Principle of automatic air brake system proportional to reduction in brake pipe pressure. If the compressed air in the brake pipes and auxiliary air C. The activated brake valve cuts the flow of air in Fig. The operation is same as that of the twin pipe from the pressure regulator and air pressure in the brake system except that the auxiliary reservoir is charged through pipes falls.
The fall in air pressure is detected by the control the D. The control valves then regulate the single pipe system. As compared to single pipe graduated flow of compressed air from auxiliary air reservoirs to brake release air brake system, twin pipe graduated release air cylinders. The brake cylinders activate the basic braking brake system is more suitable for passenger coaches. The control valves regulate the flow of air from the auxiliary air reservoirs to the brake cylinders at a pressure that is proportional to pressure drop in the brake pipes.
The straight air brake system does not have a control valve or auxiliary air reservoir in each coach as in automatic air brake system. Activation of brake valve forces compressed air from straight air pipe to brake cylinders, activating the basic braking mechanism. As the straight air pipes do not contain compressed air during normal running conditions, the brakes would fail if coaches became uncoupled.
In order to avoid this, the straight air brake system may be used in conjunction with the automatic air brake system. It can also Fig. Single pipe graduated release air brake system be avoided by using another pipe, called a main air reservoir pipe, from the first to the last coach. The air pressure in D. Twin Pipe Graduated Release Air Brake System main air reservoir pipe acts like the compressed air in the In twin pipe graduated release air brake system Fig.
The auxiliary reservoir is charged by the feed pipe at pipes or from air hoses between coaches, etc. The brake is detected and brakes are applied automatically. By this Braking system used is electric trains is electrodynamic movement the brake pipe pressure is reduced and the braking that converts the motor into a braking generator pressure differenced is sensed by the D. Air from Regenerative braking uses the generated electricity instead the auxiliary reservoir enters the brake cylinder and the of dissipating it as heat, and is becoming more common due brakes are applied.
At the time of release the air in the brake to its ability to save energy. Principle of the electrodynamic cylinder is vented progressively depending upon the traction, dynamic braking and regenerative braking systems increase in the brake pipe pressure. When the brake pipe is shown in Fig.
Principle of electrodynamic traction Fig. Twin pipe graduated release air brake system E. Advantages of Air Brake over Vacuum Brake The air brake is preferred in rail vehicles over vacuum brake due to the reasons listed in Table 1.
Weight of kg Approx kg Approx Fig. Principle of dynamic braking Equipments per wagon 4. Pressure Gradient No appreciable Steep reduction in difference in air vacuum in trains pressure between longer than m. Preparation time in Less than 40 Upto 4 Hrs yards minutes 6. Safety on down Very safe Need additional gradients precautions 7.
Overall reliability Very good Satisfactory Fig. Principle of regenerative braking. These brake mechanisms use a brake shoe that applies friction force to the disc.
The applied pressure is adjusted to control the braking force. In wheel-tread brake, the brake shoe applies friction force to the wheel tread, creating a sliding effect. High-speed trains cannot use this type of brake, because doing so may damage the wheel tread. Therefore, they use axle- or wheel-mounted disc brakes. Axle-mounted disc brakes require sufficient space to Fig.
Principle of recycled regenerated electric power accommodate therefore used in trailer bogies. Wheel- mounted disc brakes are used on motor bogies because it requires accommodating the traction motor only and having insufficient space for an axle-mounted brake.
In both systems, compressed air or oil is applied to a brake cylinder that pushes the brake lining against the disc. Brake discs are dead weight that is useful only during braking, therefore operators can install lighter discs.
During braking, they rub against each other to create a frictional force that slows down the wheel or axle. The disc is lighter in weight than conventional materials and has good heat-resistant properties. Moreover their structure is common for both axle- or wheel-mounted Fig. Transmission of breaking force from traction motors to wheels discs, achieving a much lighter disc without design. The traction motor drives and accelerates the train, during braking and it acts as an electric generator instead, forming part of a circuit that consists of a rheostat, armatures and a field system.
