A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electric motors, alternators and electric generators.
For an electric motor or generator to function, the coils
of the rotor
must be connected to complete an electrical circuit. To accomplish this, (copper
) 'slip rings
' are affixed to the shaft
, and springs
press braided copper wire
'brushes' onto the rings which conduct the current. Later, these copper wire brushes were replaced with carbon
blocks; but these blocks are still called brushes
. As the brushes are slowly abraded
, they may have to be replaced, if this is possible.
If the copper rings are split into parts with "interlaced" connections, the arrangement is called a commutator.
Metal fiber brushes are currently being developed again. These brushes may have advantages over current carbon brushes, but have not yet seen wide implementation.
Types of carbon brushes
There are distinguished basically 3 types of carbon brushes:
- brushes for automotive applications: DC current, voltage 12 - 48 Volts.
- brushes for "household" applications: AC current, voltage 110 / 220 Volts
- brushes for industrial motors: both AC and DC current of various Voltages.
Exact composition of the brush depends on the application. Graphite/Carbon powder is commonly used. Copper is used for better conductibility (rare for AC applications and not on automotive fuel pumps which run on carbon commutators). Binders
are mixed in so the powder holds its shape when compacted. (mostly phenol- or other resins, pitch). Other additives include metal powders, and solid lubricants like MoS2
. Much know-how and research is needed in order to define a brush grade mixture for each application/even for each 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 block by the powder pressed around. After this process, the brush is still very fragile and in professional jargon called a 'green brush'.
Firing of green brushes
Heat treatment of the 'green brushes' under artificial atmosphere (usually H2
). Temperatures up to 1200° C. This process is called Sintering
or Baking. During sintering/baking, the binders either burn off or carbonize and to form a crystalline structure between the carbon, copper and other Additives. Baking is followed by Graphatisation (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.
Sintering causes the brushes to shrink and to bend. They must be ground
to net shape. Some companies use additional treatments in order to enlarge durability of brush (and therefore application), for example impregnation on the running surface by special oils, resins and grease.
Manufacturing of carbon brushes requires a very high knowledge of materials and experience in mixture compositions. Very small changes in brush contents by just a few percent of components by weight can significantly change the properties of brushes on their applications. There are just a handful of brush developing companies in the world, which are mostly specialized on certain types of brushes.
Carbon brushes are one of the least costly parts in an electro motor. On the other hand, they usually are the key part which delivers the durability ("life-time") and performance to the motor they are used in. Their production requires very high attention to quality control and production process control throughout all steps of the production process.