Torque (torque) describes the ability to rotate an object around a certain axis. The basic relationship can be written as T = F × r, which is the force times the length of the moment arm. Because it contains both the dimensions of "force" and "length", the units of torque are usually written in compound forms such as N·m, lbf·ft, and kgf·cm.
In modern engineering and electromechanical documents, N·m is the most common main unit. It belongs to the International System of Units (SI) and can be easily used in conjunction with other formulas such as force, length, power, etc. Smaller torques will be seen as mN·m, larger torques will be seen as kN·m, which makes the magnitude expression more intuitive.
In maintenance, manufacturing and historical materials, kgf·m and kgf·cm are still very common. They come from the "kilogram force" system and are especially likely to appear in old equipment, Asian manuals, simulation torque wrenches and assembly training materials. For small fasteners, kgf·cm or lbf·in are often preferred because the numerical values read more accurately.
In North American data, lbf·ft and lbf·in appear frequently. Engine, transmission, off-road, and tool manuals prefer lbf·ft, while electronic equipment, small fasteners, and precision assembly more often use lbf·in. When encountering expressions such as lb-ft and lb-in, it usually refers to the torque unit context of pound-force foot / inch.
Unit conversion can only solve the problem of "how to change the ruler for the same torque value", but it cannot replace the judgment of the assembly process. Whether the actual locking is reliable is also related to the thread specifications, lubrication status, contact surface friction, tightening sequence, retightening process and angle method requirements.
