Vacuum Brakes

[Stefan Andrzejewski]

Can the experts please tell us how vacuum brakes on a train work. My understanding is that when the Loco is uncoupled, the brakes are on until there is vacuum in the pipes to release the brakes. I have seen Loco changes where the new Loco's pull forward to check the coupling. Without the pipes connected the train stands firm. Thank you.

[John Ashworth]

Stefan, you might like to look at http://www.friendsoftherail.com/phpBB2/ ... =45&t=5685, which has some discussion of vacuum brakes.

There's a good explanation here

I'm also attaching an animated diagram that shows the vacuum brake cycle.

The brakes depend on a difference in air pressure on the two sides of the piston in the brake cylinder. Most vehicles have two brake cylinders. When the driver creates vacuum in the train pipe (from the locomotive through all the vehicles) it creates vacuum on both sides of the piston. To apply the brake, air is allowed into the train pipe. A ball valve closes so that the volume above the piston is separated from the volume below; air pressure now pushes the piston up, applying the brake. To release the brake, a vacuum is created in the train pipe. The pressure on both sides of the piston equalises and gravity causes the heavy piston to drop, releasing the brakes.

So you are basically correct. Creating vacuum releases the brakes. Destroying the vacuum applies the brakes. When the loco is uncoupled the vacuum pipe is uncoupled so the vacuum in the train pipe (below the piston) is destroyed and the brakes remain on. You're right also that the loco will often pull forward to check the coupling before the vacuum pipes are coupled, and the load stands firm.

However the vacuum brake is not sufficient to hold vehicles indefinitely. The vacuum on the vacuum reservoir side (top) of the pistons will slowly leak away, allowing the pressure on both sides of the piston to equalise and thus releasing the brakes. So if the vehicles are to be left for any length of time, other forms of braking must be used - either the handbrakes must be applied on some or all of the vehicles, or wooden scotches must be placed under the wheels.

The brakes on a stationary vehicle can also be released by pulling the strings or wires on each vacuum cylinder, which destroys the vacuum on the vacuum reservoir side of the piston, again equalising the pressure on both side of the piston so the piston drops and the brakes are released. This should not be done unless another form of braking is already in use (handbrakes or scotches) or unless the vehicles are coupled to a locomotive.

I hope that's clear, and that it's correct (it's late at night so apologies for any errors!). I'm sure Gabor, Steve and many others can add to it, correct it or make it clearer.

[Greg Hart]

let me try..

When air is admitted to the train pipe, the brakes come on. When creating vacuum air is exhausted from the train pipe, creating a vacuum. If the driver applies brakes or the pipe between coaches break, air is admitted to the train pipe. Some or all of the vacuum will be destroyed in the process. So air travels through the pipe from the front end to the back, so the brakes start applying from the front first and gradually gets to the back.. So If you pull or brake a pipe off and quickly replace it on the dummy when uncoupling a loco, air would of only traveled through 1 , 2 or 3 coaches applying minimal brakes, the rest of the train will still have almost full vacuum (if there is new pipes along the train and no leaks). So when uncoupling, its vital to "listen" to hear all the vacuum being destroyed (yes, you can hear it I promise) or wait about 5 - 10 seconds before putting the pipe on the dummy, thus making sure that all the brakes would of or should be applied.. depending on the length of train of course..

hope it kinda helps
cheers

[Steve Appleton]

John's explanation is spot on.

To add some more detail:

Of course the brakes are only as effective as the maximum vacuum initially created. Which is why, as a matter of course after coupling on the loco and before setting off, a 'brake test' is always conducted. In this, the amount of train vacuum is measured at the rear-most vehicle in the train. This has to be above a certain minimum level (at least 51 kPa - just over half an atmosphere. On most FOTR trains the vacuum level is normally much greater, usually 60+ kPa. The loco's ejector is set at 64 kPa maximum). On longer trains, due to inevitable small leaks in hose couplings, etc, it can be difficult and take some time to get to that minimum value.

In addition, to ensure that the vacuum pipes are coupled right through the train, the brake pipe is removed from the last dummy. Air is allowed into the train pipe and a visual check is made that the brakes fully apply on at least the last vehicle and that the driver also sees a drop in vacuum level at the locomotive. To now be able to release the brakes on the last vehicle (and indeed by this time the whole train) the driver is forced to recreate the vacuum before he can move off. All of this ensures that the vacuum is sufficient, that there are no excessive leaks and that the brakes are coupled through the train. In addition it ensures that in the worst case, even if only the last vehicle parts from the train, its brakes will apply.

