How TorqueWorks

Torque is the force that tends to turn or rotate things. You generate a torque any time you apply a force using a wrench, such as tightening the lug nuts on your wheels. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which turns the lug nut. English units of torque are pound-inches (lb-in.

08/01/2001


Torque is the force that tends to turn or rotate things. You generate a torque any time you apply a force using a wrench, such as tightening the lug nuts on your wheels. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which turns the lug nut.

English units of torque are pound-inches (lb-in.) or pound-feet (lb-ft); the SI unit is the Newton-meter (Nm). Torque units contain both distance and force. To calculate torque, multiply the force by the distance from the center. In the case of the lug nuts, if the wrench is a foot long, and you apply 200 lb of force, you generate 200 lb-ft of torque. If you use a 2-ft wrench, you only need to apply 100 lb of force to generate the same torque.

A car engine creates torque, and uses it to spin the crankshaft. The combustion of gas in the cylinder creates pressure against the piston, forcing it down. The force is transmitted from the piston to the connecting rod, and from the connecting rod into the crankshaft. The point where the connecting rod attaches to the crankshaft is some distance from the center of the shaft. The horizontal distance changes as the crankshaft spins; therefore, the torque also changes.

You might be wondering why only the horizontal distance is important in determining the torque in the engine. When the piston is at the top of its stroke, the connecting rod points straight down at the center of the crankshaft. No torque is generated in this position, because only the force that acts on the lever in a direction perpendicular to the lever generates a torque.

If you have ever tried to loosen really tight lug nuts, you know a good way to make a lot of torque is to position the wrench so that it is horizontal, then stand on the end of the wrench. You are applying all of your weight at a distance equal to the length of the wrench. If you position the wrench with the handle pointing straight up, and then stand on the top of the handle (assuming you could keep your balance), you would have no chance of loosening the lug nut. You might as well stand directly on the lug nut.

Torque comparison

To understand further how torque works, compare two different types of engines. One engine is a turbo-charged Caterpillar C-12 diesel truck engine. This engine weighs about 2000 lb, and has a displacement of 732 cu in. The other engine is a highly modified Ford Mustang Cobra engine, with a displacement of 280 cu in. It has a supercharger and weighs about 400 pounds. They both produce a maximum of about 430 hp, but only one of these engines is suitable for pulling a heavy truck.

The Caterpillar engine produces 1650 lb-ft of torque at 1200 rpm, which is 377 hp. At 5600 rpm, the Mustang engine also makes 377 hp, but it only makes 354 lb-ft of torque. If you put a gear reduction of 4.66:1 on the Mustang engine, the output speed would be 5600/4.66 rpm, or 1200 rpm, and the torque would be 4.66 x 354 lb-ft = 1650 lb-ft — exactly the same as the big Caterpillar engine.

You might wonder why big trucks don't use small gas engines instead of big diesel engines. In the scenario above, the big Caterpillar engine is loafing along at 1200 rpm, nice and slow, producing 377 hp. Meanwhile, the small gas engine is screaming along at 5600 rpm. The small gas engine will not last very long at that speed and power output. The big truck engine is designed to last years, and to drive hundreds of thousands of miles each year it lasts.



Common units of torque

SI: Newton-meter (Nm) 1 Nm = 0.737 ft lb

English: Pound-inches (lb-in.)1 lb-in. = 0.113 Nm

Pound-feet (lb-ft)1 lb-ft = 1.356 Nm



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