The brushes used for machines are divided into five classes according to the material with which it is made they are

1. Metal Graphite brush
2. Carbon Graphite brush
3. Graphite brush
4. Electro – Graphite brush
5. Copper brush

The maximum current density at brush contact varies from 0.5A/sq.cm to 23A/sq.cm for copper.For large current machines working at low voltage copper brushes are employed.Lubrication should be done properly else it may cut the commutator rapidly leading to high wear and tear.Graphite and carbon graphite brushes are self lubricating and therefore it is used very widely.

Even for soft brushes there is wear but it takes place gradually.Though the mica segments between the commutator doesn’t wear easily but it reduces the effective contact with the segments and it may result in sparking and damage to the commutator.So to reduce this the micas ‘undercut’ a level below the commutator surface using a narrow milling cutter.

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