Torque is a fundamental concept in physics and engineering that describes the rotational equivalent of linear force. It's what causes objects to rotate around an axis when a force is applied. From opening a door to the operation of car engines, torque is all around us. In this guide, we'll explore what torque is, how it's measured, and its many applications in everyday life and industry.
What is Torque?
Torque (τ) is a measure of the force that can cause an object to rotate about an axis. It's calculated as the product of the force (F) and the perpendicular distance (r) from the axis of rotation to the line of action of the force: τ = r × F × sin(θ), where θ is the angle between the force vector and the lever arm. The SI unit of torque is the newton-meter (N·m), which is dimensionally equivalent to the joule, but torque is a vector quantity, not an energy measurement.
Key Concept:
The effectiveness of a force in causing rotation depends on both the magnitude of the force and the length of the lever arm. This is why wrenches have long handles—the longer the handle, the greater the torque you can apply with the same amount of force. This principle is also why door handles are placed far from the hinges.
Units of Torque
SI Units
- Newton-meter (N·m): Standard SI unit
- Kilonewton-meter (kN·m): 1,000 N·m
- Millinewton-meter (mN·m): 0.001 N·m
Other Common Units
- Pound-foot (lb·ft): 1.3558 N·m
- Pound-inch (lb·in): 0.11298 N·m
- Kilogram-meter (kgf·m): 9.8067 N·m
- Dyne-centimeter (dyn·cm): 10⁻⁷ N·m
Types of Torque
Static Torque
Torque that does not produce angular acceleration.
Example: Tightening a bolt
Dynamic Torque
Torque that produces angular acceleration.
Example: Car engine accelerating
Reaction Torque
Torque that is equal in magnitude but opposite in direction to another torque.
Example: Helicopter tail rotor
Torsional Torque
Torque that causes twisting of an object.
Example: Drive shaft in a car
Torque Conversion Table
| Unit | N·m | lb·ft | lb·in | kgf·m |
|---|---|---|---|---|
| 1 N·m | 1 | 0.7376 | 8.8507 | 0.1020 |
| 1 lb·ft | 1.3558 | 1 | 12 | 0.1383 |
| 1 lb·in | 0.1130 | 0.0833 | 1 | 0.0115 |
| 1 kgf·m | 9.8067 | 7.2330 | 86.7962 | 1 |
Measuring Torque
Torque Wrench
A tool used to apply a specific torque to a fastener.
Used in automotive and mechanical work
Strain Gauge
Measures torque by detecting the strain in a shaft or component.
Used in industrial applications
Optical Torque Sensor
Uses light to measure the twist in a shaft and calculate torque.
Used in research and development
Applications of Torque
Automotive
- Engine performance
- Wheel lug nut tightening
- Drivetrain components
- Brake systems
Manufacturing
- Assembly line processes
- Robotic arms
- Conveyor systems
- Packaging equipment
Aerospace
- Engine and propeller systems
- Flight control surfaces
- Landing gear mechanisms
- Satellite orientation
Medical
- Surgical tools
- Prosthetic devices
- Dental equipment
- Rehabilitation machines
Did You Know?
The Bugatti Chiron Super Sport 300+ produces a staggering 1,600 N·m (1,180 lb·ft) of torque from its quad-turbocharged W16 engine, making it one of the most powerful production cars in the world. In contrast, the massive Wärtsilä-Sulzer RTA96-C marine diesel engine, used in container ships, produces an incredible 7,603,850 N·m (5,608,310 lb·ft) of torque at just 102 RPM—enough to power a small town!
Torque in Everyday Life
Common Torque Values
- Bicycle pedal: ~35-55 N·m (26-41 lb·ft)
- Car wheel lug nuts: ~90-120 N·m (66-89 lb·ft)
- Spark plugs: ~20-30 N·m (15-22 lb·ft)
- Bottle cap: ~2-4 N·m (1.5-3 lb·ft)
- Door hinge screws: ~1.5-2.5 N·m (1.1-1.8 lb·ft)
Human Torque
- Average human grip: ~50 N·m (37 lb·ft)
- Elbow flexion: ~75 N·m (55 lb·ft) for males, ~45 N·m (33 lb·ft) for females
- Knee extension: ~250 N·m (184 lb·ft) for males, ~150 N·m (111 lb·ft) for females
- Back extension: ~350 N·m (258 lb·ft) for males, ~200 N·m (148 lb·ft) for females
Practical Tip:
When working with fasteners, always use a torque wrench to apply the correct amount of torque. Overtightening can strip threads or damage components, while undertightening can lead to loosening and potential failure. Always refer to the manufacturer's specifications for the correct torque values, and make sure to use the right units (N·m, lb·ft, etc.). For critical applications, consider using a torque-angle gauge for more precise tightening, especially for components like cylinder head bolts where proper torque sequencing is crucial.