How to Replace a Generator Alternator Bearing

Signs of a worn alternator bearing

A worn alternator bearing usually makes itself known through sound and feel. The most common symptom is a rumbling, grinding, or screeching noise coming from the generator end of the machine, not the engine. The noise often changes with engine speed and may get louder once the generator has been running for a while and the bearing warms up.

Vibration is the second sign. If you notice an unusual shaking at the rear of the generator, or if the output voltage fluctuates for no obvious reason, the rotor may be off-center because the bearing has excessive clearance. See Generators with Wattage Meters for ways to monitor output while troubleshooting.

A failing bearing also increases drag on the rotor, which can make the engine sound labored at idle. If you suspect the bearing, stop using the generator. A badly worn bearing can seize or allow the rotor to contact the stator, which can damage the alternator windings and lead to an expensive repair.

Safety and preparation

Before you open the alternator, take the same precautions you would for any generator service. Shut off the engine, disconnect the spark plug wire, and disconnect the battery if the generator has electric start. Turn the fuel valve to the off position and let the engine cool completely.

Work outdoors in a well-ventilated area, away from any building openings. The U.S. Consumer Product Safety Commission (CPSC) warns that generator exhaust contains carbon monoxide, an odorless, colorless gas that can be deadly. Never run a generator indoors or in a garage, even with doors and windows open. See Generator CO Poisoning Risks and Safe Placement Distances and Generator Safety Checklist for more on safe operation and maintenance. The Environmental Protection Agency (EPA) also stresses that generators and other gasoline-powered equipment are sources of CO that can build to deadly levels indoors.

Gather tools before starting: a socket set, a bearing puller, a press or a large bench vise with bearing driver set, a torque wrench, and a can of penetrating oil. Have a clean workbench and a container for small parts. You will also need the correct replacement bearing. Check the generator's parts diagram or the bearing's stamped part number to order the right one.

Removing the rotor

Start by removing the alternator end cover. On most portable generators, this is the sheet-metal or plastic shroud at the rear of the generator housing, held by several screws or bolts. Set the fasteners aside in a labeled container.

Disconnect the stator leads from the terminal block or the voltage regulator. Mark each wire with tape to make reconnection easier. You do not need to remove the stator to replace the bearing, but you may need to move it aside to access the rotor. See Generator AVR or Inverter if you are also working on the regulator.

The rotor is typically held by a center bolt or nut on the shaft. Use a socket and breaker bar to loosen it, keeping the rotor from spinning. If the shaft turns, slide a screwdriver through the fan or use a strap wrench. In some models, a small retaining ring or snap ring holds the rotor in place. Remove it with a snap ring pliers before trying to pull the rotor.

Once the fastener is loose, use a puller to remove the rotor from the shaft. A three-jaw puller works well. Place the puller center screw against the end of the shaft and pull evenly. Do not hammer on the rotor; this can bend the shaft or damage the windings.

Pressing out the old bearing

With the rotor free, you can see the bearing seated on the shaft. Most alternator bearings are a light press fit. To remove one without a press, use a bearing puller with arms that grab the inner race. If the bearing is stuck, apply penetrating oil around the inner race and wait 15 minutes before trying again.

If you have a shop press, support the rotor on a flat plate so the bearing hangs freely, and press the shaft out of the bearing. Do not press on the outer race or the balls; this can damage the bearing and make it hard to remove. For a bench vise, use a bearing driver or a socket that matches the inner race to push the bearing off.

Inspect the shaft and the housing bore once the old bearing is out. Look for scoring, rust, or pitting. A damaged shaft can quickly ruin a new bearing. If the shaft is worn, the alternator may need professional attention. Check the stator for burn marks or broken wires while you have access, and see How to Test a Generator Rotor Winding for Continuity if you want to verify the rotor windings.

Pressing in the new bearing

Clean the shaft and the housing bore with a lint-free cloth. Do not use grease or oil on a sealed bearing; it comes pre-lubricated. If the new bearing is a shielded or sealed type, press it in with the shield facing the outside, matching the orientation of the old one.

Press the bearing onto the shaft using a press or a bearing driver that contacts the inner race only. Do not hammer the bearing directly. If you must use a hammer, place a socket that fits the inner race over the bearing and tap the socket, not the bearing. Drive the bearing until it seats firmly against the shaft shoulder.

If the bearing mounts in the housing instead of on the shaft, press it into the housing first, then slide the rotor into place. Make sure the bearing is square to the bore; a cocked bearing will bind and wear quickly.

Reassembly and first run

Reinstall the rotor on the shaft. Apply a small amount of anti-seize compound to the shaft before sliding it home, and torque the center bolt or nut to the manufacturer's specification. Reconnect the stator leads to the terminal block or regulator, matching your wire labels.

Replace the end cover and secure it with the fasteners you removed. Reconnect the battery and the spark plug wire. Before starting the engine, rotate the rotor by hand to make sure it turns freely and does not rub against the stator. If it drags, stop and check that the bearing is fully seated and the cover is not binding.

Start the generator and run it with no load for a few minutes. Listen for the old noise. A new bearing should be quiet, with only the normal engine and exhaust sound. Check the output voltage with a multimeter while running. If the voltage is stable, the repair is complete. See How to Load Test a Generator to verify full operation under a load.

What to pick for your use

Questions

How do I know the bearing is the cause of the noise?

If the noise comes from the alternator end, changes with engine speed, and continues after the engine is warm, it is likely the bearing. Remove the end cover and spin the rotor by hand. A worn bearing will feel rough or make a clicking sound. If the noise disappears when you disconnect the load, it may be electrical, not mechanical.

Can I replace the bearing without a press?

Yes. A bearing puller can remove the old one, and you can install the new one by driving a socket that fits the inner race with a hammer. The key is to always apply force to the inner race only. Repeatedly driving a bearing by the outer race can dent the balls and shorten its life.

Should the new bearing be sealed or open?

Match the type that was originally installed. Most portable generator alternators use sealed ball bearings. Sealed bearings need no maintenance and keep out dust. Open bearings require periodic greasing and are less common in consumer generators. Check the old bearing's code before ordering.

How long does the job take?

Plan for two to four hours for a first repair. Removing the cover and rotor is straightforward, but pressing the old bearing off and the new one on carefully takes time. If the rotor is corroded to the shaft, you may need extra time with penetrating oil.

What if the noise continues after the replacement?

A new bearing that still rattles points to another issue, such as a bent rotor shaft, a worn stator, or an out-of-balance flywheel. You can check the rotor windings with a multimeter. If the problem persists, the alternator may need professional service or replacement.