Cranes are construction machines designed to lift heavy loads and move them safely across a work site. Different crane types solve different engineering problems, such as reaching high floors, traveling over rough ground, or making quick lifts from a road-ready vehicle. Understanding crane families helps students connect simple physics ideas like force, torque, balance, and stability to real construction technology.
A comparison lineup of cranes shows why no single crane is best for every job.
Key Facts
- Torque = force x lever arm, so a load farther from the crane creates more tipping effect.
- Load moment = load weight x radius, and it must stay below the crane rating.
- Tower cranes are best for tall buildings because they provide height and long horizontal reach.
- Mobile cranes are road-ready and useful for short-term lifts, but they need outriggers for stability.
- Crawler cranes spread weight over tracks, giving strong stability on soft or rough ground.
- A crane's rated capacity usually decreases as boom length or lifting radius increases.
Vocabulary
- Boom
- The boom is the long lifting arm of a crane that positions the load away from the machine.
- Jib
- A jib is an extra arm or extension that increases a crane's reach, often at an angle from the main boom.
- Counterweight
- A counterweight is a heavy mass placed opposite the load to help balance the crane against tipping.
- Outriggers
- Outriggers are extendable supports that widen a mobile crane's base and improve stability during lifting.
- Load Radius
- Load radius is the horizontal distance from the crane's center of rotation to the center of the suspended load.
Common Mistakes to Avoid
- Treating maximum capacity as constant is wrong because crane capacity decreases when the load radius or boom length increases.
- Ignoring the counterweight is wrong because crane stability depends on balancing moments, not just on the strength of the cable.
- Using a mobile crane without considering outriggers is wrong because the tires alone may not provide a wide or stiff enough support base for heavy lifts.
- Confusing crawler cranes with tower cranes is wrong because crawler cranes move on tracks at ground level, while tower cranes are fixed or climbing structures built for vertical construction.
Practice Questions
- 1 A crane lifts a 12,000 N load at a load radius of 8 m. What is the load moment in N m?
- 2 A mobile crane has a safe load moment limit of 180,000 N m. What is the greatest load it can lift at a 15 m radius?
- 3 A job site must lift steel beams to the 25th floor of a building for several months. Explain why a tower crane may be a better choice than a truck-mounted mobile crane.