...by Marshall Brain.

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)

**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.