Vacuum brake on WR/SR

I wonder if someone would be kind enough to put me right on the following : The vac brake on a WR train runs with 21 inches of vac. However on the SR they run with 18 inches of vac ?
Have I got this right or am I talking a load of @?x&
I have searched this on google but without any luck.
Many thanks

 

You are correct. GWR used a higher vacuum than other railways

 

No you are not correct - ex GWR runs at a train pipe vacuum of 25ins & the others at 21ins.

The GW version does give a noticeably more powerful brake.

 

I was always taught 25 for GWR and 21 for everything else.

 

I thought that was the case I just wanted to check I was correct. Family discussion today.
Many thanks for your reply

 

I seem to remember that the vac gauge in the guards van on the SR system must show a min of 18 inches of vac ?

 

There is an allowable reduction at the guards van but I cannot recall the exact figure - yours is probably correct. That only applies to goods workings (lots of vac connections & hence possible small leaks) but not to passenger trains.

 

Am I correct in thinking that if you change engine from GW to anyone else you have to pull all the strings on every coach to reset the vac cylinders?

 

Yup :/

 

Yes but only enough to release the reservoir side by 3 or 4" so no need to completely release.

 

A random thought but maybe the confusion arises from 18" and 21" being the most common sizes of vacuum cylinders? (But there is no connection!)

 

Personally I like to see the cylinder start to decend as dragging brakes can stall a train

 

There is no connection. The standard vac brake works at 21 in of mercury with a lower acceptable vac of 18in Hg. For the GWR vacuum is created at 25in Hgwith a low allowance of 23in Hg

 

Saw this "in action" yesterday at West Somerset. Incoming service hauled by a Hall was being prepared for departure with the S&DJR 7F, so 25" on the way in and 21" on the way out. With a mix of GW and other locos on site the WSR guards must be very used to this particular duty, especially at gala events.

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It is not really an onerous job but actually easiest trackside rather than platform side. On SVR easiest place is Kidderminster where there is a wide distance between platform and engine lines. Most awkward is Bridgnorth where the gap between the tracks is narrow. You are only allowed to go trackside if you arrange a block of the other line with the bobby. This is not allways possible so you have to knel down on the platform by each cylindert grovel under the solebar for the string. The position of the string is indicated by s white star on the solebar.

At the start of a duty as you cant be sure what vac is in the cylinders you pull all the strings as part of your train inspection, having first ensured that the handbrake is on

 

i would think you would destroy the vacuum completely not partially because as someone else said a partial release can result in dragging brakes, its a common problem having to pull the strings, if your changing from or to a GW steam engine, a question to GWR c&w, do you test your vac cylinders to 21 inches of mercury or 25 ? if your motive power tends to be former GW types?

 

Just a clarification - you only need to pull the strings when going from GW to LMS,LNER,SR AND BR. Going the other way it is not necessary as the 25in created by GW will release the 21 previously created

 

Before its recent overhaul, 53808 was set to create a vacuum of 25" so it fitted in with all the GWR locos and we never had to pull strings in everyday use. Now we also have 44422 it's not the odd one out!

On the WSR, guards check in the on-train diary at the start of the day to see what loco worked the train last to decide whether or not to pull the strings. We also keep pulling until the cylinder is completely empty.

 

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It is not a criticism but I would not rely on paperwork which may not have been updated. There is also a gooly trap with 08s/09s/11. Some have a adjuster to exhaust 25in. Saves pulling strings when putting a set away. The problem I encountered was although the train engine was 21in the gronk was still set for 25 and luckily noticed when my set was shunted to release the loco

 

GWR brakes normally run at 25" all others at 21", trains may be permitted to run as low as 17" and owing to leakage, the reading in the van may be lower than on the loco. When swapping between a 25 and a 21 inch loco the cylinders need to have the "Tops knocked off" although if you are not sure how long to pull the string for then simply draining and starting again is the safest policy.
The issue of dragging brakes is not so much that you could bring a train to a stand, but you could skate a wheel, causing the wheel set to be scrapped, or the fact the brake is dragging could over heat the tyre and cause the tyre to slip or become loose.