orifice if I remember correctly, so it takes a while. Some yards use "yard air" to keep the cars charged up until the locos "units" arrive for this reason.

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rmgill
Chaser
Posts: 84
Re: Question about air brakes
 
« Reply #9 on: Jan 10th, 2010, 11:30am »
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Trackmobile has a very decent set of animations showing how railroad brakes function. There are some specifics that are related tot he trackmobile itself, but overall the basic principles are shown.  
 
http://www.trackmobile.com/trainairani.htm


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