The Mutual Improvement Classes of the old steam railways still continue for today's preservation volunteers. This is one of a series of posts from notes of talks given by Jan. To find them all, select label 'MIC'.
1. History and theory
Before the coming of railways, horse drawn wagons didn't need much braking. Such brakes as were provided tended to be a piece of wood which rubbed against the tread of a wheel. The increased weight and speed of trains meant that braking became important. It's hard work to start any wheeled vehicle because Sir Isaac Newton's laws of mechanics say that you need to supply energy to change the state of rest. The required energy is proportional to the product of the mass and the square of the speed change. But you may have noticed that, with a railway vehicle, it takes much less effort to keep it moving. In theory, Newton's laws say a body should keep going at uniform speed without any further work being done. But in any practical vehicle, there's friction between the turning and stationary parts of the vehicle. This rubbing generates heat which uses up the energy of motion until the vehicle stops (the energy of motion or kinetic energy is converted into heat energy), unless you keep supplying energy to make up these frictional losses.
Steam locomotives were so powerful and the frictional losses relatively so small, that it's possible for an engine to haul lots of wagons. Thus, energy of motion is large because the mass is large. The unexpectedly high speed of early trains means that the energy of motion is large, by the square of the speed. In other words, a train travelling at 20 mph has four times the energy of motion of the same train travelling at 10 mph. That train will not stop until all the energy of motion has been converted into heat. This is why driving a locomotive is different from, say, driving a car. The energy of motion of 7029 'Clun Castle' is proportional to mass times square of speed. At 15 mph, the 135 ton locomotive has the same energy of motion as a 1-ton car travelling at 175 mph.
So braking technology suddenly became important, particularly on trains conveying passengers. A guard or brakeman would travel on the last vehicle, provided with some form of handbrake which could be applied as required. Great Western locos still have a distinctive-toned separate brake whistle to allow the driver to let the guard know that braking assistance is required. To increase the braking effort available, every fourth or fifth coach would be provided with a brakeman and handbrake. This simple arrangement gave rise to some terrible accidents either when the driver lost control of the train or when a coupling broke on a rising gradient and the rear part of the train rolled backwards towards the following train. Armagh 1889 was one of the most famous accidents of this class. Even today, catch points are provided on rising gradients so that breakaway wagons rolling backwards will be deliberately derailed, rather than letting them accelerate towards a following train.
Towards the end of the nineteenth century, the Railway Inspectorate gave up its long-standing campaign seeking voluntary co-operation by the railway companies in improving braking systems and changes made it a legal requirement for passenger trains to have an 'automatic' brake. The three requirements were that this brake be effective on all vehicles on the train, be capable of application by driver, guard and (through the communication cord) passengers and be automatically applied to both halves of a divided train. Back in 1875, brake trials of competing designs, carried out at Newark, had shown that designs based on the use of vacuum or compressed air were capable of meeting the requirements. Both systems came into widespread use, principally the vacuum system in Britain and the Empire and the air system elswewhere. British railways now use the air brake, but in the steam era the vacuum brake was the most common and that's the pattern we will look at in more detail in section 4 below.
2. Scotches
If vehicles have no operative brake, they must be properly secured when left on a siding, by the use of wooden scotches. One scotch is placed on either side of a wheel so as to prevent movement in either direction. Alternately, an unbraked vehicle may be coupled to an adjacent vehicle which has effective handbrakes.
3. Handbrakes
The simplest braking system is the handbrake found on most wagons. A cast iron brake shoe can be forced against the tyre of one or more wheels by pressing a long brake lever downwards. When the handbrake is applied on a moving vehicle, the friction between brake block and tyre as the wheel revolves generates heat, slowing the movement. The cast iron is softer than the steel tyre, so the brake block is worn away with use. Once stopped, an applied brake will make it harder for the vehicle to move again. Adjustments are provided in the brake linkage to allow wear in the brake block to be compensated, but eventually the complete block needs to be replaced. The block is usually fixed to the rigging with a wedge shaped cotter secured by a split pin, to facilitate exchange.
To apply the brake, the lever is dropped, pressed down hard and secured in that position by pushing a pin (attached to the wagon by a chain) through one of a series of holes. The harder the brake lever is pushed down, the greater the retarding force. A wooden 'brake stick' is used to apply the desired downward pressure. The square end of the brake stick is placed over the brake handle and the end of the brake stick is located securely under a convenient part of the wagon (such as the spring), allowing you to push down the rounded end of the brake stick with one hand and insert the pin with the other. Do not use a shunting pole as a lever, or be tempted to climb on the brake lever and use your weight to depress the brake lever.
