Experimental Mechanics / A Course of Lectures Delivered at the Royal College of Science for Ireland

by Robert S. Ball

687. We have examined the essential features of the going parts of the clock; to complete our sketch of this instrument we shall describe the beautiful mechanism by which the striking is arranged. The model which we represent in [Fig. 102] is, as usual, rather intended to illustrate the principles of the contrivance than to be an exact counter-part of the arrangement found in clocks. Some of the details are not reproduced in the model; but enough is shown to explain the principle, and to enable the model to work.

688. When the hour-hand reaches certain points on the dial, the striking is to commence; and a certain number of strokes must be delivered. The striking apparatus has both to initiate the striking and to control the number of strokes; the latter is by far the more difficult duty. Two contrivances are in common use; we shall describe that which is used in the best clocks.

689. An essential feature of the striking mechanism in the repeating clock is the snail, which is shown at b. This piece must revolve once in twelve hours, and is, therefore, attached to an axle which performs its revolution in exactly the same time as the hour-hand of the clock. In the model, the striking gear is shown detached from the going parts, but it is easy to imagine how the snail can receive this motion. The margin of the snail is marked with twelve steps, numbered from one to twelve. The portions of the margin between each pair of steps is a part of the circumference of a circle, of which the axis of the snail is the centre. The correct figuring of the snail is of the utmost importance to the correct performance of the clock. Above the snail is a portion of a toothed wheel, f, called the rack; this contains about fourteen or fifteen teeth. When this wheel is free, it falls down until a pin comes in contact with the snail at b.

Fig. 102.

690. The distance through which the rack falls depends upon the position of the snail; if the pin come in contact with the part marked i., as it does in the figure, the rack will descend but a small distance, while, if the pin fall on the part marked vii., the rack will have a longer fall: hence as the snail changes its position with the successive hours, so the distance through which the rack falls changes also. The snail is so contrived that at each hour the rack falls on a lower step than it does in the preceding hour; for example, during the hour of three o’clock, the rack would, if allowed to fall, always drop upon the part of the snail marked iii., but, when four o’clock has arrived, the rack would fall on the part marked iv.; it is to insure that this shall happen correctly that such attention must be paid to the form of the snail.

691. a is a small piece called the “gathering pallet”; it is so placed with reference to the rack that, at each revolution of a, the pallet raises the rack one tooth. Thus, after the rack has fallen, the gathering pallet gradually raises it.

692. On the same axle as the gathering pallet, and turning with it, is another piece c, the object of which is to arrest the motion when the rack has been raised, sufficiently. On the rack is a projecting pin; the piece c passes free of this pin until the rack has been lifted to its original height, when c is caught by the pin, and the mechanism is stopped. The magnitude of the teeth in the rack is so arranged with reference to the snail, that the number of lifts which the pallet must make in raising the rack is equal to the number marked upon the step of the snail upon which the rack had fallen; hence the snail has the effect of controlling the number of revolutions which the gathering pallet can make. The rack is retained by a detent f, after being raised each tooth.

693. The gathering pallet is turned by a small pinion of 27 teeth, and the pinion is worked by the wheel c, of 180 teeth. This wheel carries a barrel, to which a movement of rotation is given by a weight, the arrangement of which is evident: a second pinion of 27 teeth on the same axle with d is also turned by the large wheel c. Since these pinions are equal, they revolve with equal velocities. Over d the bell i is placed; its hammer e is so arranged that a pin attached to d strikes the bell once in every revolution of d. The action will now be easily understood. When the hour-hand reaches the hour, a simple arrangement raises the detent f; the rack then drops; the moment the rack drops, the gathering pallet commences to revolve and raises up the rack; as each tooth is raised a stroke is given to the bell, and thus the bell strikes until the piece c is brought to rest against the pin.

694. The object of the fan h is to control the rapidity of the motion: when its blades are placed more or less obliquely, the velocity is lessened or increased.