Connecting Rod convert piston
reciprocating energies in to the crank or
crankshaft rotating energies. Transferring
rotary motion to reciprocating motion was made possible by the
connecting the crankshaft to the connecting rod, which was described in
the "Book of Knowledge of Ingenious Mechanical Devices".
double-acting reciprocating piston pump was the first machine to offer
automatic motion, but its mechanisms and others such as the cam, would
also help initiate the Industrial Revolution.
In modern automotive internal
combustion engines, the connecting rods are most usually made of steel
for production engines, but can be made of aluminum or titanium for high
performance engines, or of cast iron for applications such as motor
scooters. They are not rigidly fixed at either end, so that the angle
between the connecting rod and the piston can change as the rod moves up
and down and rotates around the crankshaft.
The small end attaches to the
piston pin, gudgeon pin (the usual British term) or wrist pin, which is
currently most often press fit into the con rod but can swivel in the
piston, a "floating wrist pin" design. The big end connects to the
bearing journal on the crank throw, running on replaceable bearing
shells accessible via the con rod bolts which hold the bearing "cap"
onto the big end; typically there are pinholes bored through the bearing
and the big end of the con rod so that pressurized lubricate motor oil
squirts out onto the thrust side of the cylinder wall to lubricate the
travel of the pistons and piston rings.
Many-cylinder multi-bank engines
such as a V-12 layout have little space available for that many
connecting rod journals on a limited length of crankshaft. This is a
difficult compromise to solve and its consequence has often led to
engines being regarded as failures.
The simplest solution, almost
universal in road car engines, is to use simple rods where cylinders
from both banks share a journal. This requires the rod bearings to be
narrower, increasing bearing load and the risk of failure in a
high-performance engine. This also means the opposing cylinders are not
exactly in line with each other.
In certain types of engine,
master/slave rods are used rather than the simple type shown in the
picture above. The master rod carries one or more ring pins to which are
bolted the much smaller big ends of slave rods on other cylinders.
Radial engines typically have a master rod for one cylinder and slave
rods for all the other cylinders in the same bank. Certain designs of V
engines use a master/slave rod for each pair of opposite cylinders. A
drawback of this is that the stroke of the subsidiary rod is slightly
shorter than the master, which increase vibration in vie engine,
catastrophically so for the Sunbeam Arab.
The usual solution for
high-performance aero-engines is a "forked" connecting rod. One rod is
split in two at the big end and the other is thinned to fit into this
fork. The Rolls-Royce Merlin used this "fork-and-blade" style. The
journal is still shared between cylinders.
When building a high performance
engine, great attention is paid to the con rods, eliminating stress
risers by such techniques as grinding the edges of the rod to a smooth
radius, shot preening to induce compressive surface stresses, balancing
all con rod/piston assemblies to the same weight and Magna fluxing to
reveal otherwise invisible small cracks which would cause the rod to
fail under stress. In addition, great care is taken to torque the con
rod bolts to the exact value specified; often these bolts must be
replaced rather than reused. The big end of the rod is fabricated as a
unit and cut or cracked in two to establish precision fit around the big
end bearing shell. Therefore, the big end "caps" are not interchangeable
between con rods, and when rebuilding an engine, care must be taken to
ensure that the caps of the different con rods are not mixed up. Both
the con rod and its bearing cap are usually embossed with the
corresponding position number in the engine car parts block.
Engines such as the Ford 4.6
liter engine and the Chrysler 2.0 liter engine, have connecting rods
made using powder metallurgy, which allows more precise control of size
and weight with less machining and less excess mass to be machined off
for balancing. The cap is then separated from the rod by a fracturing
process. This ensures that upon reassembly, the cap will be perfectly
positioned with respect to the rod, compared to the minor misalignments
which can occur if the mating surfaces are both flat.
A major source of engine wear is
the sideways force exerted on the piston through the con rod by the
crankshaft, which typically wears the cylinder into an oval
cross-section rather than circular, making it impossible for piston
rings to correctly seal against the cylinder walls. Geometrically, it
can be seen that longer con rods will reduce the amount of this sideways
force, and therefore lead to longer engine life. However, for a given
engine block, the sum of the length of the con rod plus the piston
stroke is a fixed number, determined by the fixed distance between the
crankshaft axis and the top of the cylinder block where the cylinder
head fastens; thus, for a given cylinder block longer stroke, giving
greater engine displacement and power, requires a shorter connecting
rod, resulting in accelerated cylinder wear.
The con rod are under tremendous
stress from the reciprocating load represented by the piston, actually
stretching and relaxing with every rotation, and the load increases
rapidly with increasing engine speed. Failure of a connecting rod,
usually called "throwing a rod" is one of the most common causes of
catastrophic engine failure in cars, frequently putting the broken rod
through the side of the crankcase and thereby rendering the engine
irreparable; it can result from fatigue near a physical defect in the
rod, lubrication failure in a bearing due to faulty maintenance, or from
failure of the rod bolts from a defect, improper tightening, or re-use
of already used bolts where not recommended. Despite their frequent
occurrence on televised competitive automobile events, such failures are
quite rare on production cars during normal daily driving. This is
because production car
parts have a much larger factor of safety, and often more systematic