Balancing is crucial for smooth running and longevity in rotating machinery. A rotor converts electric and electromagnetic energy into rotational motion, where the weights are equally distributed along its axis for unhinged operation. Imbalances will occur when one side has excess weight, which leads to uneven load distributions and machine vibration.
This imbalance generates a centrifugal force through the interaction of unbalanced mass with radial acceleration, causing the rotor displacement. Among the different types of unbalances, a couple unbalance exhibits unique challenges, affects rotor stability and performance, necessitating detailed analysis and corrective measures.
In this blog, we will explore the basic imbalances found in rotating equipment. We will understand the Couple Unbalance, its effects, the importance of proper balancing and related topics.
Generally, imbalances occur when the principal inertia axis of a rotor is displaced from its rotational axis, which gives rise to unwanted vibrations and stress. There are three main types of imbalances:
- Static Unbalance: The principal inertia axis is displaced but remains parallel to the rotating axis, causing equal vibration amplitudes at both bearings with the same phase angles.
- Couple Unbalance: The principal inertia axis meets the rotating axis at the centre of the gravity, meaning the axes will not be parallel, but the rotor will remain statically balanced. This results in equal vibration amplitude at each bearing; however, the phase angles will be 180 degrees apart.
- Dynamic Unbalance: A combination of both static and couple unbalance, where the principal inertia axis is displaced in a way that neither remains parallel nor intersects the centre of gravity. This leads to unequal vibration amplitudes at both bearings and varying phase angles.
Couple unbalance is a bit more complex than static and dynamic unbalance. Take, for instance, the opposing forces, unlike the other two unbalances (static and dynamic), where the unbalanced masses cause a direct, predictable force in a single direction, a Couple Unbalance has two equal imbalances placed 180 degrees apart. Other reasons, such as no resting position, multi-plane corrections, difficulty in detecting its effect, make this imbalance even more complex.
That said, let us take a deeper dive into the concept of Couple Imbalance from our perspective. We’ll explore its causes, detection methods and corrective measures. Additionally, we will examine its impact on the performance and reliability of rotating machinery along with best practices for effective rotor balancing.
Couple Unbalance: Rotors Out of Sync?
Couple unbalance or the moment unbalance, occurs when two equal unbalances are positioned 180 degrees apart within a rotor. Unlike static unbalance, the couple unbalance cannot be spotted using knife edges as the rotor does not remain in a fixed position at rest. Instead, the opposing imbalances create a moment that causes a wobble to the rotor along the vertical axis, perpendicular or at the right angles to the rotation.
This imbalance often results from bearing planes not being perpendicular to the rotational axis. The rotor spins and generates undesired vibrations and spiralling effects. Hence, correcting Couple Unbalances need adjustments in two planes rather than one.
Understanding and Correcting Couple Unbalance.
Having a thorough understanding of the couple unbalance in rotating machinery is crucial for several reasons:
- Prevent excessive vibrations: The out-of-phase forces created at both ends of the rotor lead to excessive vibration levels in the system, affecting machine efficiency and lifespan. It is essential to detect couple imbalances promptly. If left unchecked, these could lead to excessive vibrations resulting in machinery wear and tear, costly repairs and operational shutdown.
- Efficient rotor performance: The Couple Unbalance creates uneven forces on bearings, causing excessive stress and fatigue, leading to premature wear, overheating and equipment failure. Identifying and addressing couple unbalance aids in smoother and more efficient rotor performance, reducing energy loss and enhancing overall performance and machine functionality.
- Improved safety: Constant unbalanced forces due to couple unbalances, shift the machine components, leading to shaft misalignments, loosened fasteners and equipment damage. Structural failure, unstable equipment parts, and safety hazards are inherent to the couple imbalances, making the correction of excessive vibrations quintessential.
- Reduced maintenance cost: Prompt diagnosis and correction of imbalances reduce the need for costly repairs and replacements, which will benefit long-term cost savings.
