The
Walschaert's Valve Gear
To
see a labelled diagram of
Walschaert's Valve Gear click here
With
this type of valve gear the movement is derived from two distinct
sources, as follows:
(a)
A single
eccentric or return crank, eccentric rod or return crank rod, expansion
link and radius rod which provides for the movement of the valve equal
to twice the steam port opening, the expansion link being provided for
varying the cut-off and reversing the direction of travel.
(b)
A combination
lever attached at its lower end to a union link which is connected to
the piston crosshead, the upper end of the combination lever being coupled
to both the valve spindle and the radius rod, the latter being attached
above or below the valve rod, depending upon the use of inside or outside
admission valves respectively.
Variations
of this gear are used to operate outside cylinders with the valve gear
inside as on former G.W.R. locomotives, also outside valve to operate
inside cylinders as on the later builds of former L.M.S.R. 4-6-2. Another
variation is that used on former L.N.E.R. 3-cylinder locomotives in
which the Walschaert valve gear for the outside cylinders operate the
inside cylinders through a system of levers.
The
point at which the radius rod is attached to the combination lever becomes
the fulcrum of the whole motion, and the relative movement of the two
ends of the lever must be such that the full movement of the crosshead
imparted to the lower end of the combination lever will give a movement
to the valve spindle equivalent to twice the steam lap plus twice the
lead.
This
valve gear is readily adapted to operate with outside or inside cylinders
and with slide valves and outside or inside admission piston valves.
The
arrangement of the eccentric or return crank to provide movement to
the valve beyond that already provided by the combination lever for
lap and lead steam depends on whether inside or outside admission valves
are employed.
With
inside-admission piston valves the eccentric or return crank is set
at 90° behind the crank. With outside-admission valves the eccentric
or return crank is set at 90° in front of the crank.
The
expansion link, suspended at its centre by trunnions, is oscillated
an equal amount forward and backward by the eccentric or return crank
through the medium of the eccentric rod or return crank rod. An expansion
die block slides in the expansion link and is attached to a radius rod
which, attached at one end, connects with the combination lever and
is attached at the other end to the expansion die block in the link.
The raising or lowering of the rear end of the radius rod causes the
die block to be raised or lowered in the link.
Normally
the bottom of the link is used for fore-gear and the top of the link
for back-gear working, giving a direct movement for fore gear and an
indirect movement for back gear.
When
the die block is in the centre of the expansion link for mid-gear position,
the expansion link does not transfer any movement to the radius rod.
Intermediate positions of the expansion, die block above or below the
centre of the link allow for proportional transfer of movement from
the link to the radius rod and valve.
The
combined movements of the two sections of the gear result in a total
movement of the valve equal to twice the amount of the steam lap
plus twice the port opening to steam, for each revolution of the driving
wheels.
For
more information about Walschearts Valve Gear and loads of useful
links try this excellent site http://home.new.rr.com/trumpetb/loco/
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