VACUUM BRAKE SYSTEM ( obsolete these days )

Introduction

Vacuum Brakes were used for many years before the use of the standard in place of the standard 

Air Brakes or E-P Brakes. These brakes used the vacuum in the brake chamber for release or apply of the brakes. Here is the short discription of the Vaccum Brakes.

Basics

A moving train contains energy, known as kinetic energy, which needs to be removed from the train in order to cause it to stop.  The simplest way of doing this is to convert the energy into heat.  The conversion is usually done by applying a contact material to the rotating wheels or to discs attached to the axles.  The material creates friction and converts the kinetic energy into heat.  The wheels slow down and eventually the train stops.  The material used for braking is normally in the form of a block or pad.

The vast majority of the world's trains are equipped with braking systems which use compressed air as the force used to push blocks on to wheels or pads on to discs.  These systems are known as "air brakes" or "pneumatic brakes".  The compressed air is transmitted along the train through a "brake pipe".  Changing the level of air pressure in the pipe causes a change in the state of the brake on each vehicle.  It can apply the brake, release it or hold it "on" after a partial application.  The system is in widespread use throughout the world.

Principal Parts of the Vacuum Brake System

This diagram shows the principal parts of the vacuum brake system as applied to an electric or diesel train.  Click on the name to see a description of the part.  The systems used on steam locomotives were somewhat different.

Operation on Each Vehicle

Brake Release

This diagram shows the condition of the brake cylinder, ball valve and vacuum reservoir in the release position.  The piston is at the bottom of the brake cylinder.  Note how the brake cylinder is open at the top so that it is in direct connection with the vacuum reservoir.

A vacuum has been created in the brake pipe, the vacuum reservoir and underneath the piston in the brake cylinder.  The removal of atmospheric pressure from the system has caused the ball valve to open the connection between the vacuum reservoir and the brake pipe.  The fall of the piston to the bottom of the brake cylinder causes the brake blocks to be released from the wheels.

Brake Application

This diagram shows the condition of the brake cylinder, ball valve and vacuum reservoir in the application position.  The vacuum has been reduced by the admission of atmospheric pressure into the brake pipe.  This has forced the piston upwards in the brake cylinder.  By way of the connection to the brake rigging, the upward movement of the piston has caused the brake blocks to be applied to the wheels.

The movement of the piston in the brake cylinder relies on the fact that there is a pressure difference between the underside of the piston and the upper side.  During the brake application, the vacuum in the brake pipe is reduced by admitting air from the atmosphere.  As the air enters the ball valve, it forces the ball (in red in the diagram above) upwards to close the connection to the vacuum reservoir.  This ensures that the vacuum in the reservoir will not be reduced.  At the same time, the air entering the underside of the brake cylinder creates an imbalance in the pressure compared with the pressure above the piston.  This forces the piston upwards to apply the brakes.

Two Pipe Systems

Another version of the vacuum brake used two train pipes.  The usual brake pipe operated in the conventional way but the second pipe was provided to give an additional supply to speed up the brake release.  The second pipe is called the reservoir pipe.  The diagram below shows a schematic of the system, with the reservoir pipe shown in grey.

The two-pipe system was introduced on diesel railcars where the exhauster was driven directly off the diesel engine.  Since the engine was only idling if the train was stationary, the exhauster would only be running at slow speed.  This meant that the restoration of the vacuum in the brake pipe and cylinders along the train would be very slow.  To get a rapid brake release when it was needed to start the train therefore, a "high vacuum" reservoir was provided on each car, the reservoirs being supplied from a second train pipe called the Reservoir Pipe.  These additional reservoirs were characterised by their operating vacuum of 28 Hg, as opposed to the 21 Hg used in the brake pipe and brake cylinders.

While the train is moving and the driver's brake valve is in the "Running" position, the exhauster is connected to the reservoir pipe and through the driver's brake valve to the brake pipe.  A automatic feed valve fitted between the reservoir pipe and the driver's brake valve limits the maximum vacuum passing to the driver's brake valve at 21 Hg.  This means that the vacuum in the brake pipe and brake cylinders will be limited to 21 Hg.  However, the vacuum created by the exhauster in the reservoir and high vacuum reservoirs will reach 28 Hg, as shown in the diagram above in grey.

To apply the brake, "Brake On" is selected by the driver and the brake pipe is opened to atmosphere at his brake valve.  The exhauster will continue to run and maintain the 28 Hg reservoir level.  The connection to the feed valve is closed by the driver's brake valve when it is in the "Brake On" position  A partial application can be made by moving the handle to "Lap". 

To get a release, the brake valve is moved to the "Running" position.  There is no "Release" position.  As soon as "Running" is selected, the connection to atmosphere is closed and the connection to the feed valve and exhauster opens to start restoring the vacuum.  As there is a store of "high" vacuum available in the reservoir pipe and reservoirs, the process is speeded up to give a rapid release.

Each reservoir has an automatic isolating valve between itself and the brake pipe.  This valve is set to 19 Hg and closes if the vacuum in the reservoir falls below this level.  This has the effect of preventing the reservoir from being emptied.  The volume of the reservoir is such that it can restore the vacuum for several applications and releases before it drops below 19 Hg.

Via - RAIL TECHNICAL