Double heading GWR with none GWR and brakes

I think you will find that if your GWR/BR WR engine is set up correctly, your "Train Pipe" should be 25", and your "Reservoir" should be 23". All is explained in a very good MIC book by Mr.Mathers.

Ralph.

 

Thanks Tom, I didn’t know about why the GW used a higher level of vacuum, I assumed it was to give a greater braking force. Did all air braked railways use the same pressure? I think currently it’s 72psi, at least it is with 35028 but the brake valve is put into the Overcharge position when coupling up in case the previous loco was using a different pressure.

 

Dear mods, please can you move the Vacuum brake MIC to the relevant thread in the MIC section?

 

A Swindon Engineering Society paper of 1921 includes the following :-
In closing the discussion the Chairman [Hawksworth, then Chief or Deputy Chief Draughtsman] said that though the Great Western Railway was the only Railway Company running with 25in., this was really due to the fact that no one else had got an ejector which would create that Vacuum, and as the result of experiments carried out by the G.W.R. Technical Staff, the present Vacuum Brake, as fitted by the G.W.R. Company, was far superior to any other vacuum brake fitted by a Railway Company or put on the market by a commercial firm.

 

Yes well, we all know what Mandy Rice-Davies would say about him!

 

I recall a suggestion that the accident at Grantham GNR arose because of this issue. It was suggested that GNR practice was to "pull the strings" to release the vacuum when ever there was a loco change. The reason was a fear that if the replacement loco was a bit low in the vacuum it could create there may be dragging brakes left. On this occasion, so the theory goes, after a loco change at Peterborough, the hose between the tender and the train was not connected, and because the fireman was a fitter from the works the brake test was not carried out correctly. As a consequence, the train left with only the loco brakes working.
I do not know if there has been any proof for or against the story.

 

Importantly when pulling strings you must ensure that you are venting to atmosphere else if it just goes to the train pipe it will not properly destroy the 25“.

Its easier to notice when you pull strings on a 1st gen DMU because the brakes will subsequently apply themselves on bogies that you've already pulled strings on when you pull them on other bogies.

 

Converting to Bar, 5 and 3 respectively. Interesting to think that the pressure in the system is fairly low (well compared to some pressures I've used at work).

Sent from my moto g(7) play using Tapatalk

 

Does the fun start? Yes.
GWR locos are additionally fitted with an air pump. (N.B. Although often referred to as such, these are not vacuum pumps. You cannot pump a vacuum; it is created by pumping the air out and the GWR correctly referred to them as air pumps) and this will continue to create a vacuum whilst ever the loco is moving. Another feature provided on GWR locomotives is the retaining valve. The air pump is connected to the train pipe via the retaining valve and the retaining valve is additionally connected to the vacuum reservoir on the locomotive via a third connection. The vacuum reservoir is restricted to 23” of vacuum by a second (reservoir) relief valve. Within the valve body is a hollow spool with different sized pistons. When the brake is off the train pipe vacuum is acting on the top of the small piston and bottom of the large piston and this difference holds the spool down. The inner faces of the pistons are connected to the vacuum reservoir which is maintained at 23”by the reservoir relief valve. When a brake application is made, the change in vacuum in the train pipe reverses the forces on the spool and moves it in an upward direction. In so doing, it cuts off the connection between the air pump and the train pipe and, at the same time, connects the air pump directly to the vacuum reservoir. This ensures that the vacuum reservoir is maintained at 23” of vacuum whilst ever the locomotive is moving. Although a GWR locomotive is usually arranged to create 25” of vacuum, if coupled to a non-GWR locomotive when double heading or top and tailing, the vacuum relief valve on the other loco will restrict the vacuum in the train pipe to 21” and initially, the vacuum in the GW loco vacuum reservoir will also be restricted to 21”. However, when a brake application is made, the retaining valve will operate and isolate the reservoir from the train pipe. In such a circumstance the air pump will be creating the vacuum in the reservoir and this will now be governed by the 23” vacuum relief valve. When the train pipe vacuum is re-created, this will be limited to the 21” of the other locomotive. Thus the retaining valve will not change over and the loco brake will continue to be partially applied by 2” of vacuum, supported by the air pump. Drivers can overcome this problem by partially destroying the chamber side vacuum on locomotive and tender when the train pipe reaches 21” but this requires them to keep a watchful eye on the vacuum gauge.

It is only on the loco that there is a 23" relief valve for the chamber. Coaches and wagons do not have these so the chamber side vacuum will always be defined by the train pipe vacuum.