Prototype BR Mk.1 Brake Arrangement - the vacuum cylinders

[Dog Star]

Last week I was given sight of a drawing showing the installation of a vacuum cylinder in a Mk.1 underframe, top / side / section views in the traditional arrangement. My immediate reaction was that I had not seen such a juxtaposition of parts before... particularly the placement of the DA valve on the underframe and the inclusion of a return spring on the brake cross-shaft. The explanation of my confusion came down to period... most of my Mk.1 searching to date has been on heritage lines that had a high proportion of late build carriages compared to early build stock. So when I visited the Mid-Hants last week I sought out some of the "older" build of Mk.1s... typically those built in the mid-1950s... and the sun shone to show me that there is always something to learn about these elegant design icons.

TSO 4423 was built 1956 and has retained the original arrangement of vacuum cylinder (inboard of brake cross-shaft) and return springs (as per the drawing referenced above). The first photo shows the return spring operating on the pull-rod lever of the cross-shaft... and the spring supporting the brake pull-rod.


The vac cyl at the other end of the coach to the previous photograph. This photo illustrates a trunnion which is welded to the solebar... a later variant has the bearing portion bolted to a bracket (welded to the solebar) to aid replacement of the cylinder.


Return and support springs visible... as is (what I believe to be ) the vacuum train pipe).




How the vacuum pipe is hung from the solebar and routed under the headstock.


BSK S34947 was built in 1956 and has a similar installation of the vacuum cylinder... with the added interest of the linkage for the handbrake as can be seen in these photos. As the carriage was against a platform edge I was not able to determine if the vacuum pipe was routed as per TSO 4423 (above) at the non-dynamo end of the underframe.

 

[Heather Kay]

I'm not sure about that vac pipe running under the outside of the frames. You may recall a similar layout I found on the preserved K&ESR RMB, which interpret as refitting the vacuum braking after having been converted to only air braking at some point. I could be wrong, but I don't see the vac pipe running as you've photographed above in period photos.

 

[Bob Reid]

Just for you Graham....


You are right enough Heather - there's something funny going on with that brake pipe - it's sitting where the main steam pipe used to sit....

 

[Dog Star]

I'm not sure about that vac pipe running under the outside of the frames ... I don't see the vac pipe running as you've photographed above in period photos.

You may be correct Heather, I just do not know and an anecdote of my recent visit may be relevant here - with a reminder of how we need to understand how things change over the years. Just look at this photo for a few moments... what is not as we might expect?

Afficonados of the Mk.1 design icon are becoming familiar with the ideas of "inboard" and "outboard" vacuum cylinders and the attendant arrangement of brake gear... viz:- pull-rods above / below bogie headstock... equalised / non-equalised braking. So the fitment of Commonwealth bogies to a Mk.1 underframe with "inboard" vacuum cylinders ought to have the bells ringing because a CW bogie has normally the pull-rod over the headstock rather than underneath as can be seen in the photo - just do not ask where the pull-rod connects to the bogie fulcrum levers... unless you are going to provide the answer.

I have no evidence that the brake arrangement shown in this photo was the result of BR modification. This is not the first time that I have seen the juxtaposition of CW and inboard vac cyl., the Mid Hants Railway has at least three carriages in the same configuration. I spoke to the C&W staff about these "oddities" and none of those to whom I spoke can remember anything about the conversion... so did the coaches arrive with CWs? was the conversion done by the Mid Hants? or were the conversions done by a third party between the coaches being sold by BR and arriving at the Mid Hants?

The lesson for modellers? Probably at least two...

1/ What we see on heritage railways need not be as built / modified by BR.
2/ The design evolution of the Mk.1 carriage continues by those who might not have been born when the original design was first set down.

 

[parky]

Good to see the workings of a MK1 again. The brake system is really effective once overhauled and adjusted up. Several times I have seen drivers come to a stand half way down the platform at the AVR. Unless the loco is able to blow the brakes off quickly, you are going to stop !
From my experiance you could get any combination of setup straight from BR. Add to that the needs of the Heritage line and you can get many configurations.Having been involved in putting a TSO back from Air/ETH to Vac/Steam the pipe runs might not be as installed when it was built. From what I worked on there is the original underslung gear and the later overslung. Add to this the change of bogies for the likes of RMB's to later heavies but retaining the underslung actuation. The CW bogies never came along until after I left, but they have always looked impressive

Swapping vac pots and DA valves is a heavy and mucky job, especially when you consider what the chassis coating really is. The overhaul of the components is fairly straighforward as they are simple machines, just heavy. Once refurbed and new seals fitted not only does the coach stop well but sits with the brakes hard on for weeks unless someone pulls the strings