A crankshaft in a two-stroke engine is subjected to several types of stresses during normal operation. These stresses arise due to gas pressure, inertia forces, centrifugal forces, and torsional effects, and they influence the strength, durability, and fatigue life of the crankshaft.

Types of Stresses Acting on the Crankshaft

1. Torsional (Shear) Stresses: The turning moment produced by gas pressure acting on the piston causes torsional shear stresses in the crankpin, webs, and journals. These stresses may be further increased by torsional vibrations, particularly when the engine operates at or near resonant speeds.
2. Bending Stresses: The crankshaft is subjected to bending moments between the main bearing supports due to gas forces and inertia forces of the reciprocating parts. These bending moments cause alternating tensile and compressive stresses in the crankshaft as it rotates.
3. Centrifugal Stresses: Centrifugal forces generated by rotating masses produce:
   • Tensile stresses in the crank webs,
   • Shear stresses in the crankpin and journals, and
   • Additional bending stresses in the journals.

Generation and Variation of Stresses During One Cycle

• Inertia forces act on both rotating and reciprocating masses. For a constant engine speed, inertia forces on rotating masses remain constant in magnitude.
• Gas forces acting on the crankshaft cause alternating bending of the crank webs and crankpin. During the upper part of the crank travel, the webs tend to spread, while during the lower part they tend to close.
• Gas forces can be resolved into two components:
  • Radial component: Causes bending and twisting of the crankpin.
  • Tangential component: Causes bending of the crank webs and tensile stress in the journals due to torque transmission.
• In a two-stroke engine, the gas force is minimum at bottom dead centre (BDC), reaches a maximum at top dead centre (TDC), and then reduces again to a minimum at BDC.
• Repeated flexing of the crank webs, combined with the reaction to propeller thrust, causes alternate lengthening and shortening of the crankshaft, resulting in axial thrust stresses.

Accommodation of Stresses by Good Design and Operation

• Material selection: The crankshaft material should possess high fatigue strength, good resistance to wear and corrosion, and reliable performance under all operating conditions.
• Grain flow control: Continuous and favourable grain flow improves the strength and fatigue resistance of the material.
• Manufacturing process: Forging is preferred as it produces a sound workpiece free from sub-surface defects.
• Design geometry: Sharp changes in cross-section are avoided to prevent stress concentration points.
• Balancing: Proper balancing of the engine’s rotating and reciprocating masses reduces dynamic stresses.
• Vibration control: Installation of vibration dampers and detuners helps control torsional vibrations and associated stresses.