Construction machines can lift, push, dig, and carry heavy loads, but they must stay balanced while doing it. A machine tips when its weight and load create a turning effect that is not supported by its wheels, tracks, or outriggers. The center of gravity shows where the machine’s weight acts as if it were concentrated at one point.
Understanding this idea helps operators choose safer positions, loads, and slopes.
Key Facts
- A machine is stable when the vertical line through its center of gravity falls inside its base of support.
- A machine tips when the line of action of weight falls outside the base of support.
- Weight is calculated by W = mg, where m is mass and g is about 9.8 m/s^2.
- Turning effect is calculated by torque = force x perpendicular distance, or τ = Fd.
- A wider base of support increases stability because the center of gravity can move farther before the machine tips.
- Raising a load raises the combined center of gravity and usually makes the machine less stable.
Vocabulary
- Center of gravity
- The point where an object’s weight acts as if all its mass were concentrated there.
- Base of support
- The area under and between the contact points that support a machine, such as tires, tracks, or outriggers.
- Line of action
- The straight vertical line in the direction of gravity through the center of gravity.
- Torque
- A turning effect caused by a force acting at a distance from a pivot point.
- Tipping point
- The condition when the line of action of weight reaches or passes the edge of the base of support.
Common Mistakes to Avoid
- Ignoring the load’s effect on the center of gravity is wrong because the machine and load act as one combined system.
- Assuming a heavier machine is always safer is wrong because stability depends on weight location, base width, slope, and torque, not just total weight.
- Thinking tracks or tires support the machine at only one point is wrong because the full contact area helps define the base of support.
- Using horizontal distance incorrectly in torque problems is wrong because torque uses the perpendicular distance from the pivot to the force’s line of action.
Practice Questions
- 1 A front loader has a weight of 80,000 N acting 0.6 m behind the front axle. A load in the bucket weighs 20,000 N and acts 1.8 m in front of the front axle. Taking the front axle as the tipping pivot, compare the stabilizing torque and tipping torque. Does it tip?
- 2 An excavator has a combined center of gravity 1.2 m from the left edge of its track base. The track base is 3.0 m wide. How far can the center of gravity shift left before the line of action reaches the edge of the base of support?
- 3 A telehandler is stable on level ground with its boom low, but becomes unsafe when the same load is raised high on a slope. Explain how the center of gravity, base of support, and line of action change the tipping risk.