A crane can lift loads that are much heavier than its own wheels could safely support by themselves. Outriggers make this possible by creating a much wider base around the machine. When the support base is wider, the crane can resist tipping from the turning effect of the load.
This matters because safe lifting depends on both the weight being lifted and where that weight acts relative to the crane.
Each outrigger pushes a stabilizer pad against the ground, spreading force over a larger contact area. The crane is stable when the combined center of gravity of the crane and load stays inside the support polygon made by the outrigger pads. If the load moves too far out, the tipping moment can exceed the resisting moment from the crane's weight and outrigger base.
Ground strength, pad size, boom angle, and load distance all affect whether a lift is safe.
Key Facts
- Torque or moment is calculated by τ = Fd, where d is the perpendicular distance from the pivot.
- A crane tips when the load moment is greater than the resisting moment.
- Widening the outrigger footprint increases the distance to the tipping edge, which increases stability.
- Pressure on the ground is calculated by P = F/A, where A is the contact area of the pad.
- Larger stabilizer pads reduce ground pressure by spreading the crane's weight over more area.
- The center of gravity must remain inside the support polygon formed by the outrigger pads for static stability.
Vocabulary
- Outrigger
- An extendable support beam on a crane that widens the machine's base during lifting.
- Stabilizer pad
- A broad plate or mat under an outrigger foot that spreads force into the ground.
- Support polygon
- The area enclosed by the contact points supporting a machine, such as the outrigger pads.
- Center of gravity
- The point where the weight of an object or system can be treated as acting.
- Tipping moment
- The turning effect that tends to rotate a crane around an edge of its support base.
Common Mistakes to Avoid
- Treating crane stability as only a weight problem is wrong because the distance of the load from the tipping edge also controls the tipping moment.
- Ignoring ground pressure is wrong because a crane can become unstable if the pads sink into soft soil even when the load chart seems acceptable.
- Assuming the tires support the lift is wrong because cranes are often designed to lift with weight transferred through the outriggers, not mainly through the wheels.
- Using a small pad on weak ground is wrong because smaller contact area increases pressure and can cause the outrigger to punch into the surface.
Practice Questions
- 1 A crane lifts a 12000 N load with its center 5.0 m from the tipping edge. What is the tipping moment?
- 2 An outrigger carries a downward force of 80000 N on a pad with an area of 2.0 m². What pressure does the pad apply to the ground?
- 3 A crane has its outriggers fully extended, but the boom swings the load outside the rectangular footprint. Explain why this creates a tipping risk even if the load's weight has not changed.