STEAM LOCOMOTIVE ENGINEMAN’S. Manual - part 23

Chapter 11: - General Engine Questions and Answers 

 

 

Q. 

In the event of the guide bars running hot, what can be 
done to overcome the trouble? 

 
A. 

Take a liner out of the back end and place it in the front end.  
This increases the distance between the top and bottom 
bars and allows freer movement of the crosshead. 

 
Q. 

How many types of slide valve are there in use? 

 
A. 

There are tow types  – the “D” slide valve and the piston 
valve.  The PB15 locomotive and ‘Kilrie’ are both fitted with 
‘D’ slide valves. 

 
Q. 

What are the parts connected with the exhaust called? 

 
A. 

Exhaust ports. 

 
Q. 

What are the ports through which steam flows from the 
steam chest to the cylinder called? 

 
A. 

Admission ports. 

 
Q. 

How is the steam exhausted from the cylinder? 

 
A. 

With the “D” valve, the exhaust cavity of the valve connects 
the steam port with the exhaust port and the steam escapes 
through the port by which it entered, it then passes, by way 
of the valve cavity, to the exhaust passage, up the blast pipe 
and chimney to the atmosphere. 

 
Q. 

How is the admission of steam cut off before the piston 
reaches the end of its stroke? 

 
A. 

In the case of a “D” valve, by having the portion of the valve 
which seats over the ports made longer than the distance 
between the outside edges of the ports, thus causing the 
valve to overlap the steam ports when central. 

 
Q. 

What events take place in a cylinder during one 
revolution of the wheel? 

 
A. 

Taking the piston as being at one end of the cylinder, steam 
is admitted to the cylinder at that end and acts on the piston, 
causing the piston to move.  The full pressure is exerted on 
the piston until the period or point of cut off when the steam 
port is closed. 

 
 

 

Chapter 11: - General Engine Questions and Answers 

 

 

  

The steam already contained in the cylinder continues to act 
on the piston until it is near the end of its stroke.  This period 
is called period of expansion and is the period in which 
advantage is taken of the property of expansion of steam.   

 
 

When the piston is near the end of the stroke, the valve 
opens the port through which the steam was admitted to the 
cylinder and allows the steam in the cylinder to escape to the 
exhaust, the valve permitting this escape of the steam to 
continue whilst the piston travels on portion of its return 
stroke. 

 
 

This is known as the period of exhaust and continues until 
the valve closes the exhaust port.  From this point the period 
of compression begins and continues until the piston has its 
return stroke. 

 
 

By the time the return stroke has been completed, the valve 
has partly opened the admission port, the amount of opening 
being the lead of the valve, causing an increase in the 
degree of compression.  If too much ‘lead’ were allowed, the 
movement of the piston at this point would be unduly 
retarded. 

 

 

 

Steam is admitted to each side of the piston alternately, the 
sequence of events on each side being the same; this, while 
steam is being admitted and expanded on one side of the 
piston it is being exhausted and compresses on the other 
side, and vice versa. 

 
Q. 

What means are adopted to enable the valve to admit 
and shut off steam to the cylinders and to permit steam 
to exhaust? 

 
A. 

By means of the valve gear, which is a mechanism that 
controls the movement of the valve and enables the engine 
to be reversed. 

 
Q. 

What types of valve gears are in use on QPSR 
locomotives? 

 
A. 

The Stephenson Link Motion and Walschaert’s Valve Gear. 

 
Q. 

Give a short description of the Stephenson Link Motion, 
as fitted to the PB15 locomotive 

 
A. 

The Stephenson Link Motion is operated by four eccentrics 
fitted to the driving axle and placed between the frames of 
the engine, they being one back gear eccentric and one fore 
gear eccentric on each side. 

Chapter 11: - General Engine Questions and Answers 

 

 

 

The fore gear eccentric rod is attached to the top of the link 
and the back gear eccentric rod to the bottom of the link.  
The link is attached by lifting arms to the weigh bar shaft, 
which is connected by means of the reach rod to the 
reversing lever. 

 
 

The link has a curved slot in which the quadrant block 
operates the radius of the slot being equal to the length of 
the eccentric rods. 

 
 

The quadrant block remains stationary; the link sliding 
around the block as it is lowered or raised according to the 
direction or travel.  The quadrant block is held attached to 
the transmission bar, which at its other end is connected to 
the inside rocker arm and is supported by the transmission 
link. 

 
 

The outside rocker arm, which is part of the same block as 
the inside rocker arm, is connected with the valve spindle, 
which in turn is connected to the slide valve by means of the 
valve yoke. 

 
 

The eccentrics are fitted to the axles such that they are 
behind the crank and that the movement of the valve in 
relation to the pisto n will cause all valve events to occur at 
the proper time. 

 
  

When the link is lowered towards the quadrant block, it will 
be under the influence of the fore gear eccentric and cause 
the engine to move forward when steam is applied. 

 
 

Refer to FIGURE 10 in Annex A. 

 
 Q. 

Give a short description of Walschaert’s valve gear, as 
fitted to ‘Kilrie’? 

 
A. 

Walschaert’s valve gear has two motions, one of which is 
derived from the eccentric arm and the other from the 
crosshead.  The two motions are combined so that full 
control of the valve is obtained. 

 
 

The eccentric arm is connected to the main crank pin with its 
centre about 90º ahead of the crank pin in the case of the 
valve with outside admission and 90º behind the crank in the 
case of a valve with inside admission. 

 
 

 

Chapter 11: - General Engine Questions and Answers 

 

 

  

The eccentric gives no angular advances and has no 
influence on the lap and lead of the valve.  The lead of the 
valve is constant and is controlled by the movement of the 
crosshead, which is connected to the radius rod and valve 
spindle by means of the combination lever and drag link.   

 
 

The reversing link oscillates about a fixed axis and the 
curvature of the link has a radius equal to the length of the 
radius rod. 

 
 

The radius rod is connected at the front end to the 
combination lever.  The point of its connection to the 
combination lever is above the valve spindle connection 
when the valve is inside admission type and below the valve 
spindle connection when the valve is the outside admission 
type. 

 
 

The motion of the eccentric arm is conveyed to the reversing 
link by the eccentric rod, which is coupled at one end to the 
eccentric arm and at the other end to the link foot. 

 
 

The quadrant block in the link is attached to the radius rod 
and the position of the quadrant block in the link determines 
the direction of travel of the engine. 

 
 

The block in the link is attached to the radius rod and the 
position of the quadrant block in the link determines the 
direction of travel of the engine. 

 
 

The block is raised or lowered in the link by means of a lifting 
arm connected to the radius rod by lifting link or a sliding 
block fitted to an extension of the radius rod.   

 
  

Usually the engine travels forward when the quadrant block 
is below the centre of the link, but sometimes, as in the case 
of the AC16 engines, the valve gear is arranged such that ht 
engine travels forward when the block is above the centre of 
the link. 

 
Q. 

What is an eccentric? 

 
A. 

An eccentric is a circular disc attached to the axle such that 
its centre does not coincide with the centre  of the axle.  
When the axle revolves the eccentric imports a motion to the 
eccentric rod, similar to that, which would be caused by a 
crank.  The motion of the slide valve is derived from the 
eccentric.