Grades 3–5 · Machines
How can a tower crane lift a load far from its tower?
A tower crane can place a heavy object far from its tall mast. A motor-driven hoist winds a cable to raise the load. On the kind shown here, a trolley carries the hanging cable along a long horizontal arm called the jib.
In the diagramA crane carries weight and turning effectsLoad farther out: greater turning effectCounterweight4 m from mastLoad8 m from mastThe mast and foundation carry the forcesCounterweight gives an opposing turning effect
The load pulls down on the jib. The farther it is from the mast, the greater its turning effect. Engineers call this a moment. Moving the same load twice as far from the mast doubles that turning effect.
A shorter arm behind the mast carries counterweights that provide an opposing moment. The mast and its strong foundation also carry forces and moments. Matching the two illustrated moments does not remove the downward weight on the foundation.
Real cranes have carefully calculated load limits that change with reach and configuration. Their design includes the arms’ own weight, movement, wind and many other effects. The picture explains the load path; its numbers do not decide what a real crane may lift.
About this explanation
Starting knowledge: Understand weight, a support and multiplication.
Sources
- Liebherr — What’s behind the type designation of Liebherr mobile construction cranes?
- Liebherr — Tower Crane Finder