Vacuum Brakes

Hi,

Does anybody know how long vac brakes are supposed to hold before releasing?

I seem to remember its about 20 minutes. Can anybody think of anything different?

cheers,

 

You should NEVER rely on any form of continuous brake after the loco has been detached.

 

it all depends on their condition I guess...the more worn the seals are, the quicker the brakes will leak off.

 

Yeah, I know that. I am a passed guard

I was just planning a pretty long fitness to run exam for a day next week as I have a feeling some are leaking off very soon indeed!!

 

I suspect that you wont find a figure in any publication aimed at operating staff, for the reason we both understand. However, there has to be some sort of spec for maintenance. There was a chap offering Mk1 manuals on here recently.

http://railways.national-preservation.com/viewtopic.php?f=43&t=18721

Could be just what you need?

 

From what I've seen it can be hours, weeks and months.

 

According to the BR 1969 manual for Mk1 coaching stock the test on the vehicle gives allowable drop of 2" over 5 minutes from a created 20-22" of vacuum in the main pipe with all the brake fittings on.

 

The leak rate is for the train pipe side, from what i rember when I used to work on VB stock the res should hold the brakes on for at least 2o mins. However if the cylinder sliding band, or rolling ring is in good order, and the release valve seals, and cylinder joint seals are in good order this should be no problem.
Other things which can cause issues with the train pipe side are the DA [direct addmission] valve and the pass com, and any of the other rubber wear in the system.
many small leaks in a vacuum system can lead to a big problem.

 

I've never been a guard, or dealt with coaches, other than hooking them on or off my engine, but I know from personal observation and chats with some of our C&W staff, that it's not uncommon to come to a set which had been stabled the previous day and which still has 18-20" in the reservoirs.

 

The (reprint of) "Handbook for Railway Steam Locomotive Enginemen" says that the driver, as part of a brake test when coupled to a train, should check the time taken from forming a vacuum of 21inches in the train pipe for the vacuum to drop to 12 inches. If 20 seconds or less there is an excessive leakage and the source of the leakage should be found.

 

MT 276 - not for coaches but the BR standard for mainline steam before privatisation gives figures for acceptable leakage rates for both train pipe and reservoir side of GW locos with vacuum brakes. It would be suprising if there was no corresponding C&W standard.

 

Slight case of Zombie Thread Resurrection...

...but this extract from BR10906 "Regulations for the Repair of B.R. Standard Locomotive Hauled Coaching Vehicles at Main Works" (1969) - ought to cover most eventualities.

View attachment Vacuum Brake Tests - BR10906.pdf

& if it doesn't, then this extract from CMS123 (1988) might:

View attachment CMS123_Ch_4.pdf

Pete.

 

If memory serves me correctly, you should blow the brake up, then put 5 inches of brake in (ie, drop to 16" if you are blowing them up to 21") hold it at that for 5 minutes, and re-check to see if the blocks are still on, re-create the brake and then put the lot in, the brake should hold for at least 15 minutes. I could check next time I'm at Bewdley if you like?

 

I always thought twenty minutes was about right. Many years ago we dug out an old ventilated van from the dock siding and took it to C&W for repairs. The following week they tried to move it and struggled, because none of them expected the brakes on such a decrepit vehicle to last for minutes, let alone days!

 

Vacuum holds up longer than air pressure. Firstly, the maximum 'pressure' applied cannot exceed atmospheric, i.e. 14.7 p.s.i. Secondly, the effect of vacuum is to draw items such as connections together, and so help seal them. Air pressure is both about four times higher and also tends to force joints apart, thus encouraging leakage.

 

Rolling rings in the cylinders seem to hold vac better than the GWR wiping seals. When prepping a set of BR standards left for a week most brakes will still be on. Prep a GW set left overnight and most will have leaked off.

Totally agree you should never rely on the automatic brake but it must be effective at holding the brakes on for suficient time to secure the stock. For example a coupling brakes at the rear of the train with the brake in the middle.Having come to a stand you find your scotches explain to the fireman who has joined you whats up. instruct your TTI to secure the door at the split to stop anyone falling out. As you walk back check no one is in the 4foot having fallen out. Apply the scotches to hold 3 mk1s at 90 ton on a 1:100 gradient. Secure the end door. Walk back as far as you think necessary to check no one is lying in the 4 foot. And the standard is 20 minutes? You then have to set a strategy. Buckeye failure - get the red coupling and drop buckeyes. I now have to set back onto the stranded 3 with the other 5 and loco and we might now be relying on 2 bits of timber if the brakes only held for 20 min. Solution bring stock back short of touching but close enough to connect vac. Split vac pipes at other end of first coach and couple pipes at split. Create and destroy vac to set brakes. Disconnect vac at the split. Buffer up to the stranded 3 and get the coupling up asap. Re connect other vac pipe. Do a continuity test and remember to then remove scotches. 20 minutes - thanks for rolling rings lasting more than 20 munutes