The Modern Clock / A Study of Time Keeping Mechanism; Its Construction, Regulation and Repair

by Ward L. Goodrich

CHAPTER XVIII.
SNAIL STRIKING WORK, ENGLISH, FRENCH AND AMERICAN.

While the majority of snail striking movements made in America are on the French system, because they are cheaper when made in that way, still this system is so condensed and so difficult to illustrate, with all its mechanism packed in a small space between the plates, that the student will gain a much better idea of the rack and snail and its principles by first making a study of an English snail striking clock, which has the whole of the counting and releasing levers placed outside the front plate, where they can occupy all the room that may be necessary. The calculation and planting of the striking train do not differ from those using the count wheel, up to and including the single toothed pinion or gathering pallet. The stopping of the train after striking is different and the counting is divided, being dependent upon four pieces acting in conjunction in an hour strike of the simplest order, which number may run to a dozen in a repeating clock.

As the count wheel system had the defect of getting out of harmony with the hands when the latter are turned backward, so the snail system has its defects, which are the displacement of the rack and failure to stop the striking in some clocks if the striking train runs down before the time side and is then rewound, and a most puzzling inaccuracy of counting, resulting from slight wear and inaccuracy of adjustment. We mention these things here because they have an influence on the construction of the clock and an advance knowledge of them will serve to make clearer some of the statements which follow.

Hour and Half-Hour Snail Striking Work.—[Fig. 104] is a view of the front plate of an English fusee striking clock, on the rack principle. The going train occupies the right and center and the striking train the left hand. The position of the trains is indicated in dotted lines, the trains having barrels and fusees as shown by the squared arbors, all the dotted work being between the clock plates, and that in full lines being placed on the outside of the front plate, under the dial. The connection between the going train and the striking work is by means of the motion wheel on the center arbor, and connection is made between the striking train and the counting work by the gathering pallet, F, which is fixed to the arbor of the last wheel but one of the striking train, and also by the warning piece, which is shown in black on the boss of the lifting piece, A. This warning piece goes through a slotted hole in the plate, and during the interval between warning and striking stands in the path of a warning pin in the last wheel of the striking train. The motion wheel on the center arbor, turning once in an hour, gears with the minute wheel, E, which has an equal number of teeth. There are two pins opposite each other and equidistant from the center of the minute wheel, which in passing raise the lifting piece, A, every half hour. Except for a few minutes before the clock strikes, the striking train is kept from running by the tail of the gathering pallet; F, resting on a pin in the rack, C. Just before the hour, as the boss of the lifting piece, A, lifts the rack hook B, the rack C, impelled by a spring in its tail, falls back until the pin in the lower arm of the rack is stopped by the snail, D. This occurs before the lifting piece, A, is released by the pin in the minute wheel, E, and in this position the warning piece stops the train. Exactly at the hour the pin in the minute wheel, E, gets past the lifting piece, A, which then falls, and the train is free. For every blow struck by the hammer the gathering pallet, F, which is really a one-toothed pinion, gathers up one tooth of the rack, C, which is then held, tooth by tooth, by the point of the hook, B. After the pinion, F, has gathered up the last tooth, its tail is caught by the pin in the rack, which stops and locks the train, and the striking ceases.

The snail, O, is mounted on a twelve-toothed star wheel, placed on a stud in the plate, so that a pin in the motion wheel on the center arbor moves it one tooth for each revolution of the motion wheel, and it is then held in position by the click and spring as shown. The pin, in moving the star wheel, presses back the click, which not only keeps the star wheel steady, but also completes its forward motion after the pin has pushed the tooth past the projecting center of the click. The steps of the snail are arranged so that at one o’clock it permits only sufficient fall of the rack for one tooth to be gathered up, and at every succeeding hour gives the rack an additional motion equal to one extra tooth. It will be seen that where a star wheel is used a cord or wire attached to A and run outside the case, so that A may be lifted, will cause the clock to repeat the hour whenever desired.

The lower arm of the rack, C, and the lower arm of the lifting piece, A, are made of brass, and thin, so as to yield when the hands of the clock are turned back; the lower extremity of the lifting piece, A, is a little wider, and bent to a slight angle with the plane of the arm, so as not to butt as it comes into contact with the pin when this is being done. If the clock is not required to repeat, the snail may be placed upon the center arbor, instead of on a stud with a star wheel as shown, and this is generally done with the cheaper class of hour striking clocks; but the position of the snail is not then so definite, owing to the backlash of the motion wheels, so that it will not repeat correctly, as the pin of the rack may fall on a slope of the snail and, besides, a smaller snail must be used, unless it is brought out to clear the nose of the minute wheel cock, or bridge if one be used.

Fig. 104. Hour and half hour snail striking work with fusee train.

Half-Hour Striking.—The usual way of getting the clock to strike one at the half-hour, is by making the first tooth of the rack, C, lower than the rest, and placing the second pin in the minute wheel, E, a little nearer the center than the hour pin, so that the rack hook, B, is lifted free of the first tooth only at the half hour. But this adjustment is too delicate after some wear has occurred and the action is then liable to fail altogether or to strike the full hour, from the pin getting bent or from uneven wear of the parts. The arrangement shown in [Fig. 104] is generally used in English work, as it is much safer. One arm of a bell-crank lever rests on a cam fixed to the minute wheel, E. This arm is shaped so that just before the half-hour the other extremity of the bell-crank lever catches a pin placed in the rack, C, and permits it to release the train and fall the distance of but one tooth. This is the position shown in [Fig. 104]. After the half-hour has struck, the cam carries the hook free from the pin in C.