There are five main groups which differ in their use of raw
material and manufacturing process:
1. Hard carbon brushes
The most important raw materials are carbons like lamp /carbon
block coke and with various types of cokes bonded with pitch , the
raw materials are processed by mixing, pulverizing , pressed and
kilned at temperatures up to 1200 C. The kilning process carbonises
the binder also and a relatively hard carbon is produced with much
better polishing qualities than electrographite & Carbon
Graphite grades. Therefore, hard carbon brushes are mostly used on
small machines with flush mica which require strongly polishing
brushes. The permissible electrical and thermal load for hard
carbon brushes is rather low and their elasticity limits their use
to machines with peripheral speeds of up to 20 m/sec.
2. Carbon-graphite brushes
Electrographite is one of the most important raw material, here and
is processed like Hard Carbons with binder pitch , pressed and
heat-treated similarly to hard carbon at temperatures to meet
specific resistivity needs. The principle field of application for
carbon graphite brushes is, therefore, intermediate between those
of electrographitic and hard carbon brushes. Their polishing
properties are better; their electrical carrying capacity is lower
than in the case of electrographitic brushes. Carbon graphite
brushes are also characterized by outstanding commutating abilities
as a result of their high resitivity & contact voltage drop.
They are used on machines where electrographitic brushes cannot be
used due to their low polishing ability, and where hard carbon
brushes are unsuitable because of their lower commutating
properties. Carbon graphite brushes with specially designed
impregnations for ‘radio interferenceare mainly used on F.H.P.
motors with mica undercut and on small thyristor fed DC machines.
3. Resin-bonded brushes
In principle, the raw material is natural crystalline graphite,
bonded with resin, which is pressed and heat treated in a special
process. The advantage of special graphite brushes is their high
contact drop and high resistance with inherent higher anistropy.
They also have good oxidation resistance. These properties are very
valuable for machines with high commutating requirements. It is to
its disadvantage, the high contact resistance causes high
electrical loss due to circulating current. This might lead to
deterioration of the resin bond. Therefore, the main field of
application for special graphite brushes covers all machines with
high commutating requirements, but with relatively low brush
current density. These are three-phase commutator motors and small
machines with mica undercut. Due to their low elasticity, special
graphite brushes are normally used only up to 35 m/sec.
4. Electrographite brushes
The raw materials like petroleum /pitch coke & lamp black are
subjected to processes similar to those described under 1 to 3
above and then further heat-treated at temperatures up to 2500C.
This converts the carbon material into a Crystalline form of
synthetic graphite, called Electrographite, a material having good
electrical and thermal conductivity. Its elasticity and
refractoriness are improved and its polishing properties are
generally lower than hard carbon or graphite brushes. Depending on
the composition of the basic materials, brushes having widely
variable characteristics can be produced with various specially
prepared impregnations . Electrographite brushes, therefore, cover
a wide range from the robust traction brush to very elastic grades
having outstanding commutating abilities. Because of all these
characteristics, electrographite grades are found in all classes of
service, though they are mainly used on large commutator machines
with high electrical, thermal and mechanical requirements.
5. Metal graphite brushes
Raw materials are metal graphite mixtures which are processed on a
powder processing te