how do airbrakes work??????

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how do airbrakes work??????
Posted by wisandsouthernkid on Tuesday, April 29, 2008 10:07 AM
can you explain to me in great detail how the air brakes on a locomotive and cars work i dont get how the last car would hold the whole train back until it is triggered???
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Posted by UPRR engineer on Tuesday, April 29, 2008 11:33 AM

http://www.railroad.net/articles/railfanning/airbrakes/index.php

Your gonna have to do some of your own hunting for answers, theres a good place to start in that link.

Theres ALOT to it.

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Posted by oltmannd on Tuesday, April 29, 2008 12:11 PM

 wisandsouthernkid wrote:
can you explain to me in great detail how the air brakes on a locomotive and cars work i dont get how the last car would hold the whole train back until it is triggered???

I think your question is more than an air brake question.

This isn't going to be very detailed, e.g. an explanation of all the inner workings of all the valve portions of a 26L brake system, but I'll try to hit all the science high points.

The air brake system uses air two ways.  One is to provide the power to move the brake shoe against the wheel.  The other is to send a control signal to regulate how much to apply the brake.  The genius of Westinghouse is that he figured out how to do both jobs with one brake pipe.

The power part comes from air stored in a reservoir on each car.  Each reservoir on each car is charged up by air supplied from the locomotive through the brake pipe.

Normally, with the train rolling along and the brakes released, the locomotive will be maintaining 90 psi in the brake pipe and the reservoirs will be charged up to 90 psi, too.

When the engineer wants to apply the brakes, he has to send a signal to each car to tell it how much brake to apply.  He does this by reducing the amount of pressure in the brake pipe.  Let's say he makes a 10 psi reduction so that the brake pipe is now at 80 psi.

The valve on each car will react to the imbalance of pressure between the reservoir and the brake pipe by bleeding some air from the reservoir into the brake cylinder until the brake pipe and reservoir are equal again - at 80 psi.

When he wants to release the brakes, he sends the signal by increasing the brake pipe pressure back to 90 psi.  This tells each car to let the air out of the brake cylinder and start recharging the reservoir back to 90 psi.

But, I think your question is more directed at the timing of all these activities.

The "problem" with air brakes is that the increase and decrease in pressure along the brake pipe is limited in how fast it can go to the speed of sound.   Sound, after all, is just an air pressure wave.  The speed of sound is roughly 1000 feet per second, so for a 6000 ft long train, the signal to apply the brakes won't reach the last car for a full 6+ seconds after the first car gets the message.

This lag, added to all the slack in a train, can cause all kinds of trouble if you don't take care how, when, where and how much you apply the brakes.  The engineer has to factor in whether the train is going up hill or down hill (or both) whether the slack is bunched up or not and whether he's trying to stop or will be doing a running release and what the conditions will be at that time in determining how to handle his train.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by wabash1 on Wednesday, April 30, 2008 8:23 AM
 oltmannd wrote:

 wisandsouthernkid wrote:
can you explain to me in great detail how the air brakes on a locomotive and cars work i dont get how the last car would hold the whole train back until it is triggered???

I think your question is more than an air brake question.

This isn't going to be very detailed, e.g. an explanation of all the inner workings of all the valve portions of a 26L brake system, but I'll try to hit all the science high points.

The air brake system uses air two ways.  One is to provide the power to move the brake shoe against the wheel.  The other is to send a control signal to regulate how much to apply the brake.  The genius of Westinghouse is that he figured out how to do both jobs with one brake pipe.

The power part comes from air stored in a reservoir on each car.  Each reservoir on each car is charged up by air supplied from the locomotive through the brake pipe.

Normally, with the train rolling along and the brakes released, the locomotive will be maintaining 90 psi in the brake pipe and the reservoirs will be charged up to 90 psi, too.

When the engineer wants to apply the brakes, he has to send a signal to each car to tell it how much brake to apply.  He does this by reducing the amount of pressure in the brake pipe.  Let's say he makes a 10 psi reduction so that the brake pipe is now at 80 psi.

The valve on each car will react to the imbalance of pressure between the reservoir and the brake pipe by bleeding some air from the reservoir into the brake cylinder until the brake pipe and reservoir are equal again - at 80 psi.

When he wants to release the brakes, he sends the signal by increasing the brake pipe pressure back to 90 psi.  This tells each car to let the air out of the brake cylinder and start recharging the reservoir back to 90 psi.

But, I think your question is more directed at the timing of all these activities.

