A. To get steam into the low-pressure cylinders before the high-pressure
engine has exhausted, some types of the Mallet compound have a live
steam pipe with a valve in the cab to admit boiler steam to the receiver
pipe and thus get the use of the front engine in starting a train. The
American Locomotive Company articulated compounds have an intercepting
valve similar to the one used in the Richmond cross compound, located
between the exhaust passage of the rear engine and the flexible
receiving pipe of the front one. This intercepting valve when in SIMPLE
position, allows the high-pressure cylinders of the rear engine to
exhaust directly to the stack instead of into the receiver, and feeds
boiler steam at a reduced pressure into the receiver pipe for the
low-pressure cylinders without giving any back pressure on the
high-pressure pistons. This increases the power of the complete
locomotive about 20 per cent. When in compound position, the
intercepting valve cuts off the supply of live steam to the receiver
pipe and forces the exhaust steam to go to the low-pressure engine
ahead.
35. Q. How is the American articulated compound changed from compound to
simple, and back to compound again?
A. To work the locomotive simple, place the handle of operating valve in
the cab to point toward the rear. This admits steam against the piston
that operates the emergency exhaust valve and opens it. Exhaust steam
from the high-pressure engine can pass to the exhaust nozzle instead of
to the low-pressure engine. The intercepting valve then moves over so
that live steam reduced to 40 per cent. of boiler pressure goes through
the receiver pipe to the low-pressure engine. To work compound, place
the handle of the operating valve to point forward. This will exhaust
the steam, holding the emergency exhaust valve open; a spring and the
pressure of the steam exhausted from the rear engine will close the
emergency exhaust valve and build up a pressure against the
intercepting valve that will open it so exhaust steam from the rear
engine will go to the forward one and at the same movement close the
reducing valve so no more live steam goes to the receiver.
36. Q. When is it necessary to use the operating valve to change the
locomotive from compound to simple, or from simple to compound?
A. When giving the engines steam to start, the intercepting valve should
automatically go to simple position until exhaust steam from the rear
engine builds up a receiver pressure that shifts the valve to compound;
if it does not, use the operating valve. When moving less than four
miles an hour or when about to stall on a grade, set the engines working
simple; changing to compound when the danger of stalling is over or the
speed is more than four miles an hour. If there is no intercepting valve
to furnish live steam to the forward engine, open the starting valve to
admit live steam to the receiver pipe and low-pressure engine.
37. Q. If in starting the locomotive the forward engine does not take
steam, what is the trouble?
A. The reducing valve may be stuck shut on account of being dirty or
stuck on the stem of the intercepting valve. In case the reducing valve
is stuck shut, the head of the dash-pot can be taken off and the valve
worked back and forth to loosen it. The intercepting valve should be
liberally oiled just before starting and occasionally during long runs
to keep it free from sticking.
38. Q. Why does the Mallet compound have more power when working simple
than compound?
A. If a starting valve is used to admit live steam to the receiver pipe
and thence to the low-pressure engine, this gives a higher pressure to
the low-pressure cylinders. If an intercepting valve is used, the open
emergency exhaust valve allows exhaust steam from the rear engine to go
direct to the stack; this takes away the back pressure of the receiver
steam from the high-pressure pistons, about 30 per cent. of the boiler
pressure, and thus adds to the power of the rear engine. The reducing
valve when feeding live steam gives about 40 per cent. of boiler
pressure to the low-pressure engine instead of the 30 per cent. it gets
from the receiver; the added power of both engines working simple is
about 20 per cent. over the compound operation.
39. Q. What is the duty of the by-pass valves on
the sides of the low-pressure cylinders? Should they be kept clean of
gum and grit?
A. These valves are connected to the steam ports at each end of the
cylinders and open to allow air and steam to pass from one end of the
cylinder to the other; away from the moving piston when the engine is
drifting. If not kept clean they may stick open; when working steam the
engine will blow badly; if they stick shut the engine will pound when
drifting.
40. Q. In what position should the reverse lever be when the steam is
shut off and the engine drifting?
A. Below three-quarters of full gear, so the valves will have nearly
full travel.
41. Q. Why should the power reversing gear of the Mallet compound always
have its dash-pot cylinder full of oil?
A. To prevent the too rapid movement of the reverse gear piston and its
damage.
42. Q. In what position should the engines stand to test for blows in
valves and piston packing?
A. Put the operating valve, or starting valve, in simple position. Spot
the engine in the proper position and test each engine for blows the
same as for a simple engine.