What is primary and secondary balancing?

Short Answer:

Primary and secondary balancing are two types of balancing used in reciprocating engines to reduce vibrations caused by moving parts. Primary balancing deals with the unbalanced forces produced by the reciprocating masses in one revolution of the crankshaft. Secondary balancing, on the other hand, occurs due to the non-uniform motion of the piston, which produces forces at twice the crankshaft speed. Both types of balancing are essential to ensure smooth engine operation and to prevent wear and tear on components.

In simple terms, primary balancing corrects the main or direct unbalanced forces due to the crank and piston movement, while secondary balancing corrects the additional unbalanced forces that occur due to the irregular motion of these parts. Achieving both types of balance ensures stable running, reduces engine noise, and increases engine life and performance.

Detailed Explanation :

Primary and Secondary Balancing

In any reciprocating engine, parts like the piston, connecting rod, and crankshaft move rapidly during operation. These moving parts produce forces that act on the engine bearings and frame. If these forces are not properly balanced, they cause vibrations, noise, and mechanical stress, which can reduce the efficiency and durability of the engine. To overcome this, engineers use a method called balancing, which ensures that the forces generated by moving parts cancel each other out as much as possible. There are mainly two types of balancing in an engine — primary balancing and secondary balancing.

Primary Balancing

Primary balancing refers to the balancing of forces that occur once in every revolution of the crankshaft. When the crank rotates, the piston moves up and down, creating an inertia force due to its acceleration and deceleration. This force is directly proportional to the mass of the reciprocating parts and the angular speed of the crankshaft.

The inertia force can be expressed as:
F = m × r × ω² × cos(θ)
where:

  • m = mass of reciprocating parts,
  • r = crank radius,
  • ω = angular velocity of the crank,
  • θ = crank angle.

This force acts along the line of stroke and varies as the crank rotates. Because it changes direction every half revolution, it produces vibrations in the engine. To counteract this, a balancing mass (counterweight) is added on the opposite side of the crankshaft to balance the centrifugal force of the rotating parts. When this adjustment reduces or cancels the primary forces, the engine is said to be primarily balanced.

Engines with multiple cylinders can be designed so that the forces in one cylinder cancel those in another. For example, in a four-cylinder inline engine, pistons move in opposite directions to balance the forces generated by each other.

Secondary Balancing

Even when primary forces are balanced, some vibration may still remain due to secondary forces. These forces occur twice during each revolution of the crankshaft. The reason is that the connecting rod is not infinitely long, and hence, the piston does not move with perfect harmonic motion. Because of this, the piston travels faster during one half of the stroke and slower during the other half, which produces additional unbalanced forces known as secondary forces.

The expression for secondary force is given by:
F = m × r × ω² × (cos(2θ) / n)
where n is the ratio of the length of the connecting rod to the crank radius.

Secondary forces are smaller than primary forces but still significant at high speeds. They act at twice the engine speed, so balancing them becomes more challenging. In multi-cylinder engines, proper arrangement of crank angles helps reduce secondary forces. For example, in a four-cylinder inline engine, pistons 1 and 4 move together, and 2 and 3 move together, helping to cancel out secondary effects.

Some advanced engines, such as inline-four and inline-six designs, use balance shafts to reduce the secondary vibrations. These shafts rotate in opposite directions to the crankshaft at twice its speed, producing equal and opposite forces to cancel the vibrations.

Importance of Primary and Secondary Balancing

Balancing both primary and secondary forces is crucial for the following reasons:

  1. Reduced Vibration: Prevents excessive shaking and improves passenger comfort in vehicles.
  2. Longer Engine Life: Reduces wear on bearings, crankshaft, and other moving parts.
  3. Improved Performance: Allows smooth operation at high speeds and increases engine efficiency.
  4. Reduced Noise: Balanced engines operate quietly with less mechanical noise.
  5. Lower Maintenance: Fewer vibrations mean fewer chances of mechanical loosening or failure.

In high-performance engines, achieving perfect balance is very important. Engineers use advanced analysis and balancing machines to ensure all rotating and reciprocating parts work smoothly.

Conclusion :

Primary and secondary balancing are key techniques used in reciprocating engines to minimize vibrations and ensure smooth engine operation. Primary balancing deals with the main forces occurring once per crankshaft revolution, while secondary balancing handles the smaller but higher-frequency forces that appear twice per revolution. Proper balancing leads to a stable, efficient, and durable engine, improving performance and reducing mechanical failures.