Understanding Field Balancing (In-Situ Balancing)

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Field balancing, also known as in-situ balancing, is the process of correcting the unbalance of a rotor while it runs in its own bearings and support structure, at or near its normal operating speed. Unlike shop balancing, where the rotor is removed and mounted on a dedicated balancing machine, field balancing is performed on-site with the machine fully assembled. It is the practical, everyday form of rotor balancing for maintenance and reliability teams, because it corrects the machine as it actually runs.

1. Definition: What is Field Balancing?

The process typically uses a portable vibration analyzer to measure the amplitude and phase of the 1× (running-speed) vibration, attach a trial weight of known mass, re-measure the new vibration response, and then calculate the required correction weight and its angular placement. Because the rotor stays in its own bearings, the result reflects the machine’s true running state rather than an idealised condition on a balancing stand.

A phase reference is indispensable: the analyser must know where the shaft is at each instant to convert a vibration peak into a heavy-spot angle. That reference comes from a tachometer triggering once per revolution, typically off a strip of reflective tape.

2. Why is Field Balancing Necessary?

While shop balancing is highly precise, it cannot account for every factor that influences 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 assembled machine. Common reasons include:

• Assembly unbalance: the final unbalance of a machine is the sum of the unbalance of all its rotating components (impeller, shaft, coupling, sheave, keys and fasteners). Field balancing corrects the unbalance of the whole assembly at once, including small shifts introduced when the machine was reassembled.
• Operational effects: unbalance can arise from conditions that only appear under normal running, such as thermal distortion of the rotor, aerodynamic forces, or hydraulic forces. These cannot be replicated on a shop balancing machine.
• Material build-up or wear: for fans, blowers and centrifuges, uneven product build-up or uneven wear causes unbalance to develop over time. Field balancing is the only practical way to correct this without a complete overhaul.
• Impracticality of removal: for very large machines — big industrial fans, turbine generators — removing the rotor for shop balancing is extremely expensive and time-consuming. Field balancing is a far more economical and faster solution, and is the basis of the in-situ criteria in ISO 21940-13.

3. The Field Balancing Process (Influence Coefficient Method)

The most common method for field balancing is the influence coefficient method, which follows a logical, repeatable sequence:

1. Initial run: the machine is run at its normal operating speed, and the initial 1× vibration amplitude and phase — the initial unbalance vector — are measured and recorded.
2. Trial-weight placement: the machine is stopped and a trial weight of 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 yields an influence coefficient, describing how much the vibration at the measurement point changes for a given unbalance at the correction location. The analyser combines this coefficient with the initial vector — using vector addition — to compute the exact mass and angle of the required correction.
5. Correction-weight placement: the machine is stopped, the trial weight removed, and the calculated correction weight permanently attached at the specified angle.
6. Verification run: the machine is run a final time to confirm the vibration has dropped to an acceptable level, per standards such as ISO 20816-1, and that the residual unbalance sits within the chosen tolerance.

Simple rotors are handled with single-plane balancing; longer rotors that exhibit a couple component require two-plane (dynamic) balancing. A trial-weight calculator helps choose a safe, effective starting mass for the first trial run.

4. Field Balancing in Practice with a Portable Analyser

In the field, the whole loop above is run with a single hand-carried instrument rather than a balancing stand. A portable two-channel analyser such as the Balanset-1A measures 1× amplitude and phase on each bearing, computes the influence coefficients automatically, and guides single- and two-plane corrections — then verifies the residual unbalance against ISO 21940-11 balance-quality grades. Working in the machine’s own bearings at operating speed, it captures the genuine running state — assembly, thermal and aerodynamic effects included — that a shop machine simply cannot reproduce. The supplied optical laser tachometer provides the once-per-revolution phase reference from a small piece of reflective tape, so no shaft preparation beyond a tape strip is needed.

5. Key Considerations and Safeguards

Field balancing demands skill and careful planning. As outlined in standards such as ISO 21940-13, safety is paramount.

• Safety: trial and correction weights must be attached securely enough to withstand the centrifugal force at operating speed, and access to the machine must be controlled while it runs.
• Prerequisites: before balancing, rule out other causes of high 1× vibration — misalignment, resonance, a bent shaft, or mechanical looseness — because balancing cannot fix a problem that is not actually unbalance.
• Instrumentation: the work requires an analyser able to measure amplitude and phase, plus a phase-reference sensor (tachometer). Repeatable measurements depend on consistent sensor mounting and a clean, reliable tachometer pulse.
• Speed stability: the machine must hold a steady speed throughout each run; drifting speed corrupts the phase data on which the whole calculation rests.

← Back to Main Index