Understanding Field Balancing (In-Situ Balancing)

Definition: What is Field Balancing?

Field balancing, also known as in-situ balancing, is the process of correcting the unbalance of a rotor while it is running in its own bearings and support structure, at or near its normal operating speed. Unlike shop balancing, where the rotor is removed and placed on a specialized balancing machine, field balancing is performed on-site with the machine fully assembled.

The process typically involves using a portable vibration analyzer to measure the amplitude and phase of the 1X (running speed) vibration, attaching a trial weight of a known mass, re-measuring the new vibration response, and then using that information to calculate the required correction weight and its angular placement.

Why is Field Balancing Necessary?

While shop balancing is highly precise, it cannot account for all the factors that influence a machine’s balance in its operational environment. Field balancing is necessary when the unbalance is caused by, or can only be corrected by, considering the entire machine assembly. Common reasons include:

• Assembly Unbalance: The final unbalance of a machine is the sum of the unbalance of all its rotating components (e.g., impeller, shaft, coupling, sheave). Field balancing corrects the unbalance of the entire assembly at once.
• Operational Effects: Unbalance can be caused by factors that only appear under normal operating conditions, such as thermal distortion of the rotor, aerodynamic forces, or hydraulic forces. These cannot be replicated on a shop balancing machine.
• Material Buildup or Wear: For machines like fans, blowers, and centrifuges, uneven buildup of product or uneven wear can cause unbalance over time. Field balancing is the only practical way to correct this without a complete overhaul.
• Impracticality of Removal: For very large machines, such as large industrial fans or turbine generators, removing the rotor for shop balancing is extremely expensive and time-consuming. Field balancing is a much more economical and faster solution.

The Field Balancing Process (Influence Coefficient Method)

The most common method for field balancing is the influence coefficient method, which follows a logical, step-by-step process:

1. Initial Run: The machine is run at its normal operating speed, and the initial 1X vibration amplitude and phase (the “unbalance vector”) are measured and recorded.
2. Trial Weight Placement: The machine is stopped, and a trial weight of a known mass is securely attached to the rotor at a known angular position.
3. Trial Run: The machine is run again at the same speed. The new vibration amplitude and phase (the “response vector”) are measured and recorded.
4. Calculation: The change in the vibration vector caused by the trial weight is used to calculate an “influence coefficient.” This coefficient describes how much the vibration at the measurement point changes for a given amount of unbalance at the correction location. The analyzer then uses this coefficient and the initial unbalance vector to calculate the exact mass and angle of the required correction weight.
5. Correction Weight Placement: The machine is stopped, the trial weight is removed, and the calculated final correction weight is permanently attached at the specified angle.
6. Verification Run: The machine is run one last time to verify that the vibration has been reduced to an acceptable level, as per standards like ISO 20816-1.

Key Considerations and Safeguards

Field balancing requires skill and careful planning. As outlined in standards like ISO 21940-13, safety is paramount.

• Safety: Trial and correction weights must be attached securely to withstand the centrifugal forces at operating speed. Access to the machine must be controlled during operation.
• Prerequisites: Before attempting to balance, other potential causes of high 1X vibration, such as misalignment, resonance, or looseness, should be ruled out.
• Instrumentation: The process requires a vibration analyzer capable of measuring amplitude and phase, as well as a phase reference sensor (tachometer).

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