Vacuum Brakes

Aim - this section provides an overview of the typical operation of the Vacuum brake system, and describes the "best" brake settings for Open Rails (OR) WAG and ENG files. Settings are based upon British Rail (BR) standards, which use a variety of systems.

To convert tons to kN = tons x 9.964 kN.

If you wish to provide any feedback on this page, please use the contact page. It would br great to have some feedback as this helps to ensure the accuracy of the information and models.

Index

Introduction

Overview of Vacuum Brake System

Operation of Vacuum Brakes

Brake Code - Structure and layout in Open Rails

Sample Brake Code - Wagon Section

Sample Brake Code - Locomotive - Compressor, Reservoir and Train system

Sample Brake Code - Train and Locomotive Control Valve

Brake Calculators

         Vehicle Maximum Brake and Handbrake Force

         Brake Pipe Volume

Test Stock

Useful References


Introduction

In the early days of operation trains usually only had brakes fitted to the tender of the locomotive, and special wagons, often called the 'brake van' which were applied by the train guard (also known as the brake man) manually, generally when the locomotive driver sounded a pre-designated whistle code. Sadly as trains were operated at faster speeds, and with heavier loads this method of braking was found to be inadequate due to the slow application and response times to stop, and at the time contributed to a number of accidents, such as the one at Abbots Ripton. As a consequence, there were numerous calls for the introduction of a continuous braking system.

Over the years a number of continuous types of air brakes have been developed and used on trains. These include air and vacuum operated brakes. More recently, electrically operated brakes have been introduced to some rolling stock. Naturally the effort to fit continuous brakes to rolling stock was spread over many years, and often started with passenger stock. In some instances with slower, less important rolling stock air brakes were never fitted throughout the lifetime of the rolling stock. In Australia there wer coal hoppers still in service up to the 1980s that did not have continuous brakes fitted to them.

The Westinghouse brake system was a major break through for the safe operation of trains. It has become the predominant type of brake system used by railways around the world. The Westinghouse system has evolved significantly since its invention in 1873, and there have been many system variations as it has been enhanced to cater for high speed and heavier trains.

Although the Westinghouse brake was the winner of two sets of brake trials in Britain, about two thirds of British Railway companies adopted the vacuum brake. Reasons being low cost, simplicity and ease of maintenance - (an ejector used to create a vacuum on a steam locomotive has no moving parts). Following the grouping of 1922 it became the standard brake of all of the four main line railway companies until superseded by air brakes in the 1970s. As well as use in Britain the vacuum brake was also exported to many British colonies and to lines in elsewhere supplied with British equipment. The vacuum brake was also the standard automatic brake used by Swedish Railways until the 1920s and by Austrian Railways until the 1930s.

There have been many detailed books written describing the design and operation of the brake system, and it is not possible to reproduce this level of detail in this page. Instead the page is designed to provide an overview of the basic features and provide some design rules of thumb that can be used to configure the ENG and WAG files of Open Rails. Naturally it goes without saying that actual prototype information should always be used as the first preference if known.

Whilst air brakes and vacuum brakes are rather different in their structure and operation, the details of what is needed will be treated on separate web pages, there are however a number of common elements. More detailed information on air brakes can be found on the Air Brakes page.

top


Overview of Vacuum Brake System

This information has been extracted from various publications, some of which may be found in the Useful References Section. Whilst a few different styles and types have been used, the following information should apply in general and give a reasonably close approximation of its operation.

In its simplest form, the automatic vacuum brake consists of a continuous pipe - the train pipe - running throughout the length of the train. In normal running a partial vacuum is maintained in the train pipe, and the brakes are released. When air is admitted to the train pipe, the air at atmospheric pressure acts against pistons in cylinders in each vehicle. A vacuum is sustained on the other face of the pistons, so that a net force is applied. A mechanical linkage transmits this force to brake shoes which act on the treads of the wheels.

