Broken garage door opener

The other morning when my wife went to open the garage door, it started to work, then the chain popped off the gear (which is on a spindle that sticks up out of the unit).
I managed to work the chain back on and the same thing happened again.
Then I noticed the tensioning bolts on the chain, so figured the chain had loosened just enough so that it was slipping off when the opener ran.
I tightened it up so that there was no slack in the chain, and figured it would work like a charm. Instead, it raised the door a few inches, like before, and then the spindle holding the gear that turned the chain just sheared right off.
The torsion springs are fine, and you can open and close the garage door by hand with no problem - so it's not like the opener should have been straining.
I'm not sure what to do now. It was cold and not too well lit in the garage, and I had my 2 yr old loose in the house, so I didn't want to spend too much time investigating, but I did not immediately see how it would be easy to open the unit and replace this spindle/gear (if in fact it can be replaced).
The other thing is it may have been something else (other than slack in the chain) that was causing the problem in the first place - and tightening the chain just transferred the effect to the spindle causing it to shear. If the problem still exists, and I just replace the spindle/gear, I could easily snap it off again.
I don't know how old the opener is. We bought the place 5 yrs ago. I did have to replace the torshion springs (one of which snapped) about 3 yrs ago.
Before I snapped the spindle off, when the chain was off the gear, it seemed to spin around just fine.
Is replacing the spindle/gear an easy (possible) job? Should I be thingking about replacing the whole unit?
Thanks.
-Jonathan
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Easy fix: http://www.garagedoorsupply.com/gear-assembly.html
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Jonathan Joseph wrote:

The part you keep calling a "gear" is properly known as a sprocket. It may have become worn so much that the chain couldn't really stay on it, especially if no one lubricated the chain once in a while.
Comparing the now snapped off sprocket with the new sprocket on the assembly you'll be buying if you decide to DIY this job will reveal if it was badly worn.
If you fix it yourself, loosen up on the chain tensioner, and with the door relased from the traveler, tension the chain just enough to keep it from sagging and touching the rail.
You could be correct in urinalysis that overzealous tightening of the chain is what broke the camel's back and snapped the shaft.
HTH,
Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat \'57 EE)
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Newer doors are MUCh safer, espically with a 2 year old around.
If your current door doesnt have a optical sensor about 6 inches off the floor across the bottom I would replace the opener.
futhermore they are a pain to install but many stores sell them installed for like 50 bucks. well worth the bucks if you ask me, espically in the cold winter
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