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

by Ward L. Goodrich

The cannon pinion drives the minute wheel, which, together with its pinion, revolves loosely on a stud in the clock plate, or on an arbor between the frames. The meshing of the minute wheel and cannon pinion should be as deep as is consistent with perfect freedom, as should also that of the hour wheel and minute pinion in order to prevent the hour hand from having too much shake, as the minute wheel and pinion are loose on the stud and the hour wheel is loose on the cannon, so that a shallow depthing here will give considerable back lash, which is especially noticeable when winding.

The hour wheel has a short pipe and runs loosely on the cannon pinion in ordinary clocks. In quarter-strike cuckoos a different train is employed and the wheels for the hands are both on a long stud in the plate and both have pipes; the minute wheel has 32 teeth and carries four pins on its under side to let off the quarters. The hour wheel has 64 teeth and works close to the minute wheel, its pipe surrounding the minute wheel pipe, and held in position by a screw and nut on the minute pipe. A wheel of 48 and a pinion of 8 teeth are mounted on the sprocket arbor with a center spring for a friction, the wheel of 48 meshing with the minute wheel of 32 and the 8-leaf pinion with the hour wheel of 64. It will be recollected that the sprocket wheel takes the place of the barrel in this clock and there is no center arbor as it is commonly understood. The sprocket arbor in this case turns once in an hour and a half, hence it requires 48 teeth to drive the minute wheel of 32 once in an hour, as it turns one-third of a revolution (or 16 teeth) every half hour. The sprocket arbor, turning once in an hour and a half, makes eight revolutions in twelve hours and its pinion of eight leaves working in the hour wheel of 64 teeth turns the hour hand once in twelve hours.

In ordinary rack and snail striking work the snail is generally mounted on the pipe of the hour wheel, so that it will always agree with the position of the hour hand and the striking will thus be in harmony with the position of the hands.

Striking Trains.--It is only natural, after finding certain fixed relations in the calculations of time trains and motion work, that we should look for a similar point in striking trains, well assured that we shall find it here also. It is evident that the clock must strike the sum of the numbers 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 78 blows of the hammer, in striking from noon to midnight; this will be repeated from midnight to noon, making 156 blows in 24 hours, and if it is a 30-hour clock, six hours more must be added; blows for these will be 21 more, making a total of 177 blows of the hammer for a 30-hour strike train. The hammer is raised by pins set in the edge of a wheel, called the pin wheel, and as one pin must pass the hammer tail for every blow, it is evident that the number of pins in this wheel will govern the number of revolutions it must make for 177 blows, so that here is the base or starting point in our striking train. If there are 13 pins in the pin wheel, it must revolve 13.5 times for 177 blows; if there are 8 pins, then the wheel must revolve 22.125 times in giving 177 blows; consequently the pinions and wheels back to the spring or barrel must be arranged to give the proper number of revolutions of the pin wheel with a reasonable number of turns of the spring or weight cord, and it is generally desirable to give the same, or nearly the same, number of turns to both time and striking barrels.

If it is an eight-day clock the calculation is a little different. There are 156 blows every 24 hours; then as the majority of “eight-day” clocks are really calculated to keep time for seven and a half days, although they will run eight, we have: 156 × 7.5 = 1,070 blows in 7.5 days. With 13 pins we have 1,070 ÷ 13 = 80 and ⁴⁄₁₃ths revolutions in the 7.5 days. If now we put an 8-leaf pinion on the pin wheel arbor and 84 teeth in the great wheel or barrel, we will get 10.5 turns of the pin wheel for every turn of the spring or barrel; consequently eight turns of the spring will be enough to run the clock for the required time, as such clocks are wound every seventh day.

Figuring forward from the pin wheel, we find that we shall have to lock our striking train after a stated number of blows of the hammer each hour; these periods increase by regular steps of one blow every hour, so that we must have our locking mechanism in position to act after the passage of each pin, whether it is then used or not; so the pinion that meshes with the pin wheel, and carries the locking plate or pin on its arbor must make one revolution every time it passes a pin. If this is a 6-leaf pinion, the pins on the pin wheel must therefore be 6 teeth apart; or an 8-leaf pinion must have the pins 8 teeth apart; and vice versa. For greater convenience in registering, the pins are set in a radial line with the spaces of the teeth in the pin wheel, as this allows us to measure from the center of the pinion leaf.

It will thus be seen that the calculation of an hour striking train is a simple matter; but if half hours are also to be struck from the train, it will change these calculations. For a 30-hour train 24 must be added to the 156 blows for 24 hours, 180 blows being required to strike hours and half hours for 24 hours. These blows may be provided for by more turns of the spring, or different numbers of the wheels and pinions, which would then also vary the spacing of the pins.

Half hours may also be struck directly from the center arbor, by putting an extra hammer tail on the hammer arbor, further back, where it will not interfere with the hammer tail for the pin wheel, and putting a cam on the center arbor to operate this second hammer tail. This simplifies the train, as it enables the use of a shorter spring or smaller wheels while providing a cheap and certain means of striking the half hours. Half-hour trains are frequently provided with a separate bell of different tone for the half hours, as with only one bell the clock strikes one blow at 12:30, 1 and 1:30, making the time a matter of doubt to one who listens without looking, as frequently happens in the night.