What field balancing is
Field balancing (also called in-place or on-site balancing) corrects unbalance with the rotor installed in its own machine, on its own bearings, running at its own speed. It avoids the cost and downtime of removing the rotor, and it accounts for the real mounting conditions that a shop balancing machine can’t reproduce.
Rotors usually need balancing in the field for reasons other than how they were made: dirt or product build-up on fan blades, erosion or wear, bearing changes that shift the center of rotation, shaft distortion, structural changes or speed increases.
How it works
- Measure the starting point. We record the vibration amplitude and phase at running speed (1×) at the bearings, and confirm that unbalance is actually the main problem.
- Run a trial weight. A known weight is attached at a known location. Because every structure responds differently, the trial weight calibrates how this machine reacts to a change in balance.
- Calculate and install the correction. From the change in vibration we calculate the size and location of the permanent correction weight.
- Verify. We re-measure and trim as needed until the 1× vibration is as low as reasonably achievable, then document before-and-after readings.
How good is good enough?
There’s no single balance standard for every machine in the field, because the vibration you measure depends on the rotor and on the mass and stiffness of the structure supporting it. As a first pass, the following guideline is widely used for filtered 1× readings taken at the bearings in the horizontal, vertical and axial directions:
| Level | Displacement, mils peak-to-peak | Velocity, in/sec peak | Acceleration, g peak |
|---|---|---|---|
| Good | 1.0 | 0.1 | 0.1 |
| Best achievable | 0.1 | 0.01 | 0.01 |
The “best achievable” level assumes ideal conditions: new bearings, straight shafts, round pulleys, no looseness, minimal misalignment and no resonance. In velocity terms, the 1× peak also shouldn’t be the dominant peak in the spectrum out to ten times running speed.
Some industries publish their own limits. Fans are the machines we balance in the field most often, and ASHRAE has long used these displacement limits:
| Fan speed | Displacement, mils peak-to-peak |
|---|---|
| Over 2,000 rpm | 1.0 |
| 1,000–2,000 rpm | 2.0 |
| 600–1,000 rpm | 3.0 |
| Under 600 rpm | 4.0 |
Other examples: cooling-tower manufacturers have used 5.0 mils measured horizontally at the base of the gearbox, and diesel-engine manufacturers have used 5.0 mils on the casting at the crankshaft centerline. Many trade groups — including the Hydraulic Institute for pumps and AMCA for fans — now judge acceptance by overall velocity, and ISO 8528-9 covers engine-driven generator sets.
These numbers are guidelines, not hard limits. The purpose of balancing is to reduce the forces that wear out bearings and damage surrounding equipment. We’ll balance to the level that makes sense for your machine, its remaining service life and the outage window you have.
Equipment we field balance
- Industrial fans and blowers
- Cooling-tower propellers
- Electric motors
- Pumps
- Compressors
- Crushers, mixers and rolls
- Machine tool spindles
- Generators and turbines