Carriage and Wagon Brake System

The diagram below shows the main parts of the vacuum brake system on a wagon.

Wagon Vacuum Brakes

The principal elements of the brake system on a wagon are as follows:

Brake pipe

The brake pipe runs the full length of the train. It provides the vacuum that operates the brakes. Most railway companies used a vacuum of 20 or 21 InHg (inches of mercury). The GWR used a greater vacuum of 25 InHg.

Vacuum Cylinder

The vacuum cylinder consists of two parts. The lower part is the "brake cylinder" and movement of the piston within the brake cylinder applies force to the brake shoes on the wagon wheels. A series of levers connect the piston to the brakes in order to provide the appropriate brake force for the weight of the wagon.

Above the brake cylinder is the "vacuum reservoir". (The equivalent of the auxiliary reservoir in an air brake system.)

Wagon Brakes

When air enters the brake pipe the difference in pressure between the underside of the brake piston and the vacuum reservoir above provides the force that operates the brakes. Some heavier carriages and locomotives had larger brake cylinders with a separate vacuum reservoir.

Air may be admitted to the pipe through the placing the driver's brake valve in the apply position or in an emergency through the guard's brake valve or by the use of the passenger alarm. If the train becomes divided then air will enter the brake pipe and the brakes will be applied automatically.

To release the brakes then a vacuum must be created in the train pipe. The ejector or exhuaster is used to do this. When the driver's brake valve is placed in the release position then the train pipe is connected directly to the ejector or exhauster or high vacuum reservoir.

Dummy Coupling

At the end of the train the flexible hose used to connect the wagons together is placed on a dummy coupling (or plug). This seals the train pipe and there is no need for a cock.

Brake System Refinements

Direct Action Valves

It takes some time for automatic brakes to come into operation along the length of a train. Following an accident at Slough in 1900, where an express overran two sets of signals the GWR designed a direct action valve to speed up the application of vacuum brakes. Direct action valves allow air to enter the brake cylinder directly if the vacuum in the train pipe falls below a certain level. Direct Action valves were fitted to all new GWR passenger carriages from 1906.

Other companies were slow to follow. The LNER introduced Direct Action valves on its streamlined high speed stock from 1935, but did not fit them to general service stock. The LMS started to fit Direct Action valves from about 1939. All post nationalisation passenger carriages were fitted with Direct Action valves. Goods wagons were not normally fitted with such valves.

Load Compensation

Some British Railways built vacuum braked goods wagons included load compensation. This took the form of a second brake cylinder so that the empty wagon operated with only one brake cylinder, but a second brake cylinder could be brought into use, by means of a lever, when the wagon was loaded.

Locomotive Brake System

A number of different braking arrangements were applied to locomotives. The diagram below shows the arrangement for a locomotive, and has been extracted from "A Manual of Steam Locomotive Restoration and Preservation" by D.W. Harvey in the early 1950s.

Locomotive Vacuum Brakes

Locomotive Brakes

Many steam locomotives fitted with vacuum train brakes, used steam brakes (or air brakes) for the locomotive and tender. Most vacuum braked diesel and electric locomotives use air brakes for the locomotive. In these cases when the locomotive was hauling a train, the locomotive brakes were controlled by changes in the vacuum in the train pipe. Some steam locomotives were fitted with vacuum brakes and these operated in a similar way to those fitted to carriages and wagons. The only difference was that there was a release valve on the locomotive and/or tender. This allowed air to be admitted to the vacuum reservoir. As a result the brakes could be released without releasing the train brakes.

Ejector(s)

Vacuum brake systems suffer from leakage to a far greater extent than air brake systems. To maintain the vacuum against leakage a "small ejector" was normally provided on steam locomotives. The driver could adjust the setting of the small ejector to adjust for leakage depending on train length. Some steam locomotives were fitted with a vacuum pump in addition to (or in place of) the small ejector. The vacuum pump was worked from the crosshead and hence could only maintain the vacuum when the train was moving. In diesel and electric locomotives the vacuum is generally maintained by an electrically driven exhauster that operates automatically. (In some diesel locomotives and multiple units the exhauster is driven by the engine crankshaft. In such cases the exhuaster operates continuosly when the engine is running, charging a high vacuum reservoir. When the engine is running at low speed a vacuum can still be created or maintained by connecting the train pipe to the high vacuum reservoir).

