This DIY repair guide explains how to replace the drive and worm gears on a chain-drive garage door opener. The worm gear, which attaches to the motor shaft, meshes with the drive gear to spin the sprocket and drive the chain to open and close the garage door. If the gears break or strip, the door won't move. Replace a broken or damaged worm and drive gear with the manufacturer-approved replacement part.
Use this procedure to replace the drive and worm gears in Craftsman, Chamberlain and LiftMaster chain-drive garage door openers.
This video explains how to replace the drive and work gears on a garage door opener.
Use a step ladder to access the garage door opener’s power cord and motor unit.
Unplug the garage door opener.
Take the garage door opener down
Position a work table near the area under your motor unit to hold the garage door opener during the repair.
Pull the emergency release rope to release the trolley from the garage door.
Shut the garage door if it's not already closed.
Remove the fastener ring from the straight door arm connected to the outer trolley.
Pull out the clevis pin while supporting the straight arm to disconnect the garage door from the outer trolley.
Lower the straight and curved arms down and rest them against the garage door. Use a helper if necessary.
Open the light cover to access the wall control and safety sensor wires connected to the motor unit.
Take a digital photo of the wall control and safety sensor wires connected to the motor unit. You'll refer to the photo later when reconnecting the wires.
Disconnect the wall control and safety sensor wires from the motor unit.
Close the light cover.
Remove the nuts, bolts, and lock washers that secure the motor unit to the ceiling bracket. Support the motor unit with one hand as you remove the last bolt from the ceiling bracket.
Carefully lower the motor unit down and rest it securely on top of the step ladder.
Climb down the step ladder, lower the motor unit down and rest it on your work table.
Move your step ladder by the garage door so you can remove the rail from the header bracket.
Remove the fastener ring from the header bracket clevis pin.
Pull the clevis pin out of the rail and header bracket while supporting the rail with one hand.
Lower the rail and rest it on a rung of the step ladder so the rail is level with the motor unit on your work table.Tip: To avoid scratching a decorative garage door, remove the nuts, bolts and washers from the curved arm and carefully rest the curved arm against the door when disconnecting the garage door from the outer trolley.Tip: You may want to have a helper assist you when taking the garage door opener down.
Disconnect the door arm from the trolley.
Remove the chain and cable assembly
Using a felt-tip marker, mark a link of chain and the sprocket tooth that meshes with it for when you reinstall the chain after replacing the drive and worm gears.
Slide the outer trolley away from the inner trolley.
Remove the outer nut and lock washer from the trolley threaded shaft and pull the threaded shaft out of the trolley.
Pull the chain off the motor unit sprocket and out of the chain spreader.
Lay the chain out on the garage floor while you complete the repair.
Disconnect the trolley threaded shaft.
Remove the gear and sprocket assembly
Remove the light covers and light bulbs from the motor unit.
Remove the screws from the chain spreader and pull the chain spreader off the motor unit.
Remove the screws from the motor unit cover and pull the motor unit cover off.
Remove the plastic helical gear retainer and pull the helical gear off.
Release the limit switch locking tabs by squeezing the sides just below the motor bracket near the helical gear.
Move the limit switch aside and let it hang by its wires.
Remove the gear and sprocket assembly mounting screws.
Pull the gear and sprocket assembly out.
Remove the mounting screws.
Remove the motor
Remove the main bearing from the motor plate. Unplug the RPM sensor wire harness.
Remove the interrupter cup from the motor shaft.
Remove the motor mounting nuts from the front of the motor plate.
Pull the motor out of the motor plate and set it beside the motor unit.
Pry off the motor shaft bearing retaining clip and pull the motor shaft bearing out of the motor plate.
Note: On some models, you'll need to loosen the set screws on the set collar and remove the set collar from the motor shaft, then remove the washer cap, spring washer, flat washer and bushing from the motor shaft before removing the mounting nuts from the motor plate and pulling the motor out.
Remove the mounting screws.
Replace the worm gear
Use a punch and hammer to drive the front roll pin out of the motor shaft.
Pull the worm gear off the motor shaft and clean the motor shaft using fine grit emery paper.
Remove the O-ring, C-ring and spacer from the motor shaft.
Install the new O-ring, C-ring and spacer on the motor shaft.
Lubricate the worm gear with grease, then slide the new worm gear onto the motor shaft, engaging the rear worm gear notches on the roll pin.
