DOI Seite / Zitierlink:
https://doi.org/10.11588/diglit.25888#0085
DRAWING OF MACHINERY.
09
varying angle of tire connecting-rod. In Fig. 3 this is
accomplished by means of parallel guides; and, in the case
now before us, by a system of jointed rods and levers,
forming what is termed a 'parallel motion, in its simplest
and most elementary shape.
This motion consists of two pairs of rods A, A', situated
in planes parallel to that of the circle of revolution of
the crank, and on opposite sides of the piston-rod, the
further extremity of each being jointed to fixed studs
o, o', while the nearer ends e and i are flexibly connected
together by a lever or link L, carrying an axis or stud c,
to which the upper end. of the piston-rod is attached (see
Fig. 6).* The nature of this motion is such that there
may be found on the line e i a point which, during the
entire stroke, does not deviate sensibly from the straight
line joining the centre O of the crank D, and the centre
of the piston-rod; this point determines the position of
the axis c which in the example before us is exactly mid-
way between the points e and i, since the rods A and A'
are of equal length. The same stud c forms also the point
of attachment for the lower end of the connecting-rod B
which, being jointed at its opposite end to the crank D,
communicates to it a continuous motion of rotation, as
before.
Fig. 5 represents, on an enlarged scale, and in a dif-
ferent position, the inner ends of the rods A, A', and the
connecting link L, so that the nature of the combination
may be more distinctly exhibited.
Fig. 6 is an end view, also on an enlarged scale, from
which it will be seen that the guiding rods and links are
double, and disposed symmetrically at equal distances on
each side of the piston-rod. In this view the student will
observe that we have supposed the links in a vertical, and
the guide rods all in a horizontal position; this is, of
course, a deviation from strict accuracy of projection, but
is adopted, and is recommended in all similar cases of
detail drawings, in order to avoid fore-shortenings and
confusion of lines. In this figure it will be seen that the
piston-rod is surmounted by a clutch or socket fixed to
it by a cotter, and adapted to receive the stud c, and that
the extremity of the connecting-rod B is formed into a
fork, which embraces the clutch, and is jointed to the same
stud by means of brasses and straps g, g. The links L, L,
are kept in their places by nuts b, b, pressing them lightly
against the collars f, f and are likewise bushed with brass
rings l, l, rivetted into their eyes, in order to diminish the
friction. The rods A, A', are jointed to them by means
of simple projecting studs with collars, in the usual way.
The nature and construction of this mechanism being
understood, we shall now proceed to point out the mode
of determining the centres of motion of the various
parts, and of representing them in any required relative
position.
In the present example, the lines joining the centre of
the crank shaft, and that of the cylinder, is a vertical
* The piston-rod is omitted in Figs. 4 and 5, in order to avoid
confusion of lines.
line; the length of the crank is in., and the distance
of its centre from the middle of the stroke of the piston is
5 feet 6 inches; therefore, set off from O to c, upon the
vertical 0 c, the latter distance, and on either side of the
point c, lay off, upon the same vertical line, the distances
o' c2, c c% each equal to the length of the crank; the part
comprised between c2 and c3 will thus represent the stroke
of the piston. Now, according to some winters on the
steam engine, the link L ought always to be in a vertical
position at the top and bottom of the stroke, so that the
strain upon the guide-rods A, A may act constantly in
the same direction, and so prevent injurious strains upon
their axes or centres of motion; t adopting this construction
in our figure, we shall proceed to determine the positions
of these axes, marked o and o'.
First, take up in the compasses half the length of the
crank (5£ inches), and from the points c3 and c2 set off
that distance upon the vertical line, in the points i3, c,
and er; then, through the point i3, draw a line, making
with the horizontal passing through c an angle of about
19° ;| the point of intersection o will be the axis or centre
of motion of the upper guide A. A circular arc described
from this point with the distance o i3, and terminated at
the points i3 and i2, will represent the path of the point i
during the entire stroke of the piston. Similarly, the
position of the axis o' of the second guide-rod A' (which
is assumed to be equal to the first), may be found by in-
tersecting the horizontal drawn through the point i2 by a
circular arc drawn from the point c or e2, with the radius o i3.
