The minimum carbon brush length is 7 mm and each motor has 4 brushes.

Usually all 4 brushes wear evenly. Therefore, check the easiest accessible brush. But in case of doubt, check all 4 brushes.
When replacement is necessary, replace all 4 brushes at the same time.

Besides the carbon brushes, also the commutator of the motor wears. Therefore it is strongly advised to preventively replace the complete motor after the carbon brushes have been replaced twice.

 

Procedure

Use a screwdiver to loosen the carbon brush holder (circle in figure). Remember the orientation of the brush and pull it out of the motor. Measure the length of the brush. When the brush length is shorter than 7 mm, replace all four brushes of that motor.

To replace the carbon brushes, loosen the motor plate and rotate the motor to gain access to the individual brushes.

When all brushes have been replaced, install the motor again and adjust the belt tension as described in the Adjustments section of the Pick & Place wagons. The belt tension is 122 ± 6 Hz.