How does the vacuum system work on Gresley coaches?

[gdb]

With photographs supplied by Mike Trice I now understand "what" pipes and fittings are under a Gresley passenger carriage... and that understanding of "what" tells me that my knowledge of "why" and "how" is lacking. Who can either point me at a description of the Gresley vacuum brake system or describe [a] the reason for the vacuum reservoir adjacent to each vacuum cylinder and the purpose and operatioin of the valve which is affixed to the bottom of the vacuum cylinder (see here for Mike Trice's photos - http://www.rmweb.co.uk/community/index. ... id__106929&).

thank you, Graham Beare

[CVR1865]

The photos are not viewable unfortuantely unless you have a login. (which I don't)

Are you referring to the arm underneath a vacuum cylinder that draws the two plates apart as the Vacuum is created?

[gdb]

The photos are not viewable unfortuantely unless you have a login.

I am not sure why there should be a problem with accessing the thread and photos because all of the threads are "open access" for viewing.

So, using the URL of http://www.rmweb.co.uk/community, find the "Modelling Zone" part of the main page, then look for the "Prototype" group of sub-forums, then select the "Prototype UK" sub-forum. The photographs can be found in a thread with the title "Gresley TK - which way round?".

Are you referring to the arm underneath a vacuum cylinder that draws the two plates apart as the Vacuum is created?

I understand the principle upon which a vacuum cylinder works - what I do not understand is the reason for the adjacent vacuum reservoir and the purpose of the gunmetal casting which is located at the outside edge of the underneath surface of the cylinder. This valve appears to be connected to a passageway which runs up the outside of the cylinder. There are two rubber hoses attached to the valve body, one hose is connected to the reservoir and the other hose may be connected to the train pipe. One of the photographs suggests that there is a lever as part of the valve body and that the vacuum release pull-cord is attached to the lever.

regards, Graham

[manna]

G'Day Gents
There are three ways to 'Destroy' the brake on a vacuum braked carriage, 1- from the engine, 2- from the brake release cord, 3-from the communication cord, your rubber pipes probably connect to one of these, sorry can't help on any other tech bits
manna

[James Brodie]

Mr Beare,
Dear Sir, may I e mail you with some Vacuum Brake sketches and explanations.
When I was off footplate duties in the late 60s I was used for teaching new entrants automatic brakes-Vac and Air.
Maybe explain some points now but tried to open photos No Go but found some interesting articles on tail Traffic and Mixed trains which were explained far better than I could have, I digress.
A small flexi pipe from the main pipe line, the guard was issued with some wooden plugs to help isolate a cylinder. Cut the flexi pipe and hammer the plug towards the train pipe opening. Then remember to pull the cord to relaese the brake from that cylinder. I'm stretching the grey matter now. If a GWR loco cameoff the train and a real sorry other companies engine came on , allthe cords had to be pulled to lower the vacuum (please accept the misnommer) as GWR worked on 23'' where other companies worked on 21'' The GW had the steam conserving device known as a vacuum pump which created the higher/lower reading.
Reservoirs were installed when small size cylinders were used maybe for space restrictions.
I do have some GER 0 gauge coaches that do not have vac Cyl's on them being Air Braked only.
When down on the GE area drivers on the EMUs were required once in their run to work the Westinghouse Brake, they got too reliant on the EP brake. Once away from Liverpool Street I would take over the driving andd preferred using the Westinghouse which the drivers then claimed for themselves in having used the alternative brake. The vaxuum Brake their proving was to apply the brake as soon as possible after getting away to get the ''feel'' of the brake.
Both brakes work on a greater pressure overcoming a lesser pressure, the air brake was greatly improved when the two pipe system came into being but if part of the train was two pipe and paart one pipe, drivers had to remember to recharge but that's another story.
Hope these remarks have been of some help and am happy to guide you further if you wish.
Jim Brodie.

[Bill Bedford]

I understand the principle upon which a vacuum cylinder works - what I do not understand is the reason for the adjacent vacuum reservoir and the purpose of the gunmetal casting which is located at the outside edge of the underneath surface of the cylinder. This valve appears to be connected to a passageway which runs up the outside of the cylinder. There are two rubber hoses attached to the valve body, one hose is connected to the reservoir and the other hose may be connected to the train pipe. One of the photographs suggests that there is a lever as part of the valve body and that the vacuum release pull-cord is attached to the lever.