The "problem" with air brakes is that the increase and decrease in pressure along the brake pipe is limited in how fast it can go to the speed of sound.   Sound, after all, is just an air pressure wave.  The speed of sound is roughly 1000 feet per second, so for a 6000 ft long train, the signal to apply the brakes won't reach the last car for a full 6+ seconds after the first car gets the message.

This lag, added to all the slack in a train, can cause all kinds of trouble if you don't take care how, when, where and how much you apply the brakes.  The engineer has to factor in whether the train is going up hill or down hill (or both) whether the slack is bunched up or not and whether he's trying to stop or will be doing a running release and what the conditions will be at that time in determining how to handle his train.

 

Great answer and the only correction i have ( and it is how i was taught ) is that the signal is 900 ft per sec for a regular brake pipe reduction and 600 feet per second for a emergency application. 

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Posted by BigJim on Wednesday, April 30, 2008 3:41 PM

Great answer and the only correction i have ( and it is how i was taught ) is that the signal is 900 ft per sec for a regular brake pipe reduction and 600 feet per second for a emergency application. 

Of course, Wabash1 meant 600 ft./sec for a service application and 900 ft./sec. for an emergency application.

Also check out this web site.
http://www.alkrug.vcn.com/rrfacts/brakes.htm

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Posted by wisandsouthernkid on Wednesday, April 30, 2008 9:03 PM
does it matter??? no cockiness involved
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Posted by BigJim on Wednesday, April 30, 2008 10:04 PM
As a matter of fact it DOES!
Now, if you have done your homework on this subject you should be able to tell me why it matters.

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Posted by caduceus on Friday, May 02, 2008 11:06 AM
Many many thanks for the link.  I love reading about the details. 
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Posted by Boomer Red on Sunday, May 04, 2008 1:22 PM
So if high pressure means "brakes released" and low pressure means "brakes applied" how do runaways happen? Shouldn't a loss of air pressure mean that the brakes would apply. Confused [%-)]
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Posted by UPRR engineer on Monday, May 05, 2008 6:20 AM

To fully answer that question will take alot of typing also, one thing that can happen, "out run your brakes". Too much speed with too many tons, you'll go to the bottom of the hill. During hoghead school on the sim, the instructor got it set up on a grade. He letter get up to 70 then dumped it.

Theres some other things that can happen that leads to a run away.

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Posted by oltmannd on Monday, May 05, 2008 8:53 AM

 Boomer Red wrote:
So if high pressure means "brakes released" and low pressure means "brakes applied" how do runaways happen? Shouldn't a loss of air pressure mean that the brakes would apply. Confused [%-)]

Yes - in theory.  That's another part of the genius of Westinghouse.  If the trainline is broken, the brakes apply. 

But, your question is really, can the brakes stop the train?

There are two typical reasons for runaways.  One is the train gets going to fast before the brakes are applied.  The other is there is insufficient braking available.

What the brakes are actually doing when you descend a hill, is disapating the potential energy (train's mass X elevation change) as heat in braking.  If the train is going down hill fast, then the rate at which that energy must be disapating is high, so the brakes and wheels can get very hot.  At some temperature, the brakes will start to fade  (ever drive a car with 4 wheel drum brakes?) and the ability of the brakes to disapate heat will exceed the rate needed to keep the speed steady or slow the train.

The second way a train can run away is insufficient braking.  After you make a running release, it takes a while for the reservoirs a good bit of time to charge back up again.  If you make another application before the reservoirs are all charged up again, the maximum braking force will be less.  If you do multiple running releases, you can wind up with viturally no braking at all.

RRs ususally have pretty strict rules about speeds on descending grade and air brake operation on grades because of this.

Other causes for insufficient braking can be too much air pressure gradient from front to rear - usually due to leaks in the train line. (rear cars won't have much braking force), too many cars with brakes cut out, angle **** left closed mid-train, leaky brake cylinders, others?

RRs also have two-way EOTs so that if an angle **** is left turned mid-train, the air can be dumped from the rear (result of a bad wreck on CN several years ago?)

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by BigJim on Tuesday, May 06, 2008 11:19 AM

So if high pressure means "brakes released" and low pressure means "brakes applied" how do runaways happen? Shouldn't a loss of air pressure mean that the brakes would apply.

One thing you need to know is, if there is no air in the system, you can't have any air brake on the car/train. If the car/train is in a yard ready to be humped, then the cars have probably had the air bled off of them. Cars left standing as such or on sidings or industrial tracks should be secured with hand brakes. The air can leak down over time and then the air brake will release. So that is another reason for runaways.

