DOI Page / Citation link:
https://doi.org/10.11588/diglit.25888#0086
70
ENGINEER AND MACHINIST’S DRAWING-BOOK.
point d\ d g will thus be the centre line of the connecting-
rod, and d 0 that of the crank.
Conversely, if the position of the crank be given, those
of the other parts may be found by simply reversing the
process above described; it is unnecessary to go further
into detail. The student will find it an interesting and
highly useful exercise to draw this subject repeatedly,
assuming various positions of the crank; and he will thus
be enabled to satisfy himself, from actual observation, that
the point of suspension of the piston-rod does not deviate
sensibly from the perpendicular line, throughout the entire
stroke.
When the point c coincides with c', in other words,
when the piston is in the middle of its stroke, the guide-
rods are nearly horizontal, as will be seen by inspection
of the figure, and the connecting-rod occupies one or other
of the positions marked d'd'.
Details of Watt’s Parallel Motion.—.
Plates XXXI. XXXII.
The subject last under notice forms an appropriate and
natural introduction to the consideration, of that most
important and admirable feature in the modern steam
engine, as usually constructed, the ‘parallel motion. This
is an ingenious combination of jointed levers and rods,
invented by the celebrated James Watt, for the purpose
of maintaining the rectilinear motion, at once of the
piston-rod and air-pump rod, under the constantly vary-
ing angular direction of the beam, which receives its
motion from the former, and communicates it to the latter.
Our Plates represent general and detailed views of a
vexy excellent and well-proportioned specimen of a parallel
motion for a steam engine of about 25 horse-power. The
various figures are drawn with the utmost accuracy and
minuteness, so as to make the form and construction of
every part distinctly intelligible; and the dimensions and
distances of the several parts are carefully marked, to
enable the student to copy this subject on an enlarged
or reduced scale, without requiring to measure from the
engraving, which should always be avoided, if possible.
Plate XXXI. Fig. 1 is a general elevation of the entire
mechanism. Here, for the sake of convenience in drawing
this view, we have assumed the position of the beam to
be such as to allow the links of the parallel motion to be
in a vertical position.
Fig. 2 is a plan, showing the relative positions of the
links, and radius, and parallel bars, &c. In order to
avoid confusion from fore-shortenings in this projection,
we have supposed the beam to be horizontal, and at the
same time the links vertical: a license which is not only
allowed but recommended in such cases.
Fig. 3. An end view, showing the distance and thick-
ness of the main links, and the mode of their attachment
to the beam and to the cross-head of the piston-rod.
Fig. 4 A section of the beam on the line 3-4 (Fig. 1),
and a general view of the arrangement of the back links,
the form and connections of the air-pump, cross-head, and
vod, and the disposition of the radius and parallel rods
(which are supposed to be sectioned on the line 5-6 Fig. 2),
upon the cross bar of the back links.
Fig. 5 represents the wrought-iron stud fixed into the
end of the beam, and forming its point of attachment
with the main links of the parallel motion. The form of
cross section of the beam upon the line 7-8 (Fig. 1) is here
also exhibited.
Fig. 6. A plan of the piston-rod cross-head.
Fig. 7. A plan of the cross bar of the back links in
Fig 4; this bar is supposed to be sectioned upon the line
9-10 of the present figure.
Plate XXXII. Figs 8 and 9. Detail views of the main
links, exhibiting their internal construction, and serving
to mark such dimensions as could not conveniently be
introduced into the previous views.
Figs. 10 and 11. Similar views of the back links.
Fig. 12 shows the form and dimensions of both ends
of the radius bars; and Fig. 13 those of the parallel bar
at the outer end, which is jointed to the piston-rod cross-
head.
Fig. 14. A diagram showing the principle of action of
the parallel motion, and the mode of finding the centre
lines of the different members of it, in any given position
of the beam or piston cross-head.
Description of the Parallel Motion.
This mechanism consists of two pairs of links, of equal
lengths, suspended from fixed centres in the beam of the
steam engine, and connected at their lower ends in such
a manner as to form a parallelogram of inflexible jointed
levers and rods; the links forming two opposite sides of
this parallelogram are so guided during the stroke of the
engine, by rods called the radius bars, that a certain
determinate point may be found in each of them, which
shall move in a line that does not deviate sensibly from
a vertical straight line. At these points the piston and
airpump-rods are connected, and are consequently con-
strained to move in a vertical rectilinear course.
The parts instrumental in producing the required recti-
linear motion are all double, and disposed symmetrically
on each side of the beam A, as will be seen by Fig. 2;
and they are connected together by the studs of the beam
and cross-heads of the piston and airpump-rods. Each of
the main links B, B, is formed simply of a wrought-iron
bar bent double, so that the two limbs are parallel to each
other; they are dressed truly on the interior and exterior
surfaces, the latter being, in the present example, turned
in the lathe. Into the upper or close end of these links
are fitted the brasses for the outer stud D of the beam,
and towards the lower extremity, which is open, are
situated those of the piston-rod cross-head E, into which
the piston-rod a is fixed by a cotter / and gibs, and by a
nut, as shown in Fig. 3. The distance between the
centres D and E is maintained by means of a small pillar e,
fitted into rectangular blocks bearing upon the conti-
guous bushes of each, and all the parts pertaining to each
link are retained in their places, and at the same time the
friction of both bearings is adjusted, by a single cotter c,
ENGINEER AND MACHINIST’S DRAWING-BOOK.
point d\ d g will thus be the centre line of the connecting-
rod, and d 0 that of the crank.
