Armengaud, Jacques Eugène; Leblanc, César Nicolas   [Hrsg.]; Armengaud, Jacques Eugène   [Hrsg.]; Armengaud, Charles   [Hrsg.]
The engineer and machinist's drawing-book: a complete course of instruction for the practical engineer: comprising linear drawing - projections - eccentric curves - the various forms of gearing - reciprocating machinery - sketching and drawing from the machine - projection of shadows - tinting and colouring - and perspective. Illustrated by numerous engravings on wood and steel. Including select details, and complete machines. Forming a progressive series of lessons in drawing, and examples of approved construction — Glasgow, 1855

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68

ENGINEER AND MACHINIST’S DRAWING-BOOK.

preceding Plates. Here A is the beam, to the extremity
of which the connecting-rod B is jointed; it differs from
that represented in Plate XXVIII., in having the stud E
fixed immovably into a cylindrical eye at the end of the
beam, instead of being in the form of a cross-head. The
brasses in which this stud works are fitted into straps
G G, which are open at one end, so that a single cotter
suffices to fix each to its place on the connecting-rod, and
at the same time, to adjust the friction of its bearings.
This peculiarity of construction is also exhibited at Fig. 2,
where the end of the beam is shown in dotted lines. The
junction of the lower end of the connecting-rod with the
crank D, is effected in precisely the same manner as we
have already described; the brasses being adjusted to
the crank-pin H so as to admit of their turning freely, but
without play, upon that centre.

To represent correctly this combination of parts at any
given point in the stroke or revolution, all that is requi-
site is to determine the positions of the various centres of
motion, and consequently the centre lines of the three
principal pieces. Supposing the following things grooved :
namely, the position of the centre C of the beam, the
length C E of its radius, the length 0 H of the crank
(equal in a steam engine to half the stroke of the piston,
and also, in the present example, to half the chord of the
arc described by the end of the beam), and the distance of
the centre of the crank shaft 0 from the horizontal line
passing through the centre G of the beam.

The horizontal line C F, drawn through the centre C,
indicates the position of the beam at the middle of its
stroke. Now, since the point of attachment E of the
connecting-rod travels through a circular arc M F N,
whose chord is equal to twice the radius of the crank, if a
distance equal to PI O be laid off on either side of the line
F C, and horizontals M a, N b, be drawn through the
points thus obtained, these lines will intersect the arc
M F N at the points M and N, which are the extreme
points of the stroke ; consequently the lines C M and C N
will indicate the positions of the beam at these points.

The position of the centre 0 of the crank-shaft will
now be readily determined by drawing a perpendicular
through the middle of that portion of the line F C, which
is intercepted between the chord and circumference of the
arc M F N; and setting off from this point I, the given dis-
tance (7 feet 5^ inches), the length E PI of the connecting-
rod must then be made equal to the distance between the
points F and 0; and a circle described from this latter
point with the radius 0 H, will of course denote the path
in which the centre of the crank-pin travels.

Let us suppose now that it is required to represent the
parts of this mechanism when the crank is in the position
0 H. The point E being throughout the entire stroke
situated in the arc M F N, let this arc be intersected by
another described from the centre H, with the distance
H E (obtained as above), and joining E H and E C, then
we have the centre lines of all the parts. Conversely, if
the direction E C of the beam be given, that of the crank
will be found by a similar process. Thus, by the help of

the dimensions marked upon the figures, this system of
parts may be drawn in any required position.

As the circular arc L 0 K, described from the centre F
with the radius E IP, cuts the path of the crank-pin in the
points L and K, then, by joining F L and F K, we obtain
the positions of the connecting-rod when the beam is
horizontal. This construction serves to demonstrate the
truth of a somewhat curious and remarkable peculiarity
in the motion of this system of parts; namely, that the
portion of the entire revolution of the crank-pin described
by it during the upper half of the stroke of the beam is
less than that described during the lower half; and that,
consequently, since the crank-pin moves through equal
spaces in equal times, the beam and piston must travel
with greater velocity through the upper than through the
lower half of the stroke.

In Fig. 3 the alternating rectilinear motion of the pis-
ton inclosed within the steam-cylinder (as exemplified
elsewhere), is communicated to a cross-head A, furnished
at each end with a suitable stud or journal, to which is
fitted the upper end of the connecting-rod B, or side rod,
as it is sometimes called in such cases. The cross-head is
constrained to move uniformly parallel to itself, by having
gun-metal guide-blocks G fitted to work smoothly and
without play within fixed upright guides, the height of
which corresponds to the length A3 E of the stroke of the
piston, which is equal to twice the length 0 a of the
crank D.

The centre 0 of the crank-shaft lies in the prolongation
of the vertical line A3 E, and at any given distance ; and
the length of the connecting-rod B must be equal to the
distance A 0, A being the middle point in the stroke of
the cross-head. A circular arc a O a, therefore, described
from the centre A, with the radius A 0, will cut the circle
of revolution of the crank-pin in the points a and a'; and
hence the lines A a and A a will indicate the positions
of the connecting-rod at the middle of the ascending and
descending strokes of the piston respectively.

It will be observed that, as in the preceding example,
the motion of the piston cannot be uniform provided that
of the crank shaft is uniform; for, if the circle of revolution
of the crank-pin be divided into any number of equal
parts, as at the points b, b1, b2, 63, &c., and arcs drawn
from each of these points as centres with the radius 0 A,
these arcs will intersect the straight line A3 E at points
A1, A2, A3, &c., which will manifestly be unequally distant
from each other, proving that the motion of the piston is
retarded in a uniform ratio towards the top and bottom
of the stroke. This is one of those important properties
of the crank which render it so admirably suitable as a
medium for the conversion of reciprocating into rotatory
motion in the steam engine.

Fig. 4 represents another combination of a somewhat
more complicated character than the preceding; it is
applicable to such steam engines as have the crank above
the cylinder (called crank-overhead engines), and is
intended to preserve the parallelism of the piston-rod
against the lateral thrusts and strains arising from the
loading ...