The long story about runaways is called "bottling the air". It's a long drawn out story. The short version is, say a trainman needs to set off a car, the train is very long and it takes a long time to pump the brakes off after coupling back up. The guys in a hurry to get off, so when making the cut, he closes both anglecocks so he won't lose the air in the cars left standing while he sets off.
When the train stopped, the enginner made a brake reduction, this created a pressure gradient in the train line. As the head end is placing the car on the spur, all hell is breaking loose out on the mainline. That pressure gradient in the trainline has been equalizing throughout the train. This equalization has caused a pressure increase on the head end of the cars and the brakes have started to release. As each car's brakes have released, the control valve has also released a small amount of air into the brake pipe. This is called the quick release feature. So on down the length of the train the brakes on each car totally release and the train has rolls away! Bad juju, eh?

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Posted by oltmannd on Wednesday, May 07, 2008 6:46 AM
 BigJim wrote:

So if high pressure means "brakes released" and low pressure means "brakes applied" how do runaways happen? Shouldn't a loss of air pressure mean that the brakes would apply.

One thing you need to know is, if there is no air in the system, you can't have any air brake on the car/train. If the car/train is in a yard ready to be humped, then the cars have probably had the air bled off of them. Cars left standing as such or on sidings or industrial tracks should be secured with hand brakes. The air can leak down over time and then the air brake will release. So that is another reason for runaways.

The long story about runaways is called "bottling the air". It's a long drawn out story. The short version is, say a trainman needs to set off a car, the train is very long and it takes a long time to pump the brakes off after coupling back up. The guys in a hurry to get off, so when making the cut, he closes both anglecocks so he won't lose the air in the cars left standing while he sets off.
When the train stopped, the enginner made a brake reduction, this created a pressure gradient in the train line. As the head end is placing the car on the spur, all hell is breaking loose out on the mainline. That pressure gradient in the trainline has been equalizing throughout the train. This equalization has caused a pressure increase on the head end of the cars and the brakes have started to release. As each car's brakes have released, the control valve has also released a small amount of air into the brake pipe. This is called the quick release feature. So on down the length of the train the brakes on each car totally release and the train has rolls away! Bad juju, eh?

ECP braking should make bottling the air a reasonable thing to do - at least for the duration of a pickup or setoff.  Wonder if they'll make it legal.....

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by geomodelrailroader on Saturday, April 08, 2017 12:37 AM

Bottling the air is dangerous and is illegal. When a car is set out both hand brakes on the car and the car's air brakes need to be applied before you close the anglecocks. If you bottle the air when the mu cable and air hose are disconnected from the train it will trigger a false brake application or a false release. This is why you do not bottle the air because under each car and locomotive are dump valves and they will open up if the anglecocks are closed. The correct way you uncoupled is you tie down the train or car and move the rear anglecocks to release and the forward anglecocks to closed this way all of the air is dumped out of the pipe and the brakes don't release on the cars it also prevents the cars attached to the locomotive from going into emergency. When attaching a locomotive to a train all the anglecocks need to be open and the FRED beacon needs to be placed on the rear. This tells the engineer that the air pressure is inside the pipe because instead of dumping the air it needs to charge the compressor on the cars. When all of the compressors are filled with air the brakes are released and the train can move. 

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Posted by BaltACD on Saturday, April 08, 2017 9:07 AM

geomodelrailroader
Bottling the air is dangerous and is illegal. When a car is set out both hand brakes on the car and the car's air brakes need to be applied before you close the anglecocks. If you bottle the air when the mu cable and air hose are disconnected from the train it will trigger a false brake application or a false release. This is why you do not bottle the air because under each car and locomotive are dump valves and they will open up if the anglecocks are closed. The correct way you uncoupled is you tie down the train or car and move the rear anglecocks to release and the forward anglecocks to closed this way all of the air is dumped out of the pipe and the brakes don't release on the cars it also prevents the cars attached to the locomotive from going into emergency. When attaching a locomotive to a train all the anglecocks need to be open and the FRED beacon needs to be placed on the rear. This tell the engineer the air pressure that is inside the pipe because instead of dumping the air it needs to charge the compressor on the cars. When all of the compressors are filled with air the brakes are released and the train can move.

The ONLY air compressors are on locomotives.  Cars have air resevoirs that provide the braking power when a reduction in trainline air pressure sends a signal to each cars Brake Valve.  The resevoirs get charged by the locomotive air compressors through the braking trainline, when in 'running' condition with brakes released the trainline and resevoir pressures are the same.  When the brakes are applied the engineer removes air from the trainline,  trainline pressure now being less than the resevoir pressure lets that much resevoir pressure work against the various rods and bellcranks to apply the brake shoes to the wheels.

