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