What is Rotational Bending Fatigue?

Rotational bending fatigue occurs when a pump shaft is subjected to repeated cyclic stresses as it rotates. These stresses arise from imbalances, misalignments, or uneven loading, causing the shaft to bend slightly with each rotation. Over time, this cyclic bending leads to the development of microscopic cracks, which can propagate and eventually result in shaft failure.

In centrifugal pumps, the shaft is constantly under stress due to the combination of rotational forces, hydraulic loads, and mechanical loads from connected components like impellers and couplings. When these stresses exceed the material's endurance limit, fatigue failure becomes inevitable.

Causes of Rotational Bending Fatigue

Several factors can contribute to rotational bending fatigue in centrifugal pumps:

1. Imbalance: An unbalanced impeller or rotor creates uneven centrifugal forces, leading to cyclic bending stresses on the shaft.
2. Misalignment: Improper alignment between the pump and motor can induce bending moments, especially at high speeds.
3. Hydraulic Forces: Uneven flow distribution or cavitation can cause fluctuating hydraulic loads, adding to the cyclic stress.
4. Resonance: Operating near the pump's critical speed can amplify vibrations, increasing the risk of fatigue.
5. Material Defects: Inherent flaws in the shaft material can act as stress concentrators, accelerating crack initiation.

Why Balancing Matters

Balancing is the process of aligning the mass distribution of a rotating component, such as an impeller or rotor, with its axis of rotation.

When a fan or compressor is unbalanced, it can lead to excessive vibrations, increased wear and tear, and even catastrophic failures. Here’s why balancing is so important:

1. Reduced Vibrations: Unbalanced rotating components create centrifugal forces that cause vibrations. These           vibrations can damage bearings, seals, and other critical parts, leading to premature failure.
2. Energy Efficiency: Unbalance forces the motor to work harder, increasing energy consumption and operational       costs.
3. Noise Reduction: Excessive vibrations often result in noisy operation, which can be disruptive and indicate underlying       mechanical issues.
4. Extended Equipment Life: Proper balancing minimizes stress on components, extending the lifespan of the fan or compressor.
5. Safety: Severe unbalance can lead to sudden failures, posing safety risks to personnel and equipment

International Balancing Standards

To ensure consistency and reliability, balancing is governed by international standards. The most widely recognized standards are set by the International Organization for Standardization (ISO), particularly ISO 1940-1 for balancing rotating machinery. These standards provide guidelines for acceptable levels of residual unbalance based on the machine’s size, speed, and application.