Reinstall the front roll pin.Tip: Some motors have the roll pin behind the worm gear on the shaft so you pull the worm gear straight off.
Remove the worm gear roll pin.
Reinstall the motor
Install the new motor shaft bearing into the motor plate and reinstall its mounting clip, using socket to push the mounting clip against the motor plate.
Push the motor into the motor plate and reinstall the motor mounting nuts.
Reinstall the interrupter cup on the motor shaft.
Apply grease to lubricate the new main bearing and install it in the motor plate.
Reconnect the RPM sensor wire harness.
Note: For motors with a set collar, push the new bushing onto the motor shaft and slide it against the motor plate, engaging the notch on the bushing in the slot on the motor plate. Install the new flat washer, spring washer and washer cap on the motor shaft. Slide the new set collar against the washer cap on the motor shaft. Hold the set collar stationary and push the motor shaft through the set collar from the back of the motor to eliminate shaft play, then tighten the set screws.
Reinstall the motor.
Replace the drive gear
Use a punch and hammer to drive the roll pin out of the drive shaft.
Pull the drive gear off the drive shaft.
Slide the new drive gear onto the drive shaft and line up the mounting hole in the drive gear with the hole in the drive shaft.
Insert the new roll pin through the gear and drive shaft. Drive the roll pin into place using the punch and hammer.
Lubricate the new drive gear with grease.
Remove the drive gear roll pin.
Reinstall the gear and sprocket assembly
Insert the gear and sprocket assembly into the motor unit and install the mounting screws.
Apply grease to the helical gear.
Push the helical gear onto the gear and sprocket assembly shaft and secure it with the retaining clip.
Reinstall the gear and sprocket assembly.
Reinstall the limit switch and motor unit cover
Position the limit switch on the motor bracket and snap the limit switch into place, engaging the limit switch drive gear with the helical gear.
Reinstall the motor unit cover and secure it with the mounting screws.
Reinstall the light bulbs and light bulb covers.
Reinstall the chain spreader, securing it with the mounting screws.
Reinstall the limit switch.
Reinstall the chain
Making sure not to twist the chain, thread it through the chain spreader and around the motor unit sprocket, matching the marked chain link to the marked tooth on the motor sprocket.
Thread the inner nut up to the top of the trolley threaded shaft and reinstall the inner lock washer on the threaded shaft.
Insert the trolley threaded shaft into the trolley hole and reinstall the outer nut and lock washer. Hand-tighten the outer nut.Tip: Have a helper push the inner trolley toward the threaded trolley shaft to apply more tension to the chain and cable, making it easier to thread the outer nut onto the trolley shaft.
Reconnect the trolley threaded shaft to the trolley.
Tighten the chain
Adjust the outer nut to position the trolley threaded shaft so the chain hangs about 1/4-inch above the base of the rail at its midpoint.
Tighten the inner nut against the trolley to secure the trolley threaded shaft.
Tighten the chain.
Reinstall the garage door opener
Reconnect the rail end to the header bracket using the clevis pin and fastener ring.
Position your step ladder under the motor unit ceiling bracket and carefully set the motor unit on top of the step ladder.
Climb the step ladder and reinstall the motor unit to the ceiling bracket using the nuts, bolts and lock washers.
Open the light cover to access the wall control and safety sensor terminals on the motor unit. Using your digital photo for reference, reconnect the wall control and safety sensor wires to the motor unit.
Close the light cover.
Reconnect the straight door arm to the outer trolley using the clevis pin and fastener ring.
Re-engage the trolley.Tip: Reconnect the curved door arm to the straight door arm using the bolts, lock washers and nut if you disconnected the curved arm from the straight arm when freeing the garage door from the outer trolley.
Reconnect the rail to the header bracket.
Reconnect electrical power
Plug in the garage door opener.
Adjust the travel limits
Run the garage door opener through a complete travel cycle to check upper and lower travel limit.
If the garage door doesn't close completely, adjust the down travel and cycle the door open and closed to test the adjustment. Continue to adjust the down travel until the door closes completely.
If the door doesn't open completely, adjust the up travel in the same manner.Tip: Repeated operation of the garage door opener during travel limit adjustment may cause the motor to overheat and shut off. Wait 15 minutes to allow the motor to cool and then continue adjustments.
Adjust the travel limits.
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