The principal braking system applied to Rail Motors is the Westinghouse Straight Air Brake. The straight air system is simple in its operation in that air, under pressure, is fed through the driver’s brake valve and through the brake system to the brake cylinder on each vehicle in the train. This causes the brake piston to move and through a series of mechanical rods and linkages applies the brakes on the train wheels. In bogie vehicles the mechanical rods and arms are arranged in such a manner that the brakes will be applied no matter how the bogie is oriented in relation to the longitudinal axis of the vehicle, while some bogie vehicles may have an individual brake cylinder fixed to the bogie.
In rail motors, there is either one or two brake shoes per wheel and these are generally made from a composite material, similar to the normal automotive type brake linings. CPH Class rail motors, however, use cast iron brake shoes.
The 42-Foot Rail Motor features a Westinghouse WL brake valve. This type is also used in tramway applications. The WL type is self-lapping brake valve and when moved into the application zone increases the air pressure in the straight air pipe in proportion to the movement of the brake valve handle. The change in air pressure in the straight air pipe, which is continuous throughout the train, causes the Relay Valve on each vehicle to operate and the brakes to apply. This valve closes the brake cylinder exhaust port and allows a corresponding amount of air pressure to flow from the supply reservoir into the brake cylinder(s). Once the pressure in the straight air pipe and the brake cylinder equalises, the Relay Valve stops the flow of air and retains the air pressure in the brake cylinder. If additional braking effort is required by the driver moving the brake valve further into the application zone then the Relay Valve permits more air to flow to the brake cylinder until the pressures again equalise. When the brake valve is moved to the release position, the air pressure in the straight air pipe drops and the Relay Valve opens the brake cylinder exhaust port permitting the air pressure to flow to the atmosphere and the brakes to release.
The straight air brake is fast in application and release for short trains, however, it is not fully automatic in operation in emergency situations such as a train parting. To provide the fail-safe automatic functionality, an emergency feature is fitted. This consists of a second, or emergency pipe, which is also continuous throughout the train. The emergency pipe is normally charged with air at emergency supply reservoir pressure. A reduction of the air pressure in the emergency pipe through any cause, such as the opening an emergency air tap, the dead man being activated, brake pipe air hoses rupturing or the train parting will operate the Emergency Valve. The action of this valve permits air from the emergency supply reservoir on each vehicle to be directed into the brake cylinder, applying the brakes and bringing the train to a stand. Once the emergency side of the system has been activated, the emergency pipe pressure must be fully restored before the brakes can be released.
The 100, 600, 620, 660, 900, 1100 and 1200 classes are fitted with Westinghouse S.E.M. air brake equipment. No.38 and the 400 Class units are fitted with a modified type of S.E.M. brake equipment which functions similarly to the S.E.M. type but is not fitted with the electro-pneumatic (EP) feature.
The main features of the S.E.M. air brake are:
In normal pneumatic operation, the S.E.M. type brakes function in a similar manner to the basic straight air type. However, one of the limitations of the straight air type is that the braking air pressure for the whole train is fed through the driver’s brake valve. In short trains this is not a problem, but in longer trains it means the brakes are slower to apply as the air has to travel the entire length of the train to give full braking effort. This also applies to the release of the brakes. This is also one of the reasons that 42-Foot Rail Motor trains are limited to a maximum of five cars in length, this being the optimum length for acceptable brake application and release times.
The electro-pneumatic (EP) feature of the S.E.M. type overcomes this delay problem, permitting trains of variable length to be operated. The EP system features a master controller in each driver’s cab and is activated by placing the 4-way changeover tap in the cab into the EP position and turning the EP master switch on. Movement of the brake valve handle permits air to flow into the brake valve side of the EP master controller, which in turn moves a diaphragm across to electrically activate the EP application train wire throughout the train. As current flows through the train wire, application magnet valves on each vehicle permit supply reservoir air pressure to flow to the brake cylinder. This occurs almost simultaneously on all vehicles and when the air pressure in the straight air side of the EP Master controller equals to amount of braking force applied at the brake valve, the master controller cuts power to the application train wire, the magnet valves close and air flow to the brake cylinders ceases. When the brake valve is moved to the release position, the air pressure in brake valve side of the EP master controller is reduced and the diaphragm moves in the opposite direction to electrically activate the EP release train wire. This in turn activates release magnet valves on each vehicle that opens the brake cylinder exhaust port and permits the brake cylinder air pressure flow to atmosphere and release the brakes. In the event that the EP system fails the normal pneumatic brake will still operate the brakes.
The Express Passenger Train (XPT) is fitted with Westinghouse Brake’s Westcode braking system. This system is an electro-pneumatic type with automatic air standby designed to give seven stages of brake application. The seven stages are applied by energising or de-energising three train wires. The three wires can be likened to three weights of 1, 2 and 4 and are used in seven different combinations to give the seven different pressure settings as detailed below.
In this system, there are three magnet valves on the brake unit of each vehicle and these are used to admit air pressure to the three diaphragm control chambers of the Relay Valve. The Relay Valve delivers a specific proportion of brake cylinder air pressure for each of the seven braking combinations. Each of the three magnet valves is controlled by the corresponding train wire that is energised or de-energised in a coded sequence by movement of the driver’s brake controller from the release to the full service application positions. The relay valve delivers air pressure from the auxiliary reservoir on each vehicle to the brake cylinders via a triple valve.
In normal braking, the automatic brake is locked out and all braking is done by the EP system. The automatic brake control is superimposed over the EP system on the driver’s brake controller and in the event of an electrical or other problem causing the EP brake to fail, the air brake will automatically take over. The Westcode system also features an emergency feature similar to that used in the straight air system consisting of a pipe throughout the train.
The Endeavour, Xplorer and Hunter rail cars are all fitted with a Davies and Metcalfe EBC/5 EP anti-slide pneumatic disc braking system. This is a basic straight air brake system which is supplemented by a hydrodynamic brake system in the transmission to reduce the wear on the brake system. The system also incorporates a facility that eliminates sliding or skidding during braking in adverse weather conditions. The Voith T 311 bre (Endeavour/Xplorer Classes) and T 312 bre (Hunter Class) transmissions fitted to these vehicles are equipped with a Voith KB260/r hydrodynamic retarder. This retarder brakes the vehicle from high speed with a low wear factor and keeps it at constant speed during downhill travel. The hydrodynamic retarder utilizes the oil from the turbo transmission with the braking energy being converted into heat and dissipated through the engine cooling water system by way of a heat exchanger. The maximum continuous braking power during hydrodynamic braking is dependent on the size of the cooling system. In the high speed range, brake power above the continuous braking power can be sustained for short periods of time for deceleration braking. The automatic air brake works in conjunction with the hydrodynamic retarder to provide effective train braking effort. |
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