Torque, also called moment, is the turning effect of a force around a pivot or fulcrum. In construction machines, torque explains how a crane boom lifts a load, how an excavator bucket digs, and how a drill bit twists into material. The same force can create a larger turning effect when it acts farther from the pivot.
Understanding torque helps operators and engineers keep machines powerful, efficient, and stable.
For a boom lifting a load, the force is mainly the weight of the load, and the distance is the perpendicular distance from the pivot to the line of action of that weight. A longer boom or a load farther from the machine creates a larger moment that can tip the machine if it is not balanced. Counterweights, wide tracks, outriggers, and safe load charts are used to control these moments.
Torque is also important in rotating machines because motors must provide enough turning force to move heavy parts without overloading.
Key Facts
- Torque or moment measures the turning effect of a force about a pivot.
- τ = F × r, where τ is torque, F is force, and r is perpendicular distance from the pivot.
- Moment = Force × perpendicular distance from pivot.
- The SI unit of torque is the newton meter, written N m.
- A force causes maximum torque when it acts at 90 degrees to the lever arm.
- For rotational balance, clockwise moments = counterclockwise moments.
Vocabulary
- Torque
- Torque is the turning effect produced by a force acting at a distance from a pivot.
- Moment
- Moment is another name for torque, especially when describing forces that make objects rotate or tip.
- Pivot
- A pivot is the fixed point or axis around which an object can rotate.
- Lever arm
- The lever arm is the perpendicular distance from the pivot to the line of action of the force.
- Counterweight
- A counterweight is a mass placed on the opposite side of a pivot to reduce tipping and balance a load.
Common Mistakes to Avoid
- Using the boom length instead of the perpendicular distance from the pivot, which is wrong because torque depends on the shortest distance to the force line, not always the full arm length.
- Forgetting to convert mass to weight, which is wrong because the force from a hanging load is F = mg, not just the mass in kilograms.
- Ignoring the direction of rotation, which is wrong because clockwise and counterclockwise moments must be compared separately for balance.
- Assuming a machine is safe just because the load is not too heavy, which is wrong because a lighter load can still create a dangerous moment if it is far from the pivot.
Practice Questions
- 1 A crane lifts a 4000 N load at a perpendicular distance of 6 m from the pivot. Calculate the torque about the pivot.
- 2 An excavator bucket applies a 2500 N digging force at a perpendicular distance of 1.8 m from the bucket pivot. What moment does it create?
- 3 A crane moves the same load from close to the tower to the far end of the boom. Explain how the torque changes and why this affects tipping stability.