The reservoir is needed to hold the vacuum on the topside of the brake piston when the brakes are applied. The valve on the underside of the cylinder controlled the vacuum in the cylinder. When the brakes were applied it allowed air in to the cylinder below the piston. When the brakes were released it allowed the vacuum on each side of the piston to be equalised allowing the piston to fall under its own weight.

[52A]

Vacuum on the Grate Western was 25", they always had to be different, but Gresley Was Right!

[coachmann]

Shame 21 inch or 25 inch wasnt standardised. I have seen GW coaches martialled behind the engine in trains of LMS and LNER stock. Would the difference in vacuum be the reason for this, it being easier to maintain 25 inches at the front of the train?

[52A]

The vacuum was standardised in the mid 60s I think it was, to 21" during operation "Vacuum Change". The vacuum is set from the governor on the loco so all GW locos, and their allocated diesels, had to have their governors reduced to the 21" standard. You could mix vehicles on a train provided that the release cords had been operated on any vehicles that had been running on a higher vacuum.

[coachmann]

You could mix vehicles on a train provided that the release cords had been operated on any vehicles that had been running on a higher vacuum.

Ah, so changing engines like removing a GW from a passenger train and installing an LMS loco meant operating the release cords on any GW vehicles that might be in the train. No doubt not too many ex GW coaches remained in regular service by the mid 1960s when operation vac change too place.

In fact there was a drive to scrap Big Four design coaches at this time. I recall traveling in 1st class comfort from Wales to Manchester in a downgraded LMS coach in the late summer of 1965, and it seemed a tragedy that such coaches were being withdrawn for scrap.

[James Brodie]

Mr Coachman Dear Sir, Coaches could be mixed this was dictated by the CWDs . The reason for cords being pulled were only on trains being worked in by GW engines to lower the stock to other companies working levels. When changing from LNE or LM etc no problem as when the GW engine came onto the train he created the higher level and no danger of dragging brakes.
The working levels are shewn in the appropriate appendices where at the front end it could be 21'' and at the rear not lower than say 18'' but there was a table to guide you.
Jim brodie.

[James Brodie]

letter 3-20pm all vehicles would have their cords pulled not just GW as they would all be running on the 'higher' vacuum.Jim.

[James Brodie]

We lost a coach to scrap in 1954 because it needed a left handed vacuum pipe replaced. Middlesbrough men worked the Guis' G5 out of the bay at Mbro , the fireman hung on the two vacuum pipes, two bell connections and the steam heater pipe BUT forgot to couple up the engine screw coupling to the coach. They gaily set of until the pipes wouldn't stretch any more hence for the sake of a broken vac pipe (the train regulator controlling pipe )
JB.this beautiful NER Clerestory DBC was condemned!

[Mr Bunt]

Shame 21 inch or 25 inch wasnt standardised. I have seen GW coaches martialled behind the engine in trains of LMS and LNER stock. Would the difference in vacuum be the reason for this, it being easier to maintain 25 inches at the front of the train?

No. Because the vacuum relief valve on the loco would stop it from creating excessive vacuum in order to prevent the flexible pipes between the vehicles collapsing. When GWR stock was added to a train run by a proper railway its brakes had to be bled off (sometimes called "pulling the strings") and then vacuum re-created before the train started again, otherwise the brakes on the GWR vehicles wouldn't release.

This kind of problem also arose sometimes (but not often) when the train engine was being changed on a real railway and the one coming off had a badly calibrated relief valve (allowing it to pull, say, 23 inches of mercury) but the replacement had one that was accurate, and therefore pulling a lower but correct vacuum.

Relief valves had to be kept clean and regularly lapped in, otherwise the ingress of dirt would impair their operation.

As a point of general interest, the RH&DR has always run on 25 inches and loco relief valve problems are frequent because they are so small and close to ground level. No one has yet invented scale size dirt to clog them with

[coachmann]

When GWR stock was added to a train run by a proper railway

Thanks for the full description of how things worked.
Larry