Short Answer:

Dynamic balancing is the process of balancing a rotating or moving body by ensuring that there is no unbalanced force or couple acting on it when it rotates. It is achieved by adjusting the mass distribution so that both the centrifugal forces and couples due to rotation are balanced in all planes of motion.

In simple words, dynamic balancing ensures smooth and vibration-free operation of rotating parts like shafts, rotors, turbines, and wheels. It improves machine life, reduces noise, and prevents wear of bearings caused by unbalanced rotating masses.

Detailed Explanation :

Dynamic Balancing

Dynamic balancing refers to the condition when a rotating system or body is in complete balance while it is in motion. It means that the net dynamic forces and moments acting on the system during rotation are zero. This type of balancing is very important for machines having parts that rotate at high speeds, such as engines, turbines, rotors, flywheels, and propellers.

When any rotating body has uneven mass distribution, the center of mass does not coincide with the axis of rotation. This causes centrifugal forces that produce vibrations, bending stresses, and noise during rotation. Dynamic balancing corrects these unbalances by properly distributing or adding small correction weights so that the resultant centrifugal forces and moments cancel out.

Dynamic balancing ensures that the machine runs smoothly without any lateral vibration or wobbling and that the load on bearings is uniformly distributed.

1. Principle of Dynamic Balancing

Dynamic balancing is based on the principle of centrifugal force. When a rotating mass moves in a circular path, it experiences a centrifugal force acting radially outward. The magnitude of this force is given by:

where,

•  = Centrifugal force (N)
•  = Mass of the rotating particle (kg)
•  = Radius of rotation (m)
•  = Angular velocity (rad/s)

If several masses are rotating in different planes, the resultant of all centrifugal forces and the resultant couple due to these forces must be zero for complete dynamic balancing.

Thus, the conditions for dynamic balancing are:

1. The vector sum of all centrifugal forces = 0
2. The vector sum of all couples due to these forces = 0

When these two conditions are satisfied, the body is said to be dynamically balanced.

2. Difference Between Static and Dynamic Balancing

• Static balancing deals with balancing the body at rest, ensuring that its center of gravity lies on the axis of rotation. It eliminates unbalanced forces but not unbalanced couples.
• Dynamic balancing, on the other hand, ensures that both unbalanced forces and couples are eliminated while the body is in motion. It considers mass distribution along multiple planes of rotation.

Therefore, dynamic balancing is an advanced form of balancing that provides complete balance in rotating machinery.

3. Process of Dynamic Balancing

The process of dynamic balancing involves the following steps:

1. Detection of Unbalance:
   The rotating part is mounted on a balancing machine. Sensors detect unbalanced forces and vibrations during rotation.
2. Measurement of Unbalance:
   The amount and position of the unbalance are measured in terms of mass and angular position in each plane of rotation.
3. Calculation of Correction Mass:
   Using vector and graphical methods, the required balancing weights and their positions are calculated to counteract the unbalanced forces and moments.
4. Correction of Unbalance:
   Balancing weights are added or material is removed from specific points on the rotating part to achieve balance.
5. Verification:
   The rotor is rechecked by running again to confirm that the unbalance has been minimized or eliminated.

This process is performed using dynamic balancing machines that operate on mechanical or electronic principles.

4. Dynamic Balancing of Several Rotating Masses

When several masses  rotate in different planes at the same angular velocity , their individual centrifugal forces act at different angular positions.
The vector sum of these forces and the couples formed by their forces and distances from a reference plane must both be zero for dynamic balance:

where,  is the distance between the planes of rotation.

By using vector diagrams or analytical methods, the magnitude and position of correction masses are determined to ensure complete dynamic balance.

5. Importance of Dynamic Balancing

Dynamic balancing is extremely important for the smooth operation and long life of rotating machinery. Its major advantages include:

1. Reduction in Vibration:
   It minimizes vibrations caused by centrifugal forces and couples, ensuring smooth running.
2. Increased Bearing Life:
   Proper balancing prevents uneven bearing load and wear, extending the life of bearings.
3. Improved Efficiency:
   Machines run more efficiently with reduced energy loss due to vibration and friction.
4. Noise Reduction:
   Eliminates mechanical noise caused by unbalanced rotation.
5. Safety:
   Prevents structural failures due to excessive vibration and dynamic stresses.
6. Higher Speed Operation:
   Dynamically balanced rotors can safely operate at high rotational speeds without instability.

6. Practical Applications of Dynamic Balancing

Dynamic balancing is widely applied in various mechanical and industrial systems, such as:

• Automobile engines: To balance crankshafts and connecting rods for smooth operation.
• Turbines and compressors: To prevent vibration and bearing damage during high-speed operation.
• Fans and blowers: To maintain stable air delivery and minimize noise.
• Flywheels and pulleys: To ensure uniform rotation without wobbling.
• Electric motors and rotors: To achieve smooth, vibration-free performance.
• Aerospace components: To balance aircraft propellers and jet engine rotors for stability.

7. Methods of Dynamic Balancing

1. Graphical Method (Force and Couple Polygon):
   The forces and couples are represented as vectors, and polygons are drawn to find the balancing masses graphically.
2. Analytical Method:
   Mathematical equations are used to calculate the exact magnitude and angular positions of the balancing masses.
3. Experimental Method:
   Balancing machines with sensors and electronic detectors automatically identify unbalanced positions and suggest corrections.

Conclusion:

Dynamic balancing is the process of correcting mass distribution in rotating bodies so that no unbalanced forces or moments act during rotation. It ensures that the resultant centrifugal forces and couples are zero, leading to smooth, stable, and efficient operation. This process is essential in all high-speed rotating machinery such as turbines, engines, and rotors to reduce vibration, prevent mechanical wear, and ensure long service life. Proper dynamic balancing enhances both the performance and safety of mechanical systems.