Auxiliary Engine Bottom-End Bolts – Causes of Failure, Design Safeguards, and Maintenance Control

Bottom-end bolts (connecting rod bolts) are among the most highly stressed components in an auxiliary engine. Even when the engine is operating under normal and correct conditions, these bolts are continuously subjected to complex and fluctuating stresses. For this reason, their ultimate mode of failure is almost always fatigue, and such failure is inevitable over long service periods unless properly controlled by design and maintenance.

1. Why Bottom-End Bolts Fail Under Normal Operating Conditions

(a) Initial Tensile Stress (Preload)

• When the bolt is tightened during assembly, it is deliberately stretched to create a high tensile preload.
• This preload is the primary working condition of the bolt and serves to clamp the two halves of the connecting rod securely around the crankpin.
• The bolt therefore operates permanently under high tensile stress. The service loads do not replace this stress; rather, they fluctuate around it.

(b) Fluctuating / Alternating Stresses During Engine Operation

• During every engine cycle, the bolt is subjected to repeated cyclic loading caused by combustion forces and inertia forces of reciprocating parts.

(i) Power Stroke

• High combustion pressure forces the piston downward.
• The connecting rod is heavily compressed.
• The big-end housing tends to distort.
• This distortion may cause the bolts to bend slightly outward, introducing bending stress in addition to tensile stress.
• The bolt experiences increased tensile loading during this phase.

(ii) Exhaust and Suction Strokes

• Inertia forces dominate as the piston changes direction.
• The reciprocating mass tends to continue moving, creating tensile loading in the connecting rod.
• At certain points (especially near TDC), the entire tensile load may be carried by the bolts.
• This produces additional cyclic tensile stress.
• Bolts may bend inward during this phase.

Since this loading occurs every revolution, the bolts experience millions of stress cycles, even under normal engine speed.

(c) Shear Stress

• The two halves of the connecting rod have a natural tendency to separate due to dynamic forces.
• The bolts resist this separation.
• This resistance introduces shear stress in addition to tensile and bending stresses.

(d) Combined Effect – Fatigue Failure

The bolt is therefore subjected to:

• Constant tensile preload
• Fluctuating (alternating) tensile stress
• Bending stress
• Shear stress

Even though these stresses remain within design limits, the repeated cyclic loading leads to:

1. Initiation of microscopic cracks (usually at stress concentration points),
2. Progressive crack propagation,
3. Final sudden fracture.

Thus, bottom-end bolts ultimately fail due to metal fatigue, even under normal operating conditions.

2. Design Features Incorporated to Inhibit Failure

To delay fatigue failure and increase service life, manufacturers incorporate several important design features.

(a) Increased Bolt Length

Bottom-end bolts are made as long as practicable.

• Greater length increases elasticity.
• The bolt behaves more like a spring.
• Stress is distributed over a larger length.
• Stress fluctuations are reduced.

This improves fatigue resistance.

(b) Reduced Shank Diameter (Waisted Bolt Design)

The shank diameter is made slightly smaller than the thread root diameter.

This ensures:

• Maximum stress occurs in the smooth shank instead of the threads.
• The smooth surface is less prone to crack initiation.
• Stress distribution is more uniform.
• The bolt can stretch elastically in a controlled manner.

(c) Generous Fillet Radius

A large rounded fillet is provided between the bolt head and shank.

This:

• Eliminates sharp corners,
• Reduces stress concentration,
• Minimizes crack initiation at critical junctions.

(d) Rolled Threads (Not Cut Threads)

Threads are produced by rolling rather than cutting.

This:

• Improves grain flow,
• Introduces compressive surface stresses,
• Produces rounded thread roots,
• Reduces stress concentration.

As a result, fatigue strength is significantly improved.

(e) High-Quality Alloy Steel

Bolts are manufactured from high tensile, fatigue-resistant alloy steel.

Such materials provide:

• High endurance strength,
• Good toughness,
• Resistance to crack propagation.

(f) High Surface Finish

Smooth surfaces reduce:

• Surface defects,
• Micro-notches,
• Stress raisers.

This delays fatigue crack initiation.

(g) Alignment Collars

Small collars or precision fits ensure proper alignment of the bolt within its hole.

This:

• Prevents shifting,
• Reduces secondary bending,
• Minimizes friction damage.

3. Effect of Maintenance on Bolt Failure

The service life of bottom-end bolts is heavily influenced by maintenance practices. Incorrect maintenance can drastically reduce fatigue life, while correct procedures can significantly extend it.

(A) How Maintenance Aggravates Failure

Failure tendency increases when:

• Bolts are over-tightened (causing plastic deformation),
• Bolts are under-tightened (leading to joint separation),
• Incorrect preload is applied,
• Tightening sequence is not followed,
• Specified lubricants are not used,
• Old or stretched bolts are reused,
• Improper tools damage threads,
• Bolts are hammered during fitting,
• Landing surfaces are dirty or uneven.

Incorrect preload is especially dangerous:

• Under-tightening increases stress fluctuation.
• Over-tightening reduces elastic range.
• Both conditions significantly reduce fatigue life.

(B) How Maintenance Inhibits Failure

Failure risk is reduced by:

• Strict adherence to manufacturer’s torque values,
• Tightening in correct sequence and stages,
• Using approved tightening methods such as:
  • Turn-of-nut method,
  • Hydraulic tensioning,
  • Specified torque procedures,
• Applying correct lubricant to threads and contact faces,
• Replacing bolts after specified running hours or whenever removed (as per maker’s instructions),
• Conducting regular Non-Destructive Testing (NDT),
• Ensuring proper seating surfaces.

Correct preloading ensures the bolt operates within its elastic limit and minimizes stress variation during operation.

4. Checks to Be Carried Out During Maintenance

During overhaul, the following inspections are essential:

(i) Visual Inspection

Check for:

• Corrosion,
• Surface cracks,
• Necking,
• Deformation,
• Thread damage.

(ii) Crack Detection (NDT)

Carry out:

• Magnetic Particle Inspection (MPI),
• Dye Penetrant Testing,
• Sound test (light hammer tap to detect internal cracks).

(iii) Length Measurement

• Measure bolt length.
• Compare with manufacturer’s specified limits.
• Detect permanent elongation (plastic stretch).

Any bolt exceeding allowable elongation must be renewed.

(iv) Thread Inspection

Inspect both:

• Bolt threads,
• Connecting rod threads.

Ensure they are:

• Clean,
• Undamaged,
• Free from burrs,
• Properly lubricated before assembly.