Drive Shaft Failure of Z-Drive

Propulsion System

Suzanne Higgins

Background

• Bunkering vessel “MV Southern Valour”

• Commissioned in 2008 in Singapore

• Operating in Cape Town harbour since August 2008

Z-Drive System

• Bunker barge powered by z-type propulsion system

• Power transmitted to bevel gears of above-water gear box

• Horizontal gears redirect power vertically to power

transmission shaft

• Drives bevel gear of underwater gear box

• Power redirected to horizontal propeller shaft

Z-Drive System

• Underwater gear box attached to steering tube

• Worm wheel mounted on steering tube

• Worm shaft engages with worm wheel

• Steering motor attached to worm-geared steering drive

• Two propellers

Failure

• Catastrophic failure of

drive shaft in splined area

• 2400 hours of service

• One Eighty appointed to

determine cause of failure

Materials Testing

• Need three tests to fully classify steel

• Spectrographic analysis: Low alloy

carbon steel AISI 4340

• Microstructure: Quench and

tempered; no anomalies.

• Hardness: HV 300

• Material properties used to determine

ultimate tensile strength for fatigue

calculations

Fracture surfaces

• Original fracture surface

• Characterised by beach marks

• Indicative of fatigue failure

• Angle of fracture surface

indicates failure due to bending

stress

• Fractured further on cutting

Fracture surfaces

• Fracture surface characterised

by beach marks

• Indicative of fatigue failure

• Angle of fracture surface

indicates failure due to

tangential stress

• Crack along root of spline

• Width of spline inconsistent

Fracture surfaces

• Fracture of remaining piece of shaft

on cutting

• Fatigue failure due to torsional stress

Fracture surfaces

• Single area of initiation

• A: Bending stress – Normal operation

• B: Tangential stress – Under normal

operating conditions, spline acts as a

uniform shaft. In this case, due to

inconsistent spline width, it acts as a

gear i.e. The force of the mating part

worked to open up the spline. Only

occurs due to deformation

• C: Torsional stress – Normal operation

Fatigue

• Fatigue is slow and insidious propagation of cracks under cyclic

loading.

• Two requirements: stress and cycles.

• Low cycle fatigue typically 103/4 cycles.

• High cycle fatigue typically >106 cycles

• Shaft rotates and each rotation represents one stress cycle.

• Number of cycles of drive shaft calculated as:

Cycles = Input rpm x Gear ratio x (Service hours)/60

= 1.88 x 107 cycles

• Category of high cycle fatigue

Fatigue • Endurance limit is the stress limit below which fatigue will not occur for

infinitely may cycles.

Stress limits

• For high cycle fatigue, endurance limit is 20-50% of UTS

• Material properties used to find UTS

• Endurance limit further modified to take into account various factors:

• Surface factor

• Size factor

• Stress concentration factor

• Temperature factor

Stress limits

Operating Condition Failure stress

Ultimate Tensile Strength 1000 MPa

Endurance limit 200 –500 MPa

Endurance limit - modified 54 –134 MPa

Bending moment calcs

• Need to determine bending moment at point of failure.

• Two known forces calculated from torque using power and speed

data.

• Distances a,b,c,d measured on site.

• Three unknown reaction forces at bearings

Bending moment calcs

• Two equations

• Sum of forces: F1+F2=R1+R2+R3

• Moment equilibrium: F2a+R2b+R3(b+c)-F1(b+c+d)=0

• Statically indeterminate system of degree 1

• Method of superposition – The combined effect of the loads is the sum

of the loads acting separately.

• Compatibility equation – Sum of deflections is zero at reaction force

• Designate one of reactions as redundant force (R2)

R2

=

+

y

y’

Bending moment calcs

• Step 1:

• Choose R2 as redundant force and remove from system

• Two unknowns and two equations

• Determinate system

• Solve for deflection at R2 position due to applied loading (y)

Bending moment calcs

• Step 2:

• Remove applied forces F1 and F2

• Solve for deflection at R2 position due to redundant force (y’)

• Two equations, three unknowns

• Deflection expression in terms of R2

R2

Bending moment calcs

• Step 3:

• Deflection at R2 is sum of the deflection due to applied

loading and deflection due to redundant force.

• Deflection at R2 = 0

• Use compatibility equation to solve for R2:

y+y’ = 0

Bending moment calcs

• Step 4:

• Original bending moment diagram

• Only two unknowns R1, R3

• Determinate system

• Resolve all forces

• Step 5:

• Calculate bending moment at point of failure

Deflection calculations

M/I = σ/y

• Bending moment at point of failure known.

• Second moment of Inertia calculated from spline diameter:

I = π(do4 – di

4))/64

• Know minimum stress required for fatigue failure (modified

endurance limit)

• Can either substitute maximum deflection values to determine if

stress levels are high enough for failure.

• Do not know maximum deflection value.

Deflection calculations

• Calculate deflection required for failure at each stress level

and determine if it is reasonable.

Stress type (Mpa) Deflection required (cm)

Yield stress 13.63

Endurance limit 2.90

Modified endurance limit 0.45

• Damaged spline leads to fatigue failure.

Visual inspection

• No signs excessive wear on

bearings or other parts

Visual inspection • Large dent of Kort nozzle (houses

propeller)

Visual inspection

• Using typical material properties, calculated that

force in order of 20kN is required to cause

deformation.

• Given the vessel’s tonnage and speed, feasible

that this could be a result of a collision during

operation.

Possible Sources of Fatigue

Fatigue source Consequence Evidence found

Cavitation Vibration on the drive assembly No pitting on propeller

surface

Misalignment Bending moment causing wear on

bearing

No excessive wear on

bearings

High stress event Localised damage on shaft creating

initiation sites for fatigue under normal

operating conditions

Damage on Kort nozzle

Conclusions

• Shaft has multiple fracture surfaces characterised by beach

marks indicating failure by fatigue.

• Single initiation site indicating high stress event, such as knock

to the Kort nozzle.

• High stress event resulted in deformation of spline, causing it to

act as a gear.

• Tangential force acts to open up spline.

• Likely that this occurred first.

• Opening of this crack increased stress concentrations.

Conclusions

• Fatigue cracks initiated under bending and torsional stress,

present under normal operating conditions.

• As cracks propagated, load transferred to intact areas,

increasing stress.

• Stress becomes greater than the UTS of the material.

• Remaining areas fail by fast fracture.

THANK YOU!