A friend's garage door opener wasn't opening the door although the motor was running. I offered to try and repair it. After all, it already wasn't working; what was the harm?
After opening the cover the problem was obvious. The nylon gear which drove the shaft that drove the sprocket (which is a gear that is driven by or drives a chain), was split in half. I looked up the manufacturer on the 'Net, then the model number and ordered a replacement kit. I was so relieved that it came with a detailed instruction book!
Here's what I learned about this garage door opener:
The limit switches are very simple. As the drive shaft turns, it drives a screw upon which is mounted a moving contact. As the screw turns, it moves the contact to one end or the other. One screw on each side of the moving contact has the other end of the contact. As soon as the moving contact meets the end contacts -- the door stops. Simple!
There is a rate sensor that measures the rate at which the motor is spinning. This was also simple. A cap on the motor shaft had a series of four slots and openings, which passed through a sensor with a tiny emitter and sensor. If the door gets blocked, the motor can't turn. If the motor can't turn and the unit has not hit one of the limit switches, and the door is moving down, the controller reverses direction. If the door is moving up, it stops. Simple!
There isn't any other way to get a retaining pin out other than with a hammer. The sprocket drive shaft holds the drive gear in place with two retaining pins. These fit tightly into place. I had to drive the lower pin out in order to slide the new drive gear into place and the only way to do that was with a hammer. As soon as the pin was flush with the shaft, I used a punch to continue driving the pin through the shaft. After the new drive gear was in place, it was hammering once again to get the pin back into position.
Disassembly & Reassembly
Check the orientation of things as you disassemble them. I made two good choice and missed one opportunity. Before unplugging drive motor power connectors, I confirmed that the diagram in the instruction booklet was accurate and took a picture of the setup with my phone. When re-assembling the unit, I had all the documentation that I needed. However, I didn't notice the tabs in the top plate that holds the drive shaft in position and ended up re-installing that plate rotated 1/3 around. I didn't notice the mistake until nearly the very last step of re-installing the protective plastic cap over the sprocket and drive chain. It didn't make sense to disassemble the entire unit at that point, so we're living with my error. Had I taken a picture of that unit before disassembling it, I would have had the necessary reference.
Clean grease isn't so bad. Dirty grease is wicked icky, but the new clean grease was easy to put on and I didn't mind cleaning up afterwards.
End of the story: the garage door works perfectly now. And I had a lot of fun taking the durn thing apart and putting it together!