Air Brakes / Air Pressure

[JDW]

Maybe this is a really silly question, but I'll ask it anyway.  After reading about two recent incidents in the US/Canada of trains running away, I'm thinking I've got it all wrong.  I always assumed that air pressure was used to keep brakes OFF, and that a loss of air pressure would cause them to stay ON.  But speculation/reports suggest that the causes were/may have been that air leaked out of the system, and thus allowed the trains to run away, because the handbrake wasn't on.  Can that be?  Have I missed something?  Is the US different to the UK?

 

JD

[Dagworth]

Read and digest http://en.wikipedia.org/wiki/Railway_air_brake

 

Andi

[JDW]

  Never thought of Wikipedia, good call Andi, that does make a bit more sense now!  Thanks.

[roythebus]

No air in brake cylinders means no brakes applied! Certain multiple unit stock with Westcode type air brakes have spring-loaded parking brakes similar to those used in road transport. Under certain conditions the spring loaded parking brake will apply. But not on goods stock!

[Lady_Ava_Hay]

It seems that the technology is still evolving and that because locos and rolling stock last a long time in service, it is impossible to upgrade and that the systems fitted require both human input and careful checking for each trip. It would take an army of men to check each train each trip and of course Sodd's Law applies.

 

The main difficulty with all brake systems is the need to allow for shunting manoeuvres without undue delay in pumping up or evacuating the brake system. I suspect this might be the main reason for having integrated trains that are semi-permanently coupled together. One wonder what happens if only one wagon fails in service and, indeed, if the driver would actually know. One also then falls into idle speculation as to how many failures does it take to be noticeable to the driver and whether that is fatal or not.

[40044]

There seems to be a lot of confusion about what the purpose of the automatic air brake actually is. It is intended to slow or stop a moving train and is more or less foolproof for this purpose. As far as I'm aware, it has never been intended to hold a stationary train indefinately, hence why handbrakes are provided for most freight wagons along with clear instructions that the air brake should never be relied upon to secure a train. Granted, some stock will sit with the automatic brake applied for weeks or even months whereas other stock will 'leak off' in a matter of hours.

[The Bigbee Line]

Air brakes are a marvel of simplicity. 

 

There are numerous interesting sub threads eg the introduction of EP (electro pneumatic) brakes on the Southern Railway.

 

When shunting there is a choice of 'pulling the strings' and shunting unfitted, or shunting on air.  Pros and cons with both options.

 

Regarding the period that a brake application will hold the wagon.  Older systems will stay applied until leakage past seals etc, lets the brake release.  Some newer wagons are designed to leak off (very slowly) eg thrall built wagons for EWS

[Beatty 139]

I wouldn't go as far as to say air brakes are a marvel of simplicity, I think credit goes to the nice British Automatic Vacuum Brake.

 

Mr Westinghouse's automatic air brakes needs quite a few more bits to make it work and even more to make it work well but unlike with vacuum where you are limited to only 1 bar operating pressure, so AVB was never going to have a future in the modern world.

[roythebus]

Mr. Westinghouse  apparently commented on the vacuum brake as "20 inches of soggy suck".

[Il Grifone]

Mr. Westinghouse  apparently commented on the vacuum brake as "20 inches of soggy suck".

 

Then he would, having a vested interest!

 

Actually the 'pull' is dependant on pressure and piston area. The GWR used rather more than 20 inches and the locomotives of inferior railways were unable to pull them off.

 

http://www.southdevonrailwayassociation.org/Brake-Cylinder.html

[roythebus]

Working on the KESR where we have a mixture of GW and proper engines, I am aware of the problem!!

[6Y99]

Working Vac stock was right pain waiting ages for the wagons to blow up making sure the pipes had the pins in & are tight listen for leaks and having to so careful on gradients you didn't work your brake off. Air Brakes are far better and now with modern freight stockmuch more responsive

[The Stationmaster]

In this case it's nothing more than the chamber side values as a result of the higher running vacuum train pipe level. Once the chamber side values were equalised with the locomotive operating on a lower train pipe figure then all was well after a locomotive change. Anyone remember the pulling of strings during W.R. to other region locomotive changes? It can even happen with Automatic Air Brake systems because of slight differences between locomotive settings operating at 5 Bar running pressure. 21" Hg was the running vacuum used on all B.R. Regions except the Western subject to a 0.5" Hg tolerance either way. As much as I have an affection for the vacuum brake in its various forms and was one of a decreasing band of Fitters entrusted with maintenance of the remaining equipment on locomotives in the later years pragmatism draws me to the air brake because of the more positive control that a driver has over both application and release.    

 

It was far simpler when changing over to a Western engine - just blow the brake off that little bit more and away you went!  And to be honest on a reasonable weight train a well adjusted set ov vacuum braked stock was quite satisfying to handle - a set  with new blocks or well worn blocks and a good bit of slack was basically an interesting challenge but you could brake very accurately with a vacuum brake on a single application/release if you knew you marks.

[roythebus]

As one who was very used to working with the straight Westinghouse brake on the W&C in the 1980's, you quickly got VERY used to it! It took skill.

 

One Sunday I was rostered to do a film job there, Dempsey and Makepeace (remember them?). I had to run a train into Bank and stop at a set point, the stop line painted on the platform. I did this as a rehearsal. The film director asked if I could do it again, stopping at the line on the platform. Which side of the line do you want me to stop? He thought I was joking...

