In the technical characteristics of engines and structures equipped with engines, the mysterious nm parameter constantly appears as a unit of torque. If everything is clear with horsepower, even on an intuitive level, the horse is the horse, then here some difficulties may arise.
Archimedean lever
The well-known scientist Archimedes once uttered the famous phrase: "Give me leverage, and I will turn the Earth." We can say that it was this phrase that served as the beginning of the birth of the unit of measure of torque. As you know, the planet Earth is somewhat heavy so that a person, even such a respected and famous as Archimedes, could turn it over. The key is the use of leverage, which allows to increase the force of influence on the object by orders of magnitude. A lever is virtually any object that can rotate freely around a fulcrum. If the fulcrum is exactly in the middle of the lever, with the same forces applied at each end of the lever, the whole structure will stand still. The situation will change only when the fulcrum is shifted to one side. This is best seen in the figure below.
It's spinning
As you can see, the lever spins around the fulcrum, making an incomplete revolution. The ratio of the applied force to the long arm of the lever and the received force on the short arm is the basis of the units of torque. The ratio is very simple: the forces multiplied by the length of the corresponding lever arm must be equal. The law of conservation of energy always works. This principle of action can be extended to a pair of gears of different diameters, and generally to any aggregates of mechanisms of different diameters interacting by rotation, which, in fact, are the shoulders of conditional levers.
Torque
Now you can take the rotating shaft of the engine. The radius of the motor shaft is a conditional lever, and when it rotates, a force arises directed perpendicular to the axis of rotation. This is shown schematically in the following figure.
Here, R is the radius of the shaft, and F is the vector of the force generated by the rotation of the shaft. As with a conventional lever, their product (R * F) will be the moment of force, or torque. Since, in accordance with the international system of units, the force is measured in Newtons and the distance is in meters, the unit of torque is the Newton meter, or nm for short.
However, there are other notations. Sometimes, to measure the force, it is not Newtons that are used, but kilograms (kgf), then this value can be converted into a "classic" using a coefficient. 1 kgf per meter is 9.81 nm. In countries that do not use the metric system, pound feet are used as the unit of measure for the motor torque. It sounds unusual, but nonetheless. 1 lb-ft is 1.36 nm. There is a relationship between power, speed, and torque generated. She is very simple. Power is equal to the product of the rotational speed and torque divided by the coefficient. The coefficient depends on the units of torque and other specified values.
If we are talking about horsepower, kgf per meter and revolutions per minute, this coefficient is 716.2, for nm and kilowatts - 9549. In the public domain there are appropriate calculators. The technical specifications usually indicate the torque measured directly on the motor shaft.