GW locos, vac brake problems.

[Bomag]

At the weekend I was given a cab ride for my birthday, with much hilarity from the family it was a GW 4-6-0, don't ask me which as they look the same.  Apart from being driven from the wrong side of the cab it had some problems getting the brakes off a rake of 6 VB Mk1s, such that they were going to swap it for the other engine in steam for the next run after pulling the brake valves on all the coaches.  I have read passing references to problems with GW locos and vac brakes, it is not something I have ever read up on. There does not seem to be an existing explanation on the forum, but can somebody explain what the difference is?  

[The Johnster]

Not a problem Bomag, a specification.  Standard vacuum brakes work at a 21" pressure, by which I mean that 21" of vacuum is required to release them.  GW vacuum brakes work at a 23" pressure, which means that when a non GW loco is replaced on a train by a GW one or vice versa, the brakes on each individual vehicles have to be isolated out of use by pulling the cord which equalises pressure on both sides of the vacuum cylinder (this also has to be done to prepare the vehicles for loose shunting) and the brakes 'reset' by being blown off by the new loco.  On a 60 wagon fitted freight, it takes some time and is no fun at all on dirty night...

 

The GW's justification for this, which continued into BR days with ex GW locos, was that the extra pressure enabled the brakes to be released more quickly after being applied, thus giving the driver better control of his train.  This is an arguable point, but 21" vacuum worked perfectly well for everyone else!

 

All GW locos are the same except when they are different...

[Sam Moss]

Strictly speaking, pulling the strings only releases the brakes, it doesn't isolate them. Once fully released, the loco creates vacuum and recalibrates the cylinder to either 21 or 25 inHg depending on the loco. 

To isolate a vacuum cylinder you either need to take the DA valve to bits (difficult when the wingnut and split pin hasn't been removed for the last 25 years) or by taking off the flexi supply pipe to the DA valve and bunging the end with a cork/tape. You'd then just pull the strings and reset that cylinder and the brakes are off. Obviously the amount of permitted isolated cylinders depends on a particular railways rule book but generally they should always have at least one fully working and according to the current NR rule book, a complete swinger is allowed as long as its within braked vehicles, never an end vehicle!

[jim.snowdon]

The problem occurs only when a non-GW loco takes over a train from a GW loco. On arrival, the vacuum (reservoir) side of each brake cylinder will be at 23"; if the loco taking over can only generate 21" of vacuum in the brake pipe, the pressures in the brake cylinders will not equalise, so the brakes will still remain partially applied, to the tune of the 2" pressure difference between the reservoir and brake pipe sides of each brake cylinder. Result, dragging brakes. 

 

In the reverse case, all that happens when a GW loco releases the brakes, the vacuum in the reservoir side of each brake cylinder is increased from 21" to 23" via the non-return valve built into each cylinder. With the pressures equalised across the cylinders, the brakes are fully released. Result, happiness.

 

 The solution when taking over a train from a GW loco is to destroy the vacuum in the reservoir side of each cylinder by pulling the release strings. That does not isolate the brake cylinder, but simply lets air into the reservoir side of the cylinder, releasing the brake on that vehicle (or that end of the vehicle where there is more than one cylinder, as on bogie carriages). Permanently isolating a vacuum brake cylinder requires it to be disconnected from the brake pipe.

 

As to why the GWR adopted 23" vacuum, I would suggest that it was probably (a) because the higher level of vacuum would give a higher force per brake cylinder on a size for size basis and (b), because they could, having chosen to use vacuum pumps in preference to steam ejectors for maintaining the brake pipe vacuum. Their locomotives still had to be fitted with ejectors to create the vacuum in the first place, but they did not need to be continually operated, unlike the situation on other railways. Getting the brakes released quickly is only a matter of generating vacuum more rapidly, for which the GWR developed the four-cone ejector. Significant in all of this is the guiding hand of one George Jackson Churchward, an engineer who believed very much in doing things properly.

 

Jim

[The Stationmaster]

First let's get the number right - the GWR and BRWR for many years, until steam was eliminated, worked on a standard vacuum of 25 inches (of mercury).  It is obviously not a 'pressure' because it is a measure of vacuum

 

The generally given reason for Churchward adopting 25" was in order to obtain a quicker and more positive application of the brake - as already explained in the final paragraph of JIm Snowdon's post above.  Jim is of course also absolutely correct in explaining what needs to happen when a loco working on GWR vacuum level is replaced by one working on the standard (21") level - you pull the strings on every vehicle in the train in order to destroy the vacuum (= release the brakes) and then recreate the vacuum at the lower level.  If this isn't done the replacement loco would not be able to fully release the train brakes and the brakes would drag (it sometimes happened!)

