The ___________ must be mechanically true, the unit in good balance, and the ________ in good shape and properly adjusted

• A. Bearings
• B. Comm/Brushes
• C. Cables
• D. Gears

Answer: (B)

In a DC machine, smooth operation depends on precise mechanical condition and proper current collection. The commutator surface must be mechanically true, meaning it runs straight and true with the shaft so each brush contacts the correct segment uniformly as the rotor turns. The rotor assembly should be in good balance to minimize vibration and bearing wear during operation. The brushes need to be in good shape and properly adjusted to maintain solid, even contact with the commutator, which ensures reliable current transfer and minimizes arcing and wear. The other options don’t fit the same way: they don’t pair with the idea of a rotating surface needing true alignment, a rotating unit needing balance, and contact components needing adjustable, maintained condition in this context.