천장 크레인 사용 등급(FEM / ISO) 해설

Crane duty classification defines how hard a crane will work over its lifetime. Specifying the correct class is critical: under-specify and the crane wears out early; over-specify and you pay for capacity you don't use.

What the duty class describes

The duty class combines two factors: the state of loading (how often the crane lifts near its rated capacity) and the utilisation (how many operating hours or lifting cycles over its life).

The A1–A8 scale

Chinese GB standards use classes A1 to A8, which align closely with FEM working groups (1Am–5m) and ISO M-classes (M3–M8):

• A3–A4 — occasional, light duty. Maintenance and low-usage workshops.
• A5 — regular, medium duty. General manufacturing and warehousing.
• A6 — frequent, heavy duty. Busy fabrication and machine shops.
• A7 — continuous, heavy duty. Foundries and steel handling.
• A8 — continuous, near-capacity duty. Casting and molten metal cranes.

Why it matters

Duty class drives the sizing of the hoist, motors, brakes, wheels and structure. A crane running at A7 needs stronger components and larger safety margins than the same-capacity crane at A4. It also determines maintenance intervals and expected fatigue life.

How the two factors combine

The state of loading is graded Q1 to Q4 (FEM calls these L1–L4). Q1 means the crane rarely approaches rated capacity; Q4 means it lifts near capacity almost every cycle. The utilisation class runs U0 to U9 and counts total lifting cycles over the crane's design life — U4 is roughly 125,000 cycles, U6 around 500,000.

Cross the two and you get the group. A crane that lifts light loads very often can land in the same class as one that lifts heavy loads occasionally, which is why "how heavy" alone never determines the answer.

Worked example

A 10 t crane in a fabrication shop lifts an average of 4 t, makes 20 lifts per shift, one shift a day, 250 days a year, for 20 years. That is 100,000 cycles at roughly 40% of capacity — Q2/U4, which lands at A5. Move the same crane to three shifts and it reaches 300,000 cycles and steps up to A6: same capacity, same span, materially different hoist and structure.

What changes when the class goes up

• Hoist mechanism — a larger gearbox, a bigger rope drum and a higher rope safety factor.
• Motors — a higher cyclic duration factor (from 25% to 40% or 60% ED) and more starts per hour.
• Brakes — larger thermal capacity, and often a second, redundant brake from A7 upward.
• Wheels and rails — larger wheel diameter to keep contact stress within fatigue limits.
• Structure — the girder is checked against fatigue, not just static strength.

The cost of getting it wrong

Under-specifying is the expensive mistake. An A4 crane pushed into A6 service does not fail dramatically — it wears. Brake linings need replacing every few months, gearboxes run hot, rope life collapses and, eventually, fatigue cracks appear at the girder-to-end-carriage welds. By then the downtime has cost far more than the correct crane would have.

Over-specifying is merely wasteful, but it is not free: a heavier crane raises wheel loads and may force runway strengthening you did not budget for.

Typical classes by industry

• Warehousing and logistics — A3–A5.
• Manufacturing and fabrication — A5–A6.
• Power and energy, grab and coal handling — A6–A7.
• Steel and metallurgy, ladle and casting cranes — A7–A8.

How to specify it

Tell your supplier: the average load as a fraction of rated capacity, the cycles or hours per day, and the expected service life. From these, the correct FEM/ISO class follows. Put the agreed class in the contract — it is the single line that most affects what you actually receive.

When in doubt, share your process and we will calculate it for you. Every overhead crane and gantry crane we build is sized to a stated class, and the calculation is part of the documentation package.