What it is
Unbalance is one of modern industrial machinery's most recognizable, pervasive, and fixable issues. It occurs when the center of mass of a rotating component (such as a rotor or impeller) is not aligned with its axis of rotation, i.e. the rotational center of gravity no longer aligns with the rotational center of mass. This misalignment causes uneven distribution of mass, resulting in vibration and potential damage to the machinery. Unbalance can lead to various operational problems, including reduced equipment lifespan, increased maintenance costs, and safety hazards.
Why it Happens
As machinery operates over time, rotating elements that are tight will loosen. This allows more movement to occur, creating more looseness, and becomes a repeating, self-destructive cycle. Process-based buildup on rotors and material degradation (typically from foreign debris or prolonged cavitation) make up the majority of unbalance problems.
Types of Unbalance
Unbalance can be classified into three main types: static, couple, and dynamic.
- Static Unbalance:
- Static unbalance occurs when the center of mass of a rotating component is not aligned with the axis of rotation in a single plane.
- This type of unbalance causes a continuous force that can result in constant vibration and can be felt as a consistent shaking or wobbling of the equipment.
- Common causes include uneven distribution of material or manufacturing defects.
- Couple Unbalance:
- Couple unbalance occurs when the center of mass of a rotating component is displaced parallel to the axis of rotation but in opposite directions on two sides of the rotor.
- It generates a moment or torque that tends to tilt the equipment.
- Couple unbalance often results from uneven material distribution or misalignment of components.
- Dynamic Unbalance:
- Dynamic unbalance is the most complex type and occurs when the center of mass is not aligned with the axis of rotation in multiple planes.
- It leads to both force and moment components, causing complex vibrations and potentially severe damage.
- Common causes include irregular shape, uneven wear, or accumulation of dirt and debris.
For more details, you can read through the ISO standard covering unbalance conditions, although you should make sure you have a pot of coffee, a dictionary, and a fidget spinner on hand before browsing.
Common Causes of Unbalance
- Accumulation of Debris: Dust, dirt, or other foreign materials can accumulate on rotating parts, causing unbalance.
- Manufacturing Defects: Inadequate machining or material defects during the manufacturing process can lead to unbalance.
- Wear and Tear: Over time, components may wear unevenly, leading to unbalance.
- Uneven Material Distribution: Variations in material density or mass distribution within a component can result in unbalance.
- Misalignment: Incorrect assembly or improper installation can introduce unbalance.
- Eccentricity: This occurs when the center of mass of a rotating object is not located precisely at the center of the object's rotation but is instead shifted to one side. This displacement can happen in any direction or plane perpendicular to the axis of rotation.
How To Prevent it
Although some cases of misalignment will never be preventable (for example, a person accidentally drops a wrench in a machine while repairing it), there are other ways to keep unbalance at bay.
- Quality Control: Ensure proper quality control during the manufacturing process to minimize manufacturing-related unbalance.
- Regular Maintenance: Implement routine maintenance and inspections to identify and address wear and tear issues before they lead to unbalance.
- Cleanliness: Keep rotating components clean and free of debris to prevent imbalance due to foreign materials.
- Precision Assembly: Follow precise assembly and alignment procedures during installation to prevent misalignment-related unbalance.
- Commissioning: Trim balancing during commissioning should be accepted practice at any manufacturer looking to avoid unbalance.
How to Correct it
Static and Couple Unbalance: To correct these types of unbalance, weights can be added or removed strategically from the rotating component. This process is called balancing and is typically done using specialized balancing equipment.
Dynamic Unbalance: Correcting dynamic unbalance is more challenging and may require specialized analysis and adjustments. It often involves both adding/removing weights and addressing other contributing factors, such as component wear or irregularities.