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The drive shaft on a bunkering vessel failed catastrophically after 2400 hours of service. The bunker barge is powered by a z-type propulsion system (Figure 1). Power is transmitted from the power input flange to the bevel gears of the above-water gearbox. These horizontal gears then redirect the power to flow vertically to the power transmission shaft. This shaft drives the bevel gear of the underwater gearbox so that the power is redirected to flow horizontally to the propeller shaft, on which the propeller is mounted.

Figure 1: Z-type propulsion system
  1. Above-water gearbox
  2. Bevel gears
  3. Input flange
  4. Worm wheel
  5. Supporting tube
  6. Steering tube
  7. Power transmission shaft
  8. Underwater gearbox
  9. Bevel gears
  10. Propeller shaft
  11. Propeller housed in Kort nozzle
  12. Worm shaft
  13. Steering drive
  14. Steering motor


An investigation was required to determine the root cause of the failure of the shaft.


The fracture surface shows beach marks, indicative of a fatigue failure (Figure 2). The angle of the fracture surface suggests failure due to bending stress.

Figure 3: Secondary fracture surface
Figure 3: Secondary fracture surface
Figure 2: Shaft fracture surface
Figure 2: Shaft fracture surface

During cutting, the shaft suffered an additional fracture at a 45° angle, characterised by beach marks on the fracture surface, indicative of torsional fatigue failure (Figure 3). The width of the spline was found to be inconsistent along the length of the specimen, narrowing towards the fracture surface. A crack extending along the root of the spline is evidence of stress in the tangential direction


Compositional analysis and hardness testing show the material to match most closely to quench and tempered AISI 4340 steel. This was used to determine the mechanical properties to be used in the fatigue calculations.

The fractured pieces of the shaft, directions of stresses leading to fatigue, and the area of fatigue initiation, are shown in the Figure 4 and described in Table 1.

Figure 4: Crack path
Figure 4: Crack path

Table 1: Stress types

Designation Stress type Stress origin
A Bending stress Stress due to bending under normal operating conditions
B Tangential stress Stress resulting from the spline acting as a gear rather than a uniform shaft i.e. The force of the mating part worked to open up the spline. This would only occur if the spline was deformed.
C Torsional stress Stress due to torque under normal operating conditions


No signs of excessive wear were found on the bearings or any other related parts. A large dent was found on the Kort nozzle, which houses the propeller (Figure 5).

Figure 5: Dent on Kort nozzle
Figure 5: Dent on Kort nozzle

In order to cause the level of deformation seen, a force in the order of 20kN (2.04 metric tons) is required. It is feasible that this force could occur as a result of a collision.

Based on the running hours and the given input speed, the number of cycles fall into the category of high cycle fatigue (>106 cycles). The surface, size, and stress concentration factors result in a modified endurance limit of 54 – 134 MPa.

The bending, tangential and torsional fatigue failures originated in a common location on the same side of the spline. The shaft rotates and thus any imbalance in the system should result in multiple fatigue initiation sites around the circumference of the spline. Indications are that a high stress event occurred in the form of a knock to the kort nozzle, which resulted in localised damage to the spline. This resulted in areas of stress concentration, which lowered the modified endurance limit into the range of the operating stress.  Therefore the normal operating stress became sufficient for fatigue cracks to initiate from the stress concentration sites.

Under normal operating conditions, the spline would act as a uniform shaft. However, if the spline was sufficiently deformed, it would behave as a gear and the tangential force would act to open up the spline. There is evidence of this process in the cracking of the spline. Under normal operating conditions, the shaft is subject to both bending and torsional stress. Thus it is reasonable that fatigue could cause fracture in the directions of these stresses.

It is likely that the tangential fracture occurred first. The opening up of this crack created increased stress concentrations, allowing fatigue cracks to initiate by torsional and bending stress.

As the cracks propagated, the load was transferred to the intact areas, thereby increasing the stress. When the stress became greater than the ultimate tensile strength of the material, the remaining areas failed by fast fracture.


  • Failure is indicative of fatigue in the tangential, bending and torsional directions.
  • A high stress event in the form of a knock to the Kort nozzle would have caused sufficient damage to the spline to initiate fatigue.
  • The damage to the spline resulted in areas of stress concentration sufficient to modify the endurance limit to such an extent that it fell within the range of operating stress. This caused the fatigue cracking to initiate under tangential, bending and torsional stress. Crack propagation eventually led to failure by fast fracture.
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