Electricity is consumed by the main resistor, which converts kinetic energy of the train into heat and acts as a brake. Regenerative braking uses the same type of circuit; however the electricity generated by braking is not consumed by rheostat.
It is transmitted to the overhead wire. The flow of this electricity is controlled by a controller under the pantograph that opens and closes within fraction of time. Electrodynamic brake systems are economical to use because they do have friction elements, as in mechanical brake systems. The regenerative braking system is even more economical because the electricity regenerated from Fig. Therefore they cannot be used as emergency brakes. In an electrodynamic braking system, the braking force of the traction motor is transmitted to the wheels via gears Fig.
Principle of wheel-mounted disc brakes Fig. Principle of electromagnetic brake Fig. An on field view of electromagnetic brake VI. In the case provided with sufficient distance in which to stop. Rigging can often be complex, especially under a passenger car with two blocks to each wheel, making a total of sixteen.
Rigging requires careful adjustment to ensure all the blocks operated from one cylinder provide an even rate of application to each wheel. If you change one block, you have to check and adjust all the blocks on that axle. The operation of the brake on each vehicle is controlled by the "triple valve", so called because it originally comprised three valves - a "slide valve", incorporating a "graduating valve" and a "regulating valve".
It also has functions - to release the brake, to apply it and to hold it at the current level of application. The triple valve contains a slide valve which detects changes in the brake pipe pressure and rearranges the connections inside the valve accordingly. It either:. The triple valve is now usually replaced by a distributor - a more sophisticated version with built-in refinements like graduated release. Operation on Each Vehicle. Figure 3: Brake Release: This diagram shows the condition of the brake cylinder, triple valve and auxiliary reservoir in the brake release position.
Brake Release: The driver has placed the brake valve in the "Release" position. Pressure in the brake pipe is rising and enters the triple valve on each car, pushing the slide valve provided inside the triple valve to the left.
The movement of the slide valve allows a "feed groove" above it to open between the brake pipe and the auxiliary reservoir, and another connection below it to open between the brake cylinder and an exhaust port. The feed groove allows brake pipe air pressure to enter the auxiliary reservoir and it will recharge it until its pressure is the same as that in the brake pipe.
At the same time, the connection at the bottom of the slide valve will allow any air pressure in the brake cylinder to escape through the exhaust port to atmosphere. As the air escapes, the spring in the cylinder will push the piston back and cause the brake blocks to be removed from contact with the wheels.
The train brakes are now released and the auxiliary reservoirs are being replenished ready for another brake application. Figure 4: Brake Application: The condition of the brake cylinder, triple valve and auxiliary reservoir in the brake application position. Brake Application: The driver has placed the brake valve in the "Application" position. This causes air pressure in the brake pipe to escape.
The loss of pressure is detected by the slide valve in the triple valve. Because the pressure on one side the brake pipe side of the valve has fallen, the auxiliary reservoir pressure on the other side has pushed the valve towards the right so that the feed groove over the valve is closed. The connection between the brake cylinder and the exhaust underneath the slide valve has also been closed.
At the same time a connection between the auxiliary reservoir and the brake cylinder has been opened. Auxiliary reservoir air now feeds through into the brake cylinder.
The air pressure forces the piston to move against the spring pressure and causes the brake blocks to be applied to the wheels. Air will continue to pass from the auxiliary reservoir to the brake cylinder until the pressure in both is equal.
This is the maximum pressure the brake cylinder will obtain and is equivalent to a full application. Figure 5: Lap: The purpose of the "Lap" position is to allow the brake rate to be held constant after a partial application has been made. Lap: When the driver places the brake valve in the "Lap" position while air is escaping from the brake pipe, the escape is suspended.
The brake pipe pressure stops falling. In each triple valve, the suspension of this loss of brake pipe pressure is detected by the slide valve because the auxiliary pressure on the opposite side continues to fall while the brake pipe pressure stops falling.
The slide valve therefore moves towards the auxiliary reservoir until the connection to the brake cylinder is closed off. The slide valve is now half-way between its application and release positions and the air pressures are now is a state of balance between the auxiliary reservoir and the brake pipe. The brake cylinder is held constant while the port connection in the triple valve remains closed. The brake is "lapped".
