Train Brakes

Aim - this section provides an overview of the operation and physics of typical brakes that might be fitted to a train.

To convert tons to kN = tons x 9.964 kN.

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Index

Introduction

Overview of Westinghouse Air Brake System

Net Braking Ratio (Braking Percentage)

Effect of Brake Shoe Friction

Generic Air Brake Design Information

Overview of Vacuum Brake System

Operation of Vacuum Brakes

Overview of Steam Brake System

Useful References - Air Brakes

Useful References - Vacuum Brakes

Introduction

In the early days of operation, some trains, such as the "Rocket" did not have brakes applied to them. In order to stop the driver put the reverser into reverse and used this "reverse" steam pressure in the cylinder to bring the train to a stop.

Robert Stephenson invented the steam brake in 1833, which was applied to the locomotive and the tender. Hence after this time, trains typically had brakes fitted to the locomotive and tender, and special wagons, which were often called the 'brake van'. The brakes in these wagons 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. For example, in Australia there were 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 breakthrough 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.

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Overview of Westinghouse Air Brake System

This information has been extracted from various Westinghouse and NSWGR publications, some of which may be found in the NSWGR Documents 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.

Overview of Brake Systems

In the early days of railway operation, the locomotive and brakevan were the only vehicles fitted with brakes. Train operation required the locomotive driver and brakeman to co-ordinate their activities to control the train. After seeing a wreck in about 1867, a 22 year old George Westinghouse patented a "fail-safe" air brake system. If the pressure in the train brake pipe drops, either because the engineer applies the brakes or the brake pipe is broken for some reason, then the brakes are applied by air pressure from a reservoir in each car. The brakes can only stay open if the compressor is pressurizing the line.

During 1877 some locomotives and passenger cars were imported into the NSWGR from North America fitted with Westinghouse air brakes. From the 1880s Westinghouse Air Brakes were adopted as standard for NSWGR passenger cars. Some older cars were fitted with through air pipes, but no brake equipment. By 1904 most passenger stock was fitted with air brakes. Goods stock followed a similar process. Some privately owned coal hoppers, right up to their retirement in the late 1970s, did not have air brakes fitted. Over the years various refinements have been made to the Westinghouse brake. The overview on this page provides a simplified description of the brake operation and design. For more detailed information refer to the useful links section at the bottom of this page.

Passenger and Freight Car Brake System

The diagram below shows a typical Westinghouse train brake system for an older style passenger or freight wagon, and has been extracted from an Australian Westinghouse publication of the early 1950s.

Wagon 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 and supplies air pressure to the brake systems on each wagon, as well as provides relevant control signals to operate the brakes. Typically the normal operating pressure in the brake pipe is 75 to 80psi on passenger trains and 60 to 70psi on freight trains.

Brake cylinder

Applies force to the brake shoes on the car wheels. The brake cylinders are selected to provide the appropriate brake force for the weight and speed of the wagon.

In some instances for small stock, such as 4 wheel wagons, combined brake cylinder and reservoirs were used. For larger stock separate reservoirs and brake cylinders were typically employed.

Auxiliary reservoir

The auxiliary reservoir was used to supply air to operate the brake cylinder. Typically the auxiliary reservoirs and brake cylinders proportioned that an equalization pressure of 50psi will, be obtained from 70psi auxiliary reservoir pressure (brake pipe pressure). This typically equated to a triple valve ratio of 2.5. In some instances supplementary reservoirs were installed to provide additional air capacity.

Triple Valve

The triple valve has three duties to perform:

An emergency application of the brakes is bought about by the rapid reduction in pressure of the brake pipe. It should be noted that guard of the train had a brake pipe release valve in the brakevan, which could be opened to drop the brake pipe pressure, and stop the train in the case of an emergency.

Brake System Refinements

Over the years a number of enhanced features have been added to the braking systems, and these include the following.

Grade Control

The purpose of the grade control valve is to slow down the rate of exhaust of the brake cylinder finally retaining a small amount of pressure, thus enabling a recharge or the braking system to be obtained on heavy grades without the necessity of using hand brakes to prevent the train accelerating at too fast a rate.

Older wagons typically had no grade control or had manually operated valves. Typically the Grade Control Valves had the following three positions on them:

Load Compensation

The purpose of using this equipment is to use normal braking force when the vehicle is empty, but to provide additional braking force when the vehicle is loaded. Loaded wagons can use larger braking forces as the Adhesive weight of the wagon is higher, and wheel skid is less likely to happen. If the same loaded wagon braking forces were used on empty wagons, it is highly likely, due to the light adhesive weights of the wagon, that wheel skid would occur.

