What is a balancing machine?
A balancing machine is a measuring rig that spins a rotor at a controlled speed, measures the force or vibration at its supports and turns it into "this many grams at this angle" for each correction plane. Motor rotors, fan and pump impellers, pulleys, cardan shafts, rolls, crankshafts and turbine rotors are balanced on these machines in production and during overhaul.
A balancing machine has four main parts: the supports (bearings) that carry the rotor, the drive, the vibration or force sensors with a reference angle sensor, and the measuring unit that shows the result as vectors.
Horizontal balancing machines
The rotor turns horizontally on two supports, usually roller carriages or V-blocks, carried on its journals or its own bearing seats. Motor and generator rotors, fan and pump rotors on shafts, rolls, cardan shafts and crankshafts are balanced on horizontal machines.
Capacities range from rotors of a few grams to turbine rotors of tens of tonnes. Besides rotor mass, the maximum swing diameter, the bearing span and the journal diameter range must also fit the machine.
Vertical balancing machines
The rotor sits on an adaptor on a vertical spindle. This suits disc-type parts without a shaft: pulleys, flywheels, brake discs, clutch plates, grinding wheels and fan impellers balanced before they are fitted to a shaft. Loading and correction are quick, which suits series production.
Most vertical machines are single-plane; two-plane models handle wider parts. Any unbalance or centring error of the adaptor adds straight to the result, so adaptor tolerance and indexing (measuring again after turning the part 180°) matter.
Hard-bearing and soft-bearing machines
In hard-bearing machines the supports are stiff and measure the force created by the rotor's unbalance. The machine runs far below the resonance of its suspension. Once the rotor dimensions (plane distances and radii) are entered, the machine is permanently calibrated; a new rotor type needs no trial weight. This is the most common type in industry.
In soft-bearing machines the supports are flexibly sprung and displacement is measured; the machine runs above the resonance of its suspension. It gives high sensitivity on very light rotors but needs a trial-weight calibration for every new rotor type.
Drive types
- Belt drive: a belt wrapped around the rotor body turns it. No connection to the shaft end is needed and the drive adds little unbalance. Suits rotors with a cylindrical surface (motor rotors, rolls).
- End drive: the rotor is turned from its shaft end by a universal joint shaft. Used for heavy rotors, fan impellers with high windage and irregular bodies a belt cannot wrap. The joint shaft's own unbalance must be compensated.
- Self-drive: a complete electric motor or turbo set runs on its own bearings under its own power, showing the true state of the assembled unit.
- Vertical spindle drive: on vertical machines the part turns with the adaptor on a motorised spindle.
What to check when choosing a machine
- Mass capacity: the rotor mass must sit between the machine's lower and upper limits. A light rotor on a very large machine loses sensitivity.
- Maximum diameter and bearing span: the rotor's swing diameter and the distance between supports must fit.
- Balancing speed range: rigid rotors can be balanced at low speed. On fan impellers, windage and the drive power needed limit the speed.
- Umar and URR: under ISO 21940-21 the minimum achievable residual unbalance (Umar) and the unbalance reduction ratio (URR) are verified with a test rotor. Compare them with the target tolerance.
- Measuring unit: polar vector display, component resolution, weight splitting, tolerance check and reporting.
- Correction method: adding mass (screws, clips, welding) or removing it (drilling, milling, grinding); automatic correction stations in series production.
Balancing machine or field balancing?
A balancing machine is the right route when the rotor can be removed and the part is being made or overhauled. But a rotor balanced on a machine can still vibrate on site: coupling and pulley fitting, temperature, bearing clearances, foundation stiffness and resonance at service speed are not present on the machine.
Field balancing is done without removing the rotor, in its own bearings and at its real service speed. It removes the time spent on dismantling, transport and reassembly, and also corrects unbalance coming from the assembly. It is also used as the final check on site for rotors balanced in a workshop.
How to use
Enter the rotor mass, outside diameter and width (bearing span for rotors on a shaft).
Choose how the rotor is carried on the machine: on a shaft between bearings, overhung impeller or a disc without a shaft.
Enter the service speed and the G grade for the application (e.g. G6.3 for fans and pumps, G2.5 for motor rotors).
The recommended machine type, capacity class and target Umar are a yardstick for comparing machine quotes or choosing a workshop.
Theory & formulas
Uper = 9549 · G · m / n — Uper in g·mm, G in mm/s, m in kg, n in rpm. Specific unbalance eper = Uper / m (µm).
Centrifugal force F = U · ω², with ω = 2π·n/60. The force of a given unbalance grows with the square of speed, so the signal measured at the low balancing speed of a machine is small and the machine's sensitivity (Umar) decides what tolerance is achievable.
The minimum achievable residual unbalance Umar (ISO 21940-21) must sit well below the tolerance. Common practice is Umar ≤ Uper / 4; otherwise measurement uncertainty eats up the tolerance.
Narrow rotors (b/D ≲ 0.25) at moderate speed can usually be balanced in a single plane; longer rotors need two-plane (dynamic) balancing. The recommendations on this page are rules of thumb widely used in industry.
Frequently asked questions
Should I choose a hard-bearing or a soft-bearing balancing machine?
Workshops and production lines that balance many different rotors prefer hard-bearing machines: they are permanently calibrated, so changing rotors needs no trial weight. Soft-bearing machines have an edge on very light, sensitive rotors but need a calibration for each rotor type.
How do I size a balancing machine?
The rotor mass should fall inside the machine's mass range, preferably with some margin below the upper limit. Check maximum diameter, bearing span, journal diameter and required drive power as well. The machine's Umar should be below a quarter of the target tolerance (Uper).
Why does a rotor balanced on a machine vibrate on site?
The machine only corrects the rotor's own unbalance. Eccentric coupling or pulley fitting, key clearance, bearing and housing tolerances, a loose foundation, misalignment and a resonance close to service speed are new vibration sources on site. Then a field measurement and, if needed, field balancing are required.
When is single-plane balancing enough?
For disc-type rotors that are narrow compared to their diameter (roughly b/D ≤ 0.25) at moderate speeds, single-plane (static) balancing is usually enough. On longer rotors unbalance forms a couple (moment unbalance), which only two-plane (dynamic) balancing removes.