Piston
(top) and connecting rod from typical automotive engine
A connecting
rod is a shaft which connects a piston to a crank or crankshaft in a reciprocating engine. Together with the crank, it forms
a simple mechanism that converts reciprocating motion into rotating motion.
A connecting rod may also convert rotating motion into
reciprocating motion, its original use. Earlier mechanisms, such as the
chain, could only impart pulling motion. Being rigid, a connecting rod may
transmit either push or pull, allowing the rod to rotate the crank through both
halves of a revolution. In a few two-stroke engines the connecting rod is only
required to push.
Today, the connecting rod is best known through its use in
internal combustion piston engines, such as automobile engines. These are of a distinctly different design from earlier
forms of connecting rod used in steam engines and steam locomotives.
INTERNAL
COMBUSTION ENGINES
Three
different connecting rods, of which the left and the aluminum center, the
connecting rod to the right (for endothermic engine) in steel, the left
connecting rod (for endothermic engine) has the modular head and the foot
equipped with a bushing, the central rod has the oil drip rod equipped with
pats.
In
modern automotive internal combustion
engines, the
connecting rods are most usually made of steel for production engines, but
can be made of T6-2024 and T651-7075 aluminum alloys (for lightness and the ability to absorb high impact
at the expense of durability) or titanium (for a combination of lightness with strength, at
higher cost) 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. Connecting rods, especially in racing engines, may be
called "billet" rods, if they are machined out of a solid billet of metal, rather than
being cast or forged.
SMALL END AND BIG END
The small end attaches to the piston
pin, gudgeon pin or wrist pin, which is currently most often press fit into the connecting rod but can swivel in the piston,
a "floating wrist pin" design. The big end connects to the crankpin (bearing journal) on the crank throw, in most engines running on replaceable bearing shells accessible via the connecting rod bolts which hold the bearing
"cap" onto the big end. Typically there is a pinhole bored through
the bearing on the big end of the connecting rod so that pressurized lubricating motor oil squirts out onto the thrust side of the cylinder wall to lubricate the travel of the pistons and piston rings. Most small two-stroke engines and some single cylinder four-stroke engines avoid the need for a pumped
lubrication system by using a rolling-element
bearing instead,
however this requires the crankshaft to be pressed apart and then back together
in order to replace a connecting rod.
ENGINE WEAR AND ROD
LENGTH
A major
source of engine wear is the sideways force exerted on the piston through the
connecting 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 connecting 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 connecting 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.
STRESS AND FAILURE
Failure of connecting rod is one of the most common causes
catastrophic engine failure.
The connecting rod is under tremendous stress from the
reciprocating load represented by the piston, actually stretching and being
compressed with every rotation, and the load increases as the square of the
engine speed increase. 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 over-revving of
the engine. In an unmaintained, dirty environment, a water or chemical
emulsifies with the oil that lubricates the bearing and causes the bearing to
fail. Re-use of rod bolts is a common practice as long as the bolts meet
manufacturer specifications. 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 auto parts have a much
larger factor of safety, and often more systematic quality control.
HIGH-PERFORMANCE ENGINES
When building
a high-performance engine, great attention is paid to the connecting rods,
eliminating stress risers by such techniques as grinding
the edges of the rod to a smooth radius, shot peening to induce compressive surface stresses (to prevent
crack initiation), balancing all connecting rod/piston assemblies to the same
weight and Magnafluxing to reveal otherwise invisible
small cracks which would cause the rod to fail under stress. In addition, great
care is taken to torque the connecting 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 connecting rods, and when rebuilding an engine,
care must be taken to ensure that the caps of the different connecting rods are
not mixed up. Both the connecting rod and its bearing cap are usually embossed
with the corresponding position number in the engine block.
POWDER METALLURGY
Recent
engines such as the Ford 4.6 litre engine and the Chrysler 2.0 litre 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, which results in an
uneven mating surface due to the grain of the powdered metal. 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.
COMPOUND RODS
Articulated connecting rods
Multi-bank engines with many cylinders, such as a V12 layout, have little space available for 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 (Sunbeam Arab, Rolls-Royce Vulture).
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 engine types, 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. 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 increases vibration in a vee engine,
catastrophically so for the Sunbeam Arab.
BMW 132 radial aero engine rods
Radial engines typically have a master rod for one cylinder and
multiple slave rods for all the other cylinders in the same bank.
Fork and blade rods
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 journal is still shared between
cylinders. The Rolls-Royce Merlin used this
"fork-and-blade" style. A common arrangement for forked rods is for
the fork rod to have a single wide bearing sleeve that spans the whole width of
the rod, including the central gap. The blade rod then runs, not directly on
the crankpin, but on the outside of this sleeve. The two rods do not rotate
relative to each other, merely oscillate back and forth, so this bearing is
relatively lightly loaded and runs at a much lower surface speed. However the
bearing movement also becomes reciprocating rather than continuously rotating,
which is a more difficult problem for lubrication.
A likely
candidate for an extreme example of compound articulated rod design could be
the complex German 24-cylinder Junkers Jumo 222aviation engine, meant to have unlike an X-engine layout with 24 cylinders, possessing six cylinders per bank only four cylinders per bank, and six banks of cylinders, all
liquid-cooled with five "slave" rods pinned to one master
rod, for each "layer" of cylinders in its design. After building
nearly 300 test examples in several different displacements, the Junkers firm's complex Jumo 222 engine turned out to be a
production failure for the more advanced combat aircraft of the Third Reich's Luftwaffe which required aviation power plants of over 1,500 kW
(2,000 PS) output apiece.
WORKING