_THE STEVENS VALVE-GEAR._
DESCRIPTION OF MOTION
This motion has been designed by A. J. Stevens, General Master
Mechanic, Central Pacific Railroad, to overcome the well-known
objections to the link-motion,--viz., letting the steam escape early in
the stroke when the engine is running hooked up; also, the closing of
the exhaust-port early, leading to excessive compression. The motion is
developed from the Walschaert motion, well known in Europe, and applied
to some extent to narrow-gauge engines in this country.
ARRANGEMENT OF THE MOTION
[Illustration: FIG. 27.]
In the Stevens motion two valves are employed, one at each end of the
cylinder, as can be seen in Fig. 27. The valves have supplementary
passages for steam and exhaust, being an improvement on the Allen
valve. Valves without the supplementary exhaust-port have been designed
by Mr. Stevens for his engines, and they are shown in two positions
in the supplementary figures 2 and 3 [p. 288]. These valves closely
resemble the Allen valve; but, in operating, steam is admitted to, and
exhausted from, the cylinder through the same passage. This arrangement
provides double area of admission and double area for exhaust
steam,--an important consideration, especially with high piston speed.
VALVE MOVEMENT
The valves are actuated by two motions,--one taken from a single
eccentric, the other derived from a connection with the cross-head. The
single eccentric is used to give both forward and backward motion, and
is set in the proper angle to the crank to produce motion in either
direction. For reversing the motion of the valve, a curved rocker-arm
_R_ (Fig. 27) is used, on which moves a sliding-block _r_. Attached to
the sliding-block is the link _H_, which connects with the lap-and-lead
lever _D_. The lower end of this lever is attached by the link _d_ to
the cross-head, from which the lever receives an oscillating motion.
This lever is suspended by a hanger from any convenient part of the
engine. Projecting from one side of the lap-and-lead lever are two
pins, to which are connected the valve-stem rods _m_ and _n_. These
pins are set upon the lever at points between 90 and 180 degrees apart,
so as to give a differential movement to the valve, which can not be
obtained when the pins are set opposite on the lever.
VALVE-STEMS AND STUFFING-BOXES.
Each valve has its own rod, and separate connection with the lever.
In most instances the stem of the back valve is made hollow, and the
stem of the forward valve is passed through it, so as to avoid the use
of more than one stuffing-box upon the steam-chest; but in several
instances they have been fitted up with two stuffing-boxes, the
separate valve-stems working side by side.
HOW MOVEMENT OF VALVE IS GOVERNED.
The valve being coupled up by independent rods to the lap-and-lead
lever, they will move together, and in the same general direction, from
the action of the eccentric. But besides this movement, common to both,
each valve is controlled and acted on by the angular position of the
two pins, which results from the rotary or oscillating movement of the
lap-and-lead lever upon its center; and each valve, instead of having
a uniform rate of travel, has a variable movement. The degree of this
variable movement is governed, both by the distance of the pins from
the center upon which the lever works and the length of the lever, and
can be increased or diminished by changing the position of one or the
length of the other in construction. The horizontal movement of the
valve received from the lap-and-lead lever while the same is at mid
travel, is very slight, while the piston or the cross-head is traveling
very rapidly. By this slow movement of the valve, steam is retained
in the cylinder until the piston has nearly completed its stroke;
and, on the other hand, the exhaust is kept open. As the cross-head
approaches either end of the stroke, the action of the valves is very
much accelerated, receiving, as they do, their motion from the combined
action of the lap-and-lead lever and the eccentric.
HOW EXHAUST LEAD IS CONTROLLED.
The exhaust lead is controlled by the pin in the lap-and-lead lever,
which is on the center, or horizontal, when the cross-head is at either
end of the cylinder. The steam lead is controlled by the pin that is
vertical, or above the center of the lap-and-lead lever, when the
cross-head is at either end of the stroke. Both steam and exhaust
lead are uniform at all points of cut-off. By this arrangement of the
valve-gear and valves, steam can be cut off at any point of the stroke
(by moving the sliding-block toward the center of the rocker-arm), and
is retained, if desired, to the last inch of the stroke. In receiving,
the steam is evenly distributed at all points of the cut-off.
There are about as many pieces to this gearing as to the ordinary
link-gear; but it is considerably cheaper in construction, and much
more durable; while it does away with two eccentrics, and their straps
and connections.