Braking With Confidence

In the early days of railways only the locomotive was braked, the brake was applied by the driver or fireman leaning on a pole that applied a wooden block to some of the wheels of the engine or its tender. As speeds and loads increased this became wholly unsatisfactory with numerous serious accidents caused by inadequate braking.

What happens when the brakes are applied?

A train in motion has energy input by the power plant (what ever from that takes) that accelerated the train from rest and keeps it moving. When the driver wants to slow the train the energy of the train is has to be removed, as energy cannot be lost it has to be converted from motion energy to some other form. The measure of the ability of a train to convert the motion energy on braking is referred to as its brake force. The brake force available is dependant on a number of factors; the weight, and speed of the train, the amount of friction available between the brake blocks and the wheels, the amount friction available between the wheels with brakes and the track, and condition of the track itself.

In short the more braked wheels and the greater the friction between the wheels and brake block and the wheels and the track (also improved by increased weight) the better.

So what happens to the energy?

The motion energy converts to a number of different forms when the brakes are applied. The precise conversion depends a lot on the material used to make up the pad of the brake block. A common conversion is to heat energy, and wooden brake blocks used to catch fire so other materials are now used. A modern brake block is designed to convert nearly all of the train’s motion energy by wearing away the surface pressed against the wheel (the smell that used to be apparent with heavy braking on HSTs and other MkIII coaches was vaporised brake pads).

An automated system.

To improve braking early railway companies started increasing the number of wheels fitted with brakes, this was frequently done by adding a brake van from which the guard could apply brakes independently from the driver. However an inattentive guard or a driver giving unclear signals could result in uncoordinated braking which could itself be dangerous.

The first attempts at increasing the number of wheels with brakes coordinated by the driver used metal rods to actuate all the brakes on each vehicle. However as these linkages had to allow the individual wagons and coaches to go curved track they were fitted with knuckles or pivots which made them unreliable.

As it is relatively straight forward to raise a vacuum using steam and a vacuum can be piped using flexible hoses without loss a scheme was eventually developed which allows coordinated braking under the control of the driver. The scheme works like this; The vacuum is used to pull against a spring which tries to hold the brake block on the wheel. When enough vacuum is applied the brakes are released. Conversely if the vacuum drops the springs pull the brakes back on to the wheel.

The driver (and guard) has a brake control valve this allows air to enter the brake pipe in controlled amounts, the faster the air is allowed to enter the pipe the quicker the brakes are applied. In shutting the valve the vacuum generator is then able to create the vacuum again and release the brakes.

In a diesel locomotive the vacuum generator is called an exhauster and is driven by an electric motor. The class 50 has two exhausters, one mounted in the number two end beside the door into the engine room just behind the bulkhead and under the traction motor blower. The other is also towards the number two end near the auxiliary generator.

So why does a Class 50 have air brakes too?

The vacuum brake is limited by the difference in pressures between a perfect vacuum (near impossible to obtain in a laboratory let alone on a train) and atmospheric air pressure, this means that it can only work against springs up to a limited strength. As the force with which the brake block is applied affects the braking efficiency (increased friction forces) it would be an advantage to use more force on the brake blocks.

In addition a distributed system would allow quicker application and release than is possible with the vacuum scheme outlined above. So a new scheme developed using air rather than vacuum. The air brake system works completely differently to vacuum, the brakes are held off the wheels by springs and applied by the air pressure, and the air pressure is stored on each vehicle locally and two brake pipes are used.

With an air brake system one brake pipe (yellow) is pressurised by an air compressor to 7 bar, this provides a constant air supply to the brake system on each vehicle. Each vehicle has its own air reservoir which is charged by the feed pipe. Another pipe (red) is used to control the brakes this normally sits at 5 bar. On each vehicle there is a balance diaphragm which is maintained on one side at a constant 5 bar and the other side is connected directly to the control brake pipe.

When the driver moves the locomotive’s air brake controller away from the running position the pressure reduces from 5 bar in the control air brake pipe (red). The diaphragm then moves as the pressure one side is now greater than the other. This movement pushes a separate control valve open in proportion to the pressure difference allowing air from the vehicle’s reservoir to enter the brake cylinder. This in turn pushes the brake block on to the wheel. The greater the pressure difference across the diaphragm the greater the air pressure applied to the break cylinder and hence the greater the force applied by the brake pad.

If the red brake pipe breaks and looses pressure the brakes are applied as above, however if the yellow pipe breaks and looses pressure a pressure sensor shuts off the brake pipe to prevent pressure loss from the reservoir and applies the brakes. Hence the air system is fail safe.

In fact for diesel or electric locomotives the exhauster used to create a vacuum is more complicated than the vacuum generator fitted to a steam engine. In comparison the compressor for an air brake system requires less maintenance, is less bulky and provides more efficient braking. Vacuum braked trains on the main line are therefore becoming increasingly rare and tend to be infrastructure freight services using older wagons.

The air compressors on a class 50 are mounted on the under-frame of the locomotive between the fuel tank and the number one end bogie on each side. When in charge of an air braked service both compressors run. However when hauling a vacuum braked train only one compressor is needed to supply the class 50’s braking system, to even out the wear this is selected according to the direction of travel.

Braking a train.

There are four different brake settings these are used to match the braking characteristics of the locomotive to that of its train. These are Air Passenger, Air Freight, Vacuum Passenger and Vacuum Freight. In broad terms freight trains brake more slowly than passenger services, so these settings adjust the rate at which the air is feed into the locomotives brakes, to prevent the train bunching or pulling unnecessarily on the couplings.

The drivers controls

The driver has two brake controls, the locomotive brake and the train brake.

The locomotive brake operates into the locomotives own air brake system and applies the locomotives brakes only. This valve is totally proportional with an infinite set of possible brake settings. This brake is used when the locomotive is running light or when controlling an unfitted freight (one without through brakes on all wagons).

The train brake operates the brakes on the whole train including the locomotive. 6 possible settings; release, running, initial, full service emergency, and shut down. "Release" speeds up the vacuum exhauster to speed up the release of the brakes on the train, or it increases the control air pressure to 5.4 bar to equalise the pressures out more rapidly ensuring that all brakes in the train release together. "Running" is the brake released position. "Initial" applies a limited brake force to gather the train together to aid smooth stopping. "Full Service" applies maximum brake block force. "Emergency" applies full brake force at a quicker rate than "Full Service". "Shut Down" is used to allow the driver to shut down the braking system at this end of the loco, i.e. when changing ends etc.

The other drivers control that affects the braking system is the driver safety device or dead mans peddle. If this is released for more than a preset time limit the train brakes are applied at the emergency setting. This is sometimes enhanced (as in 50044) by a driver vigilance device which requires the driver to cancel a bleeper within a set time regardless of the peddle position or the brakes are applied.

Totally independent of the air and vacuum brake systems is the parking brake. This is a mechanical system operated by a wheel mounted on the bulkhead wall behind the seats in the cab. Turning the wheel pulls the brake cylinders of the bogie under the cab to apply the brakes on the middle wheels.

Class 50 brakes.

The braking systems on the class 50 where designed by the Westinghouse Brake and Signal Company and are predominantly the same as other English Electric built dual brake locomotives of their era.

The Author would like to thank Neil Morgan for assistance in writing this article.