Friction torque is the force between two objects that causes one of them to rotate around an axis. It shows how much force results from one object spinning the other after accounting for the force lost to friction between the objects. For example, when a player throws a bowling ball down an alley it does not glide along on one side but rather spins in a circle as it rolls. The friction from the floor making contact with the ball creates the torque, or the twisting force, and the ball spins around its own axis. This axis is not real, but can be thought of as an invisible line going directly through the center of the bowling ball around which it rotates.
Torque is a force that acts on an object, such as the floor of the bowling alley acting on the bowling ball. Traditionally, a force pushes an object forward or pulls it down. Instead of pushing or pulling, torque refers specifically to a force that causes another object to rotate. If a rope were tied around a wheel and a heavy object placed on the end of the rope, gravity would pull the object downward, representing a pulling force, while the weight of the object would turn the wheel, causing the twisting force represented by torque.
Automobiles take advantage of torque to make the wheels on a car turn. An engine creates power that moves the pistons inside it up and down. This force only pushes and pulls, however, and cannot turn the wheels by itself. To fix this problem, the pistons rotate a crankshaft which in turn rotates the flywheel that connects to the car's transmission. All these parts eventually transfer the rotating motion from the crankshaft to the car's wheels, which then start rolling forward.
The friction torque action on an object is the result of the resistance caused by the friction between two objects subtracted from the original amount of force applied. When a bowling ball rolls along the floor, the friction between the floor and the ball slows it down some. Its total friction torque, the speed at which it's turning, is the original force when you threw it minus the forces such as the friction with the floor that slow it down. Torque measurements are expressed in newton meters or pounds-feet.
Effects caused by friction torque can create problems in machines that need to turn precisely and not slip. To control the friction torque in these machines, torque limiters are often used. Friction torque limiters create a snug fit between the moving parts to ensure they rotate at the desired speed. This is especially important in manufacturing equipment and related machines.