The
Stephenson Valve Gear
Stephenson
valve gear as fitted to GWR locomotives
This
type of valve motion employs two eccentrics, fitted to the crank axle,
for each valve, one eccentric for fore and one for back-gear working.
The
backward and forward movement of the eccentrics is transmitted through
the eccentric rods to a slotted link known as the expansion link, the
fore-gear eccentric rod being coupled to the top and the back-gear rod
to the bottom of the link. The links are suspended from a common reversing
shaft by lifting links and may be raised or lowered at will from the
reversing gear in the cab through the medium of the reversing rod.
.
Fitted
in the slot of the expansion link is a die block, which is connected
to the valve spindle by an intermediate valve rod. . When the link is
lowered to bring the fore-gear eccentric rod into line or almost in
line with the intermediate valve rod or spindle rod, the movement of
the eccentric is transferred to the valve. Conversely, if the link be
raised, the movement of the back-gear eccentric rod will be transferred
to the valve. With the link placed so that the die block is in the centre
of the link, the mid-gear position, the link simply oscillates about
the die block with a to and fro movement equal to the steam lap plus
the lead of the valve, from its central position. The full travel
in mid-gear position is equal to twice the steam lap plus twice the
mid-gear lead.
.
Intermediate
positions of the die block in the link will allow for a variation of
valve travel, according to the position of the reversing gear, varying
the cut-off of steam to the cylinders and making use of the expansive
property of the steam.
With
the arrangement of the Stephenson link motion the lead of the valve
increases as the gear is "notched up" to a maximum at mid-gear
and a minimum at full forward or full backward gear. The increase of
lead at early cut-off positions is advantageous at high speeds.
Stephenson
valve gear for outside admission piston valves
With
outside-admission valves (slide and piston) actuated directly by Stephenson
valve gear, the 'position of the eccentric on the crank axle is 90°
plus the angle of advance in front of the crank for each direction of
travel.
With
inside-admission valves operated directly, the respective eccentrics
follow the crank by 90° less the angle of advance.
If a rocking shaft, which reverses the direction of movement, is interposed
between the inside-admission valve and the valve gear, the eccentrics
are set as mentioned in the first example.
