STEAM LOCOMOTIVE ENGINEMAN’S. Manual - part 27

Chapter 12: - Westinghouse Brake 

 

 

12.5 

NO 4 DRIVER’S BRAKE VALVE 

 
 

Note: – In tracing the flow of air through the brake valve, 
the main reservoir must be charged and the isolating 
cock open, allowing air from the main reservoir to flow 
past the isolating cock, up the pipe, and through the 
passage in the body to the top of the rotary valve. 
 

 

Main reservoir air will then occupy the space above the valve 
irrespective of the position of the brake valve handle, which 
controls the flow of air according to that position. 

 
  

Refer to FIGURE 16 in Annex A. 

 
12.6 

OPERATION OF THE NO 4 DRIVER’S BRAKE VALVE IN 
THE FIVE POSITIONS OF THE HANDLE 

 

1. 

Release and Charging 

 
 

When the brake valve handle is in this position, 
main reservoir air flows through the main supply 
port in the valve to the blind cavity in the body, 
from which it then flows in two directions. 

 
 

In one direction, air flows through the short cavity 
in the rotary to the equalising charging port, 
which is connected with a passage across the 
body leading to the top of the equalising piston, 
to the small drum, and to the black hand on the 
pressure gauge.   

 
 

In the other direction, the air flows under the 
bridge into the large cavity of the rotary valve, 
and thence down the train pipe supply port to the 
train pipe and the under side of the equalising 
piston, thus giving equal pressure throughout the 
system. 

 
 

Air also flows to the top of the equalising piston, 
to the small drum,  and to the black hand on the 
pressure gauge through the excess pressure 
port in the rotary valve and preliminary exhaust 
port in the body. 

 

Chapter 12: - Westinghouse Brake 

 

 

2. 

Running Position 

 
 

The main reservoir air on top of the rotary valve 
flows through the excess pressure ports in the  
valve and body to the feed valve, which governs 
the flow of air, and thence through a passage in 
the body leading to the train pipe and the under 
side of the equalising piston. 

 
 

From there it flows up the train pipe supply port 
in the body into the large cavity of the rotary 
valve, which conducts it down the equalising port 
to the top of the equalising piston, to the small 
drum, and to the black hand on the pressure 
gauge. 

 
 

When the train pipe pressure becomes equal to 
the setting of the feed valve, the  latter will be 
closed, cutting off main reservoir pressure to the 
train pipe. 

 
3. 

Lap Position 

 
 

In the lap, or neutral position, all ports are closed 
and communication between the main reservoir 
and the train pipe is cut off, the main reservoir 
pressure being on top of the rotary valve only, 
and the train pipe pressure below. 

 
4. 

Service Position 

 
 

When the rotary valve is in this position, the long 
narrow groove in the valve communicates with 
the narrow groove in the seat, which leads to the 
main exhaust.   

 
 

This connection permits air from the chamber 
above the equalising piston and from the small 
drum to flow up the preliminary exhaust port in 
the body and through the grooves in the seat 
and valve to the atmosphere.   

 
 

This reduction in pressure above the equalising 
piston causes train pipe pressure under the 
piston to become the greater, and the piston is 
forced up, unseating the exhaust valve on the 
piston stem.   

Chapter 12: - Westinghouse Brake 

 

 

  

The train line air then escapes at the secondary 
exhaust until the pressure below the piston is 
less than that on top; when the piston is forced 
down, closing the secondary exhaust valve and 
cutting off the escape of the air from the train 
pipe. 

 
5. 

Emergency Piston 

 
 

In this position, the large cavity in the rotary 
valve connects the train pipe supply port with the 
main exhaust, causing a rapid reduction of train 
pipe pressure and the brakes to be suddenly 
applied with the maximum power. 

 
 

The air on top of the equalising piston and in the 
small drum escapes through the preliminary 
exhaust port via the narrow grooves in the rotary 
valve and its seat to the main exhaust, the 
reduction of pressure being registered on the 
gauge. 

 
 

FIGURE 16 shows a plan and sectional view of a 
No 4 automatic brake valve. 

 
12.7 

CM 6 FEED VALVE 

 

12.7.1 

PURPOSE OF CM 6 FEED VALVE 

 
 

The purpose of the Cm 6 feed valve is to govern 
the pressure in the train pipe irrespective of 
variations in the main reservoir pressure, 
provided that the main reservoir pressure does 
not fall below the pressure of which the feed 
valve has been set. 

 
 

With the brake valve in the running position, 
main reservoir air flows through the excess 
pressure ports in the rotary and body of the 
Driver’s brake valve, into a passage leading to 
the feed valve. 

 
 

It enters the feed valve and flows down a 
passage past the back of the supply valve, which 
is closed in its normal position, to a passage 
leading to the regulating valve, which is open in 
tis normal position. 

 
 

 

Chapter 12: - Westinghouse Brake 

 

 

  

The air passes the regulating valve, flows 
through a passage to the supply piston chamber, 
and, acti ng on the back of the supply piston, 
moves it to the left. 

 
 

The piston engages the supply valve stem and 
unseats the supply valve, opening 
communication between the main reservoir and 
train pipe. 

 
 

Train pipe air flows down a passage in the body 
to the diaphragm chamber and, when the 
pressure on the diaphragm exceeds the setting 
of the adjusting spring, the spring is compressed 
and the diaphragm moved to the left, thus 
permitting the regulating valve to be closed by 
the action of the regulating valve spring. 

 
 

Upon the closing of the regulating valve, 
communication is shut off between the main 
reservoir and the supply piston chamber. 

 
 

The pressure on both sides of the supply piston 
is equalising by air flowing between the spindle 
of the regulating valve and its guide.  The supply 
valve is then reseating by its spring, and 
communication is closed between the main 
reservoir and the train pipe. 

 
 

When the train pipe pressure is again reduced, 
the tension of the adjusting spring overcomes 
the pressure against the diaphragm and the 
diaphragm is moved to the right, engaging the 
spindle of the regulating valve and unseating the 
valve. 

 
 

Main reservoir air again flows to the supply 
piston chamber, and the operation previously 
explained is repeated. 

 
12.7.2 

METHOD OF TESTING THE CM 6 FEED 
VALVE 

 
 

The rotary valve must not be blowing through.  
The automatic brake valve handle is placed in 
the running position and the pump started.  The 
Black Hand on the gauge should advance with 
main reservoir pressure and stop at 70 lb, thus  
indicating that the feed valve is working freely 
and is adjusted correctly.