 

I think the change to 25" was one of the cionsequences of the 1900 Slough collision which also saw the introduction on the GWR of direct admission valves on passenger stock.

[avonside1563]

To be accurate, pulling the string does not destroy the vacuum in the brake cylinder, it equalises the vacuum on both sides of the piston thereby allowing the piston to fall under its own weight and release the brakes. It is only necessary to pull the strings until the piston decends, not until you have completely destroyed the vacuum in each cylinder and this can be undertaken with the loco attached and creating vacuum, once you have ensured that loco and brakevan/coach handbrakes are on.

 

Vacuum cylinders are always mounted vertically and the piston sits at the bottom of the cylinder when the brakes are off, therefore to apply the brakes you allow atmospheric pressure (roughly 14.7psi or 1 bar) in the cylinder below the piston which has the effect of pushing the piston up to apply the brakes.

[Mark Saunders]

10 minutes ago, avonside1563 said:

Vacuum cylinders are always mounted vertically and the piston sits at the bottom of the cylinder when the brakes are off, therefore to apply the brakes you allow atmospheric pressure (roughly 14.7psi or 1 bar) in the cylinder below the piston which has the effect of pushing the piston up to apply the brakes.

 

Not always as the VB Covhops they were mounted sideways to activate the Disk Brakes!

 

Mark Saunders

[Siberian Snooper]

39 minutes ago, The Stationmaster said:

 

 

The generally given reason for Churchward adopting 25" was in order to obtain a quicker and more positive application of the brake - as already explained in the final paragraph of JIm Snowdon's post above.  Jim is of course also absolutely correct in explaining what needs to happen when a loco working on GWR vacuum level is replaced by one working on the standard (21") level - you pull the strings on every vehicle in the train in order to destroy the vacuum (= release the brakes) and then recreate the vacuum at the lower level.  If this isn't done the replacement loco would not be able to fully release the train brakes and the brakes would drag (it sometimes happened!)

 

 

I was on a steam special hauled by KGV down the Welsh Marches, on arrival at Newport the king detached and was replaced by a 47 on the England end of the train, one coach didn't have the string (s) pulled, upon arrival at Bristol we were stopped and the offending coach was shuntted out,  we lost about an hour whilst all this took place.

 

 

[jim.snowdon]

50 minutes ago, Mark Saunders said:

 

Not always as the VB Covhops they were mounted sideways to activate the Disk Brakes!

 

Mark Saunders

And at least one diagram of GW hopper ballast wagons where there was no room to accommodate it in the usual position.

 

Jim

[The Johnster]

8 hours ago, The Stationmaster said:

First let's get the number right - the GWR and BRWR for many years, until steam was eliminated, worked on a standard vacuum of 25 inches (of mercury).  It is obviously not a 'pressure' because it is a measure of vacuum

 

The generally given reason for Churchward adopting 25" was in order to obtain a quicker and more positive application of the brake - as already explained in the final paragraph of JIm Snowdon's post above.  Jim is of course also absolutely correct in explaining what needs to happen when a loco working on GWR vacuum level is replaced by one working on the standard (21") level - you pull the strings on every vehicle in the train in order to destroy the vacuum (= release the brakes) and then recreate the vacuum at the lower level.  If this isn't done the replacement loco would not be able to fully release the train brakes and the brakes would drag (it sometimes happened!)

 

I think the change to 25" was one of the cionsequences of the 1900 Slough collision which also saw the introduction on the GWR of direct admission valves on passenger stock.

Quite right of course, Mike, 25".  Another senior moment to add to my already impressive list, no idea where I got 23" from!

[The Stationmaster]

8 hours ago, avonside1563 said:

To be accurate, pulling the string does not destroy the vacuum in the brake cylinder, it equalises the vacuum on both sides of the piston thereby allowing the piston to fall under its own weight and release the brakes. It is only necessary to pull the strings until the piston decends, not until you have completely destroyed the vacuum in each cylinder and this can be undertaken with the loco attached and creating vacuum, once you have ensured that loco and brakevan/coach handbrakes are on.

 

Vacuum cylinders are always mounted vertically and the piston sits at the bottom of the cylinder when the brakes are off, therefore to apply the brakes you allow atmospheric pressure (roughly 14.7psi or 1 bar) in the cylinder below the piston which has the effect of pushing the piston up to apply the brakes.

If the brake has been applied, which it obviously has to be in order to stop the train, there will surely already be air on the underside of the piston and the only remaining  vacuum will be that 'above' the piston.  Don't forget the vacuum pipe will have vented to atmosphere when it was uncoupled and will remain vented to atmosphere (i.e. the vacuum will have been destroyed), and the brakes will be applied, until another engine is attached and the vacuum is recreated.