All steam locomotives had one or two ejectors. These were used to create a vacuum in the train pipe. Locomotives with two ejectors had a large ejector to quickly release the brakes and a small ejector to help maintain the vacuum against leakage. Some locomotives also had a vacuum pump which helped to maintain the vacuum when the train was in motion.

Driver's Brake Valve

The driver's brake valve controls the train brakes. Most steam locomotives had simple ON / OFF valves. In the ON position the brake valve was open and air was able to enter and apply the brakes. In the OFF position the valve was closed to the atmosphere and the brake pipe connected to the ejector(s). More modern brake valves have a lap position or may be self lapping allowing the driver to select and maintain a given vacuum.

Some brake valves integrated the operation of the (large) ejector with the operation of the brake valve. These had three positions: ON / RUNNING / OFF. The ON position was used to apply the brake. The RUNNING position connected the brake pipe to the ejectors and allowed the driver to adjust the (small) ejector to compensate for leaks or release the brakes after a service stop. The OFF position operated the (large) ejector to provide a quick release of the brakes.

top


Net Braking Ratio (Braking Percentage)

The "net braking ratio" (NBR) of a vehicle is defined as the ratio of the total force applied to the brake block (shoe) on the wheels, proportional to the total weight of the vehicle. This relationship is used to determine the brake force needed to be applied to the brake shoes on the wagon.

For a more details refer to the information provided in conjunction with Air Brakes.

top


Effect of Brake Shoe Friction

The amount of force applied to the wheel by the brake shoe will be impacted by the friction of the brakeshoe. For a more details refer to the information provided in conjunction with Air Brakes.

top


Operation of Vacuum Brake Systems

Train braking systems were required to stop large loads, and therefore were not designed to stop trains in short distances. Additionally the time to release and apply the brakes could take several minutes. Therefore it was important that the driver operated the brakes system as efficiently as possible to achieve the best performance as possible.

Making Service Stops

Station stops should not be made by a violent application of the brake, but by a destruction of vacuum of, say, from 5 to 10 inches, which should be recreated slowly as the train comes to rest, by placing the handle in the running position.

By having the vacuum nearly restored at the end of the stop, "jerking" is prevented, and the brake may released without the use of the large ejector.

Southern Railway (from Southern Railway Magazine Vol 7 1929):
Station stops should not be made by a heavy application of the brake, but by a destruction of the vacuum of from 5 or 10 to 15 inches. This should be recreated as the train comes to rest by placing the handle in the "running position".

LNWR instructions to Engine-drivers 1921:

5. STOPPING -

  • The Vacuum Break must be used for the ordinary stoppage of the train by the Engine-driver, who must apply it gradually, and not suddenly, nor with full force, except in the case of an emergency.
  • Just before coming to a stand, the Vacuum Break must be eased off by partially re-creating the vacuum so as to prevent a rebound of the vehicles, or undue strain on the couplings. The Steam Break must not be used independently of the Vacuum Break.
  • Steam must not be applied to move the train forward after the Breaks have been applied, either slightly of fully, until the Breaks have been released throughout the train.

Unless you have a very long train or a very leaky vacuum brake system you will rarely need to use the large ejector on steam locomotives or the release / exhauster speed up position on diesel or electric locomotives.

Coupling and Uncoupling

Before attaching or detaching any vehicles you should make sure that the vacuum has been completely destroyed in the train pipe. Afterwards you will need to restore the vacuum in the train pipe (and vacuum reservoirs). This should be done by using the (large) ejector on full power or by placing the brake handle into the release position. When sufficient vacuum has been created you should return the brake to the running position or turn off the large ejector.

