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Brake continuity test on vacuum braked stock


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Simple - how were they undertaken?

 

I've never dealt with vacuum stock, so watching Great Rail Restorations the other day got me thinking about how a brake test is conducted with vac stock.

 

I've done some research, and if my understanding is correct, the vacuum pipe at the end of the train must be kept on the dummy fitting in order to create a vacuum. There's no end cock? So, if my understanding is correct, how does the person at the back of the train cause the pressure to rise in the pipe?

 

With air brakes, dropping the tap at the rear of the train causes a howling gale as 5 bar drops to zero and the released brakes grab hold of the wheels. I can't fathom how you'd do, effectively, the reverse with a vacuum braked train? Google doesn't seem to help much

 

Many thanks

 

Jo

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There is a simple check that were do.

 

With full vacuum the brakes will be off.

 

Take the bag off the rear vehicle dolly, need any damn good pull, the vacuum will drop to zero and the brakes will apply.

 

Replacement of the bag on the back will get the vacuum back to whatever the locomotive is creating

 

This is how we do it

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Remembering to have first tested the loco before going onto the train to prove the vacuum on the engine. 

 

Once coupled on the handbrake is applied, the guard walks up observing all the brakes (should) be hard on. Once at the engine the driver creates a full vacuum, and the guard walks back the length of the train checking all the brakes have come off. Full vacuum on the loco, does not automatically  mean the brakes are all off, just the engine or part of the train. 

 

At the rear they signal for vacuum to be destroyed and then takes the bag off the back and asks for the driver to create again. Satisfied the brakes are on and vacuum has not been created the test is then finished. 

 

Duncan

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It is the same but in reverse the Driver latches out the Vacuum or uses the brake test cock depending on locomotive trapping at least 16” of mercury in the vacuum in the pipe. The Guard or Shunter checks the brakes have released on the appropriate number of vehicles and then destroys the vacuum by removing the pipe off the dummy plug or using the valve, checks brakes have applied and the ensures the vacuum is not recreated before signalling the Driver to recreate the vacuum and checking for brake release!

 

No real difference!

 

Mark Saunders

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where there is no brake van with a brake valve/gauge to carry out the test, the guard must "ease the rear hosepipe off the dummy coupling and ensure there is an in-rush of air.".

Presumably in this case it would be the driver who notes the vacuum gauge during and after the test.

 

(source 1972 General Appendix. not sure about modern methods)

 

EDIT: everybody else beat me to it while I was posting!

Edited by keefer
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The bag can be pulled off the dummy stop with a sharp jerk; a human being can overcome 21" of vacuum fairly easily.  There is a spring clip on a chain that fits through the casting on the end of the bag and the dummy to prevent it falling off during a full brake application; this also secures bags (hoses, but we always called them bags) together between vehicles.  When the brake is released by the creation of vacuum by the loco's vacuum pump or ejector, the suction holds the system together without need for the clips.  Spares are carried in the van, and a wise guard has a couple in his satchel.

 

You cannot always see clearly that brake shoes is disengaged from the wheel when the brake is released, and a kick will reveal that there is a little play there.  You can also hit 'em with a brake stick and listen to the noise it makes.  The valve in the guard's van is called a setter, the red painted thing with a lever you pull up to admit air to the system; you then check that the vacuum has dropped on the guage.

 

A continuity test must be carried out when the loco couples to the train, and at any time that the vacuum is destroyed by a loco change, running around or shunting to attach or detach vehicles.  On a part fitted freight, or any other situation where the person carrying out the test has to go in between vehicles, it is essential that the driver is fully aware of what is going on, and advisable to removed the bag from the dummy so he can't move the train while you are in there.  A shunting pole lent against the wagon or red light shining foward at night/FFS is an indication to him that you are in between the vehicles.  

 

If you are watching a loco running around a continuously braked train at a model railway exhibition, air or vacuum, and the loco unceremoniously sets back on to the stock and then continues to unceremoniously shove it up the platform without stopping, please call the operator to account in the most vociferous and publicly embarrassing fashion you can.  This never happens in real life because the brakes, with the vacuum or air pressure destroyed, are hard on and the stock cannot move.  The brakes must be connected and blown off before the train, which must now be coupled to the loco, can move, and it is usual for the continuity test to be carried out at this time; about 90 seconds in reality if everybody knows what they are doing.

Edited by The Johnster
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Thanks for the in depth replies there guys, doesn't sound a million miles off how we do a brake test with air brakes then. Taking the bag off the dummy is the equivalent to dropping the tap then, and everything else is broadly the same.

You can tell a shunter's boots from a driver's...the toes show wear from repeated kicking the blocks over many brake tests!

 

Thanks again

 

Jo

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Yes, the principle is exactly the same, except vacuum sucks and air blows.  I found this out one day on Barry Dock as a Canton goods guard in the 70s; coal, very poor quality stuff at that, was being imported from Poland for Didcot power station and I'd gone down with a 47 to pick up a string of MGR's directly off the wharf, work to Swindon for relief and cushions home.  The ship was being unloaded by dockside cranes fitted with grabs, not the most suitable method, and a good bit had probably gone in the dock; there was a massive mess of dusty coal all over the place.  We picked up a shunter at Cadoxton, and I went back to the last wagon to do the brake test while the shunter coupled on.  I could hear, and see, the brakes releasing as the loco blew them off, got to the back, and opened the red tap.  There should have been a rush of escaping air at 80psi (don't know what that is in bar), but nothing happened.  