Suppose now the guide-rod A to be in any given posi-
tion, as o i, and let us proceed to determine the relative
positions of the other parts of the mechanism. From
the point i, with the distance c c2 (equal to the length of
the crank), draw an arc cutting in e the path described
by the end of the guide-rod A', the straight line e o' will
be the centre line of that rod. Join e i, and bisect it in
the point c; this will determine the position of the axis
of the cross-head, from which the piston-rod is suspended,
and it will be found to be upon the perpendicular 0 c.
From c as a centre with the distance 0 c (from centre of
stroke to centre of crank shaft), as radius, draw an arc
cutting the circle of revolution of the crank-pin in the
t See Tredgold on the Steam Engine. Art. “ Parallel Motion.” This
dictum, however, is not generally followed in practice; it being usual,
and we think decidedly preferable, to make the piston-rod move in
a line bisecting the versed sine of half the arc of vibration of the
beam, or guide-rod A in the present instance. By this arrangement,
although the strain upon the guide-rods certainly changes its direc-
tion, its total amount is very greatly reduced.
J It should be observed that the motion imparted to the piston-
rod by a combination of rods working upon the principle here illus-
trated is only an approximation to a truly parallel motion, and that
the deviation becomes very sensible if the length of stroke be unduly
increased, or the length of the guide-rods unduly diminished. It has
been ascertained that the arc described by the extremity of each
guide-rod should not exceed 40°, because beyond that limit the point
c begins to deviate sensibly from the perpendicular; and on this ac-
count it is necessary to make the guide-rods of a proportionate
length. In the example we have given, the arc described by the
guide-rods is 38°, and the length of the rods, determined according
to the method pointed out in the text, is the smallest that can pos-
sibly be allowed, in order to obtain a good effect.
09
varying angle of tire connecting-rod. In Fig. 3 this is
accomplished by means of parallel guides; and, in the case
now before us, by a system of jointed rods and levers,
forming what is termed a 'parallel motion, in its simplest
and most elementary shape.
This motion consists of two pairs of rods A, A', situated
in planes parallel to that of the circle of revolution of
the crank, and on opposite sides of the piston-rod, the
further extremity of each being jointed to fixed studs
o, o', while the nearer ends e and i are flexibly connected
together by a lever or link L, carrying an axis or stud c,
to which the upper end. of the piston-rod is attached (see
Fig. 6).* The nature of this motion is such that there
may be found on the line e i a point which, during the
entire stroke, does not deviate sensibly from the straight
line joining the centre O of the crank D, and the centre
of the piston-rod; this point determines the position of
the axis c which in the example before us is exactly mid-
way between the points e and i, since the rods A and A'
are of equal length. The same stud c forms also the point
of attachment for the lower end of the connecting-rod B
which, being jointed at its opposite end to the crank D,
communicates to it a continuous motion of rotation, as
before.
Fig. 5 represents, on an enlarged scale, and in a dif-
ferent position, the inner ends of the rods A, A', and the
connecting link L, so that the nature of the combination
may be more distinctly exhibited.
Fig. 6 is an end view, also on an enlarged scale, from
which it will be seen that the guiding rods and links are
double, and disposed symmetrically at equal distances on
each side of the piston-rod. In this view the student will
observe that we have supposed the links in a vertical, and
the guide rods all in a horizontal position; this is, of
course, a deviation from strict accuracy of projection, but
is adopted, and is recommended in all similar cases of
detail drawings, in order to avoid fore-shortenings and
confusion of lines. In this figure it will be seen that the
piston-rod is surmounted by a clutch or socket fixed to
it by a cotter, and adapted to receive the stud c, and that
the extremity of the connecting-rod B is formed into a
fork, which embraces the clutch, and is jointed to the same
stud by means of brasses and straps g, g. The links L, L,
are kept in their places by nuts b, b, pressing them lightly
against the collars f, f and are likewise bushed with brass
rings l, l, rivetted into their eyes, in order to diminish the
friction. The rods A, A', are jointed to them by means
of simple projecting studs with collars, in the usual way.
