my craftsman garage door opener motor hums but the chain doesn't move at all

Diagnosis

The humming sound you hear is the electric motor attempting to rotate, but the mechanical link between the motor's output shaft and the chain drive has failed. In the Craftsman 54930, this is almost always caused by the failure of the main drive gear (part #41A4253). This gear is made of a nylon-plastic composite designed to act as a "sacrificial" part; it is engineered to strip its teeth if the door hits a major obstruction to prevent the motor from burning out. Over time, however, the plastic degrades, becomes brittle, and the teeth simply shear off. You can confirm this diagnosis by unplugging the unit, removing the plastic motor housing, and looking inside the gear assembly. If you see "plastic snow" (white or grey plastic shavings) scattered throughout the gearbox or if the gear teeth are visibly missing or flattened, the gear is stripped. If the gear looks pristine but the motor still hums without turning, the issue is likely electrical—specifically a failed start/run capacitor that cannot provide the initial torque needed to move the assembly.

How to Fix It

To replace the stripped drive gear and restore function to your opener, follow these precise steps:

1. Disconnect the power cord from the 120V outlet and manually disconnect the garage door from the opener using the red emergency release cord to remove all tension from the chain.
2. Remove the screws securing the plastic motor cover and the end caps of the drive assembly. Carefully lift the cover to expose the gearbox.
3. Remove the motor mounting bolts and slide the motor assembly slightly away from the gear housing to create clearance.
4. Extract the stripped drive gear. Use a needle-nose plier to remove any remaining plastic fragments or "shavings" from the worm gear (the metal spiral shaft) and the sprocket. It is critical that the metal worm gear is cleaned thoroughly with a rag and degreaser to ensure the new gear seats perfectly.
5. Install the new gear kit (Part #41A4253). Align the gear teeth precisely with the worm shaft. Ensure the gear is seated fully and flush against the housing; any gap will cause the new gear to strip prematurely.
6. Apply a small amount of lithium-based white grease to the gear teeth and the worm shaft. Do not over-grease, as excess lubricant will attract dust and grit.
7. Reinstall the motor assembly, tighten all mounting bolts, and replace the plastic housing.
8. Plug the unit back in and run the door through one full cycle to ensure the travel limits are still correctly set.

If That Didn't Work

• Check the Start/Run Capacitor. If the gear is intact but the motor hums and refuses to turn, the capacitor (usually a cylindrical component located near the motor) has likely leaked or lost its capacitance. Look for a bulging top or oily residue on the casing. Replace it with a capacitor matching the exact microfarad (uF) and voltage rating specified on the original part.
• Inspect the Sprocket and Chain Tension. If the gear is turning but the chain isn't moving, check the drive sprocket. If the sprocket is slipping on the shaft or the chain has jumped the track, the motor will hum under the load of the stalled chain. Tighten the set screws on the sprocket or realign the chain.
• Verify the Limit Switch Alignment. If the limit switches are misaligned or stuck, the motor may be receiving a "stop" signal immediately upon starting, causing a momentary hum before the logic board cuts power. Clean the switch contacts with electronic contact cleaner.
• Test the Motor Windings. Use a multimeter to check for continuity across the motor windings. If you find an open circuit or a short to ground, the motor windings have burned out, and the entire motor assembly must be replaced.

Heads Up

• Avoid using heavy-duty industrial grease on the plastic gears; use only high-quality lithium grease. Heavy greases can cause the plastic to soften or attract debris that acts like sandpaper on the gear teeth.
• Check your door's balance every six months. If the door is too heavy (due to a broken spring), it puts excessive torque on the plastic drive gear, significantly shortening its lifespan.
• Never attempt to "patch" a stripped gear with epoxy or glue. The torque required to lift a garage door will instantly shear any adhesive repair.
• Always ensure the emergency release cord is tucked away and not tangling with the chain, as a snagged cord can cause the motor to strain and strip a brand-new gear.