There is a class of clocks which have the count wheel tight on the arbor, outside the clock plate. Many of them are on much tighter than they should be. In such a case take an alcohol lamp and heat the wheel evenly, especially around the hub; the brass will expand twice as much as the steel and the wheel may then be driven off without injury.
[Fig. 96] shows another typical American eight-day train, made by the Gilbert Clock Company, and striking the half hours from the train. Here we notice, on comparing with [Fig. 92], that there are many points of difference. First the notches on the count wheel are twice as wide as they are in [Fig. 92]. This means that half hours are struck on the train; this will be explained later. Next there are two complete sets of notches on the wheel, which shows that the wheel turns only once in twenty-four hours, whereas the other makes two revolutions in that time. There are no teeth on the count wheel, so that it must be fast to its arbor, which is that of the great wheel and spring, while [Fig. 92] has a separate stud and it is loose. The wheel being on the spring arbor and going once in 24 hours, there must be one turn of spring for each 24 hours which the train runs. There is no pin wheel in [Fig. 96], but instead of this two pins are cut out of the locking cam to raise the hammer tail as they pass. There are also two locking notches in the locking cam. The cams on the center arbor are stamped out of brass sheet, while those of [Fig. 92] were of wire.
Fig. 96. Half hours struck on the train.
Turning to the enlarged view in [Fig. 97] and comparing it with [Fig. 93], we find further differences. The levers K and J are here made of one piece of brass, while the others were separate and of wire. The lifting lever, H, is flattened at its outer end in [Fig. 93], while in [Fig. 97] it is bent at right angles and passed under the count hook, A. The hook, C, [Fig. 97], is added to the arbor, B, as a safety device, in case the locking hook should fail to enter its slot in the cam, D. It is shown as having just stopped the warning pin in [Fig. 96]. There is but one hammer tail, G, and the hammer stop acts against the stud for the hammer-spring, instead of against the bottom of the front plate, as in [Fig. 92].
The first important difference here is in the position of the count hook, A. In Figs. [92] and [93] the hook must be exactly in the middle of the slot, or there will be trouble. In trains striking half hours from the train, we must never allow the hook to occupy the middle of the slot, or we will have more trouble than we ever dreamed of. In this instance the count hook must enter the slot close to (but not touching) the side of the slot when the clock stops striking; then when the half hour is struck the count wheel will move a little and the hook must drop back into the same slot without touching; this brings it close to the opposite side of the same slot and the next movement will land the hook safely on top of the wheel for the strokes of the hour. [Fig. 96] shows its position after striking the half hour and ready to strike the hour of two. [Fig. 97] shows it dropping back after striking two.
Fig. 97. Half hour strike on the count wheel.