Another important part in an alternator are the brushes, each alternator part has its roll to play and brushes are no different. Below are some interesting facts to give you a detailed look at what brushes are and how they work.
All rotating electrical devices (starters and alternators) in today’s automotive market have one general problem in common : How to maintain a continuous current flow to a constantly rotating center component. Armatures in starters and rotors in alternators need to be moved at various revolutions per minute on demand. At certain applications, the alternator must maintain field current connection while with standing engine RPM exponentially.
The problem was first encountered and solved by the Cleveland area inventor, Charles F. Brush. Brush, likened to other inventors such as Thomas Edison, built a 60 ft. wind powered turbine in the backyard of his Euclid avenue mansion in 1877. The unit was a 60 ft high, 56 ft diameter wind turbine with a 20ft diameter output shaft. Brush Electric Company merged with other conglomerates which later formed General Electric.
CARBON/GRAPHITE/ COPPER COMPOSITION
The exact blend of metallurgy 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 where non spark components are necssary). Binders, such as 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. Extreme research and experience is necessary to arrive at a final recipe for a brush mixture for each specific application. Most manufacturers agree the higher the copper content, the better the continuity and wear factor. The higher the carbon or graphite content, the more arc and spark suppression for smoother tracking and life expectancy for higher voltages.
FORMING THE MIXTURE IN DIES
The brush compound is compacted in a die and then pressed either mechanically or hydraulically. During this process, the copper-wire (shunt wire) can be inserted automatically through a hole in the upper punch and fixed into the compound mixture by the powder pressed around the wire. After this is completed, the brush is still very fragile and susceptible to crumbling. It is referred to at the factory as a “green” brush and must be carefully handled while transferring to the next step. Binders may be used for stability purposes.
BAKING OR “SINTERING”
The brushes are then heat treated or fired in extreme high temperature with an artificial atmosphere. Hydrogen or Nitrogen is used during this step. Temperatures of 1200C can be reached. It is during this process, that any binders that were used for previous stability will either burn up or crystallize to form a permanent compound. This initial baking is followed by various heat treatments. The exact heat treat curve can be a closely guarded secret among brush manufacturer competitors, as this step is unanimously regarded as the most contributing quality control factor.
The baking process can cause shrinking or imperfections such as bending in the product.
Grinding is needed to bring the brush back to specified measurements. Also many companies are known to use grease, oils, or resins to impregnate the surface of the brush to increase life expectancy and overall durability while in service.
The demands and requirements of today’s brush manufacturing facilities are indeed a highly demanding and rigorous environment. Specialized attention to quality control, and an in depth knowledge and experience with chemistry and metallurgy is paramount. The brush in a motor or generator is probably the lowest cost component, yet the most common and most key to the overall functionality and life expectancy to all rotating electrical devices around the world.