         

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Posted by RME on Saturday, April 08, 2017 12:52 PM

The correct way you uncoupled is you tie down the train or car and move the rear anglecocks to release and the forward anglecocks to closed this way all of the air is dumped out of the pipe and the brakes don't release on the cars it also prevents the cars attached to the locomotive from going into emergency.

In the interests of fair notice, I would point out that this is the same poster who resurrected a similarly 'zombie' thread, on the etymology and use of the term 'foamer', to enlighten us with these pearls of wisdom:

The word Foamer is an insult to people who have Autism. F stands for an expletive we can not say, O stands for Overreactive, A stands for Arrogant, M is for Mentally Retarded, E is for Eccentric, and R is for Railfans. It is a bad word and it should not be in anyone's vocabulary. The acronym is as bad as the word and nobody should say it ever.

The thing about posts like these is that people new to the forums might read this sort of yivshish and think it represents actual railroading experience or knowledge.

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Posted by samfp1943 on Sunday, April 09, 2017 1:35 PM

IF the OP is really interested in finding out an answer...Bang Head I suggest he uses the link Big Jim posted for Al Krug's site.

link @ http://hm.evilgeniustech.com/alkrug.vcn.com/rrfacts/rrfacts.htm

 Then, he can start Part1 with this link @ http://hm.evilgeniustech.com/alkrug.vcn.com/rrfacts/brakes.htm

There is a wealth of information on Al's site, and we owe a Thanks to Paul North for uncovering its latest "home on the web' !Bow

Sam

 

 


 

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Posted by geomodelrailroader on Sunday, April 16, 2017 3:11 AM

I never intended to start an argument on that post. The acronym of foamer is an insult. We will fans on the West Coast don't like it and neither do the crews or police. It is East Coast slang and it is also European slang which should not be in the vocabulary of the Western United States. The east coast of America and all of Europe go through cities where are the buildings are right next to the tracks. Out west we do not have that problem the railroad just tell us to stay away from tracks and we have Operation Lifesaver out here back east and in Europe they don't neither do they in Canada or Mexico. Foamer is a pregitive term and when it is used as an acronym it is an insult. 

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Posted by BaltACD on Monday, April 17, 2017 4:32 PM

The easily 'butt hurt' don't make it - either in employment on railroads or among railfans.  Insults make the trains run - all levels from the man with the spike mall to the board room  - insults make it all happen.

         

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Posted by spsffan on Monday, April 17, 2017 5:57 PM
Hey, I was born and raised in California, will be turning 55 on Thursday, and am proud to be a foamer!
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Posted by CandOforprogress2 on Monday, April 17, 2017 9:58 PM

So in major yards like CSX Cumberland why is there "Yard Air"?

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Posted by BaltACD on Tuesday, April 18, 2017 1:42 PM

CandOforprogress2
So in major yards like CSX Cumberland why is there "Yard Air"?

When a train arrives at a hump yard, such as Cumberland, the Car Department as a part of their arrival inspection of the train bleeds the air off all cars in the train so that the train can be switched going over the hump.  The cars are free rolling and their speed is managed by the retarders that are a part of the hump operation.  There are specific rules that apply to the first cars that are placed on a clear Bowl track as to securing them.  Once a track for a outbound train has been 'closed out', the Car Department will begin coupling all air hoses, attaching Yard Air and making a Class 1 air test of the track (Class 1 is where the brake application is 'walked' and inspected to insure there are no cars with excessive brake piston travel, the brake release is also walked to insure that all brakes release.)  After this inspection is performed the track cannot be in a position to be 'off air' for over 4 hours or the inspection procedure must be repeated.

In some hump yards the above work may be done in one of the Bowl tracks (where cars get switched to) or it may be done on a Departure track after several Bowl tracks have been coupled together to form the Outbound train.  Some Hump yards depart Outbound trains from Bowl tracks, some don't.  Each Hump yard has it's own unique set of operating procedures based on the physical characteristics of the facility.

         

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Posted by CandOforprogress2 on Tuesday, April 18, 2017 3:18 PM

In Cumberland the Yard is quite long. We have WB "Departure Tracks" that start in Downtown Cumberland behind the Holiday Inn and I am sure that there are some WB tracks in the yard proper 2 miles east. There are compressor stations that feed air to these tracks. There has been a ongoing problem with townspeople crossing live tracks and live trains where these trains sit waiting for power because there are nieghboorhoods that the railroads here have cut Cumberland into quarters. The Beer Store is on the wrong side of the tracks and the people who need beer live on the other side of the yard. Getting a Ped Bridge is needed but a nightmare to work with the railroad the FRA and any buaracracy in general

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