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Operational Information
The Medium Speed 4 Stroke Trunk Piston
Engine
The Crankshaft
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The Crankshaft for a medium speed 4 stroke diesel
engine is made from a one piece forging. |
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First the billet of 0.4% carbon steel is heated in
a furnace It is then moved to the forging presses |
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In the hydraulic forging press the crankshaft throws
and flanges are formed. |
A Guide to how the Crankshaft Throws
are Forged
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The crankshaft is locally heated to a white heat
where the webs are desired to be formed. The crankshaft is then
compressed axially to form the start of the webs |
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Sets of hydraulic presses are then used to form the
crankpin journal and webs.
This method of forging gives the crankshaft continuous grain
flow. This is where the grain structure follows a path parallel to
and along the journal, bends round along the line of the web,
round through the crankpin, and back down the second web before
turning again to follow the journal. Continuous grain flow gives
the crankshaft better fatigue resistance.
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The forgings are then machined, stress relieved, and
the radii at the change of section cold rolled.
If the crankshafts are to be surface hardened they are made of
a steel alloy known as nitralloy (a steel containing 1.5%Cr, 1% Al
and 0.2% Mo)
The crankshaft is heated to 500ºC in ammonia gas for up to 4
days. The nitrogen dissociates from the ammonia gas and combines
with the chromium and aluminium to form hard nitrates at the
surface. The molybdenum refines the grain structure at the still
tough core. |
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Fillet Radii
At the change of section between journal and web and web and
crankpin, fillet radii are machined so
there is not a sharp corner to act as a stress raiser. These radii are
cold rolled to remove machining marks, harden the surface and to induce
a residual compressive stress, again to increase fatigue resistance.
Re-entrant fillets are sometimes employed; This allows for a shorter
crankshaft without compromising on bearing length.
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Oil Holes in Crankshafts.
Unlike the crankshafts for slow speed 2 stroke crosshead engines,
which lubricate the bottom ends by sending the oil DOWN the con rod from
the crosshead, the crankshaft for the medium speed trunk piston engine
must have holes drilled in it so that oil can travel from the main
bearing journals to the crankpin and then UP the con rod to lubricate
the piston pin and cool the piston. If the surface finish of the holes is not good,
then cracks can start from the flaws.
At the exit points on the crankpin, the holes must be smoothly
radiused. So that the crankshaft strength is not compromised the holes
should be positioned horizontally when the crank is at TDC.
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