Why Brush Grade Selection Matters
Carbon brushes are a consumable, but choosing the wrong grade is one of the most common causes of premature commutator wear, excessive brush consumption, and motor failures in locomotive traction applications. A brush that is too hard wears the commutator; a brush that is too soft wears too fast and causes commutator filming problems.
Key Selection Parameters
1. Current Density
Current density (A/cm²) is the most important electrical parameter. For EMD D-Series motors, typical brush current densities run 8–14 A/cm² at rated load. Match the brush grade to the rated current density range specified by the manufacturer. Exceeding the brush's rated current density causes overheating and rapid wear.
2. Peripheral (Surface) Speed
Commutator peripheral speed at rated RPM determines friction and wear behaviour. EMD traction motor commutators typically run at 25–45 m/s at full speed. High-speed applications require grades with lower coefficient of friction — typically electrographitic or graphite-impregnated grades.
3. Operating Humidity
Carbon brushes require a thin film of water vapour on the commutator surface for proper lubrication. In very dry climates (desert operations in the Middle East or high-altitude Andean routes), standard brush grades can cause severe commutator wear because the moisture film does not form. Specify humidity-compensated grades with built-in lubricating additives (copper-graphite or MoS₂-doped grades) for these environments.
4. Altitude
At high altitude, lower atmospheric pressure reduces the water vapour available at the brush-commutator interface. For operations above 2,000 m, select a grade specifically rated for high-altitude service.
5. Duty Cycle
Heavy freight locomotives with sustained high-current draw need grades with higher thermal resistance. Switcher (shunting) locomotives with frequent start-stop cycling need grades that handle repeated high-inrush current events.
Common Brush Grade Families
| Grade Family | Composition | Best For |
|---|---|---|
| Electrographitic (EG) | Synthetic graphite | High-speed, high-current traction — most common for EMD |
| Carbon-Graphite (CG) | Natural graphite + carbon | Medium-speed, lower current applications |
| Copper-Graphite (CuG) | Copper + graphite | High-current density, dry/arid environments |
| Metal-Graphite (MG) | Metal powder + graphite | Very high current density, low-voltage applications |
Signs of Wrong Brush Grade in Service
- Rapid brush wear — grade too soft or current density too high
- Commutator grooving — grade too abrasive or too hard
- Black filming on commutator — grade producing excessive carbon deposits; switch to a harder grade
- Commutator burning / threading — arcing due to improper grade or incorrect spring pressure
- White/light patches on commutator — insufficient film formation; humidity too low or wrong grade for environment
Supco Canada Brush Stock
Supco Canada stocks multiple electrographitic and copper-graphite grades for EMD D77, D78, D87, D90, and D100 motors, as well as GE traction motors. Our technical team can cross-reference your existing grade to a compatible alternative and advise on grade upgrades for specific operating environments.