Older wagons typically had no grade control or had manually operated valves. Typically Load Compensation Valves had the following two positions on them:

Modern stock tends to have automatic load compensation equipment which uses a proximity switch or pressure switch to differentiate between load and unloaded wagons.

Naturally, as wagons without load compensation used the unloaded (tare) weight for calculating braking forces, special care needed to be taken by the driver, as the braking power of the train was less than that for a loaded train with load compensation.

Wheel Slip Protection (WSP)

Wheel Slide Protection, which is described in the following extract from a XPT (High Speed Train) manual has been added to a number of modern trains.

"During braking wheelslide control is effected throughout the train by additional equipment on each vehicle. In the piping to each pair of brake cylinders are fitted electrically operated dump valves. When axle rotations which are sensed electrically, differ by a predetermined speed the dump valves are operated releasing brake cylinder pressure to both axles of the affected bogie.

Dump valve operation will cease when differences in axle rotations arewithin specified limits or the axle accelerates faster than a specified rate. The dump valve will only operate for a maximum period of seven seconds after which time it will be de-energised and the dump valve will not re-operate until the train has stopped or the throttle operated.

Dump valve operation is prevented under the following conditions:-
(i) When the Power Controller is open.
(ii) When Brake Pipe Pressure has been reduced below 250 kPa (36.3 psi)."

Some system were able to disable condition ii) above, and operate down to all brake pipe pressures.

If you wish to see WSP in operation then have a look at this film.

Miscellaneous Brake System Impacts

Train Pipe Leakage

The brake system tends not to be a perfectly closed system, and it is possible for air leaks to develop in the system. The most likely place for leakage is in the couplings due to the seals becoming damaged. Air leakage can result in a slow decline in air pressure in the train brake pipe, which will increase the application of the brakes over time. In most brake positions this leakage is compensated for, and air from the reservoir is used to replace air which leaks out of the system. In older brake systems, such as the A-6-ET system, the LAP position isolated the train brake pipe from the reservoir which prevented air leakage being compensated for, and thus the train brake pipe slowly decreased in pressure. In newer systems the LAP position is often self-lapping, i.e. it compensates for the air leakage.

Most railway companies require the crew to undertake leakage tests when marshalling a train, and this test is deemed a success if the air leakage is less then 5psi/min.

Locomotive Brake System

The diagram below shows a typical Westinghouse train brake system for a locomotive, and has been extracted from an Australian Westinghouse publication of the early 1950s.

Locomotive Brakes

The brake system on the locomotive consisted of two separate components:

The principal elements of the train brake control are as follows:

Compressor

Supplied compressed air as appropriate for the operation of the train brake.

Main Reservoir

Provide air storage for operation of the train brakes. Typically the pressure in the main reservoir was maintained at 100psi.

Drivers control stand

Typically NSWGR steam locomotives were fitted with A-6-ET (Engine and Tender) brake equipment, whereas older diesels were fitted with A-7-EL brake valves. This equipment allowed independent operation of the locomotive and tender brake compared to the rest of the train. The locomotive brake may be applied or released, in whole or in part, with or independently of the train brakes, and this without regard to the position of the locomotive in the train. This allowed the driver some flexibility in applying light brake applications or in some circumstances holding the locomotive and tender brakes on after releasing the train brake.

The brake valve stand contained two brake valves, an automatic brake (controls all cars in the train) and an independent brake which controlled the locomotive and tender brakes only. A typical brake valve stand is shown in the diagram below, with the automatic brake on the left hand side, and the independent brake on the right hand side.

Brake Stand

The operating positions of the A-6 and A-7 brake valve are shown in the diagrams below. The Automatic brake valve (Train brake) is shown on the left hand side of the diagram, whilst the Independent brake valve (Engine Brake) is shown on the right side of the respective diagrams.

A6 Brake ValveA-6 Brake Valve.

A7 Brake ValveA-7 Brake Valve.

Automatic brake valve (Train Brake)

The automatic brake valve, which controlled the train brakes had 5 positions of operation as follows:

Independent brake valve (Locomotive and Tender Brake)

The independent brake valve, which controlled the engine and tender brakes had 5 positions of operation as follows:

When releasing the brakes, a "graduated" approach can be used, so that small increases in brake pipe pressure will result in a small release of the brakes.

More Info

If you want information on other types of brake valves, such as the Westinghouse 26-L or 24-RL models, see alternate brake valves page.

SME (also called SEM) Brake System

The SME is essentially a straight air-brake equipment having an automatic emergency feature. The simplicity of the straight air brake is available for ordinary service operation, while the addi- tional safety features of the automatic application of the brake is provided in case of a break-in-two, bursting of a hose, etc. The system is designed for use on short trains of not more than two cars in length.

