A Practical Course in Position Adjusting
By HAROLD C. KELLY
Member of the Technical Board, U.H.A. of A.
A watch may be mechanically perfect, that is, its construction from barrel to balance may be as exact as human skill knows how to make it; and yet, in spite of such perfection there will be a variation of from fifteen to thirty seconds in twenty-four hours between some two vertical positions due to the condition of the balance spring alone. In watches that are less perfect, the error is frequently as high as forty seconds or more.
Action of the Balance Spring and Its Poise Error-The oscillation of the inner portion of the spring corresponds very nearly to that of the collet, that is, when the collet travels three-fourths of a circle the first coil in the center travels nearly an equal distance. Thus it is evident that each of the several coils, as they tend to become more distant from the center, will travel a shorter path until the movement ceases altogether at the regulator pins. If the coils are marked in a straight line from collet to pins, we would readily observe the distance traveled by the several coils and the extent of their path under different arcs of motion.
It is impossible to poise a spiral spring. Therefore it is at once evident that it is the unpoised inner portion of the spring when acted upon by the pull of gravity that causes position error in the vertical positions. The effect is similar to that of a balance which is out of poise. Slightly varied effects under different arcs of motion result, however, due to the fact that the greater mass of the unpoised inner portion of the spring vibrate a shorter path than does the balance proper.
Experimental Demonstration-A demonstration entirely at the command of every watchmaker is to take several watches and run them, first with the figure one up and following with the figures two, three, four, etc.; continuing the experiment around the dial with all figures up, running the watches in each of the twelve positions for twenty-four hours and taking note of the rate in each position.
Table I shows the result in an experiment at stated above, using four popular makes of American watches. All watches were in excellent condition with the balances perfectly poised, fitted with Breguet balance springs and the grades ranged from fifteen to twenty-one jewels.
The arc of motion of the balance of all watches was about 540 degrees when fully wound and a little more than 450 after twenty-four hours of running.
An examination of the balance spring showed that the fastest rate always occurred at the time when the middle of the first half of the inner most coil happened to be up. This error, due to the oscillation of the center of gravity of the inner portion of the spring, is called the natural error and it is unavoidable, we can, however, make such alterations so as to limit the fullest manifestation of the error in three ways. They are as follows:
(1) Pinning the spring at the collet in such a position that the natural error would be the least detrimental to the uniform rate of the watch.
(2) Reducing the natural error by the application of correct terminal curves both outside and inside.
(3 ) Neutralizing the effect of the natural error by counter-poising the balance.
These corrections will now be considered in the order stated above.
The Breguet Balance Spring on Position Error - It is natural that the reader would inquire as the effect of the Breguet spring with correct terminal as compared with the ordinary flat spring on position error. Experiments have demonstrated that the Breguet spring does reduce the variation in the vertical positions, but only to a small degree, proving that the position error is due primarily to the oscillation of the center of gravity of the inner portion of the spring.