A crane stays upright when the turning effect of its own weight and counterweight is greater than the turning effect of the lifted load. This balance matters because even a slow lift can become dangerous if the load is too heavy or too far from the crane. Outriggers widen the crane’s base, which moves the tipping line farther away and increases stability.
The goal is to keep the combined center of gravity inside the support area at all times.
Tipping is controlled by moments, which are forces multiplied by their distances from a pivot point. For a mobile crane, the pivot can be an outrigger pad or tire contact point on the side closest to the load. The load chart tells the operator the maximum safe load for a certain boom length, boom angle, and working radius.
Wind, soft ground, swinging loads, and uneven setup reduce the safety margin, so stable lifting depends on both physics and careful operation.
Key Facts
- Moment = force x perpendicular distance from pivot
- A crane tips when the load moment is greater than the stabilizing moment.
- Load moment = load weight x working radius
- Stabilizing moment = crane weight x distance of its center of gravity from the tipping line
- Outriggers increase the support base and move the tipping line outward.
- The combined center of gravity must stay inside the crane’s support polygon.
Vocabulary
- Tipping line
- The tipping line is the edge of the support base around which a crane would rotate if it began to overturn.
- Center of gravity
- The center of gravity is the single point where the weight of an object or system can be treated as acting.
- Working radius
- The working radius is the horizontal distance from the crane’s rotation center to the center of the lifted load.
- Counterweight
- A counterweight is a heavy mass placed on the crane to create a stabilizing moment against the lifted load.
- Load chart
- A load chart is a manufacturer’s table that gives the maximum safe load for specific crane configurations and lift conditions.
Common Mistakes to Avoid
- Ignoring the working radius, which is wrong because the same load becomes more likely to tip the crane as it moves farther from the rotation center.
- Assuming outriggers only stop sinking, which is wrong because they also widen the support base and increase the stabilizing moment.
- Using the maximum crane capacity for every lift, which is wrong because rated capacity changes with boom length, boom angle, radius, and setup conditions.
- Forgetting swinging or wind-blown loads, which is wrong because sideways motion can shift the effective center of gravity and create extra tipping moment.
Practice Questions
- 1 A crane lifts a 12,000 N load at a working radius of 8 m. What is the load moment about the tipping line?
- 2 A crane has a stabilizing moment of 180,000 N m. If the load is placed 10 m from the tipping line, what is the largest load force that would exactly balance this moment?
- 3 A crane is stable with its outriggers fully extended on level ground. Explain why retracting the outriggers while keeping the same load and boom position can make the crane unsafe.