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A Practical Course in Position Adjusting, Part 2

From The American Horologist magazine, October 1938

A Practical Course in Position Adjusting, Part 2

Member National Technical Board, U. H. A. of A.

THIS paper treats on position error as related solely to the balance spring and of the effect of gravity which is an ever present force acting upon the innermost coils. The nature of this action is such that a positive position is produced.

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.

In watch Number 1 the rate was fastest at the time when the figure eleven was up; in watch Number 2 the rate was fastest at the time when the figure three was up. The watches Number 3 and Number 4 had definite fast positions also, and in all watches the slow position was opposite or nearly opposite that of the fast position.

  1    0  +4  +1  +3
  2   -4  +5  +1   0
  3   -6  -8  -3  -2
  4   -8  +3  -3  -5
  5  -11   0  -7  -7
  6  -10   2  -5  -8 
  7   -5  -6  +1   0
  8   -1 -10  +2  +1 
  9   -2 -12  +2  +5 
  10   0  -3  +3  +5 
  11  +4   0  +5  +6 
  12   0  +3  +4  +8 

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 innermost 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 enOj' 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 enol' would be the least detl'imental to the uniform rate of the watch.
(2) Reducing the natural error by the application of conect 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 Proper Pinning at the Collet  - When fitting new balance springs to pocket watches, certain pinning points should be observed if the best position rates are to be expected. The proper pinning at the collet is shown in Figs. 3 and 4. The first half of the innermost coil tends upward as it leaves the collet in the direction of pendant up, producing a fast pendant up rate. It does not matter if the spring tends to the left as shown in Figure 3 or tends to the right as shown in Figure 4 for it can be readily seen that, in either case, the middle of the first half of the innermost coil stands in the direction of pendant up. When the spring is pinned as stated above, the pendant right and pendant left positions will have a slower but a nearly equal rate, that is, if the balance spring is properly centered and vibrates concentrically. The greater part of the natural error will show up only in the pendant down position and since a watch in practical usage is seldom if ever subjected to this position it follows naturally that the pendant down error is of little importance. 

Reducing the Natural Error - It was stated in the first part of the paper that finely con tructed watches would vary from fifteen to thirty seconds in twentyfour hours between some two vertical positions and watches that are less perfect, the error would be forty seconds or more. If the natural error is shown to be more than thirty seconds in twentyfour heiurs, the excessive variation is usually due to want of perfection of the inner terminal of the spring. A slight eccentric motion at the inner terminal will cause a greater variation than would be the case if the spring was perfectly true. Thus it is clear that balance springs should always be faultlessly trued at the collet and the reader should take particular note of the fact.

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.

Table II shows the results of an experiment using both the flat and Breguet springs. The watch selected for this example was an eighteen size, fifteen jewel grade fitted with a flat spring. The first column of the Table shows the rate with the flat spring and the second column shows the rate with the same spring after it was made over into a Breguet with correct terminal. The watch was running in each position for twenty-four hours.

             Flat Spring Sees.   Breguet Spring Sees.
  Pendant up        +7              +7
  Pendant right     +5              +3
  Pendant left      -6              -2
  Pendant down      -8             -10

Counter-poising the Balance - If the most perfect terminal curves do not produce the desired results, counter-poising may be tried. A general rule for the alteration is as follows: Reduce the weight on the lower side of the balance in the position that is slow. It is assumed that the balance has a good motion and that at no time does the arc of motion fall below 450 degrees during the twentyfour hours that the watch is under observation. This is important if success in counter-poising is to be expected. It should further be understood that any alteration of the poise of the balance should be practised only to a limited extent, otherwise a most unsatisfactory and erratic rate will result. Usually just a slight touch of the screw head file will reduce the natural error as much as five to ten seconds in twenty-four hours.

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