 

Another bit of skill with the Westinghouse was to stop in the same platform at Bank close enough for the central buffer on the leading coach to spring forward and leave a greasy mark on the stop block, but not so the wheels tripped the "run by" treadle to indicate the train had hit the blocks! They never did work out how this could happen!

 

However, with the Westinghouse, you only had 3 brake applications before you run out of air in the auxiliary reservoir on the coaches! It was possible to stop an 8SUB with just the brakes on the back 4 doing the work, due to the time lag of application and release. With a 12COR I'm told it was possible to break the coupling with the back 4 dragging their brakes.

[Il Grifone]

Working Vac stock was right pain waiting ages for the wagons to blow up making sure the pipes had the pins in & are tight listen for leaks and having to so careful on gradients you didn't work your brake off. Air Brakes are far better and now with modern freight stockmuch more responsive

 

Surely leaks are a problem with the air brake - probably even more so due to the higher pressure differential?

 

(I have no direct experience with either and thus no preference either way.)

[Dagworth]

Surely leaks are a problem with the air brake - probably even more so due to the higher pressure differential?

 

(I have no direct experience with either and thus no preference either way.)

Other way around, with the higher pressure you can afford to lose a little and you've still got plenty.

 

Andi

[Il Grifone]

What I meant was that the higher pressure would be more likely to cause leaks in the first place. I would agree that a given pressure drop would have less effect.

[The Stationmaster]

The leakage question is really one of gas volume but just to put a little more flesh on something that has already been mentioned: a given leak in the vacuum train pipe will more likely lead to problems whereas a similar leak in an air train pipe can be tolerated as stated above. With vacuum operation we are looking at the evacuation of the train pipe to 21" Hg. (Western and D.M.U. release pipe excepted). Given that a complete vacuum is 29.92" Hg. and in achieving that we are only looking at the removal of one atmosphere or bar (14.696 p.s.i. in old money) in the case of a complete vacuum and rather less in the train pipe. With such a small difference between atmospheric air pressure and vacuum train pipe pressure any leak will make the maintenance of the required vacuum much more difficult than a leak from the air train pipe with a running pressure of 72.5 p.s.i. ie. 5 bar or  a fraction short of 5 x atmospheric pressure even before the capicity held in reservoirs is taken into account. As Apprentices we discussed this topic to death almost with the instructors in the old C.M. & E.E. training school at Doncaster during our introduction to the subject as part of the basic brakes course where messrs Rowett and Taylor would hammer home such principles. In other words a fair leak would have to develop before the system's ability to cope would be compromised. For all of this however it still goes very much against the grain when a coal train rumbles past with air hissing away merrily from a train pipe cock or coupling between wagons.      

Even more worrying when a train parts and keeps going because the compressor power is more than enough to keep the brakes from applying (Class 56s are very good at air output, even if they are somewhat patchy in respect of other things).

[stadman]

I can't imagine any compressor being able to overcome a leak caused by an open valve on the train (Division), equally the brake application would cause loss of traction power.

[roythebus]

The loss of train pipe pressure will not neccessarily lead to los of traction power until the pressure gets below that at which the low air control governor operates, typically about 35 psi IIRC!

 

On the old EP underground stock, it was always possible to go in reverse with low or no air pressure; this was part of the driver's fault finding process to see if the control governor was defective.

[The Stationmaster]

Perhaps rather more related to the internal state of the air train pipe than the ability of the compressor itself. Bearing in mind that the compressor does not directly supply air to the train pipe but is effected via the Driver's Brake Valve/Brake Control Unit as applicable both of which tend to restrict the rate at which air passes into the train pipe in any case, notwithstanding the effect of chokes which may be fitted elsewhere to protect against rapid air pressure loss. We did have a test carried out on certian Examinations (on Locomotives at least) whereby the Train Pipe Hose would have blank couplings fitted, one with a small drilling to simulate leakage and another with a large hole; the system was expected to be able to overcome the effect of small drilling but not of the large, even under release/overcharge conditions. If the leakage could be overcome with the large drilling and even more so if the Air Train Pipe was directly opened to atmosphere then one was generally looking for a restriction somewhere along the train pipe which was preventing the equal loss of pressure for the full length of the pipe.

 

Certianly worrying none the less, especially given that these conditions didn't just magically appear overnight on the eve of an Examination although I would have at least expected the rudimentary brake test carried out during train preparation to have given a hint that something was not quite right in most cases.

 

If, in the event of a division of train, a portion could continue without enforced brake application and all train equipment was demonstrably in good order at the time then we must surely be looking at a design problem which should have been modified out many years ago, and that does worry me.   

It did happen back in the 1970s  (and the Driver involved was something of a press-on merchant and a bit of a pr1t) but there was no doubt at all about the brake continuity as the wagon which broke away duly stopped, on a falling gradient as it happened, although the 'have you lost a wagon?' 'phone call from a bemused lineside resident to our TOPS office was a bit embarassing - as was the fact that the now tail lamp less train went past 3 signalboxes before the situation was confirmed.

 

The loco was a Class 56 - as already noted - and they were apparently well known for their very impressive compressor power.  The Driver did own up to noticing a blip in the brake pipe pressure and he put the brake into boost but he also came nearly to a stand to hand in a token and then accelerated after that (probably with the brake in boost but he wasn't saying).  We recreated it with teh same loco and train and it did indeed have enough power to blow-off the brakes.

 

Such breakaways weren't unknown on vacuum braked trains but this is, I think, the only example I know if with an air braked train.