(a) A crankshaft is statically balanced if its centre of gravity lies on the polar axis of its journal.

When the crankshaft is placed on knife edges (pivot supports), it should remain stationary in any position without rotating. If it rotates and settles in a particular position, it indicates that the centre of gravity is offset from the polar axis. A statically balanced crankshaft ensures that the centre of gravity aligns with the centre of rotation.

Achieving Static Balance:

• The sum of all moments around the centre of rotation must be zero in any angular position.
• Counterweights are used to balance the moments to achieve this condition, ensuring smooth and stable operation.

(b) Dynamic balance involves the balancing of both unbalanced inertia forces and their moments in a rotating crankshaft system.

Even if a crankshaft is statically balanced, it may still experience imbalance during rotation due to inertia forces caused by rotating and reciprocating masses, such as the crank mechanism and connecting rods. These inertia forces generate vibrations, couples, and moments, which can impact the foundation and engine performance.

Achieving Dynamic Balance:

• Counterweights are mounted opposite to the crank throws to counteract the forces and minimize vibrations.
• Large main bearings are placed between crank throws to stabilise the crankshaft, reducing oscillations and optimising power delivery.

(c) Torque Reaction Couple

A torque reaction couple arises when a piston exerts lateral forces on the liner during the power stroke, generating opposing forces in the crankshaft and engine frame.

During the power stroke, the piston moves downwards and applies a lateral force to the liner to push the inclined connecting rod, creating a reaction couple.

This couple comprises two forces:

• One force acts on the engine frame opposite to the crankshaft's rotation.
• The second force is proportional to the piston force.

In a perfectly balanced engine, the reaction couple remains constant because all pistons exert equal forces. If one piston exerts a different force, it causes an imbalance in the reaction torque, leading to vibrations that are more pronounced at certain speeds.