 

When the vacuum is released it is released from above the piston so the piston will fall because the vacuum is no longer there to hold it.  The only equality between the vacuum on either side of the piston will be that it is zero on both sides provided you make sure it is fully destroyed when you pull the string.  And if you don't fully release the vacuum you will like as not finish up with brakes rubbing when the change loco recreates the brake (i.e. recreates the vacuum).  You can't tell what level of vacuum remains in any cylinder so the way we were taught was to always fully destroy the vacuum/release the brakes when pulling the strings and that was what we ensured was done (or should be done  - see 'Siberian Snooper's post above)  on any occasion when it was necessary to pull the strings.  I've known brakes having to be released when diesel locos were changed because the one coming off had been exhausting the brake system rather better than the one which went on - unusual but it occasionally happened (and off went a loco for a full vacuum test).

[Bomag]

Thanks for all the various bits of information; if the GW wanted better brakes is there a reason why they did not go for air brakes?

 

Our family loco (1924) had air from new, https://en.wikipedia.org/wiki/GER_Class_L77#/media/File:LNER_Class_N7.jpg  its quite handy having a dual braked loco, the only pity is its only fitted with steam heating!

Edited September 2, 2019 by Bomag

[LMS2968]

Air brakes aren't necessarily 'better' in so far as braking force is concerned. The force is dependent on the size of the operating cylinder - which is bigger with vacuum as the 'pressure', as Mike has said, is limited to 14.7 p.s.i. - and the respective lengths and angles of the linkages. It is possible to give a stronger vacuum brake than an air brake, but whatever the system, you're limited by the coefficient of friction between the wheels and rail. An air brake does have advantages over vacuum, such as faster operation, but a stronger brake isn't necessarily one of them.

[Bucoops]

As I understand it the GER opted for the Westinghouse system because of the speed of operation. For their suburban services with frequent station stops and  low top speeds it made a lot of sense. It held off electrification for many years

[The Stationmaster]

10 hours ago, Bomag said:

Thanks for all the various bits of information; if the GW wanted better brakes is there a reason why they did not go for air brakes?

 

Our family loco (1924) had air from new, https://en.wikipedia.org/wiki/GER_Class_L77#/media/File:LNER_Class_N7.jpg  its quite handy having a dual braked loco, the only pity is its only fitted with steam heating!

A good reason for not going to air brakes was money.  While railway workshops could no doubt have manufactured suitable pumps there would still have been a licence fee to Mr Westinghouse for use of his designs.  On the contrary the vacuum brake normally involved no licence or purchase fees as long as you used your own design of brake valve and ejectors.

 

Once a Railway had committed to one system or the other the cost of changing to the other would become increasingly become prohibitive - that was the big reason why BR hang back from changing from vacuum to air as the universal braking system on hauled stock.  The GWR refined its vacuum brake, and increased its power and speed of application, relatively soon after the Slough collision involving no major changes in many of the basic components but basically by adding one or two items manufactured in house at no licence fee expense to itself but getting a more powerful and quicker acting continuous brake as a result.

[Wickham Green]

20 hours ago, jim.snowdon said:

And at least one diagram of GW hopper ballast wagons where there was no room to accommodate it in the usual position.

 

Jim

..... not to mention some GW brake vans - before they found a place for the cylinder between the veranda sandboxes.

[avonside1563]

On 02/09/2019 at 21:33, The Stationmaster said:

If the brake has been applied, which it obviously has to be in order to stop the train, there will surely already be air on the underside of the piston and the only remaining  vacuum will be that 'above' the piston.  Don't forget the vacuum pipe will have vented to atmosphere when it was uncoupled and will remain vented to atmosphere (i.e. the vacuum will have been destroyed), and the brakes will be applied, until another engine is attached and the vacuum is recreated.

 

The strings are usually used when a loco is creating vacuum and the brakes haven't released fully, either way the effect of pulling the string is to equalise both sides of the piston, whether its vacuum or atmosphere.

[roythebus]

there was a similar problem in BR days with air brakes. Around 1976/77 the standard air brake pressure was increased from BR standard of 70psi to the European standard of 72.5psi. Converted locos had a yellow dot on the brake gauges to warn the driver. I spent many hours helping the guard pull strings at York and Kings Cross during the change-over period!

[rab]

I note the OP had generated a lot of discussion regarding various forms of braking and the problems caused by the GW higher level of vacuum, but it  was actually stated that in this case it was a GW loco. 

Much of the following discussion was about swapping a GW loco for a non GW loco, but as far as I am aware, a GW loco should not have a problem releasing brakes set by a non GW Loco. Am I missing somethibg obvious here?

[jim.snowdon]

True, but the obvious conclusion would be that the particular GW loco in question wasn't managing to generate sufficient vacuum to get the brakes released after taking the train over from another GW loco. Why not is another question, but it would point to an ejector defect, hence the crew's desire to swap it for another available locomotive. It's been an interesting discussion, though, despite the deviations.

