In This Article
- 01Where the rotor magnet enters the balance budget
- 02What the balance grade asks of the parts
- 03Where the unbalance actually comes from
- 04One-piece rings against segmented rings
- 05What to put on the drawing
- 06Correction, and why the magnet is the wrong place to take material off
- 07Specifying the magnet side of a balance spec
- FAQFrequently Asked Questions
Key Takeaways
- ◆Unbalance is mass times radius, and on a surface-mounted PM rotor the magnet sits at the largest radius on the shaft. The same gram of asymmetry costs more residual unbalance in the magnet than in the lamination stack.
- ◆For rigid rotors, ISO 21940-11 relates balance grade and speed through e_per = G/ω. G2.5 at 30,000 rpm is about 0.796 µm.
- ◆The allowance falls in inverse proportion to speed. The same balance grade at 30,000 rpm permits a tenth of the eccentricity it permits at 3,000 rpm.
- ◆Segmented rings carry mass asymmetry that one-piece sintered rings do not have: segment-to-segment scatter plus adhesive bond line thickness variation, distributed around the circumference in whatever pattern the assembly fixture produced.
- ◆The radial-ring TDS, Rev A of 31 July 2026, states OD 20 to 75 mm, ID 15 to 68 mm, wall 2 to 7 mm and height 5 to 50 mm, with ±0.05 mm standard dimensional tolerance within the qualified route. These limits are not freely combinable. Confirm the drawing, grade, pole pattern and individual features before quotation. The separate integrated-Halbach route states ±0.02 mm on critical features, subject to feature and drawing confirmation.
- ◆Specify permitted balance-correction locations away from the magnet and its corrosion barrier.
Where the rotor magnet enters the balance budget
Rotor unbalance depends on mass distribution about the rotation axis. Magnet wall variation, segment mass variation, adhesive and mounting datums can all contribute. On a surface-mounted rotor, magnet material is relatively far from the axis, so its asymmetry can have a material effect. Allocate a balance budget to the complete assembly, including sleeve and correction features.
What the balance grade asks of the parts
For rotors with rigid behaviour, ISO 21940-11:2016, with its 2022 amendment replaces ISO 1940-1:2003. Permissible residual specific unbalance is e_per = G/ω. With G in mm/s and speed n in rpm, e_per in µm is approximately 9550G/n. G2.5 at 30,000 rpm gives 0.796 µm.
Multiply by rotor mass in kg to obtain residual unbalance in g·mm: a 1 kg rotor gives 0.796 g·mm. This is the total rotor allowance, before allocation to correction planes. Select the required grade, speed and plane allocation for the actual machine; a loose magnet has no independent rotor balance grade.
Where the unbalance actually comes from
Separate mass asymmetry from mounting eccentricity. Measure the finished magnet and the assembled rotor against their functional datums. Bond thickness, segment distribution and hub location can affect the result even when each loose part meets its dimensional limits. The required limits follow from the complete rotor balance budget.
One-piece rings against segmented rings
One-piece and segmented constructions have different sources of variation. A one-piece ring removes segment-to-segment and joint variation, but still needs dimensional control and correct mounting. Segmented rings require control of each segment and bond line. Compare the complete assembly’s capability and cost; no construction alone guarantees a balance grade.
What to put on the drawing
The radial-ring TDS, Rev A of 31 July 2026, states OD 20 to 75 mm, ID 15 to 68 mm, wall 2 to 7 mm and height 5 to 50 mm, with ±0.05 mm standard dimensional tolerance within the qualified route. These limits are not freely combinable. Confirm the drawing, grade, pole pattern and individual features before quotation.
The separate integrated-Halbach route states ±0.02 mm on critical features, subject to feature and drawing confirmation. Define wall variation, coaxiality and cylindricity against functional assembly datums. Confirm their individual tolerances rather than inferring them from ±0.05 mm. For a bonded rotor, specify concentricity to the shaft and measurement after assembly.
Check the current certificate for the actual manufacturing entity, address and activity. Agree PPAP level and required reports for the order; a network relationship does not certify every plant or assembly.
Correction, and why the magnet is the wrong place to take material off
Define permissible correction locations on the rotor drawing. Removing magnet material can damage the corrosion barrier, create chips and alter the magnetic field. Use designated correction features and an agreed balancing method for the actual rotor. Check the required correction planes and retention of any added mass.
Specifying the magnet side of a balance spec
Send the operating point and the rotor build and we can size the magnet's share of the balance budget while the drawing is still open, rather than after the first balance report. Engineering response is within 1 business day, pricing within 2 business days. What we need:
- ●Target balance grade and the speed it applies at, plus rotor mass if you have it.
- ●Rotor construction: one-piece ring, segmented ring, or arcs on a hub, and whether a retention sleeve is fitted.
- ●Ring geometry: OD, ID, wall, height, and which of those the mechanical package fixes.
- ●How the magnet is located and by what: shoulder, hub, adhesive, sleeve interference, and who performs that assembly.
- ●The wall thickness variation, coaxiality and cylindricity you need, with a note on which one drives your balance calculation.
- ●Correction planes available on the rotor, and where material may be removed.
- ●Continuous and peak operating temperature, since the adhesive and the temperature class get chosen against the same numbers.
- ●Volume, PPAP level, and whether you want the magnet loose or as a bonded sub-assembly.
Frequently Asked Questions
What does G2.5 at 30,000 rpm mean?
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For rotors with rigid behaviour, ISO 21940-11:2016, with its 2022 amendment replaces ISO 1940-1:2003. Permissible residual specific unbalance is e_per = G/ω. With G in mm/s and speed n in rpm, e_per in µm is approximately 9550G/n. G2.5 at 30,000 rpm gives 0.796 µm. Multiply by rotor mass in kg to obtain residual unbalance in g·mm: a 1 kg rotor gives 0.796 g·mm. This is the total rotor allowance, before allocation to correction planes. Select the required grade, speed and plane allocation for the actual machine; a loose magnet has no independent rotor balance grade.
Does a loose magnet have a rotor balance grade?
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The specified balance grade applies to the complete rotor. Allocate contributions and correction planes using the rotor design.
What dimensions should a ring drawing include?
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The radial-ring TDS, Rev A of 31 July 2026, states OD 20 to 75 mm, ID 15 to 68 mm, wall 2 to 7 mm and height 5 to 50 mm, with ±0.05 mm standard dimensional tolerance within the qualified route. These limits are not freely combinable. Confirm the drawing, grade, pole pattern and individual features before quotation. The separate integrated-Halbach route states ±0.02 mm on critical features, subject to feature and drawing confirmation.
Does a one-piece ring guarantee better balance?
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One-piece and segmented constructions have different sources of variation. A one-piece ring removes segment-to-segment and joint variation, but still needs dimensional control and correct mounting. Segmented rings require control of each segment and bond line. Compare the complete assembly’s capability and cost; no construction alone guarantees a balance grade.
Where may balance correction be made?
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Define permissible correction locations on the rotor drawing. Removing magnet material can damage the corrosion barrier, create chips and alter the magnetic field. Use designated correction features and an agreed balancing method for the actual rotor. Check the required correction planes and retention of any added mass.
Send the rotor build, balance grade, operating speed and acceptance requirements for a drawing review.
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