A tower crane is not operated «by eye»: it has limiters that cut the movement before the load or the moment exceeds what the structure can bear. The crane operator has to know which they are, how they are checked and what happens if one fails.
The limiters listed by the ITC (annex II, 2.2.13; annex III, part B, point 6)
- Lifting moment limiter and distribution moment limiter: they prevent exceeding the permitted moment (load × reach).
- Maximum load limiter: it prevents lifting more than the maximum load, wherever the trolley is.
- Hoisting travel limiter (hook height) and maximum and minimum trolley reach limiter.
- Rail travel limiter, slewing limiter, upper and lower angle limiter (luffing jib) and high speed limiter.
- Weathervane device, buffers on the trolley travel, anemometer and weight control.
Test loads (article 7)
The crane operator must have test loads accessible at all times to check the state of the crane: the rated maximum and tip loads (or at the farthest jib position on that site) and those of 10 per cent of those values. With them the limiters are tested at commissioning, at inspections and in the daily checks.
How each limiter is calibrated (annex IV, point 5.1.1)
| Limiter | Test load or travel |
|---|---|
| Lifting moment | 1.1 × tip load |
| Maximum load | 1.1 × maximum load |
| Moment, trolley moving out | distance + 10 % |
| High speed | 1.1 × P |
| Hook height | cuts out at 1.5 m from the trolley |
| Trolley | cuts out at 0.2 m from the stop, forwards and backwards |
| Rail travel | cuts out at 0.5 m between stops |
And the sentence that annex IV writes in capitals: if any limiter does not work, it is prohibited to work until it is repaired.
What the OCA checks on an erected crane (annex III, part B)
A load test of at least one hour, with repeated starts and stops of each movement over its whole travel and starts with the load suspended without rolling back; then, the operation of the limiters with the available loads. The inspection also covers the electrical installation (omnipolar switch and 300 mA residual current device on the main board, emergency switch, earthing), the cab (extinguisher, visibility, railings and ladders), the guards, the mechanisms (hooks, ropes, drums, pulleys, brakes), the plates (manufacture, loads and reaches, distances on the jib) and the documentation: manufacturer's manual, operating instruction manual, logbook, CE declaration, proof of the crane operator's qualification and maintenance contract. The operator in the test is the crane operator designated by the owner or lessee.
Royal Decree 1215/1997 requires, on any lifting machine, the rated load to be visible and the lifting accessories (slings, chains, hooks) to be marked with their essential characteristics (annex I, point 2.b).
Source: Real Decreto 836/2003, ITC MIE-AEM-2 del Reglamento de aparatos de elevación y manutención, grúas torre (tower cranes, consolidated text, in Spanish), article 7 and annexes II, III and IV · Real Decreto 1215/1997, disposiciones mínimas de seguridad y salud para la utilización de los equipos de trabajo (work equipment, consolidated text, in Spanish), annex I, point 2