BIZOMAT MAKEOS · Mechanics
Permissible Residual Unbalance (Balance Grade)
How much residual unbalance is allowed for this rotor and balance grade?
Permissible residual unbalance and specific unbalance for a rigid rotor from its mass, maximum service speed and balance quality grade G, compared with a measured value.
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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 rotating assembly being balanced. |
| Maximum service speed | rpm | Highest speed the rotor runs in service. |
| Balance quality grade | choice | Grade from the equipment standard, purchase specification or ISO 21940-11 guidance for the machine type. |
| Correction planes | choice | One plane for narrow discs; two planes for longer rotors. Unequal plane positions need the standard’s allocation rules. |
| Measured residual unbalance per plane (0 = not measured) | g·mm | Residual unbalance per plane from the balancing machine report. |
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: the rotor with everything that turns with it.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 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
- The permissible residual unbalance rises in proportion to the mass.
- Get it right
- Include fitted parts such as pulleys or fans if they are balanced with the rotor.
What this means: Maximum service speed
- What it is
- The highest speed at which the rotor runs in service, which sets how tight the balance must be.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 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.
- Get it right
- Use the maximum service speed, not the speed of the balancing machine.
What this means: Balance quality grade
- What it is
- The balance quality grade (G number) the rotor must meet. A smaller G number means a tighter balance requirement.
- Units and format
- Choose one of 7 options from the list: G 0.4; G 1; G 2.5; G 6.3; G 16; G 40; G 100.
- Example
- The form opens with “G 6.3” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- The permissible unbalance rises in proportion to the G number.
- Get it right
- Take the grade from the equipment standard, purchase specification or ISO 21940-11 for the machine type.
What this means: Correction planes
- What it is
- Whether the unbalance is corrected in one plane, for a narrow disc, or in two planes, for a longer rotor. With two planes the tool gives each plane half of the total.
- Units and format
- Choose one of 2 options from the list: One plane (narrow rotor); Two planes, equal share (CG midway).
- Example
- The form opens with “One plane (narrow rotor)” selected as a worked example. Choose the option that matches your case.
- Get it right
- The equal split is valid only when the centre of mass lies midway between the planes; otherwise use the standard’s allocation rules.
What this means: Measured residual unbalance per plane (0 = not…
- What it is
- The residual unbalance left in each correction plane after balancing, as reported by the balancing machine. Zero means no measurement has been entered.
- Units and format
- Enter a number in g·mm: g·mm. The form accepts 0 g·mm or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 g·mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the value per plane in g·mm; the tool compares it with the permissible value per plane.
Method and formulas
- Permissible specific unbalance e = G × 1000 ÷ ω (µm, = g·mm/kg), ω = 2πn ÷ 60.
- Permissible residual unbalance U = e × M = 9549 × G × M ÷ n (g·mm).
- With two planes and the centre of mass midway, each plane receives half of U.
- Example: a 25 kg rotor at 3000 rpm in G 6.3 may have 501.34 g·mm residual unbalance (20.054 µm).
Limitations
- Rigid rotors only; flexible rotors need ISO 21940-12 methods.
- Equal allocation to two planes applies only when the centre of mass lies midway between them.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the balancing tolerance on a drawing or work order
- Accept or reject a balancing report
- Compare grades for a fan, pulley or spindle part
Who it is for
- Maintenance engineers
- Machinists
- Quality inspectors
What you get
- Permissible residual unbalance (total and per plane)
- Permissible specific unbalance (eccentricity)
- Measured result against the limit
Interpreting the result
- Convert the limit into a correction mass at a radius: mass (g) = U (g·mm) ÷ radius (mm).
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 191–196. Topic reference; no source tables or recommended settings are reproduced.
- ISO 21940-11:2016 Mechanical vibration — Rotor balancing — Part 11: Procedures and tolerances for rotors with rigid behaviour — https://www.iso.org/standard/54074.html
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 balance grades
In Whirling Shafts by the Numbers
What it covers: What unbalance is, the rotating force it makes, how balance quality grades turn a rotor's mass and speed into a permissible residual unbalance, and what one lost hammer does to a hammer mill.
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