The chief features of the equipment as set forth in the manufacturer's pamphlets are as follows:

  • Straight air operation for service stops.
  • Brake cylinder release operates locally, i.e., through the emergency valve on each car.
  • Prompt service application and release operations due to design of the emergency valve.
  • Automatic maintenance of brake cylinder leakage.
  • Uniform brake cylinder pressure, independent of variations in piston travel or leakage.
  • Practically uniform compressor effort insured without the necessity of a governor synchronizing system.
  • Automatic application of the brakes in case of ruptured piping, burst hose, or parting of the train.
  • Retarded release after an emergency application, as a penalty to discourage the unnecessary use of this feature.
  • One size of emergency valve for any size of brake cylinder.
  • Possibility of conductor setting the brakes in emergency by means of conductor's valve.

Double Heading of Locomotives

When multiple locomotives are being used on the one train, regardless of where they are in the train, except for the lead locomotive, the brake Valve Isolating cocks must be closed, and both brake valves should be in the Running position. This will allow the lead locomotive to operate it brakes. It should be noted that the main reservoir on all but the lead locomotive will remain at full pressure.

Prototype NSW brake settings

In setting the brakes as accurately as possible the following prototypical information has been considered. This information has been compiled from various New South Wales Government Railways (NSWGR) publications. You may however substitute information from any other railway systems to achieve a more localised rolling stock setting.

The brakes were set up to graduate off, and but not on.

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Net Braking Ratio (Braking Percentage)

Train Brake

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.

When this is expressed as a percentage it is known as the "braking percentage" and is the figure used to express the braking value of a vehicle. In the case of goods vehicles, from older publications, the usual figure is 60 - 75% of tare (empty) weight and for passenger cars 75 - 90%.

When expressed as a formula:

Net Braking Ratio (NBR) = Brake Shoe Force (BSF) / Tare (unloaded) Vehicle Weight (W)

Thus to determine the ideal Brake Shoe Force applied to the wagon wheel brake shoe, we have the following:

BSF = NBR * W

This is the force applied by the rolling stock brake cylinder and brake levers. To find the actual force applied to the wheels, we need to take into account the co-efficient of friction of the brake shoe, thus the above figure also needs to be multiplied by co-efficient of friction.

BRF = BSF * CoF

Refer to the next section "Effect of Brake Shoe Friction" for a more detailed explanation on friction coefficient.

High values of NBR will result in high brake forces being applied to the wheels of the rolling stock, causing the wheels to "lock up" and to skid along the tracks. Various publications provide guidance on the desired values for NBR under different brake configuration scenarios and brake shoe types. The following values are examples of the values recommended:

It should be remembered that OR only models the direct brake force applied to the wheels of the wagon.

Rules of Thumb that can be used in Open Rails -

Typically, for NSWGR stock, we will assume NBR values of 60% for Goods cars, 80% for Passenger Carriages, 60% for Locomotive Tenders, and 75% for locomotives.

For NSW we can assume that most pre-1970 stock will have cast iron shoes, and thus a value of 20% for the co-efficient of friction was considered appropriate.

For more modern NSW NBR recommended values refer to pg 14 of ARTC - Freight Vehicle Specific Interface Requirements WOS 01.400.

Alternatively the Association of American Railroads (AAR) has suggested a band (maximum and minimum) set of values for new and rebuilt freight wagons in their 1999 standard, S-401-99. The suggested values are shown below:

Car Type

Loaded Net Braking Ratio

Maximum Empty Braking Ratio

Minimum Hand Braking Ratio

Minimum

Maximum

TOFC/COFC

11%

13%

38%

10%

All Other

8.5%

13%

38%

10%

It is recommended that the values be tested to ensure that they give a good braking "feel" to the wagon, and don't cause the wheels to lockup and skid on the rails.

Thus the calculation of the Brake Shoe Force might look like this:

Brake Shoe Force = {(0.6 or 0.8) * Tare (or empty) Weight (lbs)} lbf

Handbrake

The NBR for a handbrake in rolling stock specifications may be between 13 - 28% of the fully loaded weight of the vehicle. Typically we might assume a NBR figure of 20% for both Goods and Passenger Stock.