The nature and construction of this mechanism being
understood, we shall now proceed to point out the mode
of determining the centres of motion of the various
parts, and of representing them in any required relative
position.
In the present example, the lines joining the centre of
the crank shaft, and that of the cylinder, is a vertical
* The piston-rod is omitted in Figs. 4 and 5, in order to avoid
confusion of lines.
line; the length of the crank is in., and the distance
of its centre from the middle of the stroke of the piston is
5 feet 6 inches; therefore, set off from O to c, upon the
vertical 0 c, the latter distance, and on either side of the
point c, lay off, upon the same vertical line, the distances
o' c2, c c% each equal to the length of the crank; the part
comprised between c2 and c3 will thus represent the stroke
of the piston. Now, according to some winters on the
steam engine, the link L ought always to be in a vertical
position at the top and bottom of the stroke, so that the
strain upon the guide-rods A, A may act constantly in
the same direction, and so prevent injurious strains upon
their axes or centres of motion; t adopting this construction
in our figure, we shall proceed to determine the positions
of these axes, marked o and o'.
First, take up in the compasses half the length of the
crank (5£ inches), and from the points c3 and c2 set off
that distance upon the vertical line, in the points i3, c,
and er; then, through the point i3, draw a line, making
with the horizontal passing through c an angle of about
19° ;| the point of intersection o will be the axis or centre
of motion of the upper guide A. A circular arc described
from this point with the distance o i3, and terminated at
the points i3 and i2, will represent the path of the point i
during the entire stroke of the piston. Similarly, the
position of the axis o' of the second guide-rod A' (which
is assumed to be equal to the first), may be found by in-
tersecting the horizontal drawn through the point i2 by a
circular arc drawn from the point c or e2, with the radius o i3.
Suppose now the guide-rod A to be in any given posi-
tion, as o i, and let us proceed to determine the relative
positions of the other parts of the mechanism. From
the point i, with the distance c c2 (equal to the length of
the crank), draw an arc cutting in e the path described
by the end of the guide-rod A', the straight line e o' will
be the centre line of that rod. Join e i, and bisect it in
the point c; this will determine the position of the axis
of the cross-head, from which the piston-rod is suspended,
and it will be found to be upon the perpendicular 0 c.
From c as a centre with the distance 0 c (from centre of
stroke to centre of crank shaft), as radius, draw an arc
cutting the circle of revolution of the crank-pin in the
t See Tredgold on the Steam Engine. Art. “ Parallel Motion.” This
dictum, however, is not generally followed in practice; it being usual,
and we think decidedly preferable, to make the piston-rod move in
a line bisecting the versed sine of half the arc of vibration of the
beam, or guide-rod A in the present instance. By this arrangement,
although the strain upon the guide-rods certainly changes its direc-
tion, its total amount is very greatly reduced.
J It should be observed that the motion imparted to the piston-
rod by a combination of rods working upon the principle here illus-
trated is only an approximation to a truly parallel motion, and that
the deviation becomes very sensible if the length of stroke be unduly
increased, or the length of the guide-rods unduly diminished. It has
been ascertained that the arc described by the extremity of each
guide-rod should not exceed 40°, because beyond that limit the point
c begins to deviate sensibly from the perpendicular; and on this ac-
count it is necessary to make the guide-rods of a proportionate
length. In the example we have given, the arc described by the
guide-rods is 38°, and the length of the rods, determined according
to the method pointed out in the text, is the smallest that can pos-
sibly be allowed, in order to obtain a good effect.