To release the brake, press down the brake handle as described above and remove the pin, carefully remove the pressure and lift the handle fully upwards so that it can be located on the ledge provided.
On old wagons, the brake handle only works the brake blocks on one side of the wagon. On more modern wagons, pressing down the brake handle on either side applies the brake blocks to all four wheels. Either way, the brake handles on both sides of a wagon must be 'picked up' before you attempt to move it. Remember, when leaving a wagon on a siding you must apply the handbrake before you uncouple it from the engine and, when collecting a wagon, it must be coupled to the engine before the handbrake is removed.
Handbrakes may be applied by other means such as a short lever or a wheel. Usually, the wheel is turned clockwise to apply the brake ('screw down') and anti-clockwise to release the brake, but check each vehicle. On goods brake vans and the guard's compartment of passenger stock, a horizontal wheel mounted at waist height on a column rising from the floor within the vehicle is usual. This type of brake generally has a pawl and ratchet arrangement so that, once the brake is applied, it cannot be released accidentally. To release the pawl, turn the brake wheel slightly clockwise so that the pawl can be lifted up to its released position. Before applying the brake, make sure the pawl is flipped down so that it can work against the ratchet and hold the brake applied. Locomotive handbrakes are often on a column similar to a guard's brake, but with an L-shaped handle in place of a wheel. Where a chain is provided, this should be slipped over the handle after applying the brake, to prevent accidental brake release.
4. The Vacuum Brake
The automatic or continuous brake provides a brake on each vehicle which can be applied by the driver, the guard or passengers. When passengers operate the communication cord (now called the PCD, Passenger Communication Device), a partial brake application is made to alert the driver. The term 'automatic' refers to the fact that, if a train becomes divided, the brakes will automatically be applied to both halves.
Let's look at the vacuum brake in more detail. The brake blocks and brake rigging are similar to a handbraked vehicle (and may be worked by a co-acting handbrake lever or wheel) but are connected by a vertical piston rod to a piston which can move up and down in a sealed brake cylinder. This piston is made airtight in the cylinder by a rolling ring or slipping band gasket around the edge of the piston. Where the piston rod passes out through the bottom of the brake cylinder, there is also an air-tight gland. The lower section of the cylinder is connected by pipes to the flexible vacuum hoses on either end of the vehicle. If these hoses are open to the air, then there is atmospheric pressure underneath the brake piston. If the brake has been previously manually released (described below), then there's also atmospheric pressure above the piston. The weight of the brake piston and its piston rod causes the piston to fall to the bottom of the brake cylinder and this RELEASES the brake blocks, allowing the vehicle to be moved. The upper and lower parts of the brake cylinder are interconnected through a one-way ball valve which lets air flow from the upper to lower part, but prevents airflow from lower to upper half.
Before moving off, the driver will create a partial vacuum in the train pipe which runs the length of the train, with adjacent vehicles connected using the flexible vacuum hoses ('bags'). The locomotive sucks air out of the train pipe so as to reduce the pressure in the brake pipe considerably below atmospheric pressure. The partial vacuum created is registered on brake gauges on the locomotive and in the guard's van or compartment. A reading of '0' represents atmospheric pressure, '21' is the degree of vacuum used by most locomotives and '25' is the higher vacuum used by ex-Great Western locomotives.
These numbers are in units called 'inches of mercury' since Torricelli discovered a simple way of creating an almost perfect vacuum and measuring it in terms of the height of a column of mercury in a closed-end glass tube. The units are often shown as 'ins/Hg' where Hg is the chemical symbol for mercury (it's an abbreviation of the latin name for mercury). Bigger numbers represent a more-perfect vacuum.
Air is sucked out of the lower part of the brake cylinder and the ball valve allows air to be extracted from the upper part of the cylinder as well, so that the train pipe and both sides of the brake pistons are at the same partial vacuum. Assuming that the vehicle brake was originally released, as described above, the brake pistons remain in the bottom position and the brakes remain off. Leaks anywhere on the system allow air to bleed in, reducing or destroying the vacuum. The locomotive will normally carry on sucking whilst the train is in motion to compensate for minor leaks, but more serious leaks must be investigated and corrected before departure. If the engine is creating 21 in/Hg, there must be at least 18 in/Hg at the rear of the train.