Thus -

Brake Shoe Force = {0.20 * Fully Loaded Weight (lbs)} lbf

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Effect of Brake Shoe Friction

As the wheel on a wagon rotates, the amount of stopping force that can be applied will be dependent upon the brake cylinder force, construction of the brake levers, and the material type of the brakeshoes installed on the wagon. The brakeshoes will have a certain amount of friction (or adhesion) which will reduce the amount of stopping force that is actually applied to the wheel. For example, a single cast iron brakeshoe, will have a "Mean" coefficient of friction (CoF) of approximately 13% (0.13). As can be seen from the graphs below, the CoF varies with the speed of the vehicle, and at low speeds the actual CoF can be as high as 50% (or 0.5), so that the braking force calculated in the formula above will be effectively almost tripled at low speeds, compared to higher speeds. For ease of calculation the "Mean CoF" was generally used in the above formula for brake design. The diagram below demonstrates the impacts of speed, and brakeshoe type on the CoF for different brakeshoe materials.

brake shoe friction

In the diagram, the curves shown represent the following:

In addition to the train speed, the force applied to the brake shoe, and the temperature of the shoe can also impact the friction between the wheel and the brake shoe. Trains drivers are encouraged not to make severe brake operations, and frequent operations in order to attempt to maintain brake shoe temperatures within reasonable bounds, to limit the impact of high temperatures on there operation. The graph below shows the impact on cast iron brakes for different speeds and pressures.

brake shoe friction speed and force variation

In the early days of railway operation cast iron brakes shoes were the norm, however as train speeds increased there was a transition to different brake shoe materials in an attempt to increase the "Mean CoF" of the train brakes. This transition saw the addition of phosphorus to the cast iron to improve wear and friction. Composite materials are now often used, and disc pad brakes are now used in some applications, such as high speed train operations. The graph below shows the improvement in friction, especially at higher speed for these different types of materials.

brake shoe friction different types

Thus trains travelling at higher speeds, due to the reduced values of friction, will require more force to be applied to the brakeshoe then at lower speeds. For example, if a brake cylinder force of 10,000lbf is applied to a cast iron brakeshoe with the train travelling at say 5mph, then from the diagram above with a coefficient of friction of 0.288, approximately 2880lbf of stopping force will be applied to the wheel. If the same brake cylinder force is applied at 40mph, then as the friction value has reduced to 0.142, the corresponding stopping force applied to the wheel will only be 1420lbf, or approximately 50% of the force applied at the lower speed.

This means that train drivers will need to exercise great care when their trains are travelling at high speeds, as it is possible for the train to become "uncontrollable", and not stop in the desired time to avoid an accident.

When the retarding force between brakeshoe and wheel becomes greater than the adhesion force between the wagon wheel and rail the wheel will skid, and as the CoF is high at low speeds, wheels are more likely to be skidded at slower speeds. In most cases skidding takes place when the vehicle is moved from rest with the brake applied, when, since the speed of the wheel relative to the brake shoe is zero, the co-efficient of friction is at its highest value. This would, of course, occur only due to failure of brakes to release, to a hand brake being applied, or to any other cause which may prevent the wheel from rotating.

Frequent use of the brakes also results in heating of the brake shoes which can result in a decrease in the CoF.

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Generic Air Brake Design Information

For instances were specific brake parameters are not know the following information, from "The Westinghouse Air-Brake Handbook; A Convenient Reference Book by International Correspondence Schools, published in 1913, may be used instead to ascertain an appropriate approximation for some brake parameters. Always use known information where available. In the tables below some values have been calculated based upon other values in the table, and are shown in green figures in the table.

Standard Brake Cylinder Size

The following table shows the maximum force exerted by the cylinder when the pressure in the cylinder is fully charged to 50psi. The required brake cylinder force can be calculated with the wagon brake force and the brake cylinder size calculators above. Once this is done an estimation can be made of the brake cylinders that need to be fitted to a locomotive, carriage or wagon, by comparing the kN force result with the closest relevant figure in the right hand column of the table below. If the calculated value exceeds the value shown in the right column, as may be the case for some heavy stock such as a locomotive of modern bulk hoppers, then multiple brake cylinders can be used to achieve the most appropriate result. For example, a NSW C38 class locomotive was fitted with 2 x 15" Brake Cylinders, whilst the tender had 2 x 12" brake cylinders. For multiple brake cylinders multiple the brake cylinder force by the relevant number of cylinders required, ie 2 off 12" = 50.264kN. Cars and wagons can have can have brake cylinders mounted on each bogie (truck).

Brake Cylinder Size

Force exerted @ 50psi in brake cylinder

Cylinder capacity (@ 8" travel)

(in)

(lbf)

(tonf)

(kN)

(cu in)

(cu ft)

6

1,400

0.625

6.227

226.2

0.131

8

2,500

1.116

11.120

450

0.260

10

3,900

1.741

17.348

675

0.391

12

5,650

2.522

25.132

950

0.550

14

7,700

3.437

34.251

1,280

0.741

16

10,000

4.464

44.482

1,650

0.955

18

12,700

5.670

56.492

2,085

1,206

For locomotives, the suggested size of brake cylinder used for different weights on driving wheels is as follows: