A crankshaft
related to crank
is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. In a reciprocating engine, it translates reciprocating motion of the piston into rotational motion; whereas in a reciprocating
compressor, it
converts the rotational motion into reciprocating motion. In order to do the
conversion between two motions, the crankshaft has "crank throws" or
"crankpins", additional bearing surfaces whose axis is offset
from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.
It is
typically connected to a flywheel to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the
opposite end, to reduce the torsional vibrations often caused along the length
of the crankshaft by the cylinders farthest from the output end acting on the
torsional elasticity of the metal.
INTERNAL COMBUSTION ENGINES
Crankshaft,
piston and connecting rods for a typical internal combustion engine.
Large engines are usually multi cylinder to reduce pulsations from individual
firing strokes, with more than one piston attached
to a complex crankshaft. Many small engines, such as those found in mopeds or garden machinery, are
single cylinder and use only a single piston, simplifying crankshaft design.
A crankshaft
is subjected to enormous stresses, potentially equivalent of several tonnes of
force. The crankshaft is connected to the fly-wheel (used to smooth out shock
and convert energy to torque), the engine block, using bearings on the main journals,
and to the pistons via their respective con-rods. An engine loses up to 75% of
its generated energy in the form of friction, noise and vibration in the
crankcase and piston area. The remaining losses occur in the valvetrain
(timing chains, belts, pulleys, camshafts, lobes, valves, seals etc.) heat and
blow by.
BEARINGS
The
crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings (the main bearings) held in the engine block. As the crankshaft undergoes a
great deal of sideways load from each cylinder in a multicylinder engine, it
must be supported by several such bearings, not just one at each end. This was
a factor in the rise of V8 engines, with their shorter crankshafts, in preference to straight-8 engines. The long crankshafts of the latter suffered
from an unacceptable amount of flex when engine designers began using
higher compression ratios and higher rotational speeds.
High performance engines often have more main bearings than their lower
performance cousins for this reason.
PISTON STROKE
The distance
the axis of the crank throws from the axis of the crankshaft determines the
piston stroke measurement, and thus engine displacement. A common way to increase the
low-speed torque of an engine is to increase the stroke, sometimes known as
"shaft-stroking." This also increases the reciprocating vibration, however, limiting the high speed
capability of the engine. In compensation, it improves the low speed operation
of the engine, as the longer intake stroke through smaller valve(s) results in
greater turbulence and mixing of the intake charge. Most modern high speed
production engines are classified as "over square" or short-stroke,
wherein the stroke is less than the diameter of the cylinder bore. As such, finding the proper balance between shaft-stroking
speed and length leads to better results.
ENGINE CONFIGURATION
The configuration, meaning the number of pistons and
their placement in relation to each other leads to straight, V or flat engines. The same basic engine block can sometimes be used with different crankshafts,
however, to alter the firing order. For instance, the 90° V6 engine configuration, in older days sometimes
derived by using six cylinders of a V8 engine with a 3 throw crankshaft, produces an engine with
an inherent pulsation in the power flow due to the "gap"
between the firing pulses alternates between short and long pauses because the
90 degree engine block does not correspond to the 120 degree spacing of the
crankshaft. The same engine, however, can be made to provide evenly spaced
power pulses by using a crankshaft with an individual crank throw for each
cylinder, spaced so that the pistons are actually phased 120° apart, as in
the GM 3800 engine. While most production V8 engines
use four crank throws spaced 90° apart, high-performance V8 engines often use a
"flat" crankshaft with throws spaced 180° apart, essentially
resulting in two straight
four engines running
on a common crankcase. The difference can be heard as the flat-plane
crankshafts result in the engine having a smoother, higher-pitched sound than
cross-plane (for example, IRL IndyCar Series compared to NASCAR Sprint Cup Series, or a Ferrari 355 compared to a Chevrolet Corvette). This type of crankshaft was also
used on early types of V8 engines. See the main article on cross plane crankshafts.
ENGINE BALANCE
For some
engines it is necessary to provide counterweights for the reciprocating mass of each piston and
connecting rod to improve engine balance. These are typically cast as part of the crankshaft but,
occasionally, are bolt-on pieces. While counter weights add a considerable
amount of weight to the crankshaft, it provides a smoother running engine and
allows higher RPM levels to be reached.
STRESS ON CRANKSHAFTS
The shaft is
subjected to various forces but generally needs to be analysed in two
positions. Firstly, failure may occur at the position of maximum bending; this
may be at the centre of the crank or at either end. In such a condition the
failure is due to bending and the pressure in the cylinder is maximal. Second,
the crank may fail due to twisting, so the conrod needs to be checked for shear
at the position of maximal twisting. The pressure at this position is the
maximal pressure, but only a fraction of maximal pressure.
WORKING
