How does AWS / GWR ATC apply the brakes - history

[darwins]

Good morning,

There have it seems been some changes over time in the way that BR AWS works. Can anyone provide more information on when the changes took place and if any of the intermediate steps that I will speculate upon later actually happened. Please also feel free to correct any errors in my understanding.

( 1 ) On steam locomotives -
Brake application started immediately the AWS was passed - the horn sound was made by air being sucked into the vacuum brake pipe. (Time for vacuum to fall to zero about 15 seconds if not cancelled. Driver regains control at any time warning is cancelled.) There is no means to close the regulator.

( 2 ) Dual braked diesel (Class 47 c. 1965) -
Horn sounds when AWS passed. Short delay 3-5 seconds before air or vacuum brake is applied.
Vacuum to fall to zero in about 12-15 seconds. Air brake pipe falls to 45 psi (full service) in about 12-15 seconds. Driver regains control at any time warning is cancelled. Fall in brake pipe vacuum or pressure cuts power due to action of vacuum or air governor.

( 3 ) Modern diesel loco (Class 60) -
Horn sounds when AWS passed. Short delay 3-5 seconds before air brake is applied.
ABP drops to zero, but more slowly than in Emergency application by driver's brake valve ( ? 12-15 seconds ?; Can driver regain control at any time the warning is cancelled? Fall in brake pipe pressure cuts power due to action of vacuum or air governor.

( 4 ) Modern MU with EP brakes -
Horn sounds when AWS passed. Short delay 3-5 seconds before brake is applied. Once brake application begins train must come to a halt (cl 321 cl 185) or a reduced speed (cl 170) before driver can regain control. After stopping there may be a built in delay of 40s to 120s before control can be regained.
Is the application slow as on Class 60 or rapid as with Emergency position on brake valve?


The examples that I have leave a lot of gaps about examples I don't have. Some questions:

( 1 ) Did early electric and diesel locos (vacuum brake only) work like steam in having the horn sounded by the brake actually being applied? or did they work like the class 47 example above?
(In other words were diesels always different to steam in this regard or did the change in this come with the introduction of dual brakes?)

( 2 ) I can see how a vacuum or air governor can operate a relay to cut power on an electric or diesel electric loco. Was there any way to cut power with mechanical transmission (first generation dmu) or hydraulic transmission (class 35, 42, 52 etc)?

( 3 ) Did the change from reducing the ABP to 45 psi to 0 psi come with the change to air brakes only (and UIC standard 5.0 bar) - e.g. Classes 87 / 56 or did it come later?

( 4 ) Did the system on older EP braked EMU stock work as the modern system does or was the driver able to regain control at any time by cancelling?

Examples and links to examples welcomed.

 

[The Lad]

2) DMU gearboxes go to neutral, throttle to idle due to loss of electrical feed to the controlling air valves.

 

[Taunton]

The GWR AWS ("ATC"), on GWR locomotives, would indeed overcome full regulator and full vacuum ejector. It was cancelled by lifting a small lever. By mechanical design this did not end the air admission until it was returned back down again, this was to stop drivers thwarting it by tying it with string in the Up position. It was a nice design that failed safe even if the batteries were flat.

40 years later, when the design was applied to BR Standard locos (by Derby) operating on the WR, for whatever reason, apparently based on the trial AWS system, it was not as competently designed, and the train could continue if not noticed - and the hoot was not as loud. This was the cause of the serious overturning accident to the Britannia-hauled express at Milton near Swindon in 1955, the ATC had operated but the loco was able to continue unhindered. Appalling.

 

[edwin_m]

The GWR AWS ("ATC"), on GWR locomotives, would indeed overcome full regulator and full vacuum ejector. It was cancelled by lifting a small lever. By mechanical design this did not end the air admission until it was returned back down again, this was to stop drivers thwarting it by tying it with string in the Up position. It was a nice design that failed safe even if the batteries were flat.

Did GWR ATC even need batteries? I was under the impression that the opening of the brake valve was driven mechanically by the lifting of the shoe, the admission of air to the vacuum operated the horn, but if the shoe was electrically live this was the source of the power to ring the bell and cancel the brake (presumably closing an electric valve in series with the mechanical one).

 

[Nym]

The Class 60 applies a penalty brake for an AWS, TPWS or Vigilance application by means of the main computer rack.

The application is made by the opening of all brake application, holding etc valves by forcibly removing power from the brake rack, this cannot be restored until the train is at standstill and has been for a period of time, without means of the AWSIS / VIS switches.

 

[coppercapped]

Did GWR ATC even need batteries? I was under the impression that the opening of the brake valve was driven mechanically by the lifting of the shoe, the admission of air to the vacuum operated the horn, but if the shoe was electrically live this was the source of the power to ring the bell and cancel the brake (presumably closing an electric valve in series with the mechanical one).

The GWR ATC did use batteries on the locomotive or railcar.

There is a good description which can be found here. A short extract reads:

The heart of the locomotive equipment was an electromagnet with two windings that held up an armature when either winding was energized, keeping a valve to the vacuum brake pipe closed. The diagram below shows the simple but effective electrical design. The circuit for one winding led through a 4V lead-acid accumulator (80 A-h capacity) on the locomotive and a switch operated by the contact shoe below the footplate (usually under the cab, but sometimes under the front buffer beam, as on the Saint class 4-6-0s), which was closed when the shoe was down in its normal position. The shoe was raised 1-1/2" (later 1")when on the ramp, and made electrical contact. When the shoe was thus raised, the switch was opened, current in the windings stopped, and the armature fell out, opening the train (vacuum) pipe. Air entering the vacuum pipe sounded a siren, consisting of a disc rotated by the inrushing air. The driver could hold the armature up by means of a 'release trigger' on the cab mechanism box and keep the vacuum pipe closed so that he could retain control of his train. This is called resetting, cancelling, acknowledging or forestalling, and established that the driver was alert. It is very important to take some voluntary muscular action at times such as these, to alert the mind and destroy a reverie. Once the armature fell out, the strength of the electromagnet was insufficent to draw it up again when the shoe left the ramp and the magnet was re-energised. Consequently, the brake application continued. The use of the brake vacuum to give the warning siren suggests that the system involved the brakes from the first, not subsequently, as stated by some authors.

GWR locos carried a largish wooden battery box often on the running board tucked underneath the curve of the boiler.
The URL leads back to the University of Denver website but the link is broken.