[48.]Short Motion Sometimes Caused by Burr on Opposite Pivot.
There are occasionally instances where a poor motion on one pivot is caused by a slight burr on the opposite pivot. This is usually due to the fact that while the burred pivot is running on its own end stone, there is space enough between the end stone and jewel to give the burr clearance, but when the position of the watch is reversed, the balance end shake allowance causes the burr to rub on the top of jewel hole and prevents perfect freedom of motion when the good pivot is downward.
[49.]Examining the Hairspring.
The hairspring may be true and level but it should be carefully examined to see that there is no possibility of touching at any point. The observation should take place during the full arc of motion of the balance, for there are some instances in which no rubbing takes place until the motion accelerates. The watch should be held at different angles and the space between the balance arm and spring, and the stud and spring, closely scrutinized for possible contact. The space between the spring and over coil at the point where the over coil rises and curves over the spring should be at least equal to the width of the coils and care should be taken to see that the over coil just before the point of rising has the usual space between it and the next coil. Either position in which the hairspring may rub will have a shorter motion and a gain in time compared to the opposite position in which there is no interference.
[50.]Exceptions in Regard to Gaining Rate and Short Motion.
Invariably the arc of motion which is the shortest will gain time compared to the opposite position which has a longer motion. There are, however, some few instances in which there are exceptions to this rule, and knowledge of these exceptions is quite valuable in preventing confusion and doubtfulness in the certainty of making specific alterations. As an example in the horizontal positions; if both end stones are perfect and the freedom of one pivot in the jewel is correct while the opposite pivot has entirely too much freedom, the motion may be somewhat shorter with the proper fitting pivot downward while the rate may be slower compared to the opposite position. This is caused by the balance describing a larger circle when the large hole jewel is upward, as the pivot is allowed to travel a greater distance from the center of the hole as it wavers from side to side during the oscillations.
When the watch is reversed the weight of the balance prevents the pivot from wobbling in the large hole and eliminates the possibility of compensating for the larger circle described by the balance in the opposite position.
The same results are possible when the freedom of both pivots is correct and when one end stone is pitted, as the pit in the stone causes a short motion when downward and prevents the pivot from having any side play whatever, while the opposite pivot enjoys full play to whatever freedom there may be and through this causing a somewhat larger circle to be described by the balance and a slower rate in time.
It should be understood that this does not refer to instances where the end stone surface is merely slightly worn, but to pittings in which the surface of the stone has been actually pierced. In most instances of slight wear the motion will be shorter and the rate fast which conforms to the general rule covering rate and motion.