The most familiar example of the use of a connecting rod is in the
steam-engine, where it is used to connect the rotating crank with the
reciprocating piston. The rod itself is made of wrought iron or steel,
and is generally circular or rectangular in section. The ends of the rod
are fitted with steps, which are held together in a variety of ways.
_Strap End._--A form of connecting rod end, which is not so common as it
used to be, is shown in fig. 41. At (_a_) is shown a longitudinal
section with all the parts put together, while at (_b_), (_c_), _(d)_
and (_e_) the details are shown separately. A B is the end of the rod
which butts against the brass bush C D, which is in two pieces. A
_strap_ E passes round the bush and on to the end of the rod as shown.
The arms of the strap have rectangular holes in them, which are not
quite opposite a similar hole in the rod when the parts are put
together. If a wedge or _cotter_ F be driven into these three holes they
will tend to come into line, and the parts of the bush will be pressed
together. To prevent the cotter opening out the strap, and to increase
the sliding surface, a _gib_ H is introduced. The gib is provided with
horns at its ends to keep it in its place. Sometimes two gibs are used,
one on each side of the cotter; this makes the sliding surface on both
sides of the cotter the same. The cotter is secured by a set screw K.
The unsectioned portion of fig. (_a_) to the right of the gib, or to the
left of the cotter, is called the _clearance_ or _draught._
[Illustration: FIG. 41.]
EXERCISE 41: _Connecting Rod End._--Make the following views of
the connecting rod end illustrated by fig. 41. First, a vertical
section, the same as shown at (_a_). Second, a horizontal section.
Third, side elevation. Fourth, a plan. Or the first and third
views may be combined in a half vertical section and half
elevation; and the second and fourth views may be combined in
a half horizontal section and half plan.
All the dimensions are to be taken from the detail drawings (_b_),
(_c_), (_d_), and (_e_), _but the details need not be drawn
separately_. The brass bush is shown at (_d_) by half elevation,
half vertical section, half plan, and half horizontal section.
The draught or clearance is 7-16ths of an inch.
_Box End._--At (_a_), fig. 42, is shown what is known as a box end for a
connecting rod. The part which corresponds to the loose strap in the
last example is here forged in one piece with the connecting rod. In
this form the brass bush is provided with a flange all round on one
side, but on the opposite side the flange is omitted except at one end;
this is to allow of the bush being placed within the end of the rod. The
construction of the bush will be understood by reference to the sketch
shown at (_b_). The bush is in two parts, which are pressed tightly
together by means of a cotter. This cotter is prevented from slackening
back by two set screws. Each set screw is cut off square at the point,
and presses on the flat bottom of a very shallow groove cut on the side
of the cotter.
The top, bottom, and ends of this box end are turned in the lathe at the
same time as the rod itself; this accounts for the curved sections of
these parts.
It is clear from the construction of a box end that it is only suitable
for an overhung crank.
EXERCISE 42: _Locomotive Connecting Rod._--In fig. 42 is shown a
connecting rod for an outside cylinder locomotive. (_a_) is the
crank-pin end, and (_c_) the cross-head end. The end (_a_) has just
been described under the head 'box end.' We may just add that in
this particular example the brass bush is lined with white metal as
shown, and that the construction of the oil-box is the same as that
on the coupling rod end shown in fig. 44. The end (_c_) is forked,
and through the prongs of the fork passes the cross-head pin, of
which a separate dimensioned drawing is shown at (_d_). Observe
that the tapered parts A and B of this pin are parts of the same
cone. The rotation of the pin is prevented by a small key as shown.
The cross-head pin need not be drawn separately, and the isometric
projection of the bush at (_b_) may be omitted, but all the other
views shown are to be drawn to a scale of 6 inches to a foot.
_Marine Connecting Rod._--The form of connecting rod shown in fig. 43 is
that used in marine engines, but it is also used extensively in land
engines. A B is the crank-pin end, and C the cross-head end. The end A B
is forged in one piece, and after it is turned, planed, and bored it is
slotted across, so as to cut off the cap A. The parts A and B are held
together by two bolts as shown. This end of the rod is fitted with brass
steps, which are lined with white metal. The cross-head end is forked,
and through the prongs of the fork passes a pin D, which also passes
through the cross-head, which is forged on to the piston rod or attached
to it in some other way.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
EXERCISE 43: _Marine Connecting Rod._--Draw all the views shown in
fig. 43 of one form of marine connecting rod. For detail drawings
of the locking arrangement for the nuts see fig. 19, page 21. Scale
4 inches to a foot.
_Coupling Rods._--A rod used to transmit the motion of one crank to
another is called a _coupling rod_. A familiar example of the use of
coupling rods will be found in the locomotive. Coupling rods are made of
wrought iron or steel, and are generally of rectangular section. The
ends are now generally made solid and lined with solid brass bushes,
_without any adjustment for wear_. This form of coupling rod end is
found to answer very well in locomotive practice where the workmanship
and arrangements for lubrication are excellent. When the brass bush
becomes worn it is replaced by a new one.
Fig. 44 shows an example of a locomotive coupling rod end for an outside
cylinder engine. In this case it is desirable to have the crank-pin
bearings for the coupling rods as short as possible, for a connecting
rod and coupling rod in this kind of engine work side by side on the
same crank-pin, which, being overhung, should be as short as convenient
for the sake of strength. The requisite bearing surface is obtained by
having a pin of large diameter. The brass bush is prevented from
rotating by means of the square key shown. The oil-box is cut out of the
solid, and has a wrought-iron cover slightly dovetailed at the edges.
This cover fits into a check round the top inner edge of the box, which
is originally parallel, but is made to close on the dovetailed edges of
the cover by riveting. A hole in the centre of this cover, which gives
access to the oil-box, is fitted with a screwed brass plug. The brass
plug has a screwed hole in the centre, through which oil may be
introduced to the box. Dust is kept out of the oil-box by screwing into
the hole in the brass plug a common cork. The oil is carried slowly but
regularly from the oil-box over to the bearing by a piece of cotton
wick.
[Illustration: FIG. 44.]
EXERCISE 44: _Coupling Rod End._--Draw first the side elevation and
plan, each partly in section as shown in fig. 44. Then instead of
the view to the left, which is an end elevation partly in section,
draw a complete end elevation looking to the right, and also a
complete vertical cross section through the centre of the bearing.
Scale 6 inches to a foot.