A mobile crane is a lifting machine built on a truck or carrier so it can drive to a construction site instead of being assembled there piece by piece. It is useful when heavy materials must be moved quickly, such as steel beams, concrete panels, equipment, or large pipes. Its main advantage is mobility combined with lifting power, which makes it common on building sites, road projects, and emergency repairs.
Understanding how it works helps explain important physics ideas such as torque, stability, center of mass, and mechanical advantage.
A truck-mounted mobile crane uses a telescopic boom that extends in sections to reach higher or farther from the vehicle. Outriggers spread from the chassis and press onto the ground to create a wider support base, which greatly improves stability during lifts. Counterweights balance part of the load’s turning effect, but the crane can still tip if the load is too heavy or too far from the center.
Operators use load charts because safe lifting capacity changes with boom length, boom angle, load radius, ground condition, and outrigger setup.
Key Facts
- Torque is the turning effect of a force: τ = Fd, where d is the perpendicular distance from the pivot.
- A crane is more likely to tip as the load radius increases, because the load torque gets larger.
- Load torque can be estimated with τload = Wload × r, where Wload is load weight and r is load radius.
- Outriggers increase the support base, which helps keep the crane’s center of mass inside the tipping boundary.
- A telescopic boom extends by sliding nested boom sections outward, often powered by hydraulic cylinders.
- Safe lifting capacity is not one number because it depends on boom angle, boom length, load radius, counterweight, and ground support.
Vocabulary
- Telescopic boom
- A boom made of nested sections that slide in and out to change the crane’s reach and lifting height.
- Outrigger
- A support leg that extends from the crane and presses on the ground to make the machine more stable.
- Counterweight
- A heavy mass added to the crane to help balance the torque caused by the lifted load.
- Load radius
- The horizontal distance from the crane’s rotation center to the vertical line through the load.
- Center of mass
- The average position of an object’s mass, which determines how its weight acts for balance and stability.
Common Mistakes to Avoid
- Treating crane capacity as a single fixed value is wrong because the safe load changes with boom length, boom angle, and load radius.
- Ignoring the load radius is wrong because a lighter load far from the crane can create more tipping torque than a heavier load close to the crane.
- Assuming outriggers make any lift safe is wrong because weak soil, poor setup, or an overloaded boom can still cause tipping or structural failure.
- Forgetting the weight of rigging is wrong because hooks, slings, spreader bars, and lifting devices add to the total load the crane must support.
Practice Questions
- 1 A crane lifts a 12,000 N load at a load radius of 8 m. What is the load torque about the crane’s rotation center?
- 2 A counterweight produces 180,000 N m of balancing torque. If the load is 15,000 N, what is the maximum load radius for equal torque, ignoring the boom’s weight and safety factors?
- 3 Explain why extending the telescopic boom outward can reduce the crane’s safe lifting capacity even if the load weight stays the same.