 

Jim

[F2Andy]

Was it ever the case that the vacuum varied along the length of the train? If you have a twelve coaches, that is 24 flexible hoses and 12 connections; that seems like a lot of potential for leaks. Would a loco would run around its train, and they would have to equalise the pressure, as what is now the back of the train is at a higher vacuum than the loco can pull, given the leaks between the two

[Blandford1969]

The outcome of the Newark brake trials of 1877 was a resounding win for Westinghouse air brakes which quite a few of the companies including the Midland had fitted at the time. 

 

William Stanier reading a paper at the Science Museum in London on the 12 October 1950 and described how the development of this brake came about “1875 the GWR adopted the continuous brake relying on vacuum holding the brake off with the ability to be applied by a communication cord operated by a passenger or the train being parted. In 1881 Dean appointed Churchward and Joe Armstrong to develop the vacuum braking system in 1876 the South Devon and Cornwall Railways were absorbed by the Great Western Railway and Churchward went to Swindon to serve the last years of his pupillage and then to the drawing office in 1877.  

 

At this time George Westinghouse came from America to introduce his air pressure brake to the English railways and it is rumoured that he visited Swindon to see Joseph Armstrong to try and get him to adopt it on the Great Western. After telling Mr. Armstrong what his brake would do and finding him not very responsive Westinghouse wound up by saying, "Well, Mr. Armstrong, you will have to have my brake." Mr. Armstrong replied that he would see about it and left it at that, but later he called in young Joe Armstrong, his son, and George Jackson Churchward and gave them the job of producing a vacuum brake. The Great Western combined vacuum brake was the result. 

 

Turning to what might have been going on to make it difficult to get the brakes off there could be a number of issues:

- The vacuum pump, which when working is very useful could be either over creating or could be creating 25 on the train pipe, but the ejector could be struggling and not able to make the full 25, maybe making 21/ 22 for example. I have seen 

- There could be leaks in the pipes or connections meaning the full vacuum could not be made to get the brakes off.

 

The fact the crew wanted to swap it suggests the ejector was struggling to maintain even 21 inches. 

 

 

[The Johnster]

Never mind 12 coaches, Andy, what about 60 vacuum fitted vans and opens!  The loco has to create vacuum all the way back to the last vehicle, despite the inevitable leaks, and maintain it, 21" or 25".  The GW used a crosshead driven vacuum pump to maintain vacuum while the train was in motion, and the large ejector of other companies' t steam locos was usually powerful enough to manage; a leak bad enough to enable the brakes to drag would need attention from a C & W fitter or the wagon detaching as a cripple if one was not available.  There may have been a greater tendency for brakes to drag and be slower to release at the rear of the train, and apply quicker, but this is not an entirely bad thing as it helps keep the couplings taut.  

 

Diesel and electric locos rely on a compressor to exhaust the vacuum, and some of the early ones were noticeably deficient compared to steam engines with long fitted trains; drivers had to learn new techniques to avoid losing time or even coming to a stand in sections.  This is one of the reasons for the adoption in the UK of a twin pipe air brake system, with a reservoir supply throughout the train to ensure a quick release of the brakes.  First generation dmus used a similar twin pipe vacuum system, and whatever else you might say about them the performance of their brakes in terms of both instant application and instant release even on long trains was superb.

[The Stationmaster]

3 hours ago, F2Andy said:

Was it ever the case that the vacuum varied along the length of the train? If you have a twelve coaches, that is 24 flexible hoses and 12 connections; that seems like a lot of potential for leaks. Would a loco would run around its train, and they would have to equalise the pressure, as what is now the back of the train is at a higher vacuum than the loco can pull, given the leaks between the two

It varied gradually decreasing (in terms of inches Hg) towards the  rear of the train.  so for (G)WR the maximum and minimum permitted levels were as follows -

 

Passenger etc trains - maximum of 25"/minimum of 23" on the loco, minimum of 21" at the rearmost brakevan.

Fully Piped/fully fitted freight - maximum of 25"/minimum of 23" on the loco gauge;

not less than 18"  the rearmost brakevan if more than 20 fitted vehicles in the train

not less that 20"  at the rearmost brakevan if 20 or fewer fitted vehicles in the train

Unfitted freights - maximum of 25"/minimum of 23" on the loco gauge 

 

The reason for the difference on freight vehicles was partly down to their greater propensity to leak.

[Blandford1969]

Of course the other thing about vacuum brakes of any type which surprises those who do driving schools is that they are not instantaneous (as the Board of Trade defined during the Brake Trials. There is always a delay to apply the brakes of about 1 second per coach for them to go on and with a single cone ejector even longer to get off again.