In short, when an unaddressed couple imbalance occurs in rotating machinery, it can lead to several critical issues. Therefore, it is vital to identify and correct these imbalances as soon as they happen to maintain the health, safety, and efficiency of any rotating machinery.
Detection and Measurement of Couple Unbalance
Identifying and quantifying the intensity of couple unbalance in rotating machinery generally involves vibration analysis, certain balancing techniques, and utilising specialised measurement tools such as accelerometers, FFT (Fast Fourier Transform) analysers, etc. Vibration analysis detects any faults in rotating machinery by tracking vibration levels and patterns, helping to identify a Couple Unbalance. Various balancing techniques, such as spinning the rotor and measuring vibration displacement, velocity, and acceleration, aid in pinpointing unbalance across different planes. Condition-Monitoring uses real-time sensor data to provide a comprehensive understanding of the rotor’s health, enabling timely maintenance and correction of couple imbalance issues.
As we saw earlier, Couple Unbalance is a specific type of rotor imbalance, which happens when two equal but opposite unbalanced masses are present in different planes of rotating equipment, resulting in rocking motion and the need for balancing in multi-planes. Couple correction requires adding two equal weights, positioned 180 degrees apart in two correction planes. The space separating these planes is referred to as the couple arm. Let us look at some of the common couple unbalances Detection Methods:
1. Vibration Analysis:
- If a rotor has unbalance, it generates centrifugal force that results in periodic vibration; meaning the vibration caused by the couple unbalance happens at the same frequency as the rotor’s rotation speed (1X Revolutions Per Minute).
- Equal Vibration amplitude at both bearings with a 180 degrees phase difference.
- More visible in rigid than flexible rotors, as shafts can deform and absorb unbalanced forces, making the unbalance complex to detect in flexible rotors.
2. Phase measurement:
- Using laser tachometer, optical sensor or strobe light, you can measure the phase angle of vibration at both the bearings.
- A couple unbalance will exhibit the phase difference close to 180 degrees between the two measurement planes.
3. On-Site Field balancing:
- In this method, vibration sensors will be installed at two bearing locations, while the machines run to collect vibration and phase data assessment.
- Trial weight method is used to identify a couple unbalance by adding a small weight in one plane and observing its effect. Adjustments are then made in the second plane accordingly. If adding weight in one plane reduces the vibration in one area but increases it in another, it indicates the presence of a couple unbalance.
4. Dual-Plane balancing machine:
- This offline method is performed when the rotors are disassembled. The rotor is then spun in a balancing machine equipped with sensors placed in two different planes. When high vibration is detected at both ends of the rotor and phase readings are 180 degrees apart, a couple unbalance is confirmed.
Correcting Couple Unbalance
- Static Balancing: is carried out when the centre of gravity remains on the rotational axis, preventing rotation due to gravity. The rotor lies idle when placed horizontally, needing no external force to keep it balanced.
- Dynamic Balancing: Balancing is secured by adding or removing weight to reduce stress on bearings. It ensures proper rotor spinning without forming extreme centrifugal force.
- Balancing Procedures and Methods: This step involves identifying heavy spots on the rotor and correcting imbalance by either removing weight through drilling, milling, grinding or adding counterweights through bolting, welding.
Proper techniques and balancing procedures reduce vibrations, prolong bearing life, and improve machine alignment, efficiency and performance.
Couple Balancing Service From Ocean
Proper rotor balancing is vital for rotating equipment performance issues such as excessive vibration, noise, heat generation, wear and shutdown. Addressing these concerns through precise balancing techniques helps extend the asset lifespan, improve operational efficiency and prevent unexpected system failures.
At Ocean, leading engineering company in Oman, we specialise in dynamic balancing, couple unbalance diagnosis and vibration analysis, making sure your equipment performs smoothly with minimal stress on components. Our expert team uses condition monitoring services to find and correct rotors, shafts, turbines, impellers and other rotating equipment imbalances with high precision.
With decades of expertise, we provide comprehensive balancing services tailored to your industrial needs. Contact us today!