Crankshaft and Its Maintenance
K Chatterjea (Singapore Maritime Academy)
MA-8101 - EK Motor
K Chatterjea
Singapore Maritime Academy
April 2023
Crankshaft and Its Maintenance
April 2023
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Aims & Learning Outcomes
Aim: Understand the construction and maintenance of crankshafts.
Learning Outcomes: At the end of this lesson the student should be able
to:
1 Explain the material used for a crankshaft
2 Describe the construction of an one-piece-crankshaft
3 Explain the inspection procedure of a medium speed engine crankshaft
4 Explain the factors that cause overstressing of a crankshaft
5 Explain the possible causes of vibration on a crankshaft
6 Identify the factors that may cause misalignment of a crankshaft
7 Describe how crankshaft bearing clearances are checked
8 Sketch and describe the procedure for checking crankshaft alignment
using deflection gauge between webs
9 Explain how to interpret the readings obtained from crankshaft
deflection
NOTE: All slides are based on [4]
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Lesson Plan for the Module
Figure: Lesson Plan for the Module
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Functions of a Crankshaft in Diesel Engines
A rotating shaft, called a crankshaft, transforms a piston’s reciprocating
action into a rotational motion. Internal combustion engines employ these
to carry out such actions. Connecting rods are joined to crankshafts
through a series of cranks and crankpins.
Figure: Diesel Engine Crankshaft [1]
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Types of Crankshafts in Diesel Engines [2]
The followings are the various types of crankshaft fitted to a diesel engine:
2-stroke large marine diesel engine
fully built-up crankshaft
semi-built-up crankshaft
welded crankshaft
4-stroke engines have one piece crank shaft.
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Processes of Construction of One Piece Crankshaft
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Steel billet is heated and worked down under steam hammer or
hydraulic forging
End of billet is first worked to form an end coupling flange and journal
Next section worked part forming two crank-webs and a pin form a
block
Third section worked on second journal
It is followed by second block forged at correct angle to the first block
Forging is continued until it is finished
Journal and sides of webs adjacent to journal are finish-machined.
Followed by machining of crankpin and inside of webs
Holes and oil passages drilled, ground smooth to reduced
stress-raisers including fillet radius at pin and journals
Crankpins and journal hardened if thin shell bearing used
Sets of hydraulic presses are then used to form the crankpin journal
and webs.
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Processes of Construction of One Piece Crankshaft
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Forging gives crankshaft continuous grain flow.
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
Forgings are machined, stress relieved. Radii at change of section is
cold rolled.
Crankshafts for surface hardened are made of a steel alloy known as
nitralloy. (a steel containing 1.5%Cr, 1% Al and 0.2% Mo)
Crankshaft is heated to 500C in ammonia gas
Nitrogen dissociates from ammonia gas combines with chromium and
aluminium to form hard nitrates at surface.
Molybdenum refines the grain structure at the still tough core
Heavy weights fitted to cranks in order to balance the reciprocating
masses
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Crankshaft Fillet Radius [3]
A crankshaft frequently has a narrow fillet radius. Crankshaft fillet rolling
is one of the often used engineering techniques to extend the fatigue life of
the crankshaft. The fillet rolling process results in the induction of
compressive residual stresses on and beneath the fillet radius surface. As a
result, the crankshaft’s fatigue life is increased.
Figure: Crankshaft Fillet Radius
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Pro & Cons of One Piece Crankshaft
ADVANTAGES:
Continuous grain flow, webs is thinner
Economy in weight, space and cost
Smaller & lighter webs since no holes
Thin crank arm, webs leads to longer journal bearing & lower specific
bearing pressures
Increase fatigue strength due to continuous grain flow. Approved by
major classification societies
Shaft lighter and thus more crank-throws in a single shaft
No shrinkfit necessary thus no consequences of slip occurs
DISADVANTAGES:
More rigid and thus less tolerant to misalignment
Difficulty in repairs ...if one section cannot be repairs by grinding or
metal spraying, whole shaft to be renewed.
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Oil Holes in One Piece Crankshaft
Oil holes are drilled in crankshaft for oil to travel from main bearing
journal to crankpin and UP the connecting rod to lubricate piston
pin/gudgeon pin and cool the piston.
