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

While this statement is a drastic oversimplification of the actual watch requirements, it does accurately describe a watch in its elemental state. The gear train simply affords a means for the mainspring to transmit its driving torque to the hands, and in order to maintain a constant rate of rotation, the driving force of the mainspring is released or “unlocked” at a regular recurring interval, and the angle of rotation which is allowed for each release period is constant. This is accomplished by several components which work together as a unit, called the escapement.
    The parts involved with maintaining the accurate speed of a watch are the escape wheel, the lever or pallet, the balance wheel, the hairspring, and the regulator or index. The balance wheel and the hairspring can be considered as a mechanical resonance, that is, the balance wheel is free to rotate back and forth and it, in conjunction with the attached hairspring, has a certain frequency or resonance at which it oscillates freely. Although it can be somewhat slightly “forced” off its natural period, it resists this quite strongly.
    The escape wheel is a specially designed wheel having hooked teeth which engage with pins on an anchor-shaped lever. The other end of the lever looks somewhat like a two-pronged fork, and the fork acts upon a pin which is attached to a steel disc called a roller on the shaft of the oscillating balance wheel.
    The requirements for railroad duty specify that the pins on the lever, and also the pin on the roller must be jewels, since these are parts which are both sensitive to friction and subject to mechanical wear. Jewels are harder and smoother than steel, although steel pins can be found in many clocks and in cheaper or lower grade watches. Railroad requirements also specify that the escape wheel and the lever must be made from high-quality steel.
    A railroad watch ticks 300 times per minute. A complete to-and-fro oscillation of the balance wheel produces two ticks. This means that for every hour that the watch runs, there are 18,000 ticks. During each balance wheel swing, the jeweled pins are impacted by or moving against hardened steel parts.
The forces that exist in a watch escapement are very minute, however if a watch runs continuously for one year, it will have produced 157,680,000 ticks, each tick being produced by the parts of the escapement, and each representing the impact or sliding of a hardened steel part against a jewel.
    The natural resonant frequency of the balance wheel and hairspring can be influenced by many diverse factors among which are; the mass and radius of gyration of the balance wheel, the modulus of elasticity of the hairspring, the effective free length of the hairspring, the viscosity of the lubricating oil, mechanical friction, and by the strength of the impulses which are given to the balance wheel by the lever, which in turn initially receives its driving force through the watch gear train from the mainspring. In considering this last sentence, we can see why operating on the correct part of the mainspring characteristic curve is necessary.

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    Another very important criterion for the railroad watch is the property of isochronism. This simply means that as the mainspring that powers the watch runs down, the speed of the watch should remain constant. If a clock or watch experiences a significant change in its driving force, it is possible for an unacceptable speed change to occur.
    Springs have a definite characteristic per-formance curve. The curve can be plotted on a graph that generally shows developed force versus deformation. As we look at the graph below, we can see that there is a portion of the curve within which the developed force is fairly constant for a considerable change or deformation of the spring.

    There are two major ways to insure isochronism in a watch. The first is to make the spring from an alloy that has reasonably constant force over a wide range of deformation, and the second and more important way is to insure operation on that part of the spring characteristic curve within which force changes very little with a change in deformation. Thus, the watch has a useful range of hours over which it may be expected to run very accurately. If not wound again, the watch will run for perhaps an additional twelve or more hours, but it will then enter a different part of the spring performance curve and isochronism can not be guaranteed. This of course necessitates that the watch has to be wound fully and preferably at the same time each day.
  Because the railroad environment could encompass wide temperature variations, all watches designated as railroad watches had to have  temperature  compensation. The  speed  of  a watch is controlled by the balance wheel. A watch is really nothing more than a spring-driven gear train, arranged so that each successive wheel in the train rotates faster than the preceding gear.