Dual Braked Locos

I remember this was brought up in a topic a while back, but can't remember which one.

Some locos (for example 6233 I believe) are air braked yet have something that allows them to haul vacuum braked stock. Can anyone just give an outline of what this is, how it works etc?

 

I did a search on "dual brake" and found this:-

viewtopic.php?f=36&t=9164&st=0&sk=t&sd=a&hilit=dual+brake&start=15

 

It is similar to the dual brake system on a diesel, I will see if I have any diagrams safe to scan that will give an idea of how that works

 

I notice 6024 is currently the only ex-GWR loco on the mainline with Air brakes. Does this work in a similar way or does it have a different set-up?

 

6024 is more like 60009 etc. It has vac brakes on the loco still which are controlled by the air system trough the DV4 proprtional valve, but the loco's own brakes are still to Swindon design

5029 is also a similar system

 

This is getting a little more complicated than I thought #-o

So how many different methods are there for locos being dual braked ?

 

Full air: Completely new system fitted with slave vacuum for train. Same principle as for dual diesels

Train air: Air system for train only plus safety equipment. Air system controls loco brakes and train vacuum brakes (when used) via proportional valve

The vac system in this case will be whatever the LMS / LNER / Southern / GWR /BR vac system was that was installed when
it was built

There is also the old railway method of doing it, where the loco's would be air braked via the train pipe and then the brakes operated by different equipment depending on the train, such as GE practice, but that might confuse the issue further!

 

Okay, imagine say a dreadnought vacuum system for simplicity's sake. Vacuum is created by the ejector and destroyed by the brake valve in the cab.

Now imagine an air brake system. operation of the valve reduces the air pressure in the pipe.

A proportional valve has two sides, the air side and the vac side. Each side has a chamber and the pipe side separated by a diaphragm linked to the valve.

the air pipe and reservoir are charged to full working pressure. The ejector on the vacuum system charges the vac pipe and the vac reservoir. With everything charged the brakes are off. If the pressure is reduced in the air brake pipe, the higher pressure in the reservoir will overcome the spring holding the valve shut and start to admit atmosphere to the vacuum pipe. The more the pressure is reduced, the further the valve will be forced open (please correct me!)

As this is a proportional valve the vacuum will reduce relative to the reduction in air pressure. The as the vacuum is decreased on the vac pipe side, the pressure on the diaphragm against the vac chamber will increase until this overcomes the spring on this side of the valve and the vacuum side of the valve will close to atmosphere stopping the application. The greater the air application the further the valve is opened, therefore the greater the reduction in vacuum needed to close the valve again, causing the proprotional application.

Because the air pressure maintains the application level, unlike the vacuum valve in the cab, this can be considered a self lapping process, i.e. the position of the brake valve determines the level of application and once reached will be held until the valve is moved.

At least that is my theoretical understanding. If someone with practical experience wants to correct it, please do!

 

I think that's more or less right, SYM. They all vary, they are all complex, but all methods have the same general objective, which is to make the train do the braking, not the engine, to eliminate compression jerking. The main variations consist of the engineering solutions to achieving this result and the proportionality. I thought LMS steam / vacuum brakes were complex until I studied the Metcalfe / Oerlikon Triple valve on the Class 25!

 

but what about loco's like 73050 that have vacuam air and steam ????????????????????

 

73050 is a completely different kettle of fish gaz as you should know.
Both the Air and Vacuum brakes will opperate the steam brake, but neither will opperate each other, Using 1800 (NVR's Thomas) as a example when I've been on the footplate and had both the Ejector and Air Pump (when it's working correctly) you can opperate the air/vacuum valve (depending on the stock that is being hauled) and it don't affect the other resivoir as they are both seperated from each other.

 

On most mainline locos that weren't originally intended to be dual braked, the brakes will be operated by the air system, which operates the vac system via the air / vac relay. The steam brakes are then operated by the vacuum using the existing braking system!!!!

 

SYM, On 73050 you don't need to use the vacuum ejector when the air pump is working to opperate the steam brake as there isn't a vac/air relay and there is 0in HG (0 inches of murcuary/vacuum).
73050 has been adapted to use either vac or air because at the NVR they haul the continental stock aswell as the Mk1's.

 

Ah appreciate that one Jamie, thanks for clarifying.

As I understand most mainline certified dual brakes use the system I have described, as the Safety equipment is always tied into the air system

 

well i assume that 73050 must be certificated to the same standards as mainline loco's as is the same safety at risk isnt it even tho u aint doin 70mph

 

73050 dose not have ATWS or black box equipment which have to be fitted to main line going locomotives. These systems are designed to operate with air braking systems and so have only been adapted to operate with Vac brakes on locos not air fitted. As there are no intensions for 73050 to go main line it has just go had an air system fitted to let it work air stock. It is truely due fitted as one system does not have to be operational for the other to work. There are several other such engines about, such as the J15, B12, several Terriers, Birch Grove....................

 

I wouldnt take Birch Grove out without a working air brake....similarly some of the terriers and even Normandy have vacuum brakes that work on the stock but not the engine. As Churchward used to say "The engine pulls the train, but the train stops the engine"

Does anyone know what system the French railways used towards the end of their steam? That had seperate brake handles for loco and train air brakes.

 

As far as I understand they had a similar system as used on modern Diesels but using only a single pipe for train braking, i.e. the loco has a direct air brake that can be either controlled by the direct valve or the brake pipe pressure.

Those you speak of with air on the loco and vac for the train, as I understand were controlled by the train pipe when operating an air braked train and when running with a vacuum train the vac system operated a direct air feed to the locos brake cylinders. Anyone who wants to make that clearer please do

 

You are right in most cases, but it’s not true for all. Most dual braked locos will have some valve or other that applies the locomotive brakes due to the application of the train braking system. But some locos have been fitted with what amounts to a handle for the train brakes. This often happens when a loco has had vacuum fitted in preservation which it did not have in working life. Normandy is one example, and I well a shunting turn during which we were rolling down towards some coaches, and the trainee driver applied the brake…then dropped the handle…the grabbed the steam brake handle at the last minute! The Vacuum ‘application’ hadn’t had any effect at all.

( I thought Birch Grove was the same, that air worked on the loco but the vacuum didn’t touch it, the coaches stopping the engine. Might be wrong though.)

As an aside, what system is used on main line steam outings, is the stock twin pipe (ie mainline/trainline as on national rail multiple units etc) or single pipe air (as on the IWSR)?

 

twin pipe is fitted to all air braked mainline steam locos