On-Site Dynamic Balancing Testing for Rotating Machinery

On-site dynamic balancing testing measures and corrects rotor unbalance in rotating equipment under real installed operating conditions, eliminating costly disassembly and workshop transport. Performed in accordance with ISO 21940 and ISO 20816 criteria by accredited third-party laboratories and condition-monitoring specialists, field balancing reduces vibration, extends bearing and seal life, and restores machine reliability.

Technical Overview

On-site dynamic balancing, also known as field balancing, is a diagnostic and corrective procedure performed on rotating machinery while the equipment remains installed in its normal operating position. The objective is to identify mass unbalance—a condition in which the principal inertia axis of the rotor does not coincide with its axis of rotation—and to reduce it by adding or removing correction mass at calculated locations on the rotor.

Unbalance is the most common cause of excessive vibration in rotating equipment. It generates a centrifugal force that rotates with the shaft, producing a dominant vibration component at the machine's running frequency (1X RPM). If left uncorrected, this force accelerates bearing wear, damages seals and couplings, loosens foundations and fasteners, degrades product quality, and can ultimately lead to catastrophic rotor failure.

Unlike workshop balancing on a balancing machine, on-site testing accounts for the actual mounting stiffness, operating speed, temperature, load, and drive conditions of the machine. This makes field balancing particularly valuable for large rotors that cannot be economically transported, for machines whose unbalance behavior changes under operating conditions, and for equipment that must return to service with minimal downtime. The evaluation and correction process is governed by the ISO 21940 series for rotor balancing, the ISO 20816 series for mechanical vibration evaluation, and corresponding national standards such as GB/T 9239 and GB/T 6075.

Testing Samples

Fans, Blowers, and Ventilation Equipment

Centrifugal fans, axial fans, induced-draft and forced-draft fans, cooling tower fans, and industrial blowers are among the most frequently balanced machines. Their large-diameter impellers accumulate dust, erosion, and wear that gradually shift the mass distribution, producing unbalance that cannot be fully corrected during manufacturing.

Electric Motors and Generators

Motor and generator rotors, armatures, and flywheels are tested for residual unbalance, particularly after rewind, shaft repair, or coupling replacement. Field balancing verifies that vibration levels remain within the limits specified by ISO 20816 for the machine class and mounting configuration.

Pumps, Impellers, and Compressors

Centrifugal pump impellers, mixer shafts, agitators, and compressor rotors are balanced on site when cavitation damage, erosion, deposits, or component replacement introduce unbalance. For these machines, balancing is often combined with an overall vibration survey to separate hydraulic or aerodynamic forcing from true mechanical unbalance.

Turbines and Large Rotors

Steam turbine and generator rotors, cooling rolls, cardan shafts, and other long or heavy rotors that exceed workshop balancing machine capacity are balanced in place using multi-plane techniques. Slow roll runout compensation and thermal growth effects are considered during measurement and evaluation.

Spindles, Pulleys, and Drive Components

Machine-tool spindles, grinding wheels, conveyor pulleys, crusher rotors, hammer mills, and large sheaves are tested and trimmed on site to achieve the tighter balance grades required for high-speed or precision operation.

Testing Items

A complete on-site dynamic balancing assessment typically includes the following measurement and evaluation items:

• Baseline vibration measurement: overall vibration amplitude in velocity, displacement, and acceleration units, recorded at bearing housings in horizontal, vertical, and axial directions.
• Frequency analysis (FFT spectrum): identification of the 1X rotational frequency component to confirm that unbalance is the dominant forcing function.
• Phase angle measurement: the angular relationship between the vibration signal and a once-per-revolution reference mark, essential for locating the heavy spot.
• Unbalance magnitude and angular position: calculated for each correction plane in gram-millimeters (g·mm) or ounce-inches, together with the equivalent correction mass at the selected mounting radius.
• Single-plane and two-plane balancing: static or couple unbalance correction for disc-shaped rotors, and dynamic two-plane correction for longer rotors with significant axial length.
• Balance quality grade verification: confirmation that the residual unbalance meets the specified grade (for example G6.3 for general industrial rotors, G2.5 for fans and medium machines, or G1.0 for precision spindles) in accordance with ISO 21940-11.
• Residual unbalance per correction plane: computed as U = 9549 × G × M / n, where G is the balance grade in mm/s, M is the rotor mass in kilograms, and n is the service speed in RPM.
• Vibration severity evaluation: comparison of final vibration levels against the zone criteria of ISO 20816 to demonstrate acceptable machine condition after correction.
• Correction application and verification runs: installation of trial and permanent correction weights, followed by trim runs until the residual vibration meets the acceptance tolerance.

Testing Methods

Preparation and Baseline Survey

Before measurement, the machine is inspected for looseness, foundation condition, coupling alignment, and bearing state, since these defects can mimic or mask unbalance. Piezoelectric accelerometers or velocity transducers are mounted magnetically or stud-mounted on the bearing housings, and a phase reference is established using an optical or laser tachometer aimed at reflective tape on the shaft, a keyway, or an existing shaft encoder signal. A baseline run records vibration amplitude and phase at the operating speed, and a spectrum analysis confirms the dominance of the 1X component.

