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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acting between the two gibs d, d, which in addition to
their ordinary uses perform, in this case, the very im-
portant function of securing the ends of the link from
springing asunder.

The bach, or air-pump, links F, F, which form the
second side of the parallelogram (the line joining the
extreme centres H and I being equal and parallel to
D E), are hung from the stud-centre H of the beam,
which, in the present example, as usual in beam engines,
is situated exactly midway between the centre C and the
extremity D of the beam; consequently the air-pump has
a stroke equal to half that of the steam-piston. The
opposite ends of the back links are jointed to the cross-
bar I (Figs. 4 and 7), to which the parallel bars J, J, and
radius bars N, N, are also attached; the former being fixed
immovably by small pins i, i, to the cross-bar, while their
opposite ends are jointed to the piston-rod cross-head E;
and the latter being adapted to work in suitable bearings
formed in the piece I. Exactly midway between the
extreme centres IT and I of the back links, is the axis K,
or centre of the bearings of the air-pump cross-head G,
which is usually bent downwards, in order to clear the
lower edge of the beam, and to the middle of which the
airpump-rod b, is attached in the usual manner. That
part of the back link which receives the brasses of the
stud centre H, and cross-head G, is constructed in the form
of an open framing of the same character as the main
links already described, the cotter g and small pillar It ,
serving as in that case, at once to secure the brasses in their
places, and to adjust their friction upon their journals;
the lower end is in the form of a plain lever, terminated
in a bushed eye (see Figs. 10 and 11).

To allow the airpump-rod b to pass downwards clear of
the cross-bar I, the latter is formed with an elongated
opening or eye in the middle, which is of sufficient dimen-
sions to allow the bar I, and consequently the links F, F,
to oscillate freely in obedience to the action of the radius
rods, without coming in contact with the rod b. The radius
rods N, NT, which are precisely of the same length be-
tween the centres as the parallel bars, and consequently as
the portion D IT of the beam, are jointed at their outer
ends to fixed stud centres o, mounted in small brackets M
bolted to the lower surface of the entablature K, K, of the
engine. * Thus the radii 0 I and C H, describing round
their respective centres 0 and C arcs of equal circles, the.
point K, intermediate between their extremities H and I,
will, as we have already seen in reference to Fig. 4, Plate
XXX., traverse a line which approximates very nearly
to a straight line, thus producing a parallel motion of the
airpump-rod, which, as we shall presently have occasion

* It is to be remarked, that in the general view, Fig. 1, the ante-
rior portion of the entablature has been supposed to be removed
for the purpose of exhibiting the parallel motion as distinctly as
possible. The bracket marked M in that figure, therefore, is not
that which sustains the fixed centre O of the radius rod N there
visible, but is intended to represent that of the rod N, which is
furthest removed from the eye. These rods and brackets being
exactly symmetrical in form, dimensions, and position, no confusion
is introduced by the transposition.

to show, is transferred to the piston-rod, by the peculiar
combination of mechanism just described.

Skeleton Diagram of the Parallel Motion.

The nature and mechanical construction of the parallel
motion being understood, it will be necessary for the
student, in order to execute a drawing of such a piece of
mechanism with ease, and with the requisite accuracy, to
be able to determine geometrically the positions of the
various centres of motion, and consequently the axes or
centre lines of the various parts. By laying down such a
skeleton diagram in the first instance, he will be enabled
to ascertain at once the relative positions and directions
of the several pieces at any given point in the stroke.

In the diagram Fig. 14, the strong lines correspond, in
length and in position, with the various working parts of
the parallel motion, as represented in the general elevation,
Fig. I; and the dotted lines denote the paths in which
they severally travel, or their positions at different points
in the stroke. The same letters of reference which in
Fig. 1 are employed to indicate the various centres of
motion are placed upon the corresponding points in the
diagram ; and to denote the various positions of the same
point the same letter is used, only with the addition of the
accents 1, and 3. In thus tracing the motion throughout
by geometrical construction, the student of mechanical
drawing will be enabled to satisfy himself, in a simple
manner, of the truth of the principles on which the action
of the parallel motion depends.

The point C denotes the centre of motion, or main
centre axis of the beam, which is represented (in the same
position as in Fig. l)t by the centre line C D. Its extre-
mity D, from which the main links B, B, are hung, tra-
vels in an arc of a circle D2, D1, D3, drawn from the centre

C, the length of which is determined by making its chord
equal to twice the length of the crank, set off at equal
distances on either side of the horizontal line C D' passing
through the centre C.

The object we have now in view is, in the first place,
to determine the positions of the links, parallel rods, and
radius bars respectively, when the beam is in the given
position C D; and secondly, to demonstrate mechanically
that the point E, representing the axis or centre of the
piston-rod cross-head, is constrained by this arrangement of
moving parts to travel in a line which does not deviate
sensibly from the vertical E E2, bisecting the versed sine
of the arc D1 D3.

Bisect C D in H, the point of suspension of the back
links; D H represents one of the long sides of the paral-
lelogram. The adjacent side, which represents the main
links B, B, must have its lower extremity in the vertical
D E2, and should be equal in length to that of the crank;
if, therefore, an arc of a circle be described from the point

D, with a distance, taken upon the scale, of 2 feet 3 inches,

f As we have before explained, we have assumed such a position
of the beam as makes the point D to be in the prolongation of the
vertical line, denoting the path of the piston-rod; but the instructions
here given apply equally to every position of the beam.
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