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Because the chain was broken so I want to repair it. There is related issue.

However after I unplugged the power and took off the chain sprocket. I tried to move the sprocket by hand. But it won't move. Is this the real reason to cause the chain broken? What I have to do?

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    Why would your chain break? You know there are actually two separte things here, the garage door proper, and the automatic opener. Very often, problems with the opener are the result of problems with the door itself. The opener can't work if the garage door is broken. Can you operate the door manually? It should open quite easily and smoothly with the opener disconnected. If you have to muscle or fight it, stop and call a garage door repairman, doors have a lot of stored energy and they can kill you. – Harper - Reinstate Monica Apr 30 '17 at 15:21
  • @Love see my answer - as this comment is too short. – Ken Apr 30 '17 at 20:11
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Inside the garage door opener there is going to be a gear reduction system to adapt the motor speed to the chain sprocket speed. This speed reduction system also increases the motor torque at the sprocket so that a relatively small motor can easily pull the chain and move the garage door up and down.

Trying to turn the sprocket directly can be difficult if the gearing system uses all normal straight gears. On the other hand if the gearing system uses a worm gear drive it would be next to impossible to directly turn the sprocket. This picture will explain a worm gear drive.

enter image description here

So you should not jump to conclusions at to if the opener is broken or not. It will require testing. Sometimes the best way to do that is to unhang the opener and chain bar and test the assembly on the garage floor.

  • Nice picture of the Worm Gear Drive that should help the OP. – Ken Apr 30 '17 at 20:13
  • @Ken - Yeah, That picture may actually be the gears from the inside of a Sears garage door opener!! – Michael Karas Apr 30 '17 at 21:13
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Which sprocket will not move by hand ? Are you referring to the sprocket on the opener ( which is where the motor is located) or are you referring to the one located at the end of the chain rail ? If the motor end - this is normal as there is most likely a worm gear arrangement - the teeth of the sprocket would be locked into the Worm Gear - it will not turn unless those teeth / worm gear is broken or the motor turns. The chain breaking means there was a lot of force on that chain - i.e the door was trying to open and it already was open. Or close and it was already closed. Disengage your Door from the chain via the PULL so the door moves freely. Close the door by hand manually - make sure it is disengaged from the Chain Rail via the pull rope. Run the Opener to Run the chain to the closed position - the chain trolley should go all the way to the closed position catch the pull trolley assembly and stop when the end stop switch is made.

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