DEAD CENTRE. ORDER OF CRANKS. CUSHIONING AND LEAD.
It is desirable at this point to clear the ground by touching for a
moment upon one or two elementary matters which, if not now explained,
might cause confusion subsequently.
_Dead Centres._—A crank is said to be “on the centre” or “on the dead
centre” when the connecting-rod and crank are in line, and this occurs
twice in every revolution, as shown in fig. 11.
[Illustration: FIG. 11.—Dead Centre—Connecting Rod and Crank in Line.]
_Right-hand Crank to Lead._—The phrases “right-hand crank to lead,” or
“left-hand crank to lead,” are sometimes used, and are confusing to
beginners; the “leading” crank is that one which leads _when the engine
is running ahead_. In most two-crank engines one of the cranks will be a
quarter of a revolution (or _less_ than _half_ a revolution, where the
cranks are not set at 90 degs.) in advance of its neighbour, and will,
therefore, _lead_ it in the direction in which it should go. Of course,
that neighbour might be said to lead the other by being _three-quarters_
of a revolution in advance (or _more_ than _half_ a revolution, where
the cranks are not set at 90 degs.), and this is where a little
confusion sometimes arises; the leading crank must always be taken as
that one which is _less than half a revolution in advance of its fellow
when the engine is running ahead_, and in locomotives and similar
engines is “left-hand” or “right-hand,” according as it lies to the left
or right of a spectator looking from behind the crank-shaft towards the
cylinders. (See fig. 12, which shows two arrangements of the driving
cranks of an English locomotive. The cylinders are supposed to be to the
right of the wheels.)
[Illustration: FIG. 12.—Order of Cranks.]
The accompanying diagram (fig. 13) will perhaps serve to make clear
without further explanation the meaning of the expression, used with
reference to three-crank marine engines, of “order of cranks, high,
intermediate, low,” or “order of cranks, high, low, intermediate,” as
the case may be.
[Illustration: FIG. 13.—Order of Cranks.]
Let us revert here to one of the points previously noted, which was that
the simple valve, when we had it in connection with an eccentric set 90
degs. in advance of the crank, would not admit steam to the cylinder at
the exact commencement of the stroke, _i.e._, when the crank is on
either of the “dead centres.”
Keeping this fact in mind, we will consider the matters of
_Cushioning and Lead._—It has long been found desirable (especially in
quick-running engines) that the motion of the returning piston should be
opposed as it arrives at either end of its stroke, so that the moving
weights of piston, piston-rod, and connecting-rod may be “cushioned,” to
prevent injurious stresses and to conduce to easy running. To use a
homely illustration, one might say that just in the same way is it
desirable, when striking out from the shoulder, to have one’s adversary
within range, that he may provide the necessary “cushioning,” and so
prevent stresses in one’s arm and possible dislocations.
This “cushioning” is provided in part by checking the exit of the
exhaust steam, as will be shown later, and in part by setting the
eccentric a little more than 90 degs. (fig. 14) in advance of the crank,
so that the valve commences to open the port to admit steam in front of
the returning piston before it arrives at the end of its stroke,
continuing to open the port as the piston comes to a rest, and is
started on the return stroke.
[Illustration: FIG. 14.]
The amount to which the port is found to be open when the piston is at
the end of its stroke is called the “lead” of the valve, and it would be
expressed as “one-eighth of an inch lead,” or “three-sixteenths of an
inch lead.” etc., as the case might be. To set this valve for lead, keep
the piston at the end of its stroke, and set the eccentric back until
the valve is opened to the desired lead; then the eccentric would be
fixed in its newly-found position, as shown in fig. 14. The radius of
the “eccentric-arm,” which is the lighter of the two radial lines on the
disc in fig. 14, is equal in length to one-half of the travel of the
valve, and therefore does not reach the edge of the disc, at which,
however, there is a short guiding-mark in the line of the eccentric-arm
“produced.” By using this guiding-mark and moving the arrow-head of the
valve over the scale on the ports, in correspondence with the movement
of the guiding-mark on the edge of the disc within the circular scale,
the valve, the main crank (C), and the eccentric can be operated in
concord, very much as if they were mechanically connected.
The explanation just given of the meaning of lead, and of its effect,
should be borne in mind, as we shall presently have to consider it in
conjunction with other matters, for, as has been said, there is
something more than “lead” employed in the production of cushioning.
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