Field Balancing of a Rotor: How It Is Done and When It Is Needed

Field balancing removes rotor unbalance in the machine's own bearings, without taking the rotor out. A vibration analyzer measures the amplitude and phase of vibration at running speed (1x) relative to a mark on the shaft read by a tachometer. A trial weight of known mass is then added, the change in the vibration vector gives the mass and angle of the correction weight, and a check run confirms the result.

When balancing is needed

Unbalance is the condition of a rotor whose center of mass does not coincide with its axis of rotation. Its vibration signs are:

• a dominant peak at running speed 1x, with few or no harmonics;
• a sinusoidal time waveform, one event per revolution;
• 1x vibration roughly equal in the horizontal and vertical directions;
• little axial vibration, except on overhung rotors;
• 1x amplitude rising with speed.

Typical causes are material buildup, wear, broken or missing parts, improper assembly or design, and thermal distortion. On fans and induced-draft fans the main cause is buildup on the blades; industry specifics are covered in the article Field balancing of fans and induced-draft fans in metallurgy and CHP plants.

When unbalance is not the cause

Before adding weights, rule out other sources of 1x vibration. Balancing only corrects unbalance, not misalignment or other faults.

Sign	What it may be
Many 1x harmonics in the spectrum	Probably not unbalance: looseness, misalignment
High 2x, increased axial vibration	Misalignment
Unstable phase, fractional harmonics	Mechanical looseness
1x does not respond to a trial weight	Bent shaft, soft foot, cracked rotor, coupling problem, resonance
Vibration changes sharply with a small speed change	Resonance near running speed

Resonance is checked with a bump test, a coast-down recording or, with a variable-speed drive, by changing speed. How to tell faults apart in the spectrum is explained in How to read an FFT spectrum.

What you need for field balancing

• One or two vibration sensors (one per bearing being measured).
• A tachometer with a reference mark on the shaft, for example a laser sensor with reflective tape: it provides the 1x phase reference.
• An analyzer with balancing software that calculates vectors and correction masses.
• Scales, a set of weights and a way to fix them (bolts, welding, adhesive, as agreed with the machine owner).

How balancing is done: the trial weight method

1. Preparation. Stop and lock out the machine, inspect the rotor, clean the impeller of deposits, check fastener tightness and bearing condition.
2. Sensors and mark. Mount the vibration sensor on the bearing housing, stick a reflective mark on the shaft and set up the tachometer. Choose one rotation direction for counting angles and mark the zero point on the rotor.
3. Initial run. At operating speed, record 1x amplitude and phase at each bearing, in the same direction and from the same mark.
4. Trial weight. Stop the machine and fit a weight of known mass at a known angle. The trial weight should change 1x vibration by at least 30 % in amplitude or 30° in phase (the 30/30 rule). Start with a small weight and double it until the change is sufficient, but never use a weight that could damage the machine.
5. Trial run. At the same speed, record 1x amplitude and phase again.
6. Calculation. The trial weight effect is the vector difference "trial run minus initial run". Correction mass = trial mass × initial amplitude / effect amplitude; the angle is the rotation that brings the effect onto the vector opposite to the initial reading. The analyzer software does this automatically.
7. Correction weight. Remove the trial weight (or leave it if the software accounted for it) and fit the permanent weight.
8. Check run. Verify residual vibration. If needed, make a trim correction using the same influence coefficient.
9. Report. Record speed, planes, weight masses and angles, vibration before and after.

ACOEM example: initial vibration 7 units, 5 units after a 100 g trial weight, trial weight effect from the vector diagram 9 units. Correction mass: 100 × 7 / 9 ≈ 78 g.

The trial weight effect should be several times larger than run-to-run scatter of the readings, otherwise the calculation will be unstable.

One plane or two

Before fitting the trial weight, compare 1x phases at the two bearings in the same direction:

Phase difference between bearings	Type of unbalance	Number of planes
About 0°	Static: both ends move together	One, near the center of mass
About 180°	Couple: ends move in opposition	Two
In between	Dynamic (the general case)	Two

One plane is usually enough for a narrow disc-like rotor (fan wheel, pulley). If on a long rotor a single-plane correction improves one bearing and worsens the other, a couple remains and two-plane balancing is needed.

Permissible unbalance per ISO 21940-11

Balance quality of rigid rotors is specified by G grades in ISO 21940-11 (which superseded ISO 1940-1). The series is G 0.4, G 1, G 2.5, G 6.3, G 16, G 40, G 100, G 250, G 630; each step is about 2.5 times coarser than the previous one. The number in the grade is the product of permissible specific unbalance and angular velocity, in mm/s.

Permissible residual unbalance is calculated as follows:

• angular velocity ω = 2π × n / 60, rad/s (n is maximum service speed, rpm);
• permissible specific unbalance e_per = G × 1000 / ω, µm (1 µm equals 1 g·mm per kg of rotor mass);
• permissible unbalance U_per = e_per × m, g·mm (m is rotor mass, kg);
• for planes roughly symmetric about the center of mass, each plane gets half of U_per.

Example: 200 kg rotor, 1480 rpm, grade G 6.3.

Quantity	Calculation	Result
ω	2π × 1480 / 60	155.0 rad/s
e_per	6.3 × 1000 / 155.0	40.6 µm
U_per	40.6 × 200	≈ 8130 g·mm
Per plane	8130 / 2	≈ 4065 g·mm
Weight at 400 mm radius	4065 / 400	≈ 10 g

Which grade to choose:

• most industrial machines are specified at G 6.3;
• high-speed machines and stand-alone electric motors may require G 2.5;
• the centrifugal pump standards ISO 5199 and ISO 9905 require major rotating components to be balanced to G 6.3;
• the final grade is set by the machine manufacturer: balance to its recommendations.

The grade is achievable only if unbalance is the only source of 1x vibration and the machine is not running at resonance.

Which instrument to balance with

• Bently Nevada vb7 — per the manufacturer's datasheet, a two-channel analyzer and balancer: simultaneous recording on two channels, two-plane balancing with two sensors over 30–60,000 rpm, laser speed sensor with reflective tape (10 cm to 2 m), Keyphasor mode, option to leave or remove trial weights, stored balancing jobs, IP65 housing. The same instrument is a route data collector and spectrum analyzer (up to 6400 lines, Fmax up to 40 kHz), so misalignment and looseness can be ruled out before balancing.
• vb Balancer — dedicated Bently Nevada balancers: vbBalancer with two channels and vbBalancer+ with four, two-plane balancing, basic spectra up to 800 lines and 5 kHz to verify the result.

KEG TRK supplies Bently Nevada instruments in Kazakhstan. Price on request.

Sources

• Bently Nevada. vb7 Portable Data Collector, Analyzer and Balancer, datasheet 116M5255 Rev. F.
• Baker Hughes / Bently Nevada. vb Balancer Series Portable Instruments, datasheet 132M3167.
• Sensemore. ISO 21940 Balance Quality Calculator.
• Sensemore. Single Plane Balancing Calculator.
• ACOEM. Balancing How To #2: Selecting a Proper Trial Weight.
• ACOEM. Balancing How To #6: Balancing Tolerances.
• ACOEM. Finding Unbalance and Misalignment on a Spectrum.
• Emerson. Vibration Analysis for Machinery Health Diagnosis.
• Europump. Pump Vibration Standards Guidelines, 1st edition, July 2013 (ISO 5199, ISO 9905).