The rating system for permanent magnets, such as Neodymium Iron Boron (NdFeB), is a technical classification defining the material’s magnetic performance. Grades, often seen as an “N” followed by a number, move beyond simple pull force to detail the inherent quality of the material. This system allows users to predict performance and compare the potential strength of different permanent magnet materials.
Key Magnetic Properties That Determine Grade
Magnet grades are determined by three physical properties derived from a material’s demagnetization curve. The first property is Remanence (\(B_r\)), which measures the residual magnetic induction left in the material after the external magnetizing force has been removed. Remanence, often measured in Gauss, represents the material’s actual magnetic field density and its potential for strong magnetic output.
The second property is Coercivity (\(H_{cj}\)), which indicates the material’s resistance to demagnetization from an opposing magnetic field or heat. A high coercivity means the magnet will maintain its field strength even when exposed to external forces that attempt to weaken it. This resistance is measured in Oersteds and is important for magnets used in dynamic environments like electric motors.
The most important factor for grading is the Maximum Energy Product (\((BH)_{max}\)), which is the mathematical product of the magnetic induction (\(B\)) and the magnetic field strength (\(H\)). This value represents the maximum amount of magnetic energy the material can store per unit volume. This product is the single figure of merit that correlates most directly with a magnet’s overall strength and is measured in MegaGauss Oersteds (MGOe).
Understanding the Standardized Rating System
The Maximum Energy Product forms the basis for the industry-standard numerical rating system, such as the “N-grade” used for Neodymium magnets. The “N” stands for Neodymium, and the number immediately following it directly correlates with the magnet’s \((BH)_{max}\) value. For example, a magnet graded N42 has a Maximum Energy Product of 42 MGOe.
This numerical value is a theoretical measure of the material’s energy density, with higher numbers indicating a magnetically stronger material. Commercially available Neodymium grades range from N35 up to N52, with N52 representing one of the strongest grades currently in mass production. The numerical grade is a standardized way to compare the inherent quality of different magnetic materials, regardless of their size or shape.
Selecting a higher N-grade, like N52 over N35, means choosing a material that can store and deliver a greater concentration of magnetic energy. This allows for the design of smaller, yet equally powerful, magnets for applications where space is limited. The number in the grade is a direct representation of the magnet material’s strength potential.
Temperature Stability and Material Designations
The letters that sometimes follow the numerical grade indicate the magnet’s thermal stability and resistance to demagnetization at higher temperatures. This stability is a measure of the material’s intrinsic coercivity, which is its ability to resist demagnetization when heated. Standard N-grade magnets, which have no letter suffix, have a maximum operating temperature of about 80°C.
Manufacturers use suffixes like M, H, SH, UH, EH, and TH to denote increasing temperature tolerance. For example, an “H” grade magnet (e.g., N42H) is certified to operate reliably up to 120°C, while an “SH” grade can handle up to 150°C. The highest temperature grades, such as EH or TH, can withstand temperatures reaching 200°C to 230°C.
Achieving this higher temperature stability often requires modifications to the magnet’s alloy composition, which can result in a slight trade-off in magnetic strength. Therefore, a magnet with a high thermal rating, like N35EH, might have a lower numerical strength rating than a standard N52 magnet. The letter suffix is important for industrial applications where the magnet will be exposed to significant heat, such as in motors or generators.
Relating Technical Grades to Practical Pull Force
While the N-grade defines the material quality in terms of its energy density (MGOe), the actual, measurable holding power is the practical pull force, measured in pounds or kilograms. This pull force is the maximum weight a magnet can hold when in direct, perpendicular contact with a thick, flat steel plate. The technical grade is only one factor influencing this real-world measurement.
A magnet’s geometry—its shape, size, and thickness—plays an equally large role in determining the final pull force. A large magnet with a lower grade, such as a thick N35 disc, may exhibit a greater pull force than a small, thin disc made from a higher grade, like N52. This occurs because the larger volume of the magnetic field material compensates for the lower energy density.
The presence of an air gap, even a tiny one caused by paint or a thin coating on the steel surface, significantly reduces the pull force. Therefore, the theoretical MGOe rating provides the ceiling for performance, but the physical dimensions and the conditions of the magnetic circuit determine the final, usable force. Consumers must consider both the grade and the magnet’s physical size when selecting a magnet for a specific task.