Double heading GWR with none GWR and brakes

If the loco over creates is that bad?

 

So long as it can create the same vacuum again then no, assuming it doesn't break something.

 

Thank you for the information. I once saw our resident 56xx 5619 with nearly 30 on the vac gauge!

 

The problem occurs if the leading loco, in charge of the brake, can only release 21in with the towed loco pump trying to maintain 25. So there is a problem with over creating

 

The pepperpot on the lead loco will keep the train pipe to 21". The problem comes if the GWR loco is at the rear of the train.

 

The problem then comes if you are running slowly, so the air pump isn't creating a vacuum, and your boiler pressure has dropped so the ejector can't make that high vacuum. A problem we on the K&ESR sometimes have with Western engines when arriving at Tenterden, having worked hard up the bank and the Fireman has let the fire run down a bit too much.

 

Yeah GWR Ejectors eat steam!

 

What an interesting thread and quite worrying. It sounds as though there is a distinct lack of maintenance of brake systems in certain quarters. when properly repaired and maintained the GWR vacuum brake is an absolutely superb system with a number of very clever features, but it does need to be maintained. The comments about welsh 56 drivers is pretty superfluous - they got by because the brakes worked, having been maintained by people who actually knew what they were doing. Comments about firemen having to be quick on the hand brake are appalling, if the reservoir side leaks that badly the engine should simply not be in traffic until the leakage is sorted.

All ejectors use steam, and will use even more if the cones are worn, they do wear out due to erosion of the cones and become inefficient, far too many people seem to believe that they last for ever. The great beauty of the GW ejector is that you do not have to use it once you get moving, our panniers will hold the brake off from below 10 mph. What the GW brake system does require however is that those people who mend the trucks at the back actually pay attention to the vacuum cylinders, pipework and hoses on the coaches behind. The pumps on our engines are perfectly capable of maintaining the 25ins on trains of 10 or 11 coaches with decent brakes and even the 4 inch pump on 9600 will hold the brakes off on 8 coaches. All you have to do is to make sure that the pump pistons and rings, piston rods, glands and valves are in good order, that there is not excessive travel on the valves and that the whole is properly lubricated. 50/50 paraffin is to be preferred, when the LNWR used the GW patent for air pumps, they used paraffin, but it was possibly more oily in those days, but definitely not the cylinder grade oil that I have seen some ignorami using. Paraffin will certainly help to keep the valves clean and free from gumming and sticking for long periods. The VR valve too needs care in maintenance, the top seat can cause leakage and some sheds took the cap off and cleaned them at every washout, putting a little lub oil in the well in the bottom cap to aid lubrication and sealing. We have seen problems with VR valves due to some pretty grotty attempts by other's at new piston rings, causing the pistons to stick.

The comments made quoting instructions for working with engines with inferior brake systems are spot on, GW on the front (the reason for the arrangement recently with 5043 and 6233) and crack the reservoir release cock to control any over creation of vacuum.

The moral of all of this is that if you are having trouble with Great Western vacuum brakes, somebody is not maintaining them properly and you need to stop using the engine until it is mended properly, they are after all BRAKES and do have a pretty fundamental function; the old saying "anybody can start an engine - it takes a bit more to stop it" is not helped at all by badly maintained brakes. If the maintenance staff do not know how to mend them properly then they should either be trained, or find someone who can mend them.

Regards
Bob

 

Quite right! It's like everything if it is looked after properly then it will work! It saddens me aswell to see that G.W.R brakes are not being maintained properly at my local railway 5619's vacuum pump does not work and it's reservoir leaks off far too quickly this should not happen in practice and I think something should be done right away because a loco need it's brakes!

 

Thread resurrection....
+1 to what Bob said.
I've driven GWR tanks that leak off the chamber side scarily fast.
Funny that no-one's mentioned Gresham and Craven Dreadnought ejectors.
There is the main air clack between the the vacuum relief valve and the train pipe.
This means you can put what vacuum you want in the train pipe and so long as the clack is in good nick the VR valve won't operate.
Makes double heading with GWR engines really easy.
You can get 25", you might have to put the handle up for a bit though.
Dreadnoughts are brilliant things.
You can service just about any component other than the main steam supply valve with the engine in steam, they are easy and intuitive to use and as long as the small jet is running you maintain your chamber vacuum.
No running for the handbrake!
It's not arcane knowledge BTW, it's in the Black Book.

 

I've driven GWR (vacuum brake fitted) tanks where the reservoir hardly leaks and there is no need to use the ejector above 3 MPH - due to the vacuum pump. I'm afraid that is down to poor maintenance rather than a design fault.

It its that scary - the loco should be stopped!

