Removing a clock mainspring barrel
Letting down the mainspring power
The mainspring let down tool allows the release of the mainspring with the fist rather than the fingers. Put the mainspring let down tool over the square post the key goes onto. With a flat screwdriver, move the click out of the way. The click has a tension spring that needs to be held back. This is what holds the mainspring so that it winds in only one direction. Slowly allow the mainspring to unwind in the fist. This method lets you control the speed of the unwind with your whole fist rather than the fingers. Unquestionably, doing this with a key will result in a painful experience on the fingers. Of course, if the mainspring is broken, there usually is no worry since there isn't much tension to begin with.
After the barrel is out
Once the power has been let down, the next step is to yank out the winding arbor shaft. The clock mainspring barrel should then fall out the side of the movement. After the barrel is removed, it can be seen the number that is on the barrel end cap. This will tell what hole end mainspring to order. You can locate the mainspring by using the hole end mainspring chart. Be sure to check the movement for any damage as a result of the mainspring breaking. In particular, inspect the teeth of the gears, as well as the arbor on the next wheel up. This could have consequently been bent.
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