BIZOMAT MAKEOS · Mechanics
Rotor Balancing Check (Grade, Plane Unbalance and Corrections)
Is this rotor within its balance grade in each correction plane, and if not, what correction masses go where?
Two-plane balancing check of a rigid rotor: permissible residual unbalance from the balance quality grade, mass and speed, split equally between two correction planes (as the balance grade tool), the unbalance in each plane resolved from up to three known or measured unbalance masses (as the two-plane balance tool), the correction mass and angle in each plane at its correction radius, the rotating force the initial unbalance puts on the bearings, and whether the corrections fit within your largest practical correction mass.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Rotor mass | kg | Mass of the rotor. |
| Service speed | rpm | Highest service speed. |
| Balance quality grade (G number) | mm/s | Balance quality grade for the machine type, e.g. 6.3 or 2.5 (your value from ISO 21940-11 / ISO 1940-1). |
| Distance between correction planes A and B | mm | Axial distance between the two correction planes. |
| Correction radius in plane A | mm | Radius at which weights are added or removed in plane A. |
| Correction radius in plane B | mm | Radius at which weights are added or removed in plane B. |
| Largest practical correction mass | g | Largest single weight you can fit or remove at the correction radius. |
| Unbalance masses (from plane A) | ||
| Unbalance 1 mass | g | Mass of unbalance 1 (0 if not used). |
| Unbalance 1 radius | mm | Radius of unbalance 1. |
| Unbalance 1 angle | ° | Angular position of unbalance 1. |
| Unbalance 1 distance from plane A | mm | Axial distance of unbalance 1 from plane A towards plane B. |
| Unbalance 2 mass | g | Mass of unbalance 2 (0 if not used). |
| Unbalance 2 radius | mm | Radius of unbalance 2. |
| Unbalance 2 angle | ° | Angular position of unbalance 2. |
| Unbalance 2 distance from plane A | mm | Axial distance of unbalance 2 from plane A towards plane B. |
| Unbalance 3 mass | g | Mass of unbalance 3 (0 if not used). |
| Unbalance 3 radius | mm | Radius of unbalance 3. |
| Unbalance 3 angle | ° | Angular position of unbalance 3. |
| Unbalance 3 distance from plane A | mm | Axial distance of unbalance 3 from plane A towards plane B. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Rotor mass
- What it is
- The mass of the complete rotating assembly being balanced: shaft, rotor body and anything fixed to it that turns with it. Take it from the drawing, the CAD model or by weighing.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 100000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Permissible unbalance per plane is proportional to rotor mass, so a heavier rotor is allowed more g·mm.
- Get it right
- Use the rotor alone, not the whole machine with its stationary housing.
What this means: Service speed
- What it is
- The highest speed the rotor runs at in service. The balance grade is defined at this speed, so it sets how much residual unbalance is allowed and the rotating force on the bearings.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 200000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher speed lowers the permissible unbalance in proportion and raises the rotating force with the square of speed.
- Get it right
- Use the maximum service speed, not the speed of the balancing machine.
What this means: Balance quality grade (G number)
- What it is
- The balance quality grade for this type of machine: the permitted product of eccentricity and angular speed, expressed as a G number. Take it from ISO 21940-11 (formerly ISO 1940-1) or the customer specification.
- Units and format
- Enter a number in mm/s: millimetres per second. The form accepts 0.1 mm/s to 4000 mm/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.3 mm/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Permissible eccentricity and unbalance are proportional to G.
- Get it right
- Type the number after the G only: G6.3 is entered as 6.3. A smaller G number is a finer balance.
What this means: Distance between correction planes A and B
- What it is
- The axial distance along the shaft between correction plane A and correction plane B, the two planes where you add or remove weight.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure between the planes where the weights actually sit, not between the bearings.
What this means: Correction radius in plane A
- What it is
- The radius in plane A, from the shaft axis to where correction weight is added or metal removed. The correction mass for plane A is its unbalance divided by this radius.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger correction radius reduces the correction mass needed in plane A.
- Get it right
- This is a radius from the axis, not a diameter across the rotor.
What this means: Correction radius in plane B
- What it is
- The radius in plane B, from the shaft axis to where correction weight is added or metal removed. The correction mass for plane B is its unbalance divided by this radius.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger correction radius reduces the correction mass needed in plane B.
- Get it right
- This is a radius from the axis, not a diameter across the rotor.
What this means: Largest practical correction mass
- What it is
- The heaviest single correction weight you can fit, or the most metal you can remove, at the correction radius. Corrections larger than this are flagged as too large.
- Units and format
- Enter a number in g: grams. The form accepts 0.000001 g to 100000 g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Base it on the space, thread size or drilling depth available at the correction position.
Unbalance masses (from plane A)
What this means: Unbalance 1 mass
- What it is
- The mass of the first known or measured unbalance on the rotor, such as a key, an offset feature or a trial-run result expressed as an equivalent mass.
- Units and format
- Enter a number in g: grams. The form accepts 0 g to 100000 g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Its contribution to plane unbalance is mass × radius, so doubling it doubles its effect.
