Western Railroad Discussion > Air brake question from novice

With all the talk of air brakes or lack of setting them I have a question. How do they work? I ask about because if a train looses the air how can the brakes release? The derailment in Canada brings this up.
I thought that once the air is out of the system the brakes would stay on. I am not sure I am asking the question correctly.

This is one of the many unanswered and confusing questions coming from Quebec. I think we all need to wait for more information to know what happened.

Posted from Windows Phone OS 7

fbe Wrote:
-------------------------------------------------------
> This is one of the many unanswered and confusing
> questions coming from Quebec. I think we all need
> to wait for more information to know what
> happened.
>
> Posted from Windows Phone OS 7


This is not my question. I thought that once the air is out of the system that is if a line is broken the brakes stay on. Hoe do they release? Is there a spring in the cylinder that releases when the air is reintroduced?

A leaking seal anywhere in the system on only one car on the train will finally equalize the pressure and the brakes are then released.

Air Brakes 101:

https://en.wikipedia.org/wiki/Railway_air_brake

The most common misperception about air brakes is that air pressure keeps the brakes from applying, so that if pressure is lost, the brakes apply. This is not true. If it were, then it would be impossible to switch railcars in a yard without charging the air after every coupling. Instead, air pressure is used to apply the brakes, and changes in pressure are mechanically detected in the system to control brake applications; a rapid loss of trainline air pressure releases other air stored in the system to apply brakes in an emergency.



Edited 1 time(s). Last edit at 07/10/13 09:19 by toledopatch.

There are several good descriptions out the of how the air brakes work. I think a gentleman by the name of Al Krug does some of the best. Try looking at
http://www.railway-technical.com/brake2.shtml

In general, each car stores up air pressure in the reservoir that is then used to apply pressure on the brakes when the pressure in the air brake line drops. With no air supplied eventually that stored air pressure will leak out.

Stephen

>[train air brakes] How do they work?

Pressurize the reservoirs on the train via the brake pipe, then let air out of the brake pipe, and the brakes on each car apply.

>I ask about because if a train looses the air how can the brakes release?

Repressurize the brake pipe or bleed the air out of each car individually.

>The derailment in Canada brings this up. I thought that once the air is out of the system the brakes would stay on.

It's possible for them to "self-release" under very special circumstances and/or leak off, what causes them to do so is the key.

>Is there a spring in the cylinder that releases when the air is reintroduced?

There is a spring, but all it does is force the piston back into the cylinder when BC air is exhausted so the brake shoes don't drag on the wheels, increasing fuel consumption and undesirable/unnecessary wear and tear on the equipment. It's not strong enough to "release the brakes" (you can actually compress slightly it by hand), and if the foundation brake rigging sticks or binds up, it won't be able to force the piston back into the cylinder.

>A leaking seal anywhere in the system on only one car on the train will finally equalize the pressure and the brakes are then released.

Too generalized and vague. Scroll through the posts here: http://www.trainorders.com/discussion/read.php?10,3122382

>The most common misperception about air brakes is that air pressure keeps the brakes from applying, so that if pressure is lost, the brakes apply. This is not true.

Well, it is true because the the key feature in the success of the automatic air brake is the constant pressurization of the Brake Pip to its operating pressure, which keeps the brakes released when you're not using them to slow or stop the train.

>If it were, then it would be impossible to switch railcars in a yard without charging the air after every coupling. Instead, air pressure is used to apply the brakes, and changes in pressure are mechanically detected in the system to control brake applications; a rapid loss of trainline air pressure releases other air stored in the system to apply brakes in an emergency.

If it "was", not "were". The way you switch cars without the air [in a yard] is to bleed the air out of them before switching so the air brake system is not used. For those occasions when you do have to switch cars "with the air", then yes, you have to make up the brake hoses and cut in the air to each car you couple to (you also have to be sure the rear angle cock on the car your hanging onto is closed at all times).



Edited 1 time(s). Last edit at 07/10/13 09:43 by EtoinShrdlu.

santafedan Wrote:
-------------------------------------------------------
> fbe Wrote:
> --------------------------------------------------
> -----
> > This is one of the many unanswered and
> confusing
> > questions coming from Quebec. I think we all
> need
> > to wait for more information to know what
> > happened.
> >
> > Posted from Windows Phone OS 7
>
>
> This is not my question. I thought that once the
> air is out of the system that is if a line is
> broken the brakes stay on. Hoe do they release?
> Is there a spring in the cylinder that releases
> when the air is reintroduced?

