Brush Basics Electricity is transferred from the battery to the commutator through the brushes. Brushes are generally made from a graphite/copper or graphite/silver blend. The graphite is used for lubrication and the copper/silver is used to carry the current. The face of the brush is cut to conform to the curvature of the commutator. Some brushes come with a serrated face which is supposed to help it seat to the armature more quickly. 
Brushes are available as soft, medium and hard. What this indirectly refers to is the percent of graphite to conductor (copper or silver). Soft brushes have a higher graphite content while hard brushes have more conductor (metal). In general for high RPM and load you should use a harder brush, and for more stop/go accelerate a medium brush is best.
Spring Power The springs are used to hold the brushes up against the commutator. To soft a spring and the brush will bounce on the comm, to hard a spring and the brush will wear quickly. Very hard springs can lower overall power due to the amount of pressure they place on the comm. 
Springs are available in various sizes, wire thickness and tensions. The most used spring tensions are the 135 and 150 degree models and they provide the best overall performance 95% of the time.
| 
