Part 4 (2/2)

With reference to the escapement, good watchmakers often have different methods of examining the various points and of making corrections and it is not of so much importance as to just how correct conditions are obtained, as it is that they actually be obtained.

Whatever the method may be it is certain that each escape wheel tooth must have positive locking on each pallet stone and that there must be positive s.p.a.ce for drop between the back of each stone and the pointed end of each escape wheel tooth. There must also be sufficient draw when each tooth and stone are locked to hold the fork against the bankings.

When the lock, drop and draw are correct it is next necessary to see that the fork length and guard pin freedom are correct.

There is only one positive method of determining as to when the fork length is correct, and this is through closing the bankings to drop.

This can be done either before or after placing the balance in the watch and merely requires turning the banking screws so that the excentric pins will close in on the fork until the fork arrives at the pins, at the same instant that the tooth drops on the pallet stone.

This eliminates any slide of the stone on the tooth beyond the actual locking and in this condition it is required that the roller jewel pa.s.s through the fork slot and out of the fork horn entirely on both sides with perfect freedom.

Should it touch on both sides of the fork, then the fork is either too long or the roller jewel is too far forward, and if it touches on one side only it may require simply equalization of the freedom. The guard pin length also must be obtained with the bankings closed to drop and should be just free from the safety roller on both sides.

When the inspection proves that these conditions have been properly provided for, it is necessary to slightly open the bankings so that there will be just a trifle of slide of each stone, on each tooth, after the locking takes place.

Extremely wide side shakes of the escape, pallet or balance pivots will sometimes cause striking of the roller jewel when conditions are otherwise correct, and these side shakes should not be very much beyond the extreme limits mentioned in this number. The fact of this feature, however, should not be construed as a recommendation that these pivots be closely fitted, for reasonable freedom is to be desired because it is positively necessary.

CHAPTER VII

THEORY AND PRACTICE

26. _Theory of Frictional Errors and the Isochronal Hairspring._

Theory teaches us in brief, that the position adjustment is made necessary princ.i.p.ally because of frictional errors. It would therefore seem that if the watch was mechanically correct there would be little or no requirement for position alterations.

We are also advised that an isochronal hairspring is one which will cause the long and short arcs of the balance to be made in equal time and that to attain this, the center of gravity of the spring must coincide with the center of gravity of the balance and that a certain pinning point is necessary in producing this result.

Now if we have a watch of correct mechanical construction and fitted with an isochronal spring it would seem that a close rating timepiece would be a.s.sured.

27. _How Theory Works Out in Practice and What Isochronism Consists of._

Practical adjusting, however, proves that such is not the case, for even when the construction and alterations produce watches as nearly correct as scientific methods can determine, there is often considerable variation in the position rates. A twenty-four hour test in any position may prove that the long and short arcs are made in equal time showing the spring to be isochronous and yet the position variations have not been accounted for. In this connection experience proves that a spring showing a perfect isochronal rate may have its collet pinning point changed, in relation to the pinning point at the stud and that through such an alteration, a correction in positions can be obtained, without in the least disturbing the perfect isochronal rate.

This indicates that the separation of the two adjustments which is possible in theory, does not hold good in practice, because a spring showing a perfect isochronal rate has been altered for the purpose of counteracting some position error and thereby producing a practical center of gravity of the balance and spring combined, instead of separately.

This may be further explained as creating an error in a spring which is supposed to be theoretically isochronous, with the idea of making it act in opposition to the position error and the combination thus obtained produces practical isochronism as well as a corrected position rate.

It is not suggested that these relative pinning points be altered for the purpose of overcoming position variation such as may be caused by dirt and gummy oil, damaged pivots, or balances that are out of poise.

The watch should be in first-cla.s.s condition and have a good motion in every position and then the alterations may be safely undertaken in accordance with the principles.

Adjusted to isochronism indicates that the watch functions uniformly during the entire twenty-four hours running. It is immaterial as to whether the rate be perfect or whether it be a gain or a loss, so long as it is uniform.

The watch is not isochronous if there is both a gain and a loss in the rate, even though the time be perfect at the expiration of twenty-four hours.

Experiment will demonstrate that watches carefully adjusted to positions will also have a very close isochronal rate. These isochronal experiments can be made by timing watches for twenty-four hours in any one of the vertical positions and noting the variation in periods of from four to twelve hours and by comparing the variation in the first period, during which time the arc of motion is long, with the variation in the latter period when the mainspring power is weaker and the arc of motion is short.

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