Stephen

 

Unless it’s the GW then it’s 25in. Many years ago at an engine change at Didcot from 5051 to a diesel we had a right performance when the diesel loco couldn’t fully blow off the brake. The guard said the engine was a failure and he knew best and wouldn’t listen to the people from the GWS who said he should go along the train and pull all the strings. In the end the GWS RO quietly jumped of the platform pulled all the strings and said now try. We got away about 30 minutes late.

 

In this situation is the brake fully released leaving the coaching stock with no brake at all, or just partly released to just get the vacuum something below 21", thus retaining some brake?

 

A vac brake works on the differential of pressure between the chamber side and the train pipe. So on most trains, in the normal running situation, the reservoir or chamber has 21" of mercury pressure, and the train pipe has 21", so there is no differential, and the brakes are held off. When you admit air to the train pipe, the vacuum in the train pipe "falls" (i.e. the pressure rises) but the vacuum in the chamber stays the same, and you get a differential which applies the brakes in proportion to how much that differential is.

Imagine a train that has stopped and the loco is removed. The chamber side still has 21" (subject to a low level of leakage (*); the train pipe is at atmospheric pressure (e.g. 0") so the brakes are held on hard because there is 21" of pressure differential either side of the piston in the brake cylinder (**). When another loco couples on, you pump out the train pipe until it is back at 21", at which point no pressure differential, the brakes are released and the train can move.

The GWR system is basically the same, except that it works at 25". But the principle is the same: in normal running, both sides are at 25" of vacuum, so no differential of pressure, and the brakes are released. Add air into the train pipe, and the brakes apply in proportion to the pressure differential.

Now imagine what happens when a GWR loco uncouples from a train, and a proper loco attaches. Immediately after uncoupling, the chamber side is at 25" and the train pipe is at 0", so the brakes are hard on. The new loco attaches and pumps out the train pipe to 21" - at which point you have a train pipe at 21", a chamber at 25" and therefore a 4" differential. That is sufficient for a moderate brake application: the brakes aren't hard on, but they are probably on hard enough that you can't start the train.

The solution is to "pull the stings" which drains the vacuum (i.e. admits air) to the chamber side. That drops the pressure in the chamber to 0"; the train pipe is also at 0", so the brakes are released. (Hence - before pulling the strings, you should ensure the handbrake is applied). The non-GWR loco then creates a brake in the normal way, pulling both the chamber and the train pipe to 21", the brakes are released and you can depart.

What about the reverse situation, i.e. attaching a GWR loco to a train previously hauled by a non-GWR loco? Well, in that situation the chamber is at 21" and the train pipe at 0". The loco attaches, started to blow up the brake and in time both sides rise to 25". So in that situation there is no problem.

(*) Over time, the chamber will leak, equalising the pressure on both sides and releasing the brakes. Which is why you can't rely on the vacuum brake to hold a set of carriages for a long period: if they are being berthed, you must apply a hand brake or scotches. But in the length of time for, say, a loco to detach and a new one to attach, the chamber side will hold up reasonably well, enough to make coupling a non-GWR loco to a rake of carriages previously hauled by a GWR loco a problem.

(**) 2" of mercury equates to about 1 psi. So on a 20 inch diameter brake cylinder (equal to about 314 sq inches), that 21" pressure differential is 10.5 pound per square inch * 314 square inches = 3,300lbs force on the piston rod, which is then translated via levers to the brake blocks). I think the primary advantage of the GWR use of a higher level of vacuum is it allowed a similar force from a smaller diameter brake cylinder, which helped packaging in the confines of a locomotive or carriage.

Tom

 

The fun really starts when you have 2 locos on the same train - one GWR and one not GWR.

 

Does it? Isn't it just a case of only one loco (usually the leading loco) creates the brake using its ejector, and the other loco (usually the train loco) leaves their brake handle in the running position, thereby not creating or destroying the brake. It is therefore the leading loco that controls what the train pipe and reservoir are set to (21 or 25) and so as long as you remember to pull the strings if the last loco to create a brake on the train ran at 25 and the new loco creating a brake runs at 21, the fact that you have two locos which would create a brake to different inches of mercury doesn't matter because only one of them is actively doing it. The other is effectively functioning the same as the coaches - it is passive in this scenario and so brakes are applied and released on it when controlled by the leading loco just like on the coaching stock.

 

Tom, you have an excellent ability to clearly explain matters of steam locomotives in a way that does not over simplify or dumb it down and yet is accessible to people of all ability and knowledge levels. Something that many people whom have tried to teach me sadly lack. Your trainees at the Bluebell are lucky to have you!

 

Tom,
Will the vacuum in the chamber not fall slightly (or pressure rise) due to the movement of the piston reducing the volume of the chamber side when the brakes are applied or train pipe disconnected?

Dick

 

In theory, yes, when pulling strings you don't need to wait until the cylinders are completely drained, although in practise you usually end up pulling them enough such that the blocks come off the wheels just to be sure. So you effectively have to act as if you've left the set with no vac brake at all, and put hand brakes on during the process.