Specification

FEM Duty Classification Explained

Two cranes can both be rated at 10 tonnes and be completely different machines. Duty class is the difference — and it is decided by how hard the crane works, not by what it can pick up.

Capacity is the number everyone asks for first. It is also the one that tells you least about whether a crane will still be working in ten years.

Consider two bays. In the first, a 10-tonne crane lifts a near-full load twice a shift — a mould change, a machine bed, then nothing for hours. In the second, a 10-tonne crane runs almost continuously, mostly at a third of its rating, with hundreds of starts an hour. Both are “10-tonne cranes”. The second one will destroy a mechanism built for the first inside a year.

Duty classification is how that difference gets written down. It is the language that lets a buyer and a manufacturer agree on the service the equipment will actually see, before anyone quotes a price.

How the class is decided

Two Inputs, Combined — Never One Alone

Under FEM 9.511, a hoisting mechanism’s duty group comes from two things considered together. This is the part that most often gets lost in a specification.

1. Load spectrum

Not the maximum load — the distribution of loads over the crane’s life. A hook that spends most of its time near rated capacity is a different proposition from one that mostly moves light loads and occasionally lifts a heavy one. FEM 9.511 distinguishes four load spectra for exactly this reason.

2. Operating time

How long the mechanism actually runs in an average working day — hook motion under power, not the hours the bay is open. A crane that is switched on for twelve hours but moving for forty minutes is a light-duty machine.

The duty group falls out of the combination. This is why “we run it about four hours a day” is not, on its own, an answer — the same four hours produce different classes depending on what is on the hook.

The notation

FEM Groups and Their ISO Equivalents

You will see two notations on datasheets, often side by side. They describe the same ladder of severity: FEM 9.511 mechanism groups, and the ISO 4301 M-classes.

FEM 9.511 mechanism groups and their ISO 4301 equivalents, from lightest to heaviest duty
FEM 9.511	ISO 4301	Broadly
1Bm	M3	Lightest duty — occasional use, light loads
1Am	M4	Light duty — the common workshop and maintenance-bay class
2m	M5	Medium duty — regular production use
3m	M6	Heavy duty — sustained production, frequent lifts
4m	M7	Very heavy duty — near-continuous, often near capacity
5m	M8	Severe, continuous service

The “broadly” column is orientation, not specification. The exact boundary between two groups depends on the load spectrum as well as the running time, which is why a single hours-per-day figure never maps cleanly onto a class. The mechanism group and the crane’s structural classification are also separate things — a crane can sit in one group structurally and its hoist in another.

The cost of a wrong class

It Fails Expensively in Both Directions

Under-specified

The visible failures come first — ropes and brakes wearing out far ahead of schedule, call-outs that cluster. Underneath that, the mechanism is consuming its design life early, and the replacement arrives years before it was budgeted. The crane was never faulty; it was answering a question nobody asked it correctly.

Over-specified

Less dramatic and easier to hide. You pay for steel and mechanism you will never use, and often carry heavier runway and structural requirements with it. It is a real cost, just one that shows up once, at purchase, instead of repeatedly.

Getting the class right at quotation stage is not a formality. It is the decision that sets what the crane will cost to own — and it is the one being made when we ask about span, capacity, lifting height and how often you lift.

Specifying it properly

What an Engineer Needs to Classify Your Bay

The heaviest load, and roughly how often it actually gets lifted
The typical load — the one on the hook most of the day
How many hours the hook is genuinely in motion per shift
Shift pattern — one shift, two, or continuous
Span, required lifting height and the emirate you are in
Whether the process is likely to intensify — duty is easier to buy than retrofit

If those answers are not to hand, the free site visit settles them — an engineer measures the bay and recommends the configuration and duty class in writing. Aerolift installs to FEM and BS EN standards, and the class we specify is the one the documentation is built around. See the MT-series hoist range for the configurations we supply, or the EOT crane range for single and double girder options.

After it is installed

Duty Class Assumes Someone Is Watching the Wear

A duty group is a prediction about how the mechanism will be used and how long it will therefore last. It holds only if the wear items are inspected on a cycle — ropes, brakes, limit switches. That is the work an Annual Maintenance Contract exists to do, and when a component does reach the end of its life, the crane spare parts range decides how long the bay stands still waiting for it.

It is also worth revisiting the class if the process changes. A bay that was light duty when the crane was bought and now runs two shifts is no longer operating in the group it was specified for.

One accountable team

Installed to FEM and BS EN. Kept Running for a Decade.

Installation

Engineered to FEM and BS EN standards, commissioned and load tested — with the documentation your compliance file needs.
Installation services
Maintenance

Quarterly inspections on a 12-month AMC, with priority call-out — for our cranes and anyone else’s.
How AMC cover works
Breakdown — 24/7

On-site diagnostics and repair around the clock, across all seven emirates.
Emergency response