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Most watches are adjusted in positions. This means that the watch has been fine tuned to maintain a certain amount of accuracy when placed in various physical positions. Positional error is the timing error or difference in rate a watch displays between these different positions. Often times after a watch has been cleaned and oiled it will run well with good balance wheel motion in one position but when changed to another position the amplitude of the balance will drop considerably. Positional error cannot be dealt with until the amplitude of the balance is acceptable in all positions. There should always be a small drop in balance amplitude when a watch is moved from a horizontal position i.e. dial up or dial down, to a vertical position. Friction is the reason for this. In the horizontal position the balance pivot is exerting most of its pressure down onto one cap jewel. If the shape of the pivot is correct and the cap jewel is in good condition then the inertia of the balance will experience little resistance from friction. An analogy of this would be a spinning top on a smooth glass surface. When changed to a vertical position both balance pivots will experience greater friction due to an increase in surface contact along the sides of the pivot from the walls of the hole jewels. The amplitude of the balance can tell us a great deal about the condition of a watch. Poor balance motion in any position tells us there is a problem somewhere that must be corrected. A watch that exhibits good balance motion in one horizontal position, say dial up, but has poor or no motion when turned over to its other horizontal position, dial down, could suffer from one or more of several possible problems. Some of these problems could emanate from somewhere in the train or may be in the balance assembly itself. If the problem is not obvious then it will be helpful to isolate the balance from the driving portion of the watch. This is easily accomplished by removing the pallet from the movement. We can now impart a few puffs from a watch blower to the balance wheel to observe the balance motion while placing the movement in various positions. If the motion is now acceptable when changing between dial up and dial down then we have eliminated the balance assembly as the problem and the problem is somewhere in the drive train or pallet assembly.
If on the other hand the motion still changes excessively then the clearance between the balance wheel and center wheel should be checked in both positions. The balance or hairspring will sometimes hit the underside of the center wheel during part of its oscillation. This can be caused by a wobbly balance wheel that is loose or crooked on the balance staff, an out of flat hairspring (if the hairspring is hitting the center wheel) a wobbly center wheel that is out of flat, a cracked upper cap jewel or a broken upper balance staff pivot. If all is well then there is either something interfering with the balance assembly such as a screw protruding through the pillar plate or balance cock etc. or is in the balance assembly itself.
Check the balance staff pivots under high magnification to see if they are bent or scored. The pivots must have a perfect shape (see pic) with a well polished surface. Any imperfection on either pivot will most likely result in balance amplitude inconsistencies between positions. If they are scored then you should either polish or replace the pivots.
The balance hole and cap jewels must also be in perfect condition. Most modern watches use synthetic sapphire jewels providing an extremely hard, highly polished bearing for the balance staff pivots to run in. A crack or chip in a hole jewel may produce a sharp cutting edge which can easily cut grooves into a pivot or worse eventually reduce the diameter of a pivot much like a cutting tool would do in a lathe. A damaged hole or cap jewel will exert a different amount of friction on a pivot in different positions resulting in amplitude variations.
Next check to see that the hairspring is true and flat. If it is out of flat (pic) it could be hitting the balance arm or the overcoil may be contacting the underside of the balance cock in one or more positions. It is sometimes difficult to see contact being made as it may occur for only an instant during each oscillation of the balance. This too will result in amplitude variations.
The hairspring must be in excellent condition. It must be true (completely concentric) with no kinks. Make certain the spring is between the regulator pins. If it has jumped out from between the pins the spring may be making contact with the outside of one of the pins when placed in one position and then move away from the pin when the position is changed. This will result in a rate increase during pin contact and a rate decrease when no contact is made. Also check the space between the regulator pins. If they are too far apart the hairspring may only hit one of the pins in one position and may make no contact with either pin when the position is changed. The spring should move back and forth between the slot making contact with both pins during a balance oscillation. If the spacing looks good but contact is made with one pin only then the spring may need a small adjustment. This can usually be done with a fine point tweezer by bending the spring in or out slightly just in front of the hairspring stud. The regulator pins must also be straight up and down. Pins that are not parallel with a hairspring my cause the hairspring to tilt up and down (wobble) during the oscillation. Again this may result in spring contact with the balance arm or underside of the balance cock.
A dirty, rusty or magnetized hairspring can also cause positional error. While the balance is oscillating the spring may stick together in one or more positions while breathing normally in others. Sticking hairsprings are very common so always check them under high magnification to determine their condition.
Another reason for periodic amplitude fluctuations could be a distorted or tilted mainspring barrel cap.
E-Mail: Lance Young
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