How to Select a Crane Duty Class: FEM 9.511 and ISO 4301 Guide

Selection and engineering

Why does the duty class matter?

Two cranes can have the same capacity: one lifts half loads a few times a day in a warehouse, the other works near full load throughout every shift on a production line. On paper both are “10 tonne cranes”, but the fatigue on gearboxes, bearings, rope, brakes and motors is completely different. The duty class turns this difference into numbers and defines the strength of the mechanisms.

Too low a class shows up in the first years as rope, brake lining, gearbox and motor failures; too high a class means paying for a heavier, more expensive crane than needed. The most common mistake we see on site is focusing on capacity and never asking how intensively the crane will be used.

Key concepts: load spectrum and operating time

Load spectrum (L1–L4)

The load spectrum describes how heavy the loads lifted over the crane’s life are relative to the rated load. FEM 9.511 defines four classes with a spectrum factor k_m:

Average daily operating time (t_m)

This is the average time per day the mechanism actually moves, not the time the crane is on site. For the hoist it is estimated as:

t_m = 2 · H · N · T ÷ (60 · V)

• H: average lifting height per cycle (m)
• N: cycles per hour (one cycle = lift and lower)
• T: working hours per day
• V: hoisting speed (m/min)

The factor 2 comes from the load being both lifted and lowered in every cycle.

FEM 9.511 mechanism group table

Combining load spectrum and average daily operating time gives the mechanism group. FEM groups correspond to ISO 4301 M groups as follows: 1Bm = M3, 1Am = M4, 2m = M5, 3m = M6, 4m = M7, 5m = M8.

Worked example

A 10 tonne crane for a steel fabrication shop: H = 4 m, N = 10 cycles/h, T = 8 h/day (one shift), V = 5 m/min.

t_m = 2 · 4 · 10 · 8 ÷ (60 · 5) = 640 ÷ 300 ≈ 2.1 h/day, which falls into the “≤ 4 h” column.

• Mixed loads (L2): mechanism group 2m (M5)
• Frequently near full load (L3): 3m (M6)

If the shop moves to two shifts, t_m rises to about 4.3 h and even with L2 the group becomes 3m. That is why future production plans should be discussed at the quotation stage, not only today’s use.

Calculate with your own values using our FEM / ISO classification tool, which also gives the crane class, estimated rope diameter and recommended motor duty.

Mechanism group vs crane class (A1–A8)

M groups (M1–M8) classify each mechanism (hoist, trolley, bridge) by operating time. A groups (A1–A8) classify the crane as a whole, especially the steel structure, by the total number of cycles and load spectrum. A crane can have an M6 hoist, M5 bridge travel and an A5 structure; this is normal.

Indicative classes for typical applications

This table is only a starting point; the exact class must be calculated with your cycles and lifting heights.

Trolley and bridge travel classes

The formula above is for the hoist. For travel motions the same logic applies using average travel distance instead of lifting height and travel speed instead of hoisting speed. Example: 10 cycles/h, 30 m average travel each way, 40 m/min speed: 2 · 30 · 10 · 8 ÷ (60 · 40) = 2 h/day. Bridge travel often runs shorter than the hoist and may come out one group lower; on process cranes where the load is always suspended during travel the opposite can be true.

Effect on rope, drum and motor selection

A higher mechanism group changes not only gearbox and bearings but also the rope safety factor, the drum and sheave diameter ratios and the motor duty. According to DIN 15020 the rope diameter is determined from the square root of the maximum rope force and a group-dependent factor; a higher group means a thicker rope and a larger drum. Heavier groups also use motors rated for higher duty cycles (e.g. 40 % or 60 %) and more starts per hour.

So if a crane selected for a low class is later used more intensively, replacing the motor alone is rarely enough; rope, drum and gearbox also become undersized.

A case from the field

At a metalworking plant, a 5 tonne crane selected for light duty on one shift started needing frequent brake lining and rope replacements a few years after production moved to two shifts. The records showed that the average daily hoisting time had doubled and most loads were close to rated capacity, i.e. the crane had moved from L2 to L3 and to a longer operating time. The solution was to renew the hoist for one group higher and convert to inverter drives. The duty class is not a one-off decision: when use changes, it must be reassessed.

Common mistakes

1. Looking only at capacity.
2. Confusing time on site with operating time: in an 8-hour shift the mechanisms often run only 1–3 hours.
3. Forgetting future growth: more shifts mean a higher class.
4. Giving every mechanism the same class: the hoist usually works harder than travel motions.
5. Ignoring the environment: temperature, dust and outdoor use affect motor duty and protection class.

Information to prepare when requesting a quotation

• List of loads and their approximate distribution (e.g. 20 % near full load, 60 % half load)
• Average lifts per hour and working hours per day
• Average lifting height and travel distance per cycle
• Shifts, working days per year and expected growth
• Environmental conditions: temperature, dust, humidity, outdoor use

How the duty class determines service life

Each mechanism group has a theoretical operating time at full load. According to FEM 9.755 it is, for example, 800 h for 1Am, 1,600 h for 2m, 3,200 h for 3m, 6,300 h for 4m and 12,500 h for 5m. Actual use, weighted by the load spectrum, is deducted from this; as it approaches the limit a general overhaul is required. Monitoring systems that record hours and loads make estimating remaining life much easier.

For a short introduction see What are crane duty classes?

Need support?

If you need help on site with the topics in this guide, the Aylen team is available 24/7. We inspect your crane, take measurements and provide recommendations in a written report. Quotation and service request form

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