In brushed motors, brushes conduct power to the commutator.
The brushes are held in place by springs.
By changing the springs` tension to increase or decrease the pressure of the brushes against the commutator, you can adjust the revolutions of the motor.
Simply put, harder spring pressure increases friction and slows down the commutator.
In general, the less tension placed on the brush, the more RPM (top speed).
The more tension, the more torque (faster acceleration).
Spring tension is measured by ounce rate.
The lower ounce rate generates less pressure than a higher ounce rate.
Choose a lower ounce rate for slightly more RPM, and a higher rate for more torque. However, harder pressure will hasten commutator wear.
The springs give only a slight range of adjustment.
If you want greater changes to RPM, for instance, you will still switch to an armature of fewer turns.