 

I then did what was probably a very foolish thing, but the alternative might have been just as foolish.  I took the red pipe off the bracket, and gave the buffer beam a hefty bash with it.   There was a bang, and 4 quadrants of a plug of fairly solid coal slurry that had been blocking the pipe whizzed past my right ear close enough to feel a slight touch, and disappeared, last seen heading for Neptune at some considerable velocity.  An inch to the left and they'd have probably removed a good bit of my head, not that I'd have missed it...

 

But it is just as well I didn't look to see what the matter was; had the plug fired off while I was looking at it I'd have had it full in the face!  Then the air blew out of the pipe in the normal fashion and I was able to complete the test, kicking the brakes on the last 2 vehicles.

 

I was a bit shaken by this, but got no sympathy from those on the engine, who frankly didn't believe me!

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Yeah, seen that happen, also on a 20 wagon Freightliner, and heard of one being blocked by ice, which had to be hit with a hammer and boiling water poured over it before the brake test could be done.  We had much colder winters in those days; I know I'm sounding a bit like an exaggerating Yorkshireman (kids today, don't know they're born, we used to walks 400 mile 'cross freezin' moor, do 12 year shift down't pit, blah blah...), but serious frosts really were much more common before the 80s, and the winters in 63, 74, 83, and 84 were genuinely severe.

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On the few occasions I had to deal with vacuum brakes on the main line (BR SR CCT's), the greatest problem I had was getting the fugging things off the back, no matter how much I turned and twisted it. I was in my mid 20's then, and not long out of management training (but I had some experience of them whilst a volunteer on the Ffestiniog), but I was not much of Ray Davies' physical guy. My greatest embarrassment of my working career (Old Dudders may disagree here) was having to watch a sixty-something station supervisor leap down off the platform (at Sittingbourne), get them apart in two seconds, leap back onto the platform and then offer me a hand to get back up. Not my greatest hour.....

 

I improved as time went on, and I was pleasantly surprised to reverse the lesson to an old hand, when at Ramsgate, but by then I had developed an arm muscle. We never called them bags either, so that must be a local thing. They were pipes (or reds), couples, pins and housings to us.

 

But as for brake tests, the descriptions above seem over-complex compared to what we routinely did on the SR. We broke the vacuum, listened for the rush, waited for the driver to confirm he had the gauge, and re-attached. There was no brake van, unless a rake of unfitted minerals for scrap, or the nuclear flask (and only the Area Inspectors were allowed to check those). We could not immediately see whether the blocks had applied as almost all tests were conducted in platforms. A walk along the off-side was perfunctory to say the least, with kicking absolutely forbidden over the third rail.

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There is a simple check that were do.

 

With full vacuum the brakes will be off.

 

Take the bag off the rear vehicle dolly, need any damn good pull, the vacuum will drop to zero and the brakes will apply.

 

Replacement of the bag on the back will get the vacuum back to whatever the locomotive is creating

 

This is how we do it

 

In BR days when I was a train guard, I always walked the length of the train kicking the brake shoes are released and that there was no dragging brake shoes.

 

And then when you got to the rear of the train, checking that there is a tail lamp in position, give the vacuum bag a tug of it dummy to apply the brakes. and walk along the last two vehicles kicking the brake shoes to confirm that they have applied. Replace the vacuum bag and give the driver the hand signal to create vacuum. On the way back to the loco, if the train had a guards van in the formation check that the correct vacuum has been created which should 21 inches of vacuum or more.

 

I hope that is a help.

 

Terry.        

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Like a lot of these sorts of procedures, it's a lot easier to do that to explain. Getting the hose/bag (I suspect, but don't know, that 'bag' was a GW thing) off the stopper, which we called a dummy, was not difficult but you had to develop a bit of a knack of twisting them.  I found it harder to attach them to the stop or another hose at first, as again a knack is required, but no great strength.  It's one of those things Geoff Capes couldn't have done without the knack!

 

Air hoses were easier because they had the taps, which we called cocks, but it was a good idea to check that they were on their holding brackets before opening the cock, as the thing would otherwise fly around like a demented snake until the pressure was exhausted; I've seen a shunter get a nasty cut over the eye from this, an old hand who should have known better, too!

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One major problem with vacuum brakes, and this still exists on heritage railways today, is when a train is hauled by a GW loco, then gets taken over by a non-GW engine.

 

The GWR for some reason used something like 23" of vacuum compared to the standard 21" vacuum used by everywhere else. so it is still necessary to manually release the brakes on each vehicle using the brake release strings on each vehicle! Otherwise the loco won't create enough vacuum to release the brakes.

 

As Mr.Westinghouse onece described the vacuum brake as "21" of soggy suck".

