A typical tower crane has the following specifications:
- Maximum unsupported height - 265 feet (80 meters) The crane can have a total height much greater than 265 feet if it is tied into the building as the building rises around the crane.
- Maximum reach - 230 feet (70 meters)
- Maximum lifting power - 19.8 tons (18 metric tons), 300 tonne-meters (metric ton = tonne)
- Counterweights - 20 tons (16.3 metric tons)
The maximum load that the crane can lift is 18 metric tons (39,690 pounds), but the crane cannot lift that much weight if the load is positioned at the end of the jib. The closer the load is positioned to the mast, the more weight the crane can lift safely. The 300 tonne-meter rating tells you the relationship. For example, if the operator positions the load 30 meters (100 feet) from the mast, the crane can lift a maximum of 10.1 tonnes.
The crane uses two limit switches to make sure that the operator does not overload the crane:
- The maximum load switch monitors the pull on the cable and makes sure that the load does not exceed 18 tonnes.
- The load moment switch makes sure that the operator does not exceed the tonne-meter rating of the crane as the load moves out on the jib. A cat head assembly in the slewing unit can measure the amount of collapse in the jib and sense when an overload condition occurs.
Now, it would be a pretty big problem if one of these things fell over on a job site. Let's find out what keeps these massive structures standing upright.