Short Answer:

Unbalanced primary forces are the forces produced in a reciprocating engine due to the primary or first-order motion of reciprocating masses. These forces act along the line of stroke and vary with the crank angle. They are caused by the inertia of reciprocating parts such as the piston, piston pin, and part of the connecting rod. When these forces are not balanced properly, they create vibration and shaking in the engine.

Unbalanced primary forces occur once during every revolution of the crankshaft. If not balanced, they result in uneven engine running, wear of parts, and uncomfortable operation. Hence, balancing of primary forces is very important in engines to ensure smooth working and longer machine life.

Detailed Explanation :

Unbalanced Primary Forces

In reciprocating engines and machines, the moving parts continuously accelerate and decelerate during operation. Due to this motion, they develop inertia forces which act in the opposite direction of their acceleration. These inertia forces are called unbalanced forces when they are not properly balanced by counterweights or by other moving parts in the system.

Among these, the unbalanced primary forces are the most important because they occur due to the primary component of acceleration of the reciprocating mass. These forces cause vibration and unsteady operation of the engine, especially at higher speeds. Understanding and controlling unbalanced primary forces are essential for designing stable and smooth-running machines.

1. Meaning of Unbalanced Primary Forces

When a reciprocating part like a piston moves inside a cylinder, its motion can be divided into two components — a primary motion and a secondary motion. The primary motion corresponds to one complete crank revolution, while the secondary motion occurs twice during each revolution. The force produced due to the primary motion of reciprocating mass is called the primary force.

If this force is not properly balanced by another equal and opposite force, it becomes an unbalanced primary force. These forces act along the line of stroke and vary in magnitude and direction as the crankshaft rotates.

Mathematically, the unbalanced primary force can be expressed as:

where,
= primary unbalanced force,
= reciprocating mass,
= angular speed of the crank,
= crank radius,
= crank angle from inner dead center.

From this relation, we can see that the unbalanced primary force depends directly on the reciprocating mass, crank radius, and speed of rotation.

2. Cause of Unbalanced Primary Forces

The main cause of unbalanced primary forces is the inertia of reciprocating parts in the engine. During every revolution, the piston must stop and change its direction twice — at the top dead center (TDC) and bottom dead center (BDC). To achieve this motion, it must accelerate and decelerate continuously, producing varying inertia forces.

These inertia forces act along the line of stroke and are transmitted to the engine frame, creating vibration. Unless another part of the machine produces an equal and opposite force, the resultant becomes an unbalanced primary force.

In single-cylinder engines, this effect is particularly noticeable because there is no other cylinder to balance the motion. In multi-cylinder engines, the arrangement of cylinders is done in such a way that the forces from one cylinder partially or completely cancel those from others.

3. Characteristics of Unbalanced Primary Forces

Unbalanced primary forces have the following key characteristics:

1. They act along the line of stroke of the reciprocating part.
2. Their magnitude varies with the cosine of the crank angle.
3. They complete one full cycle during each revolution of the crankshaft.
4. They are proportional to the mass, crank radius, and the square of angular velocity.
5. They cause vibration, noise, and shaking in the engine structure.

Because these forces act at a frequency equal to the rotational speed of the crankshaft, they are sometimes referred to as first-order forces.

4. Effects of Unbalanced Primary Forces

If unbalanced primary forces are not controlled, they can produce several harmful effects on the performance and structure of the machine:

• Vibration of the Engine: The most common effect is vibration, which can lead to uncomfortable operation and reduced efficiency.
• Wear and Tear of Bearings: The continuous reversal of forces increases bearing load and wear.
• Noise Generation: The uneven motion creates mechanical noise, especially at high speeds.
• Damage to Foundation: In large machines, unbalanced forces can cause shaking of the base or foundation.
• Reduced Machine Life: Prolonged unbalanced operation reduces the overall life of the engine and its parts.

Therefore, it is very important to minimize or balance these forces as much as possible for smoother operation.

5. Balancing of Unbalanced Primary Forces

To reduce or eliminate the effect of unbalanced primary forces, engineers use counterweights or design multi-cylinder arrangements in such a way that the forces oppose each other.

• In a single-cylinder engine, a part of the reciprocating mass is balanced by adding a counterweight to the crankshaft. This creates a centrifugal force that opposes the primary unbalanced force. However, only partial balancing is possible because complete balancing would cause other unwanted forces.
• In multi-cylinder engines, the crank angles and cylinder positions are arranged so that the primary forces from different cylinders act in opposite directions, effectively canceling each other. For example, in a four-cylinder inline engine, cylinders 1 and 4 move opposite to cylinders 2 and 3, reducing unbalanced forces.

6. Practical Example

Consider a single-cylinder vertical engine. When the piston moves upward, the inertia of the reciprocating parts creates a downward unbalanced primary force on the engine frame. When the piston moves downward, the direction of this force reverses. This continuous reversal produces vibration in the vertical direction. Adding a counterweight helps reduce the unbalanced force but cannot eliminate it fully.

In railway locomotives and compressors, proper arrangement and balancing are critical because unbalanced forces can cause shaking of the entire system.

7. Importance in Design

During engine design, engineers always calculate the expected magnitude of primary forces. They use balancing techniques to minimize them while maintaining engine strength and stability. The design of crankshafts, connecting rods, and even engine mounts takes unbalanced forces into account to ensure reliability.

Conclusion

Unbalanced primary forces are caused by the inertia of reciprocating parts during the primary motion of the crankshaft. They act along the line of stroke and occur once in each revolution. If not properly balanced, these forces produce vibration, noise, and mechanical stress. By using partial or complete balancing methods such as counterweights or multi-cylinder arrangements, these forces can be minimized to improve machine efficiency and smoothness. Proper understanding and control of unbalanced primary forces are essential in mechanical and engine design.