A brush or carbon brush is an electrical contact which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electric motors, alternators and electric generators. The lifespan of a carbon brush depends on how much the motor is used, and how much power is put through the motor. Etymology For certain types of electric motors or generators to function, the coils of the rotor must be connected to complete an electrical circuit. Originally this was accomplished by affixing a copper or brass commutator or 'slip ring' to the shaft, with springs pressing braided copper wire 'brushes' onto the slip rings or commutator which conduct the current. Such brushes arced and even welded as the commutator rotated, because the brush short–circuited adjacent segments. The cure was the introduction of 'high resistance brushes' made from graphite (sometimes with added copper). Although the resistance was of the order of tens of milliohms, they were high resistance enough to provide a gradual shift of current from one commutator segment to the next. Carbon brushes are available in four main grade categories: carbon graphite, electrographitic, graphite, and metal graphite. The term brush remains in use. Since the brushes wear out, they can be replaced in products intended to allow maintenance. During World War II, high–altitude aircraft generators had very rapid brush wear, requiring reformulated brush compounds for acceptable life. Metal fiber brushes are currently being developed. They may have advantages over current brush technology, but have not yet seen wide implementation. Manufacturing process Mixing components Exact composition of the brush depends on the application. Graphite/carbon powder is commonly used. Copper is used for better conductance (rare for AC applications). In order to maximize electrical conductivity and green strength, highly dendritic (electrolytic) copper powder is used. Binders, mostly phenol or other resins or pitch, are mixed in so the powder holds its shape when compacted. Other additives include metal powders, and solid lubricants like MoS2, WS2. Much know-how and research is needed in order to define a brush grade mixture for each application or motor. Compacting the mixture The brush compound is compacted in a tool consisting of upper and lower punch and die, on mechanical or hydraulic presses. In this step, depending on later processing, the copper-wire (called shunt wire) can be inserted automatically through a hole in the upper punch and fixed into the pressed brush