Neodymium magnets are graded not only by strength (N35 up to N52) but also by how much heat they can tolerate before losing magnetism. For motor applications, temperature class is often the deciding factor — not raw pull force.
Standard vs. SH, UH and EH grades
A standard N42 magnet is rated for continuous use up to around 80°C. Once a motor's internal temperature regularly exceeds that — common in industrial drives, automotive actuators and continuously loaded servo motors — the magnet needs a higher temperature class: SH (150°C), UH (180°C) or EH (200°C).
Using an underrated grade in a hot motor doesn't just reduce output — it can cause partial, permanent demagnetization over time, which is expensive to diagnose after the fact.
Practical rule of thumb
When specifying magnets for a new motor design, always check the magnet's expected operating temperature under full continuous load (not just ambient temperature), and leave a safety margin of at least 20–30°C below the grade's rated maximum.