Trial Weight Method

A trial weight of known mass is attached to the rotor at a known radius and angle, and the machine is rerun. The change in vibration amplitude and phase produced by the trial weight allows the influence of a known unbalance to be calculated. The original unbalance is then determined by vector calculation, and the correction mass and its angular position are derived directly from the measured influence coefficients.

Influence Coefficient Method

For two-plane and multi-plane balancing, trial weights are applied sequentially to each plane while vibration is recorded at multiple measurement points. The resulting influence coefficient matrix is solved to compute the set of correction masses that minimizes vibration at all measured locations simultaneously. This method, described in ISO 21940 and ISO 19499 balancing guidelines, is the standard approach for field balancing with modern dual-channel analyzers and balancing software.

Correction and Verification

Corrections are applied by adding mass through welding, bolting, or clamping balanced weights, or by removing material through controlled drilling, grinding, or milling. After the first correction, a verification run is performed; if residual vibration exceeds tolerance, additional trim runs are completed until the required balance grade and vibration limits are achieved. All welding or material-removal work is carried out under the site's hot-work and safety procedures, with appropriate machine guarding and lockout arrangements during weight installation.

Acceptance Criteria

Acceptance is based on two complementary criteria: the residual unbalance per plane meeting the balance quality grade specified in ISO 21940-11 or the equipment purchase specification, and the final bearing vibration velocity meeting the acceptable zones of ISO 20816 for the machine class. The results are documented in a test report containing baseline and final data, spectra, phase readings, correction masses and locations, and residual unbalance values.

Applications

On-site dynamic balancing testing is applied across industries wherever rotating machinery is critical to production:

• Power generation: ID/FD fans, boiler feed pumps, induced-draft fans, turbine-generator sets, and cooling tower fans.
• Petrochemical and chemical processing: compressors, blowers, mixers, agitators, and large pump trains.
• Cement, mining, and aggregates: crusher rotors, hammer mills, separator and exhaust fans, and conveyor pulleys.
• Steel and metallurgy: cooling bed fans, blower systems, rolling mill drive components, and cardan shafts.
• HVAC and building services: air handling units, rooftop fans, and large ventilation rotors.
• Water and wastewater treatment: aeration blowers, mixers, and large vertical pump motors.
• Pulp and paper: dryer cans, press rolls, fan pumps, and refiner discs.
• Marine and offshore: engine flywheels, ventilation fans, thruster motors, and deck machinery.
• General manufacturing: machine-tool spindles, grinding wheels, and dedicated production machinery.

Typical scenarios triggering a field balancing campaign include excessive vibration alarms after overhaul, recurring bearing failures, replacement of rotating components, product quality degradation attributable to vibration, commissioning of new or relocated equipment, and predictive maintenance programs operating under ISO 13373 condition-monitoring practices.

FAQ

What is the difference between workshop balancing and on-site dynamic balancing?

Workshop balancing is performed on a dedicated balancing machine with the rotor removed from service, and is ideal during manufacturing or overhaul. On-site balancing is performed on the assembled machine at its operating speed and installed condition, capturing the effects of mounting stiffness, coupling, load, and temperature. It avoids disassembly, transport, and re-alignment, and typically reduces downtime from days to hours.

Which balance quality grade should my rotor meet?

The required grade depends on rotor type and speed. Common reference values from ISO 21940-11 include G6.3 for general machinery and pump impellers, G2.5 for fans and gas turbine rotor assemblies, and G1.0 for precision machine-tool spindles and small high-speed armatures. The governing equipment specification or purchase contract should always be consulted for the applicable grade.

How long does an on-site balancing job take?

A single-plane fan balance can often be completed within two to four hours including setup, trial runs, correction, and verification. Two-plane or multi-plane jobs on large machines may require a full shift or longer, depending on machine access, startup time, and the number of trim runs needed to reach tolerance.

Can variable-speed or flexible rotors be balanced in the field?

Yes. Machines operating on variable-frequency drives are balanced at their dominant duty-point speed, and measurements may be repeated at several speeds to confirm stability. Flexible rotors operating above their first critical speed are handled with multi-plane or modal balancing techniques performed by experienced specialists, often in coordination with shaft vibration measurements using eddy-current proximity probes.

Are the corrections permanent, and how are they applied?

Corrections are intended to be permanent. Mass is typically added by welding balancing washers, bolting clamp-on weights, or securing setscrew weights into machined grooves; alternatively, material is removed by drilling or grinding at the calculated angle. All correction hardware is selected to withstand the operating centrifugal forces and environment of the rotor.

How is the success of field balancing verified?

Verification consists of a final measurement run after correction. The residual vibration at the 1X frequency must fall within the agreed tolerance, the computed residual unbalance must satisfy the specified balance grade, and the overall vibration must fall within the acceptable zone of ISO 20816. A formal test report documents before-and-after data for maintenance records and acceptance purposes.