Custom Ring Magnets: OD, ID, Thickness, Magnetisation and Polarity

Ring magnets can appear straightforward: a circular magnet with a hole through its centre. In practice, two rings with the same outer dimensions can produce very different magnetic fields depending on their inner diameter, thickness, material, grade, magnetisation direction and pole arrangement.

GUK Magnetics receives requirements ranging from small sensor rings measuring only a few millimetres to thin 90 mm diameter rings and multipole rings for motor applications. These enquiries demonstrate why “ring magnet” is not a complete specification.

To assess manufacturability, price and expected performance, the requirement should define the outer diameter, inner diameter, thickness, tolerances, material, coating and complete magnetisation pattern.

Key takeaway: OD, ID and thickness define the ring’s geometry. Magnetisation direction and polarity define where its usable magnetic field appears.

Outer diameter: overall fit and magnetic volume

The outer diameter normally determines how the ring fits within a housing, rotor, sleeve or other surrounding component. It should be specified as a finished dimension with an appropriate tolerance.

If the ring is fitted inside another component, the drawing should define the intended clearance or interference. Allowance may also be required for the magnet coating, adhesive, a protective sleeve and variation within the mating component.

A larger OD generally provides more magnetic material, but it does not automatically produce a proportionate increase in useful field or force. The result also depends on the ID, thickness, grade, air gap and surrounding magnetic circuit.

Inner diameter: bore, clearance and concentricity

The inner diameter may accommodate a shaft, fastener, bearing, sleeve, fluid passage, optical path or sensor. It is important to distinguish the actual magnet bore from the clearance required by the final assembly.

A ring should not automatically be specified with the same ID as the shaft it surrounds. The assembly may require allowance for adhesive, coating thickness, thermal movement, shaft runout or a separate protective sleeve.

For rotating components, concentricity between the ID and OD can be more important than the individual diameter tolerances. A ring that meets both size limits but has an off-centre bore may still create unacceptable runout or an uneven working air gap.

Thickness: space, air gap and field behaviour

Thickness is the distance between the two flat faces of the ring. It may also be described as height where the component is shown standing vertically.

For an axially magnetised ring, thickness is the magnetisation length. Increasing it changes the magnet’s aspect ratio and can improve the field at some working distances, but the benefit is not necessarily linear. Once the magnetic circuit or surrounding steel approaches saturation, additional material may produce progressively smaller gains.

The selection should therefore be based on the required performance at the real air gap—not magnet volume alone.

Tolerances, coating and finished dimensions

A drawing should identify which dimensions are genuinely critical. Applying unnecessarily tight tolerances to the OD, ID and thickness can increase grinding and inspection costs without improving the finished assembly.

Depending on the application, the relevant controls may include:

• OD, ID and thickness tolerances
• ID-to-OD concentricity
• Face flatness and parallelism
• Radial or axial runout
• Edge chamfers or radii
• Coating thickness and finished coated dimensions

Magnets are hard but brittle. Thin walls, sharp edges and large diameter-to-thickness ratios can increase the risk of chipping or fracture during manufacture, coating, transport and assembly. Small chamfers or edge radii may be preferable to perfectly sharp edges, provided they do not interfere with the magnetic circuit.

Magnetisation direction and polarity are not the same

Magnetisation direction describes the direction in which the magnetic field is established within the magnet. Polarity describes where its north and south poles are located.

A drawing stating only “north pole required” is incomplete. It should show both the magnetisation direction and the required position of the north pole.

Axially magnetised ring magnets

In an axially magnetised ring, the field runs through the thickness. One flat face is north and the opposite face is south. Drawings may describe this as “axially magnetised”, “magnetised through thickness” or “north on marked face”.

Words such as “top” and “bottom” should only be used where the drawing clearly defines the component’s orientation. Referencing a datum or marked face removes this ambiguity.

Diametrically magnetised ring magnets

A diametrically magnetised ring has north and south regions on opposing sides of its circumference. This can be useful where rotation of the ring needs to create a changing magnetic field at a fixed sensor.

If the pole axis must align with another component, the drawing should define its angular position relative to a physical datum.

