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EXPLANATION OF THE PLATES.
PLATES X. (A) and (B).
The figures of these plates are to illustrate the combinations used to produce
rectilinear motion, from motion in a circular arc.
Fig. 1. is the parallel motion used for steam boat engines. The beam A F is
below the cylinder ; from G, the end of the cross head, draw a line to A, the centre
of the axis of the beam, and it will cut the rod D B in E, and the length of the
radius bar C D may be found by art. 492.; when E B is equal to E D, the length
of the bar C D is equal to A B, and this is the best though not always the most
convenient form. The rod D' G may be at any height, provided it be parallel to
A F; and B may be at any point in A F, if the position of C is not limited.
Fig. 2. shows the most common construction for engines with the beam above
the cylinder. H is the piston rod connected at G, and C D is the radius bar.
The line G A cuts the link B D in E, the proper point for the air pump rod.
Fig. 3. shows a plan of the upper side of the beam, where C D, C D are the radius
bars; and the beam is in two parts, as is usual in large engines.
Fig. 4. is a diagram to illustrate the investigation of the properties of the
combination in its most simple form. See art. 489.
Fig. 5. is a diagram for the apparently more complicated case, when the rod is
fixed to one angle of a parallelogram. See art. 492.
Fig. 6. shows how to arrange for three piston rods to move parallel, as for
Woolf’s engine : the points of suspension must be all in the line A G ; and any
number of them may be similarly adapted.
Fig. 7. shows another arrangement for three rods at one end, and two at the
other end of the beam. See art. 495. The dotted arcs, Plate x. (B), show how
the centre C for the radius bar may be determined geometrically, by drawing a
circle through the three positions, D, d, d\ of the point of connexion.
In all the cases the corresponding points are marked by the same letters, and
therefore by referring to the investigation of Fig. 4. the relations may be traced :
the particular forms of different engine makers will be found by turning the part
upside down, altering the place of the parallel bar G D, or altering the proportions
of the parts. In every combination where the bar C D is not equal to A B, the
variation from rectilinear motion increases with the extent of the angle described.
Variations of the parallel motion are exhibited in some of the other Plates.
Fig. 8. is designed to show that the vibrating pillar motion is exactly the same
in principle, and only differs in the particular mode of combination. See art. 494.
Fig. 9. is a particular case.