Fig. 105. Rack, showing method of getting sizes of snail steps according
to distance from the rack center to the pin in the rack tail.
Division of the Hour Snail.—The length of the rack tail, from the center of the stud hole in the rack to the center of the pin, should be equal to the distance between the center of the stud hole and the center of the snail. The difference between the radius of the top and the radius of the bottom step of the snail may be obtained by getting the angular distance of twelve teeth of the rack from center to pin. See A B, C D, E F, [Fig. 105], which show the total distances for twelve steps of the snail for rack tails of different lengths. Divide the circumference of a piece of brass into twelve parts and draw radial lines as shown in [Fig. 106]. Each of these spaces is devoted to a step of the snail. Draw circles representing the top and bottom step. Divide the distance, A B or E F, [Fig. 105], between these two circles, into eleven equal parts, and at each division draw a circle which will represent a step of the snail. The rise from one step to another should be sloped as shown, so as to raise the pin in the rack arm if the striking train has been allowed to run down, and it should be resting on the snail when it is desired to turn the hands back. The rise from the bottom to the top step is bevelled off, so as to push the pin in the rack arm on one side, by springing the thin brass of the arm and allow it to ride over the snail if it is in the way when the clock is going. It should also be curved to avoid interference with the pin. Clockmakers making new snails when repairing generally mark off the snail on the clock itself after the rest of the striking work is in position. A steel pointer is fixed in the hole of the lower rack arm, and the star wheel jumped forward twelve teeth (one at a time) by means of the pin in the motion wheel. After each jump a line is marked on the blank snail with the pointer in the rack arm by moving the rack arm. These twelve lines correspond to the twelve radial lines in [Fig. 106]. The motion wheel is then turned sufficiently to carry the pin in it free of the star wheel and leave the star wheel and blank snail quite free on their stud. The rack hook is placed in the first tooth of the rack, and while the pointer in the rack arm is pressed on the blank snail, the latter is rotated a little, so that a curve is traced on it. The rack hook is then placed in the second, and afterwards in the succeeding teeth consecutively, and the operation repeated till the twelve curves are marked. There is one advantage in marking off the snail in this way. Should there be any inaccuracy in the division of the teeth of the rack, the steps of the snail are thus varied to suit it. This frequently occurs in old clocks which have had new racks filed up by hand by some watchmaker.
Reference to the drawing, [Fig. 105], will show that the rack is laid out as a segment of a wheel with teeth occupying two degrees each, with a few teeth added for safety. Fourteen to sixteen teeth are generally provided, for the following reasons: If the first tooth is used to strike the half hours, it may in time become worn so that it can no longer be stretched to its proper length. In such cases moving the pin two degrees nearer the rack teeth will allow us to use the teeth from the second to the thirteenth in striking twelve, which makes a cheap and easy repair, as compared to inserting a new tooth or making a new rack.
Weight driven snail clocks should have the weight cords of the striking side long enough so that the striking train will not run down before the time train, as in such a case the rack tail is pushed to one side by the progress of the snail (which is carried on the time train and is still running); then the rack will drop clear out of reach of the gathering pallet and when the striking train is wound that train will continue striking until it runs down, or the dial is removed and the rack replaced in mesh with the gathering pallet. This happens with short racks and with large, old-fashioned snails. By leaving a few more teeth in the rack the rack tail will strike the stud, or hour wheel sleeve, before the rack teeth get out of reach of the gathering pallet.
Many watchmakers put a stud or pin in the plate to stop the rack from falling beyond the twelfth step, to prevent troubles of this kind.
The rack tail is friction-tight on its arbor and should be adjusted so that the proper tooth shall come in mesh with the gathering pallet for each step of the snail, or irregular striking will result. Such a clock may strike one, two, three and four correctly and then strike six for five, or seven or nine for eight, or thirteen for twelve, or it may strike one or two hours wrong and the rest correctly. This is because the gathering pallet, F, [Fig. 104], does not carry the rack teeth safely past the edge of the rack hook, B, owing to the tail of the rack not being properly adjusted. The teeth should all be carried safely past the edge of the hook and then be dropped back a little as the hook engages; this is the more necessary to watch with hand-made racks and snails, or after putting in a new, and therefore larger, pin in the rack tail to replace one which is badly worn.
