Mechanical Losses:

Rotational or mechanical losses could be caused through bearing friction, and air friction (known as windage), brush friction on the commutator, that is caused through the air turbulence due to armature rotation. Careful maintenance could be instrumental in keeping bearing friction to a minimum. A Clean bearings and proper lubrication are essential to the reduction of bearing friction. Brush friction is reduced through ensuring:  proper brush use, proper brush seating and maintenance of proper brush tension.   A clean and smooth commutator also aids in the reduction of brush friction.  Within extremely large generators, hydrogen is used inside the generator for cooling; hydrogen is being less dense than air and causes less windage losses than air.