It is a multifaceted system. If ALL of the system is devoid of air then there is no air pressure to apply the brakes nor maintain the set if there was one.

If the brakes were set and the air leaked out of the trainline the brakes would remain applied. Eventually, the air in each car would leak off releasing the brakes in that car. Some cars could leak off in an hour or so, other cars might still have the brakes applied days or weeks later.

The idea that a train with the air brakes applied could have the brake pipe leak away and enough of the cars leak off their brake cylinder to allow the train to begin rolling in 15 minutes does not seem possible, in my experience. With more information we will have a better foundation to work from.

After the engineer applies the train brakes by reducing the brake pipe pressure, leakage in the brake pipe will apply the brakes harder to a certain point and hold them there beyond the point the trainline goes to 0 psi. Someone who has recently been through an engineers' training class can probably give the specific brake pipe pressure where the brake application is at maximum.

Some cars can leak off in a minute.....

This is why a sufficient number of hand brakes are required to hold the train in place. In case the air leaks off.

In a nutshell; The brake pipe feeds air into two reservoirs on each car. One is the service reservoir and the other is the emergency reservoir.

The service reservoir responds to the gradually changing pressure of the brake line, as the brake line pressure decreases it equalizes by feeding the air from the reservoir into a brake cylinder, which physically applies the brakes. By increasing the brake pipe pressure the reservoir equalizes by drawing the air out of the brake cylinder, releasing the brakes. If the air in the reservoir is drained into the atmosphere and not recharged from the brake line (no air coming from the locomotive, or the brakes on that car are cut out) the cylinder actually defaults to a released position. This is how cars can be dropped, kicked or humped, also how a train car with stuck brakes can get over the road (Cut out the car and drain the reservoir).

The emergency reservoir responds when the valves from the service reservoir cannot keep up with the rate at which the air is decreasing in the brake line. In the event of a rapid decrease in brake pressure the emergency valve opens and quickly drains all the air from the emergency reservoir into the brake cylinder.

Without a source of new compressed air (from the locomotive or wayside source) the air in the brake cylinders and reservoirs can leak out, sometimes quickly, which results in the brake cylinders releasing the brakes. The industry requires that a "sufficient number" of handbrakes be set if the train is being left unattended or without a source of compressed air just to prevent cars from rolling when the air leaks off. In the Quebec runaway there is discussion that the train cars may have been tampered with by somebody,, possibly releasing handbrakes (although the other fact is that the train was waiting for a new crew, it's possible the crew trusted the air to hold the train and did not set brakes, though I doubt it as setting handbrakes on unattended equipment is engrained into every railroader from training on.)



Edited 2 time(s). Last edit at 07/10/13 11:24 by OTG.

From what I've learned, air brakes should never be trusted when leaving a train, locomotives attached or not.

Wouldn't a few set handbrakes prevented this? There was no mention of the handbrakes being set or not set, but I bet if they had been the train wouldn't have rolled. My experience has been that when a train is "tied-down" as seems to be in this case, handbrakes are usually set on the first 5-10 (or more) cars in a train regardless if attached to a locomotive or locomotives. Obviously to prevent the cars and/or train from rolling should something happen. If the cars came uncoupled from the loco, the handbrakes should have held the cars if they had been set. I would imagine that longer the train the more handbrakes need to be set.

Lee Ryan - Rio Grande Fan
Denver, CO

Set out a single car, cut away, the brakes go into emergency. Eventually the air behind the piston will leak away because it's just a worn rubber cup type gasket around the piston. Not meant to hold the piston out forever. The spring inside will retract the piston back into the cylinder. So, chock the wheels, spin up the handbrake, couple to other equipment that has brakes, or in some scenerios, chain the car to the rail.

I think a lot of people don't understand that in the case of a train with locomotives attached, there are four braking systems in play that are somewhat independent of each other. Each rail car has an air brake system and a manual hand brake. Each locomotive also has air brakes and hand brakes. If this train was properly secured, the air brakes could completely fail and release and it should have stayed in place.

According to things I've read elsewhere in some of the railroading forums, the engineer/conductor (one-man crew) tied down (secured the hand brakes) on all five locomotives and the first ten cars. By the rules, he then should have then released the air brakes on the locomotives AND cars to make sure the train remained stationary. If not, he would have had to apply more hand brakes.

