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| © Eugene Brennan |
Torque is a quantity often encountered in the context of the specs of vehicles and power tools. A common misconception is that torque is the same thing as power, and an engine that produces more torque has more power, but that's not necessarily the case.
So what exactly is torque?
A little bit of maths and algebra required here, but it's not rocket science! If there are two forces acting opposite to each other as shown in the diagram, but not in a line, it's called a "couple". If the distance between each force F is d, then the magnitude of the couple is called the torque = Fd (F multiplied by d) Torque is a twisting force. Bigger forces means greater torque. Increasing the distance between the forces also increases torque (which is why bigger handles on gate valves or longer handles on tools give more turning force. More info on forces here:
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| ANKAWÜ, CC BY-SA 3.0 via Wikimedia Commons |
What are gearboxes for?
Gears are torque converters converting high speed low torque into lower speed higher torque (or vice versa). At its most simplest, a gearing arrangement is simply one gear wheel driving another as in the animation below.
A common misconception is that more torque = more power. However this isn't necessarily so. Reduction gearing for instance produces more torque, but it also reduces angular rotation speed at the output of the gearbox. Since power = torque x angular rotation speed, power stays the same. A gearbox is used in your car to produce variable amounts of speed and torque, depending on conditions. A low gear, producing a large torque on the wheels, which transforms into a large force pushing forwards on the axle, is necessary to get the car moving, or for it to climb hills. Without it, the engine would have to have a much larger horsepower to accelerate the vehicle from a standstill. Once a car starts moving and reaches cruising speed, a relatively small force is required to overcome friction at the wheels and drag (friction due to the vehicle moving through the air), hence the use of higher gears. Gearboxes are used as speed/torque converters in lots of other machines including windmills, power tools and winches. Pulleys of differing diameter, driving each other via belts, work in practically the same way as gearing systems, changing speed and torque.
Torque is measured in newton metres (Nm). In the US, the unit of torque is the pound-force foot (lbf · ft) abbreviated to pound-foot (lb⋅ft)
More info here:
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| Jahobr, Creative Commons CC0 1.0 Universal Public Domain Dedication via Wikimedia Commons |
Image attributions:
Wheel valve: ANKAWÜ, CC BY-SA via Wikimedia Commons.
https://commons.wikimedia.org/wiki/File:D-BW-Kressbronn_aB_-_Kl%C3%A4ranlage_028.jpg
Gear animation: Jahobr, Creative Commons CC0 1.0 Universal Public Domain Dedication via Wikimedia Commons
https://commons.wikimedia.org/wiki/File:Animated_two_spur_gears_1_2.gif