Conversely, if the position of the crank be given, those
of the other parts may be found by simply reversing the
process above described; it is unnecessary to go further
into detail. The student will find it an interesting and
highly useful exercise to draw this subject repeatedly,
assuming various positions of the crank; and he will thus
be enabled to satisfy himself, from actual observation, that
the point of suspension of the piston-rod does not deviate
sensibly from the perpendicular line, throughout the entire
stroke.
When the point c coincides with c', in other words,
when the piston is in the middle of its stroke, the guide-
rods are nearly horizontal, as will be seen by inspection
of the figure, and the connecting-rod occupies one or other
of the positions marked d'd'.
Details of Watt’s Parallel Motion.—.
Plates XXXI. XXXII.
The subject last under notice forms an appropriate and
natural introduction to the consideration, of that most
important and admirable feature in the modern steam
engine, as usually constructed, the ‘parallel motion. This
is an ingenious combination of jointed levers and rods,
invented by the celebrated James Watt, for the purpose
of maintaining the rectilinear motion, at once of the
piston-rod and air-pump rod, under the constantly vary-
ing angular direction of the beam, which receives its
motion from the former, and communicates it to the latter.
Our Plates represent general and detailed views of a
vexy excellent and well-proportioned specimen of a parallel
motion for a steam engine of about 25 horse-power. The
various figures are drawn with the utmost accuracy and
minuteness, so as to make the form and construction of
every part distinctly intelligible; and the dimensions and
distances of the several parts are carefully marked, to
enable the student to copy this subject on an enlarged
or reduced scale, without requiring to measure from the
engraving, which should always be avoided, if possible.
Plate XXXI. Fig. 1 is a general elevation of the entire
mechanism. Here, for the sake of convenience in drawing
this view, we have assumed the position of the beam to
be such as to allow the links of the parallel motion to be
in a vertical position.
Fig. 2 is a plan, showing the relative positions of the
links, and radius, and parallel bars, &c. In order to
avoid confusion from fore-shortenings in this projection,
we have supposed the beam to be horizontal, and at the
same time the links vertical: a license which is not only
allowed but recommended in such cases.
Fig. 3. An end view, showing the distance and thick-
ness of the main links, and the mode of their attachment
to the beam and to the cross-head of the piston-rod.
Fig. 4 A section of the beam on the line 3-4 (Fig. 1),
and a general view of the arrangement of the back links,
the form and connections of the air-pump, cross-head, and
vod, and the disposition of the radius and parallel rods
(which are supposed to be sectioned on the line 5-6 Fig. 2),
upon the cross bar of the back links.
Fig. 5 represents the wrought-iron stud fixed into the
end of the beam, and forming its point of attachment
with the main links of the parallel motion. The form of
cross section of the beam upon the line 7-8 (Fig. 1) is here
also exhibited.
Fig. 6. A plan of the piston-rod cross-head.
Fig. 7. A plan of the cross bar of the back links in
Fig 4; this bar is supposed to be sectioned upon the line
9-10 of the present figure.
Plate XXXII. Figs 8 and 9. Detail views of the main
links, exhibiting their internal construction, and serving
to mark such dimensions as could not conveniently be
introduced into the previous views.
Figs. 10 and 11. Similar views of the back links.
Fig. 12 shows the form and dimensions of both ends
of the radius bars; and Fig. 13 those of the parallel bar
at the outer end, which is jointed to the piston-rod cross-
head.
Fig. 14. A diagram showing the principle of action of
the parallel motion, and the mode of finding the centre
lines of the different members of it, in any given position
of the beam or piston cross-head.
Description of the Parallel Motion.
This mechanism consists of two pairs of links, of equal
lengths, suspended from fixed centres in the beam of the
steam engine, and connected at their lower ends in such
a manner as to form a parallelogram of inflexible jointed
levers and rods; the links forming two opposite sides of
this parallelogram are so guided during the stroke of the
engine, by rods called the radius bars, that a certain
determinate point may be found in each of them, which
shall move in a line that does not deviate sensibly from
a vertical straight line. At these points the piston and
airpump-rods are connected, and are consequently con-
strained to move in a vertical rectilinear course.
The parts instrumental in producing the required recti-
linear motion are all double, and disposed symmetrically
on each side of the beam A, as will be seen by Fig. 2;
and they are connected together by the studs of the beam
and cross-heads of the piston and airpump-rods. Each of
the main links B, B, is formed simply of a wrought-iron
bar bent double, so that the two limbs are parallel to each
other; they are dressed truly on the interior and exterior
surfaces, the latter being, in the present example, turned
in the lathe. Into the upper or close end of these links
are fitted the brasses for the outer stud D of the beam,
and towards the lower extremity, which is open, are
situated those of the piston-rod cross-head E, into which
the piston-rod a is fixed by a cotter / and gibs, and by a
nut, as shown in Fig. 3. The distance between the
centres D and E is maintained by means of a small pillar e,
fitted into rectangular blocks bearing upon the conti-
guous bushes of each, and all the parts pertaining to each
link are retained in their places, and at the same time the
friction of both bearings is adjusted, by a single cotter c,