In case that the rust has reached a stage far enough advanced to seriously pit the metal, good results cannot be expected from the spring even though further rusting may be prevented.
84. _Stopping by Escapement Locking When Hands are Set Backward, or When Watch Receives a Jar._
This is sometimes a very annoying trouble and while it should not occur on high grade watches at all, it does show up just often enough to cause a certain degree of unpleasantness for the owner of the watch as well as for the watchmaker.
There are two principal causes for the difficulty. One is due to the back of discharging pallet stone having a very sharp corner combined with a slightly rough edge on the back of the escape wheel teeth and when the two factors meet with some slight force, such as is caused by reversal of the train wheels the sharp corner of the stone wedges itself into the rough surface of the tooth and holds until pulled away by some small instrument. This can be remedied by removing the sharp edge of the stone on a diamond charged polishing lap and a very slight correction is sufficient.
The second principal cause is due to sharp edges on the roller jewel. First quality roller jewels always have these edges rounded, as otherwise they may wedge into the horn of the fork and often will not release through ordinary shaking of the watch.
A short guard pin can also cause the trouble by allowing the roller jewel to catch on the end of the fork horn before it enters, or the guard pin may catch on the edge of the crescent on the safety roller, but the two causes mentioned above will allow "hanging up" even when the guard pin, roller jewel and all other shakes are correct.
When the above conditions are correct and all setting connections are properly fitted, the hands may be set either forward or backward without in any way disturbing the time. There are instances, however, where the watch will stop when the hands are reversed and at times the second hand will actually turn backward although the watch will immediately begin to run as soon as the backward pressure on the hands is discontinued.
This is caused by the cannon pinion being so tightly fitted that turning it backward will require more force than that which is supplied by the mainspring. A condition of this description is more pronounced when the mainspring is nearly run down and sometimes it will happen at such times and will not occur when the spring is fully wound.