Carbon-based Brushes and Metal Graphite Brushes

Carbon-based Brushes

How are carbon-based brush materials manufactured?

They are made from a mixture of coke and graphite powders, agglomerated with pitch or resin. This powder is molded into blocks which are baked at high temperature to convert the binder into coke. These materials are not graphitized.

Main Characteristics

Carbon-based brushes commutate well due to their high resistance and provide good polishing action, while maintaining moderate contact drop. As a result of their high temperature treatment, carbon-based brushes can withstand both high temperatures and variable loads.

Main Applications

• Steel and stainless steel slip rings for synchronous machines

• Modern small machines, operating with permanent magnets, servomotors, and universal motors

• low voltage battery powered motors

Operating Limitations

• Current density in the brushes: 8 to 12 A/cm2 (50 to 75 A/in2 ) depending on the application

• Maximum peripheral speed: 60 m/s (182 ft/s)

Metal Graphite Brushes

How are metal graphite brush grades manufactured?

powdered natural or artificial graphite is mixed with a thermo-setting resin, copper powder, and/or other metal powders. The mix is then pressed and polymerized at a suitable temperature in an inert atmosphere.

Main Characteristics

Dense to very dense carbon brushes with low friction and very low contact drop, therefore operating with very low losses and high currents.

Copper-based Brush Grades

Main Applications

• low-speed, low voltage DC machines
• Medium-speed, highly-loaded AC asynchronous machines (wind turbine generators)
• Medium-speed AC synchronous motors slip rings
• High current collection systems (electrolytic treatment lines, wire annealers, galvanizing lines…)
• low-voltage current collection (military, medical, paramedical, signal…)
• Special machines
• Slip ring assemblies in rotary joints

Operating Limitations

• Current density:

• peripheral speed: up to 40 m/s (131 ft/s), depending on metal content

Silver-based Brush Grades

Silver grades have a higher conductivity than copper grades and form a special
low resistance film due to the conductivity of silver oxide. Silver grades can also
transfer low voltage current signals without degradation.

Main Applications

• Signal current transmission (thermocouples, thermometric sensors, regulation…)
• pulse transmission to rotating devices (radar, prospection…)
• Tachometer generators
• Aerospace and space applications
• Shaft grounding in a dual-grade construction

Operating Limitations

• Current density up to 50 A/cm2 (440 A/in2).
• peripheral speed: up to 30 m/s (98 ft/s)