Connecting Rods
Connecting
rods connect the pistons to the crankshaft. They must be strong enough to transmit the
thrust of the pistons to the crankshaft and to withstand the internal forces of the
directional changes of the pistons. The connecting rods (fig. 3-38) are in the form of an I-beam. This
design gives the highest overall strength and lowest weight. They are made of forged steel
but may also be made of aluminum in smaller engines.
The upper end
of the connecting rod is connected to the piston by the piston pin. The piston pin is
locked in the pin bosses, or it floats in both piston and connecting rod. The upper hole
of the connecting rod has a solid bearing (bushing) of bronze or similar material. As the
lower end of the connecting rod revolves with the crankshaft, the upper end is forced to
turn back and forth on the piston pin. Although the movement is slight, the bushing is
necessary because the temperatures and pressures are high. If the piston pin is
semifloating, a bushing is not needed.
The lower hole
in the connecting rod is split, so it can be clamped around the crankshaft. The bottom
part, or cap, is made of the same type of material as the rod and is attached by two or
more bolts. The surface that bears on the crankshaft is generally a bearing material in
the form of a split shell, although, in a few cases, it may be spun or die-cast in the
inside of the rod and cap during manufacture. The two parts of the separate bearing are
positioned in the rod and cap by dowel pins and projections or by a short brass screw. The
shell may be of Babbitt metal that is die-cast on a backing of bronze or steel. Split
bearings may be of the precision or semiprecision type.
The precision
type of bearing is accurately finished to fit the crankpin and does not require
further fitting during installation. It is positioned by projections on the shell that
match relief in the rod and cap. The projections prevent the bearings from moving sideways
and from rotary motion in the rod and cap.
The semiprecision
type of bearing is fastened to or die-cast with the rod and cap. Before installation,
it is machined and fitted to the proper inside diameter with the cap and rod bolted
together.
The connecting
rod bearings are fed a constant supply of oil through a hole in the crankshaft journal. A
hole in the upper bearing half feeds a passage in the connecting rod to provide oil to the
piston pin.
Connecting rod
numbers are used to assure a proper location of each connecting rod in the engine. They
all assure that the rod cap is installed on the rod body correctly. When connecting rod
caps are being manufactured, they are bolted to the connecting rods.Then the lower end
holes are machined in the rods.
Since the
holes may not be perfectly centered, rod caps must not be mixed up or turned
around. If the cap is installed without the rod numbers in alignment, the bore will not
be perfectly round. Connecting rod caps, crankshaft, and bearing damage will result.
In addition to
the proper fit of the connecting rod bearings and the proper position of the connecting
rod, the alignment of the rod itself must be considered. That is to say, the hole for the
piston pin and the crankpin must be precisely parallel. Equipment of suitable accuracy is
available for checking connecting rods (fig.3-39). Every connecting
rod should be checked for proper alignment just before it is installed in the engine.
Misalignment of connecting rods causes many hard to locate noises in the engine. |