As fbe said, the possibility of the entire train bleeding off in such a short period of time would be almost unheard of. Even so, sufficient hand brakes should have prevented it from moving, air or not. Unfortunately, only two scenarios seem plausible here to me. One is that the train was simply not properly secured and tested for its weight and location. Two is that vandalism or some other human act occurred releasing some or all of the hand brakes, at least enough to not be sufficient to secure the train without air although I still don't see how the entire train could have bled off so quickly. Either way, I see more rules about securing trains, especially those containing hazmat,coming at us in the future.

trainjunkie Wrote:
-------------------------------------------------------
> ... Either way, I see
> more rules about securing trains, especially those
> containing hazmat,coming at us in the future.

Maybe even some rules about where you can park them.

At least on BNSF, we had "Key" trains. These were particular types of Hazmat trains that were not supposed to be operated over 50 mph, and 10 mph in sidings. Other restrictions were how long and where these trains could be tied down and/or left unattended. I'm retired now, but I imagine this is still in effect. Wondering if these crude oil trains are Key trains.

EtoinShrdlu Wrote:
-------------------------------------------------------

> >The most common misperception about air brakes is
> that air pressure keeps the brakes from applying,
> so that if pressure is lost, the brakes apply.
> This is not true.
-------->
> Well, it is true because the the key feature in
> the success of the automatic air brake is the
> constant pressurization of the Brake Pip to its
> operating pressure, which keeps the brakes
> released when you're not using them to slow or
> stop the train.
-------->
> >If it were, then it would be impossible to switch
> railcars in a yard without charging the air after
> every coupling. Instead, air pressure is used to
> apply the brakes, and changes in pressure are
> mechanically detected in the system to control
> brake applications; a rapid loss of trainline air
> pressure releases other air stored in the system
> to apply brakes in an emergency.
--------->
> If it "was", not "were". The way you switch cars
> without the air is to bleed the air out of them
> before switching so the air brake system is not
> used. For those occasions when you do have to
> switch cars "with the air", then yes, you have to
> make up the brake hoses and cut in the air to each
> car you couple to (you also have to be sure the
> rear angle cock on the car your hanging onto is
> closed at all times).


My point was that it's not so basic as "air pressure directly keeps brakes off, loss of air and they clamp on". It's more complicated than that.

I think this thread on the Steam board seemed like a good explanation to me:

http://www.trainorders.com/discussion/read.php?10,3122382

Jim B

jimB Wrote:
-------------------------------------------------------
> I think this thread on the Steam board seemed like
> a good explanation to me:
>
> http://www.trainorders.com/discussion/read.php?10,
> 3122382
>
> Jim B


Jim i agree it spells it out, he did a very good explanation of it.It was reported that 11 hand brakes should have been applied by a railroad
employee i saw on the news.Steve(up6553)

Am I correct in understanding that modern brake valves will maintain brake cylinder pressure as long as brake pipe remains at a constant pressure after a set.

In other words, if a 20 lb set is made to drop the brake pipe to 70PSI, as long as a locomotive is keeping the bake pipe at 70PSI, the valve will keep the cylinder at the correct pressure (2.5x20PSI=50PSI) even as it slowly leaks out seals? This would allow a parked train with a partial set, to hold the brakes on.

This would be a change from older air brakes which only vented to he cylinders when the pressure changed. In which case, even with a 20lb set, the cylinders would leak off.

Scott Schifer
Torrance, CA
TinyTrains Website

"In other words, if a 20 lb set is made to drop the brake pipe to 70PSI, as long as a locomotive is keeping the bake pipe at 70PSI, the valve will keep the cylinder at the correct pressure (2.5x20PSI=50PSI) even as it slowly leaks out seals? This would allow a parked train with a partial set, to hold the brakes on. "

In between the reservoirs, brake pipe and brake cylinder is the control valve which used to be called the triple valve. It is the most complex component of the air brake system mounted on the car. It can suffer a myriad of problems causing the brakes on the car not to set or to release quickly with no increase in brake pipe pressure. This may even hold the application on the car after the engineer has initiated a release. These are three of the things which will cause the car to be set out as having failed the air test given every time a train is made up or a car is picked up. Defective cars do slip by in an inspection or fail shortly after. A temperature change after the sun goes down or as the train climbs elevation may bring a failure.

Posted from Windows Phone OS 7