Connecting Rod Bottom End Bolt
K Chatterjea (Singapore Maritime Academy)
MA-3017 - EK General
K Chatterjea
Singapore Maritime Academy
April 2023
Connecting Rod Bottom End Bolt
April 2023
1 / 15
Aims & Learning Outcomes
Aim: Understand the design principles of Bottom End Bolts and their
Maintenance.
Learning Outcomes: At the end of this lesson the student should be able
to:
1
Explain the functions of connecting rod bottom end bolts.
2
Explain the forces acting on the four-stroke trunk type engine bottom
end bolts and the concept of load reversal.
3
Explain the forces acting on the two-stroke bottom end bolts.
4
Describe the design features of bolts in moderThe failure can take
place at a much lower load than the breaking-load which is termed as
’Fatigue Failure’.n 4S and 2S cycle engines.
5
State the problems associated with bottom end bolts.
6
State the maintenance procedures of bottom end bolts.
NOTE: All slides are based on [4]
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
2 / 15
Lesson Plan for the Module
Figure: Lesson Plan for the Module
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
3 / 15
Introduction
Bolts and studs play an important role in the design and satisfactory
operation of diesel engines.
Connecting rod bolts are subjected to high and complex nature of
stresses in modern highly rated engines.
In 4 stroke engines, the nature of stresses are particularly complex
because of load reversal at the end of exhaust stroke which results in
fluctuating tensile stresses.
Bottom end bolt failures are essentially fatigue failures.
The consequences of bolt failures in running engines can be disastrous
to the man and machine in the engine room.
As operating engineers, we need to understand the processes that
affect bottom end bolts.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
4 / 15
Bottom End Bolt Design
Figure: Bottom End Bolt Designs (Left - Better)
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
5 / 15
Stresses in Connecting Rod Bolts - I
The bolts are subjected to tensile stress when they are tightened to
keep the two parts together.
In simple words, this means that bolts are stretched when tightened
to keep two components together.
The components in between are subjected to compressive stress.
This tensile stress in the bolts is quite high because tightening load
has significantly increased in modern highly marine rated engines.
This is more pronounced in 4 stroke engines because of load reversal
at the end of exhaust stroke.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
6 / 15
Stresses in Connecting Rod Bolts - II
The failure can take place at a much lower load than the breaking-load
which is termed as ’Fatigue Failure’.
It is important to note that 4 stoke trunk type engine (e.g. Pielstick
and Wartsila Main and Auxiliary Engines) bolts are subjected to much
higher Fluctuating Tensile Stress than 2 stroke engines engines. Note
the word ’Fluctuation’.
This means the load is not steady.
Any material will fail at a much lower fluctuating load than a steady
higher load.
This fluctuation can be of tensile or compressive in nature.
For example, in the bottom end bolts of 4 stoke engines, the load is
fluctuating tensile.
This means that the load is fluctuating between high tensile and low
tensile.
This can be damaging for the component.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
7 / 15
Load Reversal - I
K Chatterjea (Singapore Maritime Academy)
Figure: Load Reversal - I
Connecting Rod Bottom End Bolt
April 2023
8 / 15
Load Reversal - II
K Chatterjea (Singapore Maritime Academy)
Figure: Load Reversal - II
Connecting Rod Bottom End Bolt
April 2023
9 / 15
Load Reversal - III
With reference to Trunk Type 4 Stroke Cycle Engine shown in Slide-8,
there is a load reversal taking place in the bearings at the end of exhaust
stroke.
Normally the loads on the bearings are as follows:
Bottom half of main bearings
Top half of bottom end bearings
Bottom half of top end bearings
In 4 stroke engines. this load pattern is reversed in a cyclical nature. The
load gets transferred to the other half (opposite half) at the end of exhaust
stroke and gets back to the normal again. This continues in every cycle.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
10 / 15
Load Reversal - IV
At the end of exhaust stroke momentarily and instantaneously ,the
main bearing top half is loaded, bottom end bearing bottom half is
loaded and top end bearing top half is loaded.
This has a significant effect on the bottom end bolts because the
whole connecting rod is under tension at the end of exhaust stroke.
Bottom end bolts are subjected to additional tensile stress at the end
of exhaust stroke resulting in severe stress fluctuations during the
cycle.
Pay due attention to bottom end bolts of main engine and auxiliary
engines during crankcase inspection.
Special attention must be given to auxiliary engine bottom end bolts
because of the running condition as mentioned as above.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
11 / 15
Care of 4S Bottom End Bolt [5]
As the fluctuating stresses, mentioned before, lead to the fatigue failure of
bottom end bolts, and this is why the bottom end bolts are prone to
failure under normal condition in medium speed engines.
The following proper maintenance can minimize bolt failure:
Proper Surface Finish: The bolt is given a good surface finish to
reduce stress raisers. Bolt surface and threads must not be damaged
by rough handling.
Bolts should be tightened according to the manufacturer’s
instructions.
Tightening must be done in stages.
The landing faces of the bolt-head nut should be cleaned.
During tightening, measurement of the extension is essential for
correct stretch.
Always change the bolts in pairs.
Check the bolt for cracks, damage, etc. Check the locking of the nuts.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
12 / 15
Comparing Load on 4S and 2S Diesel Engines - I
Compressive and tensile pressures alternately act on four stroke
marine diesel engines.
The gases expand immediately after combustion, pushing the piston
downward and compressively pushing the gases until the inlet valve
opens.
Now, a tensile or centrifugal force acting on the piston and
connecting rod is directly proportional to the component mass and
the square of the velocity.
The components are released from compressive forces when the inlet
and exhaust valves both remain open and briefly overlap one another.
Additionally, the inlet valve closes during the compression stroke, and
the exhaust valve closes during the induction stroke. Although the
amount of compression is little, the piston and connecting rod are
once again under tension.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
13 / 15
Comparing Load on 4S abd 2S Diesel Engines - II
In contrast, the piston and connecting rod are always in compression
during the expansion and compression strokes of a two-stroke engine.
As the gases expand after combustion and push the piston downward,
the magnitude of compressive force is greater in the expansion stroke
compared to the compression stroke.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
14 / 15
References
[1] SAFETY4SEA. ‘Failure of Engine Connecting Rod Bolts in Four Stroke Auxiliary Engines’,
20 November 2014.
https://safety4sea.com/failure-of-engine-connecting-rod-bolts-in-four-stroke-auxiliary-engines/.
[2] linkedin.com/in/saaimshaikh, Saaim Shaikh. ‘Engine Connecting Rod — Types, Parts,
Materials, and Stresses Explained’. Ship Management Company, Vessel Management Services
— Nautilus Shipping (blog), 21 December 2022.
https://www.nautilusshipping.com/engine-connecting-rod/.
[3] Woodyard, Doug. Pounder’s Marine Diesel Engines and Gas Turbines. Amsterdam London,
2009. p.574,
[4] SMA Classnotes
Accessed 7 April 2023.
[5] ‘Connecting Rod Bolts’, 14 June 2010.
https://www.brighthubengineering.com/marine-engines-machinery/74087-operating-faults-inconnecting-rod-bolts/.
K Chatterjea (Singapore Maritime Academy)
Connecting Rod Bottom End Bolt
April 2023
15 / 15