Fretting corrosion happens when there is tiny relative motion between contacting surfaces, such as a shaft and the bearing bore, under load and vibration. That micro-movement breaks the protective oxide layer and allows wear and corrosion to accumulate at the interface. If the shaft and housing fit are loose, those micro-m motions can occur easily, promoting fretting damage. A tight interference fit on both the shaft and in the housing prevents this by holding the parts firmly together so the contact interface doesn’t move relative to itself, stopping the small motions that drive fretting. Pitting is a form of localized corrosion that typically stems from electrical effects or high contact stresses, not from micro-movement at a fit. Seizure results from excessive friction and heat or loss of lubrication causing surfaces to weld together; a tight fit alone doesn’t address the lubrication or heat factors that cause seizure. Bearing slippage arises when there’s too much clearance, allowing the bearing to slip on the shaft or in the housing. The scenario described targets fretting, where eliminating micro-motion with a tight fit is the preventive measure.

As the sleeve bearing wears, the rotor gains more end play and becomes less stiff in its mounting. That looseness allows the shaft to tilt or sag slightly toward the stator, especially under gravity and magnetic forces during operation. The result is that the minimum radial gap between the armature and the stator—the armature clearance—shortens. So, the overall tendency is for the armature clearance to drop as the bearing wears. (The movement can be uneven around the circumference, but the dominant effect is a reduction in clearance rather than an increase.)

When seating a brush, you want an abrasive that won’t leave behind conductive particles that could create a short path in the brush area. Sandpaper fits this need because it uses non-conductive paper backing and abrasive grains that are not intended to carry current into the commutator slots. Using emery paper risks leaving conductive emery particles in contact areas, which can lead to short circuits. The other options aren’t suitable: cardboard isn’t abrasive enough to true the seating surface, and glass paper can shed glass fragments and isn’t the proper choice for this maintenance step.

Verifying phase rotation is about making sure the generator’s output arrives on the bus in the same phase order as the bus. When you parallel a generator with a live bus, the voltages must rise in the correct sequence (A, B, C) so the phase relationships line up and there’s no circulating current or uneven loading that could stress equipment. Using a phase sequence indicator gives a direct, reliable check of the order of the three output lines and confirms it matches the bus. By verifying the correct sequence before closing the breaker, you ensure proper paralleling and minimize risk of damage or instability on the system. If the sequence isn’t right, you can swap two leads on the generator (or rewire) to correct it and recheck. Other methods don’t reliably reveal phase order: winding temperature doesn’t indicate rotation, listening to the hum isn’t a valid diagnostic of phase sequence, and wire color alone isn’t a dependable indicator since codes can vary. The essential practice is to confirm the phase sequence with a phase sequence indicator before connecting to the bus.