The crankshaft of an engine (either internal or external combustion) serves
to translate the
reciprocating motion of the
pistons to
rotational motion for
power output. Where possible, crankshafts
are
forged from
billet steel. Some crankshafts are
cast or built of
multiple sections bolted together due to the
higher cost of forging, however this results in greatly increased weight.
Most crankshafts in 4 cycle or diesel engines share the following design:
The shaft itself spins in the crankcase upon its main bearings. These
bearings are pressure lubricated, and one or more of these will feed oil
into a series of passages inside the crankshaft for distribution to the
connecting rod bearings and / or other main bearings. (A hole in the
connecting rod is
in turn used to feed lubricating oil to the piston and the wrist pin
bearing at the small end of the rod.)
Most crankshafts (especially those which run at high RPM) include a "sludge
trap". Due to high RPM rotation the oil passages in the crank act as a
centrifuge, meaning that deposits can accumulate in the larger radius
sections. This would eventually block flow, therefor upon entry to the
crank, the oil passes into a relatively large reservoir (meaning that it
resides there longer than elswhere) at the outer radius of revolution.
This causes particles which would otherwise clog passages to be caught in
the trap. Some engines make the trap too small and thus limit the potential
engine life (e.g. '60s BMW motorcycles need to have the sludge trap cleaned
about every 150,000 miles).
Most automotive and large stationary engines use journal bearings in both
the main and connecting rod bearing locations. The journal is defined
as the cylindrical sections which rotate inside of the bearing shell. Many
motorcycles and most 2 cycle engines use ball or roller
bearings.
Crankshaft design largely determines the vibration characteristics of the
engine. This is principally used to set the "balance factor" to obtain
the desired behavior, and may accomplished using attached counter weights
A balance factor near 1.0 indicates an engine whose primary vibration is
in line with piston motion (the engine vibrates up-down), while a value
near 0.5 results in an engine which vibrates more in a circle. Additionally,
the balance factor is set to minimize vibration at the most common operating
RPM ranges. A balance factor not well matched to the engine mounts and
frame can result in fatigue failures of the supporting structures.*
Very few crankshaft designs are adequately heavy in themselves to generate
a smooth-running engine. For this reason, most engines have an attached
flywheel which stores energy, providing smooth operation. Racing engines
use very small flywheels, which allows for substantially increased
acceleration, but which also may result in an engine which will not idle
at a speed lower than say 2-3000 RPM, which would be impractical for
daily use.
*Or in some instances of the
mechanism. A friend once told me about the
failure of a
large power generation diesel engine at a US Military
installation in
Scandanavia. It seems that the engineers skimped on
the recommended concrete footings for the engine mounts. After a few
months of operation the attachment of a (several ton)
flywheel failed. It
then fractured its case and the wall of the
facility and rolled into the
nearby
fjord.