When a locomotive working at 20 or 21 InHg vacuum is coupled to rolling stock that has previously been hauled by a (GWR) locomotive operating at 25 InHg then it will be necessary to reduce the vacuum in the vacuum reservoirs on all the vehicles before the brakes can be released.

top


Brake Code - Structure and layout in Open Rails

In OR, code to define the braking capabilities of the rolling stock is defined in a number of separate locations as follows:

  • WAG files (Non-powered stock) - contain all the braking parameters relevant to the braking equipment usually located on the wagon.
  • ENG files (Powered or Locomotive Stock) - contains all the relevant braking information for the locomotive and train as follows:
    • Wagon sub-section - contains the parameters for the locomotives "own" brake equipment
    • Engine sub-section - train & engine brake operation - contains the parameters for the compressor, general brake system, train and engine brake operation.
    • Engine sub-section - control levers - contains information for the control levers that operate the various brakes.

For consistency of overall operation, braking parameters should be set the same between all common types of rolling stock and only a small number of stock specific parameters should need to be changed.

More Info

Note: It is good practice to put notes into the ENG and WAG files to act as reminder for any assumptions made. These notes can be made with comment statements as shown below:

Comment (Assume a Brake Cylinder size of 24in)

The following sections provide a suggested brake configuration to closely approximate the model of a Dreadnought Ejector in Open Rails.

The key parameters that impact upon the performance of the vacuum braking system are described on the following web page.

Standard Vacuum Brake Parameters for ENG and WAG Files (updated January 2018)

The Brake Calculators can be used to calculate some of the relevant values that are required for entry in the WAG files.

top


Sample Brake Code - Wagon Section

The sample brake code from below is from a typical British railway carriage of the early twentieth century. Most passenger carriages in Britain did not have handbrakes. Brake carriages and fitted goods wagons would also have a handbrake. Carriages built from the 1940s onwards (or after 1906 on the GWR) would also have Direct Action valves fitted. During the 20th century the maximum vacuum specified by most railway companies was increased from 20 InHg to 21 InHg. The Great Western Railway used a vacuum of 25 InHg. Bogie vehicles generally had two brake cylinders, whereas four and six wheeled vehicles usually only had one brake cylinder.

This type of code goes into the WAG file or the wagon section of the ENG file.

Typically the lines shown in red text are the only ones that would need to be changed to suit individual wagons.

Use the
Brake Calculators to calculate some of the relevant values that are required for entry in the WAG files.

WAG File or wagon section of ENG file

Comment ( *************** Brakes - Wagon Section - General ********************** )
    BrakeEquipmentType ( "Vacuum_brake, Auxilary_reservoir" )
    BrakeSystemType ( "Vacuum_single_pipe" )
    MaxBrakeForce ( 74.0kN )      Comment ( == Braked at 70% tare mass, coefficient of friction 0.50 == )
    ORTSNumberBrakeCylinders ( 2 )
    ORTSBrakeCylinderSize ( 15in )
    ORTSAuxilaryResCapacity ( 2.25ft^3 )
    MaxReleaseRate ( 5.0 )
    MaxApplicationRate ( 5.0 )
    BrakeCylinderPressureForMaxBrakeBrakeForce ( 20.0InHg )
    BrakePipeVolume ( 1.45ft^3 )      Comment ( == Based on length of carriage plus 16 feet for bends etc == )
    ORTSBrakeShoeFriction ( 0.0 0.50 8.0 0.288 16.1 0.241 24.1 0.211 32.2 0.187 40.2 0.173 48.3 0.161 56.3 0.150 64.4 0.142 72.2 0.139 80.5 0.134 88.5 0.129 96.6 0.125 104.6 0.123 112.7 0.121)      Comment ( == Cast Iron Brakeshoes == )