If the surface finish of oil holes is poor, then cracks can start from the
flaws during operation
At the exit points on crankpin, oil holes must not have sharp edges
but to be smoothly radiused to reduce stress concentration at the
edges.
Oil hole drilled along the journal, webs crankpin in crankshaft must
not compromise the strength of the crankshaft.
The oil holes should be positioned horizontally when the crank is at
TDC to avoid the loaded region.
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Oil Holes in One Piece Crankshaft - Figure
Figure: Drilled Oil Hole in One Piece Crankshaft
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Material for Crankshsft
One Piece Crankshaft The materials for one piece crankshaft are steel.
The compositions are generally 0.4carbon steel
Fully built-up and semi-built-up crankshaft The materials for these
types of crankshaft are generally unalloyed steel of low carbon steel or
alloyed steel of low allloyed chrome and molybdenum steel.
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Forces on Crankshaft Diagram [5]
Figure: Forces on A Crankshaft
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Stresses on a Crankshaft
Crankshaft experience the following stresses during dynamic
loading:
Bending of crankshaft contribute to compressive, tensile and shear
stresses
Twisting causes shear stresses
Torsional stresses fluctuation when propeller comes out of water and
plunges back into water.
Sudden fluctuations of engine speed result in shock loading on
crankpins.
Misalignment and wear down of bearing
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Stresses on a Crankshaft - Diagram
Figure: Stresses on A Crankshaft
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Likely Causes of Crankshaft Vibration
Possible causes of crankshaft vibration
Load due to unbalanced gears contribute to torsional vibration
Load due to propeller thrust that could result in axial vibration.
Operation of engine in the range of critical speed.
Sudden shocks loads due to propeller racing in heavy weather
Misalignment of crankshaft due to wear down of main bearing.
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Likely Causes of Crankshaft Misalignment
Factors that may cause misalignment of crankshaft
Main bearing supporting the crankshaft are damaged
The lowest part of the engine structure as bedplate where crankshaft
is resting is deformed / damaged
Slack or broken tie bolts in some medium speed engine fitted with tie
bolts
Foundation bolts or holding down bolts securing the engine to the
engine seating are loosen or fracture
Chocks supporting the Engine to the engine seating are broken,
cracked or fretted
Propeller shaft bearing or stern tube bearing for the main engine are
defective or wear down.
The engine structure is weak due to corrosion and thus running
components such as crankshaft will be affected
Hull deformation especially due to grounding and fire that affect the
engine seating condition and thus the crankshaft
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Checking of Bearing Clearances
Ensure risk assessment is completed in the entry into
crankcase/confined space
In the measurement of crankpin bearing clearance, use the Engine
Instruction Manual as the guide.
The use of a feeler gauge to insert between the crankpin and crankpin
bearing are the commonly practiced.
Measure thickness of crankpin and the inside diameter of the crankpin
after it is tensioned to the correct torque. The difference between the
inside diameter of the crankpin and the crankpin diameter gives the
bearing clearance
In the measurement of main bearing clearance, the use of ‘Kjaer
‘feeler gauge is used in certain engine (see Fig in the next slide) to
measure the clearance between the journal and main bearing at the
top.
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Taking Main Bearing Clearance with Telescopic Guage [6]
Figure: Taking Bearing Clearance with Telescopic Gauge
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Crankshaft Deflection - Introduction - I [7]
Consider one section for better understanding i.e. two crank webs, a
crank pin and two journals.
If a straight length of shaft is supported at either end and is subjected
to a central load.
The effect in shaft is upper material in compression and the lower in
tension.
This effect is applicable to the section of crankshaft described above
with the bearings supporting the assembly at the journals and the
point loading being effect by the weight of the piston and conrod
assembly (ignoring other loads found operational conditions such as
combustion and centrifugal).
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Crankshaft Deflection - Introduction - II
It can be seen that the effect is to
increase the distance between the
webs at top dead centre (TDC) and
reduce the distance at bottom dead
centre (BDC). This deflection is
normally found in all crankshafts
although for smaller engines with
very rigid cranks this may be very
small. A set of measurements taken
from an engine will reveal this
deflection which should be constant
through each crank/piston unit.
