How Much Weight Can They Lift?

...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)

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.

How Tower Cranes Work?

...by Marshall Brain

Parts of a Tower Crane

All tower cranes consist of the same basic parts:

• The base is bolted to a large concrete pad that supports the crane.
• The base connects to the mast (or tower), which gives the tower crane its height.
• Attached to the top of the mast is the slewing unit -- the gear and motor -- that allows the crane to rotate:

On top of the slewing unit are three parts:

• The long horizontal jib (or working arm), which is the portion of the crane that carries the load. A trolley runs along the jib to move the load in and out from the crane's center:

• The shorter horizontal machinery arm, which contains the crane's motors and electronics as well as the large concrete counter weights:

• The operator's cab:

The machinery arm contains the motor that lifts the load, along with the control electronics that drive it and the cable drum, as shown here:

The motors that drive the slewing unit are located above the unit's large gear:

Now let's find out how much weight this equipment can handle.