Neodymium magnet segments
These magnets are quite unusual, but you will find everything in our range, including these segment-shaped neodymium magnets, where the magnetic field is perpendicular to the surface.
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Alpha angle (°)
Length L (mm)
outer radius R (mm)
Temperature resistance range
List of products
1Read the four values in the product name
A segment described as R11,30xr10,30x20°x3 has an outer radius R of 11.30 mm, an inner radius r of 10.30 mm, an alpha angle of 20 degrees and a length L of 3 mm.
Capital R is always the outer radius and lowercase r the inner one. All four are listed in the product parameters as well, so you can filter by outer radius, angle or length.
2The wall thickness is the difference between R and r
Subtract the inner radius from the outer one and you have the thickness of the magnet in the direction of magnetisation. In the example above, 11.30 minus 10.30 gives a wall of exactly 1 mm.
That value drives both the field strength and the fragility of the part. Thin-walled segments are easy to crack, so decide the wall thickness from the magnetic requirement of the assembly rather than choosing the thinnest piece that fits.
3Work out how many segments make a full circle
Divide 360 by the alpha angle. A 20 degree segment gives 18 pieces per circle, 30 degrees gives 12, 45 degrees gives 8 and 90 degrees gives 4.
Angles that are not whole divisors are deliberate: 25.71 degrees corresponds to 14 poles and 14.1 degrees to a design where the segments sit with a gap between them. If your rotor needs a specific pole count, start from that number and work back to the angle.
4Check the magnetisation direction
Segments in this range are magnetised perpendicular to the curved surface, so the poles sit on the inner and outer arcs. Mounted on a rotor, one arc faces the shaft and the other faces the air gap.
The consequence is that a segment behaves completely differently from a flat block: there is no usable pole on the flat end faces, and the piece only performs as intended when it is bonded onto a curved surface of the matching radius.
5Take the working temperature seriously
Segments are mostly used in motors, generators and rotating machinery, which run hot. That is why the great majority of the range is made from the higher SH and UH grades rated for 121 to 150 °C and 151 to 200 °C, rather than the standard 80 °C material.
Establish the real temperature inside your machine, including the rise under continuous load, and choose the grade from that figure. A grade chosen too low loses force permanently and the motor never recovers its rated output.
6Plan the bonding and retention
- Bond segments with a two-component epoxy rated for the working temperature of the machine, not a general purpose adhesive.
- Use a jig to keep the spacing even, because each segment pushes its neighbour away during assembly.
- Alternate the polarity according to the pole pattern of your design and mark each piece before you start.
- On anything that spins fast, add a retaining band or sleeve: centrifugal force loads the adhesive in shear, which is its weakest direction.
Tip: Order a few spare segments. They chip at the thin edges during assembly, and a part with a cracked corner is not worth bonding into a rotor you will then have to take apart again.
Related categories to the Neodymium magnet segments category
- Selection by temperature resistance
- Temperature resistance 41 to 80 °C
- Temperature resistance 81 to 120 °C
- Temperature resistance 121 to 150 °C
- Temperature resistance 151 to 200 °C
- Heat-resistant magnets
- Samarium magnets
- Neodymium ring magnets
- Neodymium cylinder magnets
- Neodymium block magnets
- Atypical neodymium magnets
- Cylinders with hole
- All neodymium magnets
Frequently asked questions about neodymium magnet segments
Capital R is the outer radius, lowercase r the inner radius, the degree value is the alpha angle of the arc and the last number is the length L. A segment listed as R11,30xr10,30x20°x3 has an outer radius of 11.30 mm, an inner radius of 10.30 mm, an arc of 20 degrees and a length of 3 mm.
Divide 360 by the alpha angle. A 20 degree segment needs 18 pieces, 30 degrees needs 12, 45 degrees needs 8 and 90 degrees needs 4. Angles such as 25.71 degrees are designed for a specific pole count, in this case 14, and angles like 14.1 degrees are meant to sit with a gap between neighbouring poles rather than filling the circle completely.
It is the outer radius minus the inner radius. For R11,30xr10,30 the wall is exactly 1 mm. This dimension sits in the direction of magnetisation, so it drives the field strength, and it is also the reason thin-walled segments need careful handling during assembly.
Perpendicular to the curved surface, so the poles sit on the inner and outer arcs. On a rotor, one arc faces the shaft and the other faces the air gap. There is no usable pole on the flat end faces, which is why a segment cannot substitute for a block magnet in a flat holding application.
Because they go into motors, generators and rotating machinery, where the temperature routinely exceeds the 80 °C limit of standard N grade material. Most of the range is therefore made from SH and UH grades rated for 121 to 150 °C and 151 to 200 °C. Choose the grade from the real temperature inside your machine under continuous load, not from the ambient temperature.
Bond them with a two-component epoxy rated for the working temperature of the machine, position them with a jig so the spacing stays even, and mark the polarity of each piece before assembly. On fast rotating parts add a retaining band or sleeve, because centrifugal force loads the adhesive in shear, which is its weakest direction.
Magnet segments are a component for rotors, motors and generators, and our range reflects that: outer radii from under 5 mm to over 30 mm, alpha angles from 10 to 89 degrees, lengths from 1.6 to 40 mm and, above all, grades rated mostly for 121 to 200 °C rather than the standard 80 °C material. Each product states the outer radius R, inner radius r, alpha angle, length L, coating and maximum working temperature, so you can size the pole count and the wall thickness before you order. Shapes outside the catalogue are quoted on enquiry.