K Chatterjea (Singapore Maritime Academy)
Figure: Crankshaft Deflection
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Crankshaft Deflections - Points to Note
After initial installation of the engine and alignment, a set of
deflections are taken.
These then form the datum line to which all other recordings are
measured against.
It should be noted that changes in circumstances will effect the
deflections and are not indicative of faults.
These include a) Ambient temperature b) Engine temperature
c)Vessel hull loading (hogging, sagging etc.) d) Vessel afloat, dry
docked ( again vessel hull loading can cause effects even in drydock
due to movement of blocks, which tanks are full etc.)
These effects are well known and an experienced engineer will take
into account these factors when looking at a set of recordings.
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Crankshaft Deflections - Worn Out Bearing
If a situation now occurs where a bearing becomes more worn than an
adjacent one the effects will be shown as a change in the pattern of
delections.
When the cranks is turned from BDC to TDC the weight of the
running gear causes the crank webs and crackpins to bend in such a
manner that the distance between the webs decreases, and continues
to decrease until the bearing is no longer in contact with the journal.
The deflection when the crankshaft is approaching TDC will then go
from its normal positive reading to zero and then to negative readings
at which point the assembly is supporting the weight without the
assistance from the lowered main bearing.
Thus, any changes from natural deflections can be related to main bearing
misalignement and is proportional to the differences in height of the
bearings
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Crankshaft Deflections - Worn Out Bearing Figure
K Chatterjea (Singapore Maritime Academy)
Figure: Worn Out Bearing
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Crankshaft Deflections - Measurement Procedure
These are generally taken using a spring loaded dial gauge as shown
in the figure (next Slide).
The crank webs are punch marked to ensure that the readings are
taken in the same place each time.
Five measuring points are taken - TDC, 90’ either side of TDC and
30’ either side of BDC. The latter two measurements are required as
it is not possible to measure at BDC due to the Con rod.
The measurements are always taken starting at the same starting
point.
In this case we will say Port side near BDC. The gauge is fitted and
zeroed.
Rotate the engine continuously and the readings read off during
rotation.
After the final reading the egine is rotated back to the start point.
If the reading is not zero then it indicates that the gauge is moved
and the readings are re-taken.
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Crankshaft Deflections - Measurement Procedure - Figure
Figure: Crankshaft Deflection - Measurement
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Location of Dial Gauge and Crankpin Position
Figure: Crankshaft Deflection Positions
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Values of Measurement as Positive , Negative and Zero
Figure: Crankshaft Deflection - The Values
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Sample of Crankshaft Deflection Form
Figure: Crankshaft Deflection - Sample Forms
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References
[1] Iyas, M. D. ‘Crankshaft: Parts, Function, Types, Diagram & More [PDF]’. The Engineers
Post, 27 May 2021. https://www.theengineerspost.com.
[2] ‘Cast vs Forged Crankshaft’. Accessed 8 April 2023.
https://www.dropforging.net/cast-vs-forged-crankshaft.html.
[3] Ho, Simon, Yung-Li Lee, Hong-Tae Kang, and Cheng J. Wang. ‘Optimization of a
Crankshaft Rolling Process for Durability’. International Journal of Fatigue 31, no. 5 (1 May
2009): 799–808. https://doi.org/10.1016/j.ijfatigue.2008.11.011.
[4] SMA Classnotes
Accessed 7 April 2023.
[5] ‘Marine Diesel Engines: Role and Purpose of the Crankshaft’, 25 December 2008.
https://www.brighthubengineering.com/marine-engines-machinery/20969-components-ofmarine-diesel-engines-crankshaft/.
[6] Anish. ‘Ways to Measure Main Bearing Clearance of Marine Engine’. Marine Insight, 2
March 2019. https://www.marineinsight.com/main-engine/4-ways-to-measure-main-bearingclearance-of-two-stroke-marine-engine/.
[7]Moozhikkara, Vineeth. ‘Simple Explanation Of Crankshaft Deflections,
Measurement Procedure, Readings,Curves’. MarineSite.Info (blog).
Accessed 8 April 2023.
https://www.marinesite.info/2013/10/simple-explanation-of-crankshaft.html.
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