In the traditional air brake with a triple valve, Lap does not work after a release has been initiated. Once the brake valve has been placed in the "Release" position, the slide valves will all be moved to enable the recharge of the auxiliary reservoirs.
Another application should not be made until sufficient time has been allowed for this recharge. The length of time will depend on the amount of air used for the previous application and the length of the train. Modern air braking systems have a distributor in place of the triple valve. It performs basically the same function but also includes a graduated release capability and some other features that improve brake control.
Additional Features of the Air Brake. What we have seen so far is the basics of the air brake system. Over the years since its invention, there have been a number of improvements as described below. A further description of the most sophisticated version of the pure air brake is available at my page North American Freight Train Brakes written by Al Krug.
Most air brake systems have an "Emergency" position on the driver's brake valve. This position dumps the brake pipe air quickly. Although the maximum amount of air which can be obtained in the brake cylinders does not vary on a standard air brake system, the rate of application is faster in "Emergency".
Some triple valves and most distributors are fitted with sensor valves which detect a sudden drop in brake pipe pressure and then locally drop brake pipe pressure.
This has the effect of speeding up the drop in pressure along the train - it increases the "propagation rate". Some air brake systems use emergency reservoirs. These are provided on each car like the auxiliary reservoir and are recharged from the brake pipe in a similar way. However, they are only used in an emergency, usually being triggered by the triple valve sensing a sudden drop in brake pipe pressure. A special version of the triple valve a distributor is required for cars fitted with emergency reservoirs.
As note above, a distributor performs the same function as the triple valve, it's just a more sophisticated version. Distributors have the ability to connect an emergency reservoir to the brake system on the vehicle and to recharge it.
Distributors may also have a partial release facility, something not usually available with triple valves. Figure 6: A modern distributor valve for use on freight cars. Photo: Wabtec. All of these features are achieved with no electrical control. The control systems comprise diaphragms and springs arranged in a series of complex valves and passages within the steel valve block.
Distributors with all these features will normally be provided on passenger trains or specialist high-speed freight vehicles. A problem with the design of the standard air brake is that it is possible to use up the air in the auxiliary reservoir more quickly than the brake pipe can recharge it. Many runaways have resulted from overuse of the air brake so that no auxiliary reservoir air is available for the much needed last application.
The problem can be overcome with a two-pipe system as shown in the simplified diagram in Figure 7. Figure 7: Schematic of a two-pipe brake system. The second pipe of the two-pipe system is the main reservoir pipe. This is simply a supply pipe running the length of the train which is fed from the compressor and main reservoir. It performs no control function but it is used to overcome the problem of critical loss of pressure in the auxiliary reservoirs on each car.
A connecting pipe, with a one-way valve, is provided between the main reservoir pipe and the auxiliary reservoir. The one-way valve allows air from the main reservoir pipe to top up the auxiliary reservoir. The one-way feature of the valve prevents a loss of auxiliary reservoir air if the main reservoir pressure is lost. The two-pipe system has the ability to provide a quick release. Because the recharging of the auxiliaries is done by the main reservoir pipe, the brake pipe pressure increase which signals a brake release is used just to trigger the brake release on each car, instead of having to supply the auxiliaries as well.
Two pipe systems invariably have distributors in place of triple valves. One feature of the distributor is that it is designed to restrict the brake cylinder pressure so that, while enough air is available to provide a full brake application, there isn't so much that the brake cylinder pressure causes the blocks to lock the wheels and cause a skid.
This is an essential feature if the auxiliary reservoir is being topped up with main reservoir air, which is usually kept at a higher pressure than brake pipe air.
Needless to say, fitting a second pipe to every railway vehicle is an expensive business so it is always the aim of the brake equipment designer to allow backward compatibility - in much the same way as new computer programs are usually compatible with older versions.
Most vehicles fitted with distributors or two-pipe systems can be operated in trains with simple one-pipe systems and triple valves, subject to the correct set-up during train formation.
Self lapping is the name given to a brake controller which is position sensitive, i. The closer the brake handle is to full application, the greater the application achieved on the train.
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