Constant force springs ensure the carbon brush works consistently by dependably exerting constant pressure to increase brush life and to reduce electrical wear. Carbon brushes can also be used for earth return current units to divert static and induced electric currents in a motor shaft away from the bearings, and to protect them from pitting and damage.

Constant force spring keeps carbon brush to rub against the outside wall of a rotating slip ring. As the metal ring turns, the electrical current or signal is conducted through the brush to the metal ring making the connection.