The driver applies the brake by partially destroying the vacuum in the train pipe. Operating the brake application valve deliberately allows air into the brake pipe. The further the brake valve is moved, the more air is admitted to the brake pipe. Since the engine is still sucking, altering the position of the valve alters the vacuum in the train pipe.
15 in/Hg is the normal value for initial brake application, 10 in/Hg. is the value for full service braking. Lower values are only applied in an emergency.
Allowing air into the train pipe lowers the vacuum underneath the brake piston, but the vacuum above the brake piston remains at the initial value because of the one-way valve. There is now, say, 21 in/Hg above the brake piston and 15 in/Hg below. The pressure underneath the piston is greater than the pressure above, causing the piston to rise upwards in the cylinder, pulling on the brakes.
As the driver admits more air to the brake pipe, so the differential pressure across the brake piston increases, pulling the brakes on harder. To release the brakes, the driver puts the brake valve back to 'release', stopping the inrush of air and allowing the locomotive to re-create the vacuum in the train pipe and the underside of brake pistons, which rises back to 21 in/Hg. Once again, the same partial vacuum is present on both sides of the brake piston, allowing gravity to make the piston fall and the brakes are released.
In a system as described above, the front brakes are applied before those at the rear, because of the time it takes for air admitted by the driver's brake valve to travel down the train pipe and reduce the vacuum at the rear.
This problem was overcome by the introduction of the Direct Admission Valve ('DA Valve') which is fitted adjacent to each brake cylinder. The DA Valve is a diaphragm valve actuated by the difference in vacuum between the train pipe and the lower part of the brake cylinder. When the driver admits air to the brake pipe to make a brake application, the DA Valve opens on the difference in vacuum between the train pipe and lower part of the brake cylinder. The open DA Valve allows air directly into the lower part of the cylinder until the vacuum in the lower part of the brake cylinder is the same as the vacuum in the train pipe, when the DA Valve closes. Thus, the driver's valve only supplies air to the train pipe: each brake cylinder is directly fed with air under the control of the DA valve on that cylinder and the brakes are applied more quickly. The procedure for releasing the brakes is unchanged.
When a vacuum-fitted vehicle is uncoupled from a train on which the vacuum brake has been previously operative, the train pipe will be open to atmosphere, but there will still be vacuum above the piston, causing the vehicle brake to be applied. You may not rely on this brake, as any leaks will destroy the vacuum above the brake piston and allow the brake piston to fall, releasing the brakes. The brake is said to 'leak off' so you must apply handbrakes or scotches to secure the vehicle. If the braking system is in good condition and air cannot leak past the brake piston, it may be some hours (or even days) before the brake leaks off.
Before the brake has leaked off, it may occur that the vehicle needs to be moved as part of a shunting operation, preferably without having to re-create the vacuum in the train pipe. This is achieved by manually allowing air into the upper part of the brake cylinder so as to equalise the pressures on either side of the piston, allowing the piston to fall and release the brakes. Air is admitted by operating a short lever on each brake cylinder which pushes the ball valve off its seat, letting air past. The lever is attached to a stout cord extending to either side of the vehicle, always called the 'string'. To help you find the string, a star or similar symbol is painted on the underframe adjacent to the string. To release the brake, locate the symbol, reach in and pull the cord hard enough to unseat the ball valve. Keep pulling until you can see that the brake piston is fully down. Remember that bogie vehicles have two brake cylinders and you will need to pull both strings. Vacuum-fitted locomotives are not provided with strings, but some form of release valve will be provided. You must find out the arrangements before you try to shunt a dead engine.
Vehicle brakes work with whatever degree of vacuum the engine can create, but the higher level used by ex-GW locomotives gives a higher available maximum brake effort and better tolerance to leaks before brake force is compromised. A problem arises if a GW locomotive works a train and is then replaced by a locomotive creating only 21 in/Hg. Even when the driver of the GW locomtive destroys the vacuum, 25 in/Hg remains in the upper part of the brake cylinders. When the other locomotive is connected and creates 21 in/Hg, the brakes are not properly released - the difference in vacuum between the upper and lower sides of the piston means that the brakes will be partially applied. The solution is to walk along the train, pulling each string, so that every brake is released. Then, the new engine can create 21 in/Hg which will become the new level of working vacuum in the upper part of each vacuum cylinder.
