ADVANCE OF THE ECCENTRIC.
Assume a horizontal engine, with the main crank moved into a horizontal
position so that the piston would be at, say, the left-hand end of its
stroke. Assume also that the engine is at rest, and put the valve in the
centre of its travel (as in fig. 5), when its faces exactly cover the
steam-ports and its cavity covers the bridges and the exhaust-port; the
first movement of the valve is wanted to be to the _right_, in order to
admit steam to the left-hand end of the cylinder to move the piston also
to the right, no matter whether the crank-shaft is to have negative or
positive rotation. Now, a valve of this pattern evidently will not admit
steam to the cylinder to _start_ the movement of the piston (supposing
that the engine has not yet been started), which will therefore have to
be helped by some external agency; admitting this as a point which shall
be touched again presently, place the crank next, so that the piston
would be at the opposite end of its stroke; the valve, which had to move
to the right as explained, must have come back into the position whence
it started, and be in readiness to move equally to the _left_,
afterwards returning once more to the central position by the time that
the piston gets back to the left-hand end of the cylinder. Hence we see
that, whenever the piston is at either end of its stroke, this valve
must be in the middle of its travel.
If this be granted, we may proceed further:—Having the piston at the
right-hand end of its stroke, for instance, draw a line on the disc at
right angles with the crank (see fig. 10); this will represent two
“eccentric-arms” of the necessary throw if each arm equals in length the
half-travel of the valve; either of them at present suits the position
of the valve, which is in the middle of its travel. Choose, now, the
direction in which your crank-shaft shall rotate; let us suppose, for
example, that it shall have “right-handed” rotation, and place an arrow
on the disc to indicate the direction of rotation. Under these
conditions the upper eccentric will _follow_ the crank round, operating
the valve to close the right-hand port and open the left-hand one,
admitting steam to stop the engine. Cross this upper arm out, then, for
manifestly we have to use the lower eccentric, which, going ahead of the
crank, will cause the valve to open the right-hand port when required.
But now conversely, suppose that the engine is to be run in the opposite
direction; the upper eccentric (represented by the crossed-out line) now
goes ahead of the crank, and gives the valve motion in the right
direction, consequently the lower one must now go out of use, for under
the altered conditions of working it tends to move the valve in the
wrong direction. The deduction from this reasoning is, that in whichever
direction an engine runs, the eccentric used for the time being must be
set in advance of the crank, and that advance must be _at least 90°_.
If it were less than 90° the wrong port would be opened—the port at the
opposite end of the cylinder to that at which the piston might happen to
be. This effect may be shown by purposely giving the eccentric less
advance.
The amount which the angle between the eccentric and the crank exceeds
90° is the “angle of advance”; and the distance which the valve is moved
from midposition when the piston is at the end of the stroke is the
“linear advance.”
Because of the necessity for different settings of the eccentric for
different directions of rotation of the crank, engines required to be
reversible usually have either:
(1) Means for shifting the position of a single eccentric upon the
shaft, that it may always be placed so as to lead the crank. Or
(2) Two eccentrics fixed upon the crank-shaft so that one of them is
always ahead of the crank in whichever direction it rotates,
mechanism being arranged in connection with the eccentrics, so
that the valve can be driven by the leading eccentric, or
operated by the conjoint action of both eccentrics. Or
(3) Radial or other special valve gears.
The simple form of slide valve shown in fig. 10 does not permit of the
economical use of steam, inasmuch as it allows steam at full pressure to
follow the piston for the whole of the stroke, and does not admit of the
use of its expansive properties, for the simple reason that, at the
instant the admission of steam ceases, the exhaust of the same body of
steam must immediately commence, as may be clearly seen from the
circular diagram.
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