Nature of Forces Acting on a Main Engine Crankshaft

The crankshaft is the component that converts the reciprocating motion of the piston into rotary motion. In service, it is subjected to the following forces:

1. Gas Forces

• Arising from compression of air and combustion of fuel.
• At TDC: Gas pressure acts downward through the piston. With the crankshaft supported at both ends, it behaves like a beam. The upper half of the crankpin is in compression, while the lower half is in tension.
• At BDC: The stresses are reversed; compression becomes tension and vice versa.
• Hence, the stresses are cyclic in nature, leading to alternating bending stresses.
• Gas forces can be resolved into:
  • Radial component – causes bending and twisting of crankpin and webs.
  • Tangential component – causes bending of webs and torsional stress in journals due to torque transmission.

  2. Inertia Forces

  • Due to rotating and reciprocating masses.
  • For rotating masses, inertia forces are constant in magnitude but change direction with rotation.
  • For reciprocating masses, inertia forces vary with piston position, even at constant speed.

  3. Torsional Stresses

  • Caused by alternating twisting moments due to torque fluctuations.
  • Can lead to dangerous resonance if critical speeds are encountered.

  4. Axial Stresses

  • Arise from repeated flexing of webs and propeller thrust reaction.
  • Cause lengthening and shortening of the shaft, adding cyclic axial loading.

  5. Shear Forces

  • Due to varying torque transmission and resistance offered by the propeller.

  Maintenance of Stresses within Safe Limits

  By Design:

  • Use of high tensile strength and ductile materials with good fatigue resistance.
  • Forged construction ensures continuous grain flow and eliminates weak points.
  • Surfaces of crankpins and journals are hardened for wear resistance.
  • Avoidance of stress raisers by using smooth transitions (fillets) instead of sharp changes, and avoiding dowel pins/keys.
  • Provision of axial and torsional vibration dampers to counter cyclic stresses.
  • Materials selected for wear and corrosion resistance.

  By Efficient Maintenance:

  • Avoid prolonged operation in the barred speed range.
  • Avoid thermal overloading and engine overload.
  • Conduct regular overhauls to ensure correct power balance.
  • Routine checks:
    • Crankshaft deflections to detect misalignment.
    • Pmax monitoring to verify combustion efficiency.
    • Vibration damper condition.
    • Tightening of tie bolts and foundation bolts.
  • Maintain proper lubrication and bearing alignment.
  • Gradual application of load to avoid sudden stress rise.

  Circumstances Leading to Crankshaft Overstressing

  • Improper combustion (e.g., faulty injection or valve timing).
  • Operation at critical speeds causing resonance.
  • Prolonged running in barred speed range.
  • Unequal wear between adjacent main bearings.
  • Misalignment of crankshaft or bearings.
  • Running engine with one unit misfiring or cut out.
  • Heavy weather conditions causing engine hunting or fluctuating load.
  • Increased resistance due to fouled hull or propeller.
  • Excessive crankshaft deflection.
  • Defective/incorrect VIT action leading to excessive Pmax.
  • High torsional or axial vibrations.

  Crankshaft Becoming Defective Without Being Overstressed

  • Fatigue Failure – main mechanism due to cyclic reversal of stresses, even within design limits.
  • Cracks initiate at high stress locations (fillets, journals) and propagate with time.
  • Material Defects – sub-surface flaws or improper forging may lead to crack initiation.
  • Poor lubrication – results in wear, heating, and surface damage.
  • Overheating – causes surface cracks that propagate under repeated stress cycles.