the opener motor runs but the belt just spins and the door doesn't go up, it's a liftmaster 8500 wall mount.

Diagnosis

The issue is a mechanical disconnection between the motor's output shaft and the drive pulley. In the LiftMaster 8500W, the motor rotates a steel shaft, but the belt is driven by a white plastic drive hub (part #41A4276) that slides over this shaft. When you hear the motor humming or spinning but the belt remains stationary, the internal teeth of the drive hub have stripped out or the set screw has backed out completely, allowing the shaft to spin freely inside the hub. This failure typically occurs due to "door shock"—when the door hits a hard stop or an obstruction, the motor's torque is transferred into the plastic hub, shearing the plastic internals. You can confirm this by unplugging the unit and attempting to rotate the motor shaft by hand; if the shaft turns but the pulley and belt do not move, the hub is definitively stripped. If the pulley turns but the belt slips, you are dealing with a tensioning or belt tooth failure rather than a hub failure.

How to Fix It

To resolve this, you must replace the drive hub assembly. Ensure you have a replacement kit (Part #41A4276) and a set of Torx and Phillips head drivers.

1. Power Down: Completely disconnect the 8500W from the 120V power outlet to prevent accidental activation while your fingers are near the drive sprocket.
2. Expose the Drive: Locate the plastic belt cover protecting the motor assembly. Remove the three screws securing the cover and lift it away to reveal the drive pulley and the motor shaft.
3. Remove the Old Hub: Locate the small set screw on the side of the white plastic hub. Use a hex key or screwdriver to loosen this screw. Carefully slide the stripped hub off the motor shaft. If the hub is seized due to plastic shavings, use a pair of needle-nose pliers to gently wiggle it free.
4. Clean the Shaft: Use a clean rag and a small amount of isopropyl alcohol to wipe any plastic debris or old lubricant off the motor shaft. A clean surface ensures the new hub seats fully.
5. Install New Hub: Slide the new drive hub onto the shaft, ensuring it is pushed all the way to the shoulder. Align the hub so it sits flush against the motor housing.
6. Secure the Set Screw: Tighten the set screw firmly. Do not over-torque to the point of cracking the plastic, but ensure it is tight enough to bite into the flat spot of the motor shaft.
7. Test Tension: Manually rotate the shaft one full turn to ensure the belt is tracking straight and the hub is locked. Replace the belt cover and restore power.

If That Didn't Work

• Belt Tension Adjustment: If the hub is turning but the belt is slipping, the tension spring may have lost its elasticity. Loosen the tensioning bolt on the side of the motor carriage, pull the belt tight (approximately 1/4 inch of play), and retighten the bolt. If the belt is frayed or "stretched," replace the entire belt assembly.
• Check for Door Binding: The motor may be spinning the hub, but the door may be physically jammed in the tracks. Disengage the emergency release cord and try to lift the door manually. If the door is heavy or sticks, the motor's safety sensors may be triggering a "phantom" slip to prevent motor burnout.
• Inspect the Drive Pulley: Check the larger pulley wheel at the end of the belt. If the teeth on the pulley are worn down or rounded, the hub will spin and the belt will move, but it won't "grip" the pulley to lift the door. This requires a full pulley replacement.
• Motor Capacitor Failure: If the motor sounds like it is struggling (a loud hum) but isn't actually spinning the shaft, the start capacitor may have failed. This prevents the motor from generating enough torque to move the load, mimicking a stripped hub.

Heads Up

• Avoid "Hard Stops": To prevent the hub from stripping again, adjust your travel limits so the door doesn't slam into the header or the floor. Excessive impact is the primary cause of hub failure.
• Lubrication Warning: Never apply grease or oil to the drive belt or the plastic drive hub. Lubricants will cause the belt to slip and can degrade the plastic components of the hub.
• Check Balance: Every six months, disconnect the door and check if it stays in place when halfway open. An unbalanced door puts immense strain on the 8500W's wall-mount geometry, leading to premature hub wear.
• Set Screw Maintenance: After the first 50 cycles with a new hub, power down and check the set screw for tightness, as vibration can occasionally loosen it during the initial break-in period.