<P> Metal fiber brushes are currently being developed . They may have advantages over current brush technology, but have not yet seen wide implementation . </P> <P> 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 and not on automotive fuel pumps which run on carbon commutators). 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 MoS, WS . Much know - how and research is needed in order to define a brush grade mixture for each application or motor . </P> <P> 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 block by the powder pressed around . This operation, called "tamping", is usually performed using electrolytic copper powder, possibly with silver coating for some high performance applications . After this process, the brush is still very fragile and in professional jargon called a' green brush' . </P> <P> Next follows heat treatment of the "green brushes" under artificial atmosphere (usually hydrogen and nitrogen). Temperatures range up to 1200 ° C . This process is called sintering or baking . During sintering, the binders either burn off or carbonize and form a crystalline structure between the carbon, copper and other additives . Baking is followed by graphitization (heat treatment). The heat treatment is transformed by a temperature curve exactly defined for each material mixture . Besides the mixture composition, the used temperature curve is the second big "secret" of each brush manufacturer . After the heat treatment, the brush structure is modified in a way which makes copying of the brush nearly impossible for competing companies . </P>

Why do we use carbon brushes in a machine