Stability and Load Handling
What the exam tests
Stability and Load Handling tests why machines tip and how to control load movement. Questions cover centre of gravity, load moment, tipping lines, ground pressure, wind and dynamic forces.
Core rules
- Stability depends on the centre of gravity staying inside the tipping line. The load moment is load times radius; the machine's counterweight and base create a resisting moment.
- Increasing radius, boom length or load swing increases the overturning moment. Side loads and dragging loads are especially dangerous.
- Outriggers must be fully extended and supported on suitable mats or pads. Soft ground reduces support and can cause a leg to sink.
- Keep the machine level. Avoid rapid slewing, sudden braking and hoisting that causes load swing.
- Wind acting on a large sail area increases load and can move a suspended load.
Worked example
A crane lifts 8 t at 6 m radius, giving a load moment of 48 t·m. If the same load is lifted at 8 m, the moment rises to 64 t·m. The chart must be rechecked because the increased radius reduces capacity even though the load has not changed. The operator should not slew faster to shorten the lift; that adds dynamic load.
Traps
Do not judge stability by machine weight alone. Do not lift over the side without checking the chart. Do not use outriggers on uncompacted fill without pads. Do not drag a load sideways.
Key points
- Keep the centre of gravity inside the tipping line.
- Load moment increases with radius.
- Use pads under outriggers on soft ground.
- Avoid shock, swing and side loading.