THE EFFECT OF ALTERATIONS TO VALVE AND ECCENTRIC.
_Advance of the Eccentric._—It should here be established that to any
given example of valve, ports, and eccentric (leaving the piston out of
consideration for a moment) belongs a certain cycle of operations, the
occurrence of these operations and their _duration, expressed in
fractions of a revolution of the rotating eccentric_, being constant for
a given case,[3] but the fraction of stroke traversed by the _piston_
between or during any of these operations of the valve will depend upon
_where_ in the stroke of the piston the cycle of operations is started,
taking any convenient operation as being the commencement of the cycle,
for the speed of the piston rises from _nil_ at the commencement of its
stroke to a maximum near the middle, falling again to _nil_ at the end.
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Footnote 3:
To make the idea clear one might plot these operations round a circle
representing one revolution of the eccentric, when it would be seen
that the operations of the _valve_ could be considered quite
independently of those of the piston—then it would remain to show what
the piston might do, or have done upon it, by comparing its motion
under varying conditions with the _unvarying_ operations of the valve,
in the manner explained later.
Now we may go on to consider the effect of increasing the advance of an
eccentric beyond that which it would ordinarily have (fig. 32). If we
suppose our eccentric and valve to give us a cut-off at C and an exhaust
at E, expansion lasting 0.17 of the stroke (as shown in full lines, fig.
32), and that by advancing the eccentric we get a cut-off at C^1 0.14
earlier, we must not expect to find that exhaust also will be 0.14
earlier (dotted lines, fig. 32), for although the exhaust edge of the
valve always gives exhaust at a certain constant period of time later
than steam is cut off by the steam edge, yet in that interval the speed
of the piston slackens less in this second case than in the first
example, and it therefore travels through a greater distance than in the
same time-interval taken later in the stroke in the first case. Hence,
with increased advance we get _earlier_ cut-off, with expansion of
_lengthened duration_ because _exhaust is not earlier in the same
degree_, neither are all the other operations performed by the valve,
although they each commence earlier and last longer in the stroke, lead
included, but admission, of course, excepted. _The idea may, perhaps, be
more easily grasped by keeping the eccentric and valve in view at an
unaltering standard of reference, and when it is desired to consider the
effect of advancing the eccentric, to imagine instead that it is the
setting of the main crank which is altered_—which in effect is the same
thing, and is, perhaps, easier to reason out.
[Illustration: FIG. 32.]
_Lengthening or Shortening the Eccentric Rod—Shifting the Valve on its
Spindle._—Starting with the valve in its central position, and
lengthening or shortening the eccentric rod or shifting the valve on its
spindle, or combining both operations in the same sense, the distance
between the centre of the crank-shaft and the centre of the valve is
altered, and the travel of the valve is shifted bodily to the right or
left, _i.e._, the valve when in the centre of its travel (the extent of
travel remaining of course unaltered) will be either further from or
nearer to the crank-shaft than before, while the position of the ports
remains unaltered; therefore the arrow upon the valve at its centre (see
fig. 16) lies to the right or left of the centre line of the port-face,
and, that arrow being no longer appropriate as a pointer at mid-travel
to the zero of the scale on the ports, we must adopt as our index-finger
the graduation mark made adjacent to the arrow and lying over the said
zero, in order that when the disc may indicate that the valve should
move, say, from 0 to 6 the new mark or index-finger, and not the arrow,
may be given that movement.
If the valve had previously been properly set for lead, and the
alteration shifted it ¹⁄₈ in. to the left, for example, we should find
that the lead at the left-hand end would be diminished by that amount,
and that subsequently, at cut-off, release, and compression, for both
ends of the cylinder, the valve would always be found ¹⁄₈ in. to the
left of its proper position. The result would be, as shown in fig. 33,
that: At left-hand of cylinder (see lines of dashes):—
Lead decreases } earlier, these operations
Cut-off takes place } taking place
Exhaust commences } as the valve moves
} _towards_ the left.
Compression commences later, this operation taking place as the valve
moves _from_ the left.
At right-hand end of cylinder (see lines of dots):—
Lead increases } later, because when
Exhaust commences } effecting them the
Cut-off takes place } valve is moving
} _from_ the left.
[Illustration: FIG. 33.]
Compression commences earlier, for it occurs whilst the valve is moving
_to_ the left. The alterations at opposite ends being in opposite sense.
(The normal distribution is shown by _full lines_ for comparison.) And
although the commencement of the operations is advanced and retarded by
the same amount of error, operating in opposite sense on opposite sides
of the piston, yet there is a difference in the work done upon the
opposite sides because of the commencement of the two cycles of
operation at different times, whence we infer differences in the speeds
of the piston, in the quantities of steam admitted, and in the
expansion-curves.
In some cases such a difference is desirable, and with the working
diagram suggested in Chapter I. another series of results may be
obtained by using the ordinary valve for distribution on one side of the
piston, and for the other side using a differently-proportioned valve
with a different index-pointer, as though opposite ends of one valve
were differently proportioned, as in certain large vertical engines.
_Adding Extra Lap to a Valve._—In some cases cut-off may be wanted
earlier in the stroke, permanently, with expansion of longer duration.
Under such circumstances more lap may be added to the valve. If this be
done, and the valve with its extra lap be re-set by giving the eccentric
greater advance in order to get the same lead as before (travel of valve
remaining unaltered), the steam-ports will not be opened to so great an
extent as formerly, and will close earlier, partly because of the added
lap,[4] and partly because of the increased advance. The increased
length of the valve-face will, after cut-off, confine the steam longer
in the cylinder, and it should be obvious (the effect of advancing the
eccentric having already been shown with reference to fig. 32) that in
addition to the earlier commencement and lengthened duration of
expansion, the exhaust and compression will begin somewhat earlier
because of the increased advance, and the compression will be of longer
duration because of the increased length of the valve-face.
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Footnote 4:
To assure one’s self of this, take an extreme case and imagine an
excessive amount of lap to be added. In such a case, with travel
unaltered, the ports might not be opened to steam at all, the valve
simply sliding backwards and forwards without uncovering them; now,
taking this lap off piecemeal the port would first be opened to an
exceedingly small degree and instantly shut again, the amount and
duration of opening increasing with each successive removal of lap.
_General._—It is here seen that in altering either the outside or the
inside laps with intent to affect only the steam admission with the
former, or only the exhaust with the latter, we must _perforce_ affect
other things. For instance, we alter the outside lap, with the
intention, say, of simply making cut-off earlier or later as the case
may be—but having altered the lengths of the valve-faces, they will
alter the duration of expansion and of compression, and the alteration
necessitated in the advance of the eccentric in order that the lead may
remain unaltered has its own effect in addition. This interdependence,
which is often inconvenient, is characteristic of the slide valve worked
by an eccentric, and has led to the use of valve gears and valves which
permit, more or less, of the independent regulation of some or all of
the various operations in the distribution of steam. Some of these gears
are used with a slide valve (as in the case of the Joy and other
well-known valve gears), while others operate special valves.
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