Neodymium (NdFeB) magnets are the strongest commercially available type of permanent magnet, composed of an alloy of neodymium, iron, and boron. Their superior strength results from a crystalline structure that allows for exceptionally high magnetic energy density and resistance to demagnetization. While manufacturers provide a theoretical maximum holding capacity, the actual weight a magnet can support in a real-world setting is often significantly lower.
Defining Magnet Holding Power
The strength of a magnet is measured by two distinct metrics. The primary measure is Pull Force, defined as the maximum force required to separate the magnet from a thick, flat, ferrous steel plate when the force is applied perpendicularly. This measurement provides a standardized benchmark for comparing magnet strength under ideal, laboratory conditions. For example, a 1-inch diameter N52-grade disc magnet may have a theoretical pull force exceeding 90 pounds.
A different and often more relevant measurement for everyday use is Shear Force, which is the force required to slide the magnet sideways across the surface. When a magnet is mounted vertically, the weight applies a shearing force parallel to the surface. Shear force is largely dependent on the friction between the magnet and the surface and is dramatically lower than the pull force.
The effective holding capacity in a vertical application is typically only about 15% to 30% of the rated pull force. This lower capacity is the limiting factor when supporting a load against gravity. Because shear force is highly dependent on surface conditions, manufacturers cannot provide a single, universal shear rating.
The pull force measurement represents the absolute maximum capacity under perfect circumstances and is useful for comparing magnet quality. In contrast, the shear force represents the real-world limit when suspending an object against a vertical surface. The actual weight a magnet holds is almost always dictated by its resistance to sliding, not its resistance to being pulled straight off.
Internal Characteristics Affecting Strength
The inherent maximum strength of a Neodymium magnet is determined by its composition and physical structure. The most significant factor is the Magnet Grade, indicated by the “N-rating” system, which ranges commercially from N35 up to N55. The number after the “N” represents the Maximum Energy Product (MGOe), reflecting the magnetic energy density the material can store.
A higher N-rating signifies a greater energy product, allowing the magnet to generate a stronger magnetic field and a higher pull force for the same size. For example, an N52 magnet is substantially stronger than an N35 magnet of identical dimensions. This grading system allows users to select a magnet with the ideal balance of strength and cost for a specific application. Some grades also include letters, such as N42SH, which denote enhanced temperature tolerance, affecting performance under thermal stress.
The Size and Volume of the magnet are fundamental in determining its maximum theoretical strength. A larger magnet contains more magnetic material, leading to a greater overall magnetic field. Increasing the surface area in contact with the ferrous material directly enhances the pull force. The thickness of the magnet is also important, as it ensures the magnet is magnetically saturated, generating its maximum possible field.
The Shape of the magnet influences how magnetic flux lines interact with the target surface, affecting the final pull force calculation. Common shapes include discs, blocks, and rings. The size of the contact surface area is a direct multiplier of the holding capacity, meaning a magnet with a larger face contacting the steel will hold more weight.
External Variables That Reduce Capacity
The ideal pull force rating rarely matches the weight a magnet holds in a practical setting due to several external variables. The most significant factor is the Air Gap, which is any non-magnetic distance between the magnet face and the ferrous surface. This gap can be literal air or a layer of paint, rust, paper, or plating.
The magnetic field strength decreases rapidly as the distance from the magnet increases. Even a tiny air gap of 0.1 millimeters can reduce the theoretical pull force by 30% to 50%. This dramatic reduction explains why a magnet applied to a thick layer of paint holds far less than one applied to bare steel.
The Surface Condition of the target object is important to the holding capacity. Maximum pull force requires a clean, smooth, and flat contact surface made of thick, mild steel. Rough surfaces, such as cast iron or rusty metal, create small air gaps and reduce the effective contact area, significantly lowering the holding power. If the steel is too thin, it can become magnetically saturated, preventing the magnet from exerting its full force.
The Angle of Pull dictates whether the magnet’s force is applied as pure pull force or as a combination of pull and shear. If the force is applied at any angle other than 90 degrees perpendicular to the surface, the effective load shifts toward the weaker shear capacity. This angular loading is a common reason why magnets fail to hold their rated weight in lifting or hanging applications.
Temperature is another external factor that can reduce the strength of a Neodymium magnet, sometimes permanently. Standard Neodymium grades operate up to about 80°C (176°F). Exposure to temperatures above this maximum operating limit can cause an irreversible loss of magnetic strength, meaning a standard magnet used in a hot environment will perform significantly below its rating.
Practical Holding Capacity and Safety
When considering practical holding capacity, real-world examples under near-ideal conditions are helpful. A small Neodymium disc magnet (10mm by 5mm) can hold over 4 pounds (2 kilograms) when pulled straight off a thick steel plate. For industrial purposes, a large N52 magnet block (50mm by 50mm by 25mm) can generate a pull force over 220 pounds (100 kilograms).
Safety considerations are paramount due to the extreme strength of these magnets. Neodymium magnets can snap together with enough force to pinch fingers severely, potentially causing serious injuries. Eye protection is recommended when handling larger magnets, as their brittle nature means they can chip or shatter if allowed to collide.
These powerful magnetic fields also pose risks to electronic devices and medical implants. They must be kept away from pacemakers and other implanted electronic devices, as the magnetic field can interfere with their function. Strong Neodymium magnets can also damage magnetic media, such as credit cards and older computer hard drives.