In practice, any rotating machine can be converted into a balancing machine. This only requires calibration, which is achieved by measuring the amplitude and phase of the vibration of the bearing(s) in two initial states and when applying a test mass, followed by balancing calculations. The test mass is selected based on the number of balancing planes—either one or two—along the rotor, where each can have a specific radius and a specified amount of mass applied. This calibration is valid for a specific rotational speed, which is usually the machine’s operating speed.

Field balancing services address not only the forces due to unbalance but also any forces that share the same frequency as the unbalance, including misalignment, shaft bending, eccentricity, aerodynamic forces, and more. Therefore, prior to conducting field balancing services, it is essential to ensure that the vibrations are indeed caused by unbalance rather than other forces.

Tavator Sepahan Engineering Company is one of the oldest service providers of field balancing services across various industries both domestically and internationally. The use of specialized techniques and equipment by this company enables the balancing of the most complex machines, including turbomachinery, all the way to simpler machines by its expert technicians.

What is Unbalance?

Unbalance can be considered as the eccentricity of the center of gravity of the rotor’s plane relative to the axis of rotation. This can be viewed as a large mass at a small distance. This amount of unbalance can be represented by a mass $m$ at a radius $r$ (small mass at a large distance), and a balancing mass can be installed against it or the same amount can be removed from that location to achieve balance.

Number of Balancing Planes

The number of planes that should be chosen as balancing planes varies depending on the type of rotor. Accordingly, different types of balancing can be defined as follows:

• Balancing of Rigid Rotors – Operating speed is less than the first natural frequency of the rotor.

  • Single-plane balancing

  • Two-plane balancing

  • Multi-plane balancing

• Balancing of Flexible Rotors – Operating speed is greater than the first natural frequency of the rotor.

  • Approximation with rigid rotors

  • Modal balancing (vacuum balancing)

Effect of Unbalance Forces on Vibration in Each Bearing

In balancing operations, the impact of the unbalance force from each plane (near and far planes) on the vibration of each bearing (near and far bearing) is measured. The corrective weights for both selected planes are calculated in such a way that their resultant neutralizes the initial unbalance force. This method is known as the Influence Coefficient Method. This approach is used in all modern field balancing equipment.

Static and Dynamic Balancing (Static Coupling)

Any single-plane balance can be considered a static balance, where the amount and location of the unbalance can be identified and corrected using static methods by evaluating the gravitational effects of the unbalance. However, in high-speed or heavy rotors, due to frictional resistance and other effects, precise evaluation is not possible. In such cases, it is necessary to balance the rotor dynamically by measuring the effect of centrifugal force.

Unbalances of two planes and above cannot be balanced using static methods; instead, the rotor must be balanced by measuring the effect of the unbalance force, which can cause vibration in the rotor supports.

Each two-plane unbalance is a dynamic unbalance comprising two components: coupled unbalance and static unbalance. A coupled unbalance consists of two identical unbalances with a 180-degree phase difference, located in two separate planes.

When a dynamic unbalance is balanced using static methods, only the static unbalance component is corrected, while the coupled unbalance remains. In this case, the vibrations of the two bearings supporting the rotor typically have a 180-degree phase difference.

Response to Unbalance and Balancing Calculations

In a portable balancer, amplitude and phase can be measured using both analog and digital methods. In today’s modern equipment, the use of advanced microprocessors not only allows for more accurate digital measurements of amplitude and phase but also facilitates calculations, value displays, and subsequent processing for vector analysis, storage, graphical displays, and information transfer to computers, making the balancing process significantly easier. However, familiarity with graphical methods and their use by the balancing specialist can often simplify and expedite the balancing task, even when advanced balancers are available. Furthermore, advanced complementary techniques provide users with the ability to conduct more thorough assessments of unbalance conditions. Therefore, expertise and experience in balancing operations are essential.

Field balancing in normal conditions is a specific and relatively straightforward method, but in many cases, specific considerations may need to be taken into account, requiring more experience. This is especially true for heavy and high-speed rotors, where the dynamic behavior of the rotor and machine has a significant impact on the rotor’s unbalance response.