Radially magnetised ring magnets

In a true radially magnetised ring, the inner and outer curved surfaces have opposing polarities. This is different from diametric magnetisation, where two main pole regions sit on opposite sides of the circumference.

A one-piece radially oriented ring can require a specialist production route and magnetising fixture. For anisotropic sintered magnets, the required material orientation may need to be established during manufacture rather than applied to a conventional ring afterwards.

Multipole ring magnets

A multipole ring contains several alternating north and south poles around a specified surface. The pattern may be axial, with poles on a flat face, or arranged around the inner or outer circumference.

The specification should define the total pole count, pole pitch, working surface, north-south sequence and angular position relative to a datum. Specialist patterns normally require a purpose-built magnetising fixture, which can affect tooling cost, minimum quantities and lead time.

For a fuller explanation of these patterns, see Magnetisation Directions: Axial, Diametric, Radial & Multipole.

How to specify polarity

Magnetisation direction establishes the axis or pole pattern. The drawing must then identify which face or surface should be north.

Where operators need to identify polarity during assembly, specify the marking method. This may be an ink mark, dot, engraved feature or inspection requirement, subject to the coating and application.

How ring dimensions affect magnetic performance

Changing the OD, ID or thickness alters the quantity and distribution of magnetic material. No individual dimension determines performance on its own.

• Increasing OD adds material and pole-face area but also changes the overall assembly envelope.
• Reducing ID increases wall thickness and magnetic material but reduces the available central clearance.
• Increasing thickness changes the magnetisation length for an axial ring, but the benefit depends on working distance and the magnetic circuit.
• Adding steel back iron can redirect and concentrate magnetic flux.
• Adding coatings, sleeves or adhesive increases the effective air gap between the magnet and its target.

A higher material grade is therefore not always the most efficient answer. An N52 ring can have a higher remanence than an N35 ring, but useful field and force still depend on geometry, air gap and the complete magnetic circuit.

Where performance is critical, state what must be achieved and where it should be measured. This could be flux density at a specified distance, pull force against a defined steel target, magnet-to-magnet force at a stated air gap, sensor activation distance, torque or field uniformity through the bore.

Our guide to magnet strength, pull force, gauss, Br and BHmax explains why these terms should not be used interchangeably.

Selecting the material and grade

The grade should be selected against the required working temperature, magnetic circuit and demagnetisation conditions—not simply by choosing the highest available grade.

Countersunk, stepped and non-standard rings

Not every magnet with a central hole should be treated as a simple ring. Custom features can include countersinks, stepped bores, tapered holes, radial slots, keyways, partial rings and arc segments.

Countersunk magnets require particular care because the countersink removes magnetic material and creates a thinner, more fragile region around the hole. The drawing should define the countersink diameter, through-hole diameter, included angle, depth, magnetisation direction and required pole on the countersunk face.

Permanent magnets should not be treated like conventional metal components that can simply be drilled or modified during assembly. Machining can damage the magnet, compromise the protective coating and create a significant risk of fracture.

One-piece ring or segmented assembly?

A one-piece ring can simplify assembly and mechanical balance, but it is not always the most practical manufacturing route. Large diameters, thin walls, radial orientation and complex multipole requirements can make a segmented construction more suitable.

Potential routes include a one-piece sintered ring, a bonded or injection-moulded ring, or an assembly of individually magnetised arc segments. The appropriate option depends on the required output, dimensions, temperature, pole pattern, quantity and acceptable tooling cost.

Custom ring magnet specification checklist

Provide as much of the following information as is available. A drawing is strongly recommended where tolerances, alignment, countersinks or multipole magnetisation are involved.

Example ring magnet specification

The following example demonstrates the level of information that makes a ring magnet enquiry easier to assess. It is illustrative rather than a recommended design.

This provides considerably more information than requesting a “20 mm ring magnet” and allows the manufacturing route, price and lead time to be considered properly.

Custom ring magnets from GUK Magnetics

GUK Magnetics supplies custom ring magnets in neodymium, samarium cobalt, ferrite and other permanent magnet materials, subject to the required geometry and application.

Our role is to review the magnet requirement, advise on manufacturability and identify suitable material, grade, coating, magnetisation or assembly options. Final application performance should be validated within the intended product or magnetic circuit.

Frequently asked questions