Piped Stock

The code for a wagon that has a through vacuum pipe looks like this

Comment ( *************** Brakes - Wagon Section - General ********************** )
    BrakeEquipmentType ( "Handbrake" )
    BrakeSystemType ( "Vacuum_piped " ) Comment (Wagons not fitted with vacuum brakes)
    MaxHandbrakeForce ( 2.5kN )      Comment ( Empty weight - 6.5t-uk, NBR - 0.6, Friction - 0.5 )
    BrakePipeVolume ( 0.82ft^3 )      Comment ( == Based on length of wagon plus 16 feet for bends, stands and hoses == )

Typically these types of wagons had handbrakes fitted. It was also normal operating practice to 'pin' down (or apply) a number of handbrakes on non-air trains when they were descending steep gradients to maintain control of the train. Once at the bottom of the grade the handbrakes were released.

Make sure that you test your settings with the brake tests described on the testing page.

top


Sample Brake Code - Locomotive and Train system

This type of code describes the configuration and operation of the ejector(s) or exhausters and the train braking system, and is located in the engine section of the ENG file.

Use the
Brake Calculators to calculate some of the relevant values that are required for entry in the WAG files.

Comment (*************************** Brake System *********************************************
             Included in this section - Compressor, Reservoir, Application rates, etc
             **************************************************************************************)


Comment ( == Ejector/Exhauster, Reservoir and General == )
    BrakesTrainBrakeType( vacuum_single_pipe )
    ORTSBrakePipeChargingRate ( 0.21 ) Comment (* Estimate for Gresham & Craven Dreadnought 25/20 ejector: BrakePipeChargingRateLargeEjector = 0.13 / BrakePipeChargingRateSmallEjector = 0.08 *)
    ORTSBrakeServiceTimeFactor ( 10 )
    ORTSBrakePipeTimeFactor ( 0.36 )
    TrainPipeLeakRate ( 0.04 )
Comment ( == Automatic Brake valve - Train == )
    TrainBrakesControllerMaxSystemPressure ( 20 )

Make sure that you test your settings with the brake tests described on the testing page.

top


Sample Brake Code - Train and Locomotive Control Valve

This type of code describes the controller (brake valves) in the locomotive. The example code below describes a Dreadnought controller. For more information on the BrakeControl tokens used by Open Rails to model the different types of brake valves, see Vacuum BrakeControllers page.

Comment ( *** Brake control equipment *** )
Brake_Train ( 0 1 0.2 0.2
    NumNotches ( 3
       Notch(0 0 TrainBrakesControllerReleaseStart )
       Notch(0.2 0 TrainBrakesControllerRunningStart )
       Notch(0.4 0 TrainBrakesControllerApplyStart )
       )
    )

SmallEjectorOrCompressor ( 0 1 0.1 0.5 )

Make sure that you test your settings with the brake tests described on the testing page.

top


Brake Calculators

The following brake calculators found on the Air Brake web page can be used also for Vacuum Brakes.

Vehicle Maximum Brake and Handbrake Force

Brake Pipe Volume

top


Test Stock

The following stock demonstrates the typical setup for vacuum brake systems. Two different brake controllers are demonstrated. Refer to the consist names for an indication of which controller has been used for each test locomotive.

Models by Dave Robinson, John Riddell & Darwin Smith
Physics Files by Darwin Smith (Version 1 - updated Jul 2018)

top


Useful References

Advanced Steam Traction - Train Brakes

Barrowmore - BR diagram books

Class40 Motherlist - Driving Instructions

Class 47 Training Manual

Dawlish Trains - BR cab details

Dreadnought Ejector

KESR - Brake Ejectors & Vacuum Pumps

LMS Carriage Association - The Automatic Vacuum Brake

Locodocs.co.uk - Driver's Manuals

Online Railway Cabin - Manuals

Railcar.co.uk - DMU brake system

Railway Wonders of the World

RSSB - Standards for Brake Systems

The Railway Technical Website

top