It is the number most buyers skim past and the one that decides whether a crane lasts fifteen years or fails in three. Here is what it means without the standards jargon.
What duty class measures (it is not the weight)
Duty class describes how hard the crane works, not how heavy a single lift is. It combines three things: how often the crane moves (cycles per hour), how fully loaded it usually runs, and how many hours a day it operates. A crane that lifts 20 t twice a week is lightly loaded; a crane that lifts 3 t four hundred times a shift is not.
The same idea is expressed three ways depending on the standard: A1–A7 in the Chinese classification, FEM 1Am–2m in the European one, and ISO M3–M7 internationally. Roughly: A3–A5 ≈ FEM 1Am–1Bm ≈ ISO M3–M5, and A6–A7 ≈ FEM 1Am–2m ≈ ISO M6–M7.
Which class fits which process
- A3–A4 — a maintenance crane or an installation crane. Lifts a few times a day, mostly light loads, and stands idle in between.
- A5 — the ordinary workshop crane on one shift. Intermittent work, moderate loads, several lifts an hour.
- A6 — production duty on two or three shifts. The hook is moving most of the day and the loads are a large fraction of capacity.
- A7 — continuous duty: grab cranes in waste plants, ladle cranes in foundries, cranes feeding a process where stopping the crane stops the plant.
If you are unsure, describe the process rather than guessing a class. A supplier who will not classify the duty with you in writing, and put it on the offer, is leaving the risk with you.
What actually changes with the class
Duty class is not a label added to the same machine. Going from A5 to A6 or A7 changes:
- Motor ratings, insulation class and the number of starts per hour the motor tolerates
- Reducer size and service factor — a smaller reducer on heavy duty fails at the gear teeth
- Brake size and lining area: the brake is usually the first thing to overheat on a hard-working crane
- Rope safety factor and drum diameter
- Wheel material and bearing selection
- Girder stiffness and the number of load cycles the structure is analysed for
- On grab and magnet duty: the whole structure and the rope reeving
That is why two cranes of identical capacity can differ in price by a third: they are not the same machine.
The mistake to avoid in both directions
Under-specifying a light class for heavy work is the classic error. The crane runs for a year or two, then motors overheat, reducers leak, brakes wear out and the structure starts to show fatigue. Nobody will call it a manufacturing defect, because the duty class was the cause — and no warranty covers a duty class mismatch.
Over-specifying is only money. If you buy A7 for a warehouse crane used four hours a day, you paid for drives and structure you will never use. Both mistakes are avoided the same way: describe the process honestly — loads, cycles per hour, shifts per day and what happens if the crane stops — and ask the supplier to state the class on the quotation.
Questions about your own project? Send the capacity, span, lifting height and duty and we will tell you honestly what we would specify — send an inquiry or message us on Zalo.