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I've simplified, but it seems the modern equivalent of the GW's 23" is that each loco will blow up the air to a slightly different pressure. If the loco coming off got the brake to 4.999999 bar and the replacement loco is 5.0001 bar, you may have trouble releasing the brake. The overcharge will sort this out, or pulling the string on a few wagons. I take it there was no overcharge on vacuum?

 

When doing a brake test, the official method we've been shown is to hold the pipe in one hand and carefully drop the cock with the other hand. Some wagons have mega long pipes (HTAs I'm looking at you) that can still flail around despite best efforts.

 

Other wagons (tends to be newer ones) can have cocks where you don't need to squeeze the two parts of the handle together. They have a single piece handle and it's very difficult to open them smoothly as once they hit about 40% open a spring tries to keep opening them. If you let it, it'll then snap to the fully open position. Not good when brake test a class 70 hauled train, if the tap is opened too quickly the inboard computer thinks the train has split and you have to recharge and do another test, once the loco has sorted itself out.

 

As for bags, I had a Scottish driver suggest putting the yellow bag up, when we had trouble with the main res pipe, so obviously a term used north of the border too

 

Jo

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The GWR vacuum was 25" Hg while everyone else used 21". If changing locos from a GWR one to one from a different railway, you had to pull the strings to reduce the vacuum to below 21"; you didn't destroy it as this would mean building it up again from atmospheric. The problems arose when the strings hadn't been pulled for long enough and the retained vacuum was still above 21". The train would be restarted with dragging brakes, much to the amusement of the driver!

 

The term, 'bags' applied to any hose, not just the brakes. Taking water was also termed, 'Putting the bag in.'

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The GWR vacuum was 25" Hg while everyone else used 21". If changing locos from a GWR one to one from a different railway, you had to pull the strings to reduce the vacuum to below 21"; you didn't destroy it as this would mean building it up again from atmospheric. The problems arose when the strings hadn't been pulled for long enough and the retained vacuum was still above 21". The train would be restarted with dragging brakes, much to the amusement of the driver!The term, 'bags' applied to any hose, not just the brakes. Taking water was also termed, 'Putting the bag in.'

Changing locomotives means splitting the pipes and atmospheric pressure entering the systems destroys the vacuum.

 

Pulling the strings on the cylinders is only so to balance out the control side of the cylinder and prevent dragging brakes!

 

Mark Saunders

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Changing locomotives means splitting the pipes and atmospheric pressure entering the systems destroys the vacuum.

 

Pulling the strings on the cylinders is only so to balance out the control side of the cylinder and prevent dragging brakes!

 

Mark Saunders

Yes, but unless the strings are pulled for long enough you have a higher vacuum on the upper side of the piston than on the lower, so the brake doesn't fully release. Pulling the strings effects only the vacuum at the top of the cylinder, allowing for a brake release if the bags are not connected so that the vehicle runs without the automatic brake in operation. When a vacuum is next create, it forms at the top and bottom of the cylinder simultaneously, so both are in equilibrium and the piston falls under gravity to release the brake.

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Back to 21 v 25 inches, the train brake handle has a brake overcharge position. A push past the normal release point(against a spring just for few secs) will overcharge the vac or air and then bleed off back to 21in/72.5psi. Even if the loco coming off isn't of WR origin I was always told it was good practice to overcharge the brake after a brake test. I assume hydraulics continued the 25inch(lets be different) WR tradition. This applies to locos upto cl 60. Older vac only locos had a slightly different method depending the make of vac equipment fitted(big green brake handle push down in the release position speeds up the exhausters) A driver would indicate he's ready for a brake test by a hand signal.

Edited by w124bob
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You couldn't overcharge vacuum just speed the exhausters up for a faster release. On dual braked locos this was done by holding the brake handle in release which when in air overcharged the pipe but in vac speeded up the exhausters.

To stop the distributor causing dragging brakes on the loco the air side of the distributor or triple valve was equalised by the air vac isolating valve when in vacuum

Overcharge in air is 78.5 psi of air in the train pipe which bleads down over four minutes

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The GW was trying to achieve quicker release of vacuum brakes with it's 25psi, so it wasn't just about being different; as usual with the GW, it was about being 'better', even at the cost of the inconvenience of having to isolate the brakes and blow them off again every time a 25 inch loco was changed for a 21 inch on or vice versa.  You also have to pull the strings to isolate the applied brakes to release them for shunting when the train arrived at the destination yard, a lengthy enough process as you have to do each individual wagon on what might be a 60 wagon fitted freight, and no fun if it's belting down with rain!

 

The large ejector of a steam loco can blow a train's vacuum brakes pretty quickly, but early diesel and electric locos' compressors were not as capable, and this is probably one, though by no means the only, factor leading to the adoption of air brakes, originally with a twin pipe 'train' and 'reservoir' to facilitate quicker release.  There is no inherent superiority of air over vacuum; the efficiency of the braking system is down to the pressure of the brake block on the wheel or clasp on the disc, and a very good 'instant release' twin pipe vacuum system can be found on 1st generation dmus.  A quick release enables smoother and more accurate braking, and ensures that the brakes do not drag.

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