(a) Need for a Moment Compensator in Large Two-Stroke Marine Engines

Large slow-speed two-stroke marine engines have very long strokes, which produce high inertia forces due to rapid acceleration and deceleration of the pistons.

The most problematic among these are 2nd-order vertical inertia forces, which vary with the square of engine speed. These forces:

• Create strong vertical shaking moments,
• Can excite the ship’s hull at its natural frequency,
• Lead to excessive hull vibration, noise, and structural fatigue.

A moment compensator is therefore installed to cancel out these secondary vertical forces. By producing equal and opposite inertial forces, it protects both the engine and the hull from harmful vibration and ensures smoother, safer operation.

Construction of a Moment Compensator

A typical moment compensator consists of:

• Two heavy rotating masses (elliptical or circular),
• Mounted inside a rigid casing,
• Driven by a gear train from the engine crankshaft,
• Rotating at twice the engine speed (2N),
• Rotating in opposite directions.

The masses and their phasing are precisely calculated so that their generated inertial forces match the magnitude and timing of the engine’s 2nd-order forces.

Working Principle

1. The reciprocating masses (pistons and rods) generate vertical unbalanced 2nd-order inertia forces.
2. The two compensator masses rotate at 2 × crankshaft speed, producing centrifugal forces of the same order.
3. Because the masses rotate in opposite directions, their horizontal components cancel, while the vertical components combine.
4. These vertical forces are timed such that:
   • When the engine’s inertia force is maximum upwards, the compensator generates a maximum downward force.
   • When the engine force is downwards, the compensator force is upwards.

Thus, the compensator dynamically balances the engine’s vertical inertia forces, preventing their transmission to the hull.

(b) Consequences of Failure or Incorrect Operation

Failure or incorrect functioning of the moment compensator can result in:

1. Severe Hull Vibration
   • Increased noise and crew discomfort.
   • Vibration of accommodation areas and decks.
2. Structural Fatigue
   • Repeated cyclic loading causing cracks in hull plating, bulkheads, and structural members.
3. Engine Damage
   • Excessive shaking loads on crankshaft, main bearings, and thrust bearings.
   • Potential misalignment of the main engine.
4. Foundation and Mounting Issues
   • Loosening of holding-down bolts,
   • Damage to engine seating and chocks.

Recommended Checks and Maintenance Practices

To ensure reliability of the moment compensator:

1. Lubrication Checks
   • Ensure reliable oil supply to gears and bearings.
   • Check for oil leaks and maintain correct oil levels.
2. Vibration Monitoring
   • Analyze vibration trends and PRU values.
   • Rising vibration levels often indicate incorrect timing or bearing wear.
3. Balance and Timing Checks
   • Ensure counterweights are correctly phased.
   • Confirm gear backlash and timing marks during overhauls.
4. Visual and Mechanical Inspection
   • Check gear teeth for wear or pitting.
   • Inspect bearings for clearance and temperature abnormalities.
   • Verify the integrity of the casing and mounting bolts.
5. Oil Analysis
   • Test for metal particles or wear debris from gears and bearings.
6. Alignment Checks
   • Ensure correct alignment between the compensator drive gears and crankshaft drives.