What are the common causes and effects of unbalance in rotating machines?

1 Causes of unbalance

Unbalance can be caused by various factors, such as manufacturing defects, assembly errors, material degradation, thermal expansion, corrosion, erosion, fouling, impact, or misalignment. Some of these causes are inherent to the design or fabrication of the rotor, while others are due to external influences or operating conditions. Unbalance can also be static, dynamic, or mixed, depending on the distribution and direction of the unbalanced mass.

2 Effects of unbalance

Unbalance can have negative effects on both the rotating machine and the surrounding environment. The most noticeable effect is vibration, which is the oscillation of the rotor around its axis due to the centrifugal force of the unbalanced mass. Vibration can damage the bearings, shaft, seals, couplings, and other components of the machine, as well as transmit to the foundation, structure, and piping. Vibration can also generate noise, which can be harmful to the workers and the equipment. Another effect of unbalance is heat, which is the result of the friction and deformation caused by the vibration. Heat can reduce the lubrication, efficiency, and lifespan of the machine, as well as increase the risk of fire or explosion.

4 Correction of unbalance

Unbalance can be corrected by performing a balancing procedure, which is a process of measuring and adjusting the mass distribution of the rotor to minimize the vibration. Balancing can be done either in situ or in a workshop, depending on the size, accessibility, and condition of the machine. Balancing can be done either statically or dynamically, depending on the type of unbalance. Balancing can be done either by adding or removing mass from the rotor, or by shifting the position of the existing mass.

5 Balancing methods

Balancing equipment, tools, and techniques vary, with some of the more common methods being the trial-and-error method, the influence-coefficient method, and the modal balancing method. The trial-and-error method is a simple and intuitive approach that involves adding or removing trial weights to the rotor and observing the change in vibration. This is suitable for small and low-speed machines, but it can be time-consuming and inaccurate for larger and higher-speed machines. The influence-coefficient method is a more advanced and accurate approach that involves measuring the vibration response of the rotor to known weights at different locations and angles. This can calculate the optimal weight and position for balancing, but requires sophisticated instruments and calculations. Lastly, the modal balancing method is a specialized and efficient method that involves identifying and isolating the natural modes of vibration of the rotor. This can balance the rotor at multiple speeds and planes, but requires complex analysis and software.

6 Benefits of balancing

Balancing can bring several benefits to the rotating machine and the overall system, such as reducing vibration, noise, wear, heat, and stress. It can also improve efficiency, performance, reliability, and safety while extending the service life and reducing maintenance costs. Furthermore, balancing can enhance the comfort and satisfaction of workers and customers.