- Get it right
- Enter 0 to leave this unbalance out; its radius, angle and distance are then ignored.
What this means: Unbalance 1 radius
- What it is
- The distance from the shaft axis to the centre of gravity of unbalance 1.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the axis of rotation, not from the outside surface.
What this means: Unbalance 1 angle
- What it is
- The angular position of unbalance 1 around the shaft, measured from a reference mark on the rotor in a consistent direction.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 360°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the same zero mark and direction for every unbalance; the correction angles are reported on that same scale.
What this means: Unbalance 1 distance from plane A
- What it is
- The axial position of unbalance 1, measured along the shaft from plane A towards plane B. It decides how unbalance 1 is shared between the two planes.
- Units and format
- Enter a number in mm: millimetres. The form accepts -10000 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from plane A, not from a shaft end or a bearing.
What this means: Unbalance 2 mass
- What it is
- The mass of the second known or measured unbalance on the rotor, expressed as an equivalent point mass at its radius.
- Units and format
- Enter a number in g: grams. The form accepts 0 g to 100000 g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Its contribution to plane unbalance is mass × radius, so doubling it doubles its effect.
- Get it right
- Enter 0 to leave this unbalance out; its radius, angle and distance are then ignored.
What this means: Unbalance 2 radius
- What it is
- The distance from the shaft axis to the centre of gravity of unbalance 2.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the axis of rotation, not from the outside surface.
What this means: Unbalance 2 angle
- What it is
- The angular position of unbalance 2 around the shaft, measured from the same reference mark and in the same direction as the other unbalances.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 360°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the same zero mark and direction for every unbalance; the correction angles are reported on that same scale.
What this means: Unbalance 2 distance from plane A
- What it is
- The axial position of unbalance 2, measured along the shaft from plane A towards plane B. It decides how unbalance 2 is shared between the two planes.
- Units and format
- Enter a number in mm: millimetres. The form accepts -10000 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from plane A, not from a shaft end or a bearing.
What this means: Unbalance 3 mass
- What it is
- The mass of an optional third known or measured unbalance on the rotor, expressed as an equivalent point mass at its radius.
- Units and format
- Enter a number in g: grams. The form accepts 0 g to 100000 g. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 g as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Its contribution to plane unbalance is mass × radius, so doubling it doubles its effect.
- Get it right
- Enter 0 to leave this unbalance out; its radius, angle and distance are then ignored.
What this means: Unbalance 3 radius
- What it is
- The distance from the shaft axis to the centre of gravity of unbalance 3.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the axis of rotation, not from the outside surface.
What this means: Unbalance 3 angle
- What it is
- The angular position of unbalance 3 around the shaft, measured from the same reference mark and in the same direction as the other unbalances.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 360°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the same zero mark and direction for every unbalance; the correction angles are reported on that same scale.
What this means: Unbalance 3 distance from plane A
- What it is
- The axial position of unbalance 3, measured along the shaft from plane A towards plane B. It decides how unbalance 3 is shared between the two planes.
- Units and format
- Enter a number in mm: millimetres. The form accepts -10000 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from plane A, not from a shaft end or a bearing.
Method and formulas
- Permissible eccentricity e = G ÷ ω; permissible unbalance U = e × mass, half to each plane (as the balance grade tool).
- Resolve each unbalance m r at angle a into the planes by moments about plane A (as the two-plane balance tool): U_B = −Σ m r z ÷ L, U_A = −Σ m r − U_B (as vectors).
- Correction mass = U ÷ correction radius, at the angle of the correcting vector.
- Rotating force = U ω² (U in kg·m).
- Example: grade G6.3 at 3000 rpm allows 250.67 g·mm per plane; plane A carries 2,704.2 g·mm and needs 27.042 g at 193.9°.
Limitations
- Rigid rotor below its first bending critical speed; flexible rotors need modal balancing.
- Equal split of the permissible unbalance; ISO allows an allocation by bearing distances.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Decide whether a rotor needs balancing for its grade
- Work out correction weights in two planes
- Estimate the bearing force from unbalance
Who it is for
- Maintenance engineers
- Balancing technicians
- Design engineers
What you get
- Permissible residual unbalance per plane
- Unbalance and corrections in each plane
- Bearing force from the initial unbalance
Interpreting the result
- Correct at the largest radius available: the correction mass falls in proportion.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 191–195. Topic reference; no source tables or recommended settings are reproduced.
- ISO 21940-11 Mechanical vibration — Rotor balancing — Procedures and tolerances for rotors with rigid behaviour
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Unbalance and balancing in place
In Vibration Monitoring by the Numbers
What it covers: How unbalance makes a 1× vibration, the force on the bearings, balance quality grades, single-plane balancing with a trial weight and vectors, splitting a correction, and a trial weight left on.
Method explained here · BIZOMAT original book
Balancing long rotors in two planes
In Whirling Shafts by the Numbers
What it covers: Why a long rotor needs corrections in two planes: resolving unbalances by moments, checking each plane against its grade, weighing hammer sets, and why a static balance left a couple behind.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Rotor Balancing Check (Grade, Plane Unbalance and Corrections) calculator