Horn & Whistle Magazine: Source for Horns, Whistles, Sirens, Collecting signal devices and Related Information

    The actual speed that a watch runs at is set by the regulator, a lever whose pointer moves over a graduated scale and which has an extension supporting two curb pins. The curb pins embrace the hairspring of the balance and establish its free length. Therefore, by rotating the regulator, we can change the effective length of the hairspring, which is correctly referred to as the balance spring. Shortening the effective length of the balance spring increases the mechanical resonant frequency of the balance wheel and makes the escapement run faster.
    Figure ten is a general drawing of a watch balance bridge with the speed regulator lever. In this illustration, the pointer of the lever moves over a graduated scale. In a good railroad watch, however, there is a positive means of both adjusting and holding the lever to the exact position, since even a slight movement of the regulator can make a noticeable change in the rate of the watch. For a watch to qualify as a railroad watch, it has to be accurate to within ±30 seconds per week, which is, when you consider that this applies to a mechanical timepiece designed for rough service, extremely accurate.

Figure 9, above. Detailed illustration showing how the forked end of the lever and the guard pin interact with the two rollers on the balance staff to impart motion to the balance wheel and to prevent the disengagement of the escapement. It is crucial to the functioning of the escapement that the roller jewel always enters the slot between the two diverging forked ends of the lever. If it were not for the guard pin and the safety roller, a heavy jolt to the watch could make the roller jewel hit on either right or left outside edges of the fork, preventing the watch from operating. The guard pin keeps the lever fork always in the correct relationship to the roller jewel as the balance wheel and, therefore, the two rollers rotate back and forth.

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Since sudden bumping or jarring when a watch is operating can result in a disengagement of the escapement, that is, for the roller jewel to hit on the outside edges of the forked portion of the lever, the railroad watch has a second or safety roller. The guard pin can only pass by the safety roller when it is in such a position that the small cut-out is in line with the guard pin. At any other position of the safety roller, the guard pin prevents the lever from pivoting.
    Keep in mind that the balance wheel can turn through 360 degrees during each forward and backward swing, whereas the motion of the lever is restricted to an angle of about 10 degrees. This results in an interval of perhaps a tenth of a second during each balance wheel swing during which a sudden bump to the watch could disengage the escapement. If that were to happen, the watch would immediately stop and require the attention of a watchmaker to correct. The guard pin and the safety roller prevent this and insure reliability.