Crankshaft
The crankshaft, a car
parts, sometimes casually abbreviated to crank, is the part of an
engine which translates reciprocating linear piston motion into
rotation. To convert the reciprocating motion into rotation, the
crankshaft has "crank throws" or "crankpins", additional bearing
surfaces whose axis are offset from that of the crank, to which the "big
ends" of the connecting rods from each cylinder attach.
It typically connects to a flywheel which is car parts, to reduce the
pulsation characteristic of the four-stroke cycle, and sometimes a
tensional or vibration damper at the opposite end, to reduce the torsion
vibrations often caused along the length of the crankshaft by the
cylinders farthest from the output end acting on the tensional
elasticity of the metal.
Crankshafts can be monolithic (made in a single piece) or assembled from
several pieces. Monolithic crankshafts are most common, but some smaller
and larger car parts use assembled crankshafts.
Forging and casting
Crankshafts can be forged from a steel bar or cast in ductile iron.
Now a day more and more manufacturers tend to favor the use of forged
crankshafts due to their lighter weight, more compact dimensions and
better inherent dampening. With forged crankshafts, vanadium
microalloyed steels are mostly used as these steels can be air cooled
after forging reaching high strengths without additional heat treatment,
with exception to the surface hardening of the bearing surfaces. Carbon
steels are also used, but these require additional heat treatment to
reach the desired properties. Cast iron crankshafts are today mostly
found in cheaper production engines where the loads are lower. Some
engines also use cast iron crankshafts for low output versions while the
more expensive high output version use forged steel.
Machining
Crankshafts can also be machined out of a billet. Even though the fiber
flow (local inhomogeneities of the material's chemical composition
generated during casting) doesn’t follow the shape of the crankshaft
(which is undesirable), this is usually not a problem since higher
quality steels which normally are difficult to forge can be used. These
crankshafts tend to be very expensive due to the large amount of
material removal which needs to be done by using lathes and milling
machines, the high material cost and the additional heat treatment
required. However, since no expensive tooling is required, this
production method allows small production runs of crankshafts to be made
without high costs.
Fatigue strength
The fatigue strength of crankshafts is usually increased by using a
radius at the ends of each main and crankpin bearing. The radius itself
reduces the stress in these critical areas, although since the radiuses
in most cases are rolled, this also leaves some compressive residual
stress in the surface which prevents cracks from forming.
Hardening
Most production crankshafts use induction hardened bearing surfaces
since that method gives good results with low costs. It allows the
crankshaft to be reground without having to redo the hardening. But high
performance crankshafts, billet crankshafts in particular, tend to use
nitrification instead. Nitridization is slower and thereby more costly,
and in addition it puts certain demands on the alloying metals in the
steel, in order to be able to create stable nitrides. The advantage is
that it can be done at low temperatures, it produces a very hard surface
and the process will leave some compressive residual stress in the
surface which is good for the fatigue properties of the crankshaft. The
low temperature during treatment is advantageous in that it doesn’t have
any negative effects on the steel, such as annealing. With crankshafts
that operate on roller bearings, the use of carburization tends to be
favored due to the high Hertzian contact stresses in such an
application. Like nitriding, carburization also leaves some compressive
residual stresses in the surface.
Counterweights
Some expensive, high performance crankshafts also use heavy-metal
counterweights to make the crankshaft more compact. A heavy-metal used
is most often a tungsten alloy but depleted uranium has also been used.
A cheaper option is to use lead, but compared with tungsten its density
is much lower.
Stress analysis of crankshaft
The shaft is subjected to various forces but it needs to be checked in
two positions. First, failure may occur at the position of maximum
bending. The crank may fail due to twisting, so the crankpin needs to be
checked for shear at the position of maximal twisting. The pressure at
this position is not the maximal pressure, but a fraction of maximal
pressure.