The system itself is 'dynamic' and relies on the presence of sufficient vacuum in the top section of the vacuum cylinders. Over time and with repeated brake applications this residual vacuum level will reduce unless it is routinely recreated. Any gradual reduction of vacuum will seriously reduce the brake efficiency. This is especially important on long downgrades. For this reason, drivers are warned not to 'milk' the brakes (make repeated short or partial brake applications) and to fully release the brakes at regular intervals and for long enough to maintain full train vacuum.

The vacuum brake system, although simple, good, reliable and tested by time, is indeed not 'fool proof'.

As Greg rightly says, it is indeed possible to create a 'parked' rake of vehicles whose train brakes are only partially applied. In my experience it is more usual, out of laziness if nothing else, for the shunter to uncouple the brake pipe and leave the train end dangling (thus definately fully applying the brakes) rather than to place it very quickly on the dummy. As Greg also says, the brakes nearest the uncoupling will apply first. However, with time, the vacuum drop will propogate evenly down the rake, and the brakes will all end up roughly evenly applied, albeit not fully.

What is of interest is that to lengthen the time that the brakes will remain effective, many vehicles are fitted with an extra vacuum tank or reservoir coupled to the top of the cylinder. This increases the 'volume' of the vacuum storage available at each brake cylinder and the lengthens the time before the residual vacuum drops too jmuch and the brakes 'leak-off' or become ineffective. This provides an extra safety margin for long applications on long gradients and whilst parked. It also increases the time it takes to create or restore the initial vacuum. That resevoir is shown on the diagram in John's post. Nonetheless, its provision is not universal and not all vehicles are so fitted.

[John Ashworth]

The system itself is 'dynamic' and relies on the presence of sufficient vacuum in the top section of the vacuum cylinders.

I think this is an important point that Steve makes and it is not always understood by non-railwaymen. I have seen discussions on other sites where people assume that a stationary train will always have its brakes applied as there is no vacuum in the system, and they are surprised that this is not the case. They assume that "failing to safety" means that the brakes will always be applied when vacuum is destroyed in the train pipe, whereas as Steve says, this is only true in 'dynamic' circumstances, ie when there is vacuum on the top (or reservoir) side of the piston.

So when a train is running, the brakes should always fail to safety because there is vacuum above the piston (unless the driver has inadvertently 'milked' it off, as Steve says). Any reduction in vacuum in the train pipe, from whatever cause, deliberate or accidental, will cause air pressure to push the piston upwards and apply the brakes.

However on stationary vehicles, when there is no vacuum above the piston (either because it has leaked away or because someone has pulled the strings), then atmospheric pressure (rather than vacuum) equalises on both sides of the piston, the piston drops under gravity, and the brakes are released.

[Steve Appleton]

Indeed, brake management and having the train always 'under control' are a major part of the skills set learnt by a driver. I have heard (and I am sure John has, too) several stories of near disasters, or at best close calls, that I am sure are not exaggerated involving the near loss of train control on down grades and similar circumstances.

One major flaw in the vacuum brake is that to 'recharge' the system (to recreate the maximum vacuum in the cylinders) requires the total release of the train brakes for a considerable length of time - several minutes.

Imagine the scenario. You crest the summit of 'xxx' pass and you start down the grade. It will be some 30 minutes or more at 20 to 30 km/h to get your 1000 tonne (maybe heavier) train to the bottom. You apply the brakes. Slowly, imperceptively at first, the brakes gradually become less effective. They are 'leaking-off'; the vacuum above the cylinders and in the reservoirs is reducing. You make progressively heavier brake applications until that's it, you are at maximum braking but you are still accelerating. You will never make it. You need to recharge the brakes - now. Should have done that a while back. Of course the only way to do that is to release the brakes completely for a few minutes. You are forced to do that. Dratted steam loco - no dynamic or regenerative brakes here. The train accelerates. The laws of physics take over. The kinetic energy increases as the square of the speed. 50 000 000 000 gigajoules at 10km/h, 450 000 000 000 Gjoules at 30 km/h, 1 800 000 000 000 Gjoules at 60 km/h. Finally, you are able to reapply the brakes: full-on this time. It takes many, many agonisingly long seconds before the whole train's brakes fully apply (Greg Hart's law at work here). The brakes now have some serious work to do. Will the brakes be able to arrest the train and dissipate all the kinetic energy which built up during the brake recharge period before they overheat? If so, good call. If not... err, hope and pray.

[John Ashworth]

A PowerPoint presentation showing the vacuum cycle, sent to FOTR by our old instructor Uncle Cliff.