CHAPTER VI.

TORSION PENDULUMS FOR FOUR-HUNDRED DAY CLOCKS.

As this pendulum is only found in the 400-day, or annual wind, or anniversary clocks (they are known by all of these names), it is best to describe the pendulum and movement together, as its relations to the work to be done may be more easily perceived.

Rotating pendulums of this kind—that is, in which the bob rotates by the twisting of the suspension rod or spring—will not bear comparison with vibrating pendulums for accurate timekeeping. They are only used when a long period between windings is required. Small clocks to go for twelve months with one winding have the torsion pendulum ribbons of flat steel about six inches long, making 15 beats per minute. The time occupied in the beat of such a pendulum depends on the power of the suspending ribbon to resist twisting, and the weight and distance from the center of motion of the bob. In fact, the action of the bob and suspending ribbon is very analogous to that of a balance and balance spring.

In order to get good time from a clock of this character, it should be made with a dead-beat escapement. With such an escapement there is no motion of the escape wheel, after the tooth drops on the locking face of the pallet; the escape wheel is dead and does not move again until it starts to give the pallet impulse. This style of an escapement allows the pendulum as much freedom to vibrate as possible, as the fork in one form of this escapement may leave the pallet pin as soon as the latter strikes the guard pins, as in the ordinary lever escapement of a watch, and it will remain in that position until the return of the fork unlocks the escapement to receive another impulse. B, Fig. 17, represents the escape wheel; C, the pallet; E, pallet staff; D, the pallet pin rivetted on to the pallet staff E, which works in the slot or fork H; this fork is screwed fast to the spring. The spring G is made of a piece of flat steel wire and looks like a clock hairspring straightened out. G is fast to the collar I and rests on a seat screwed to the plate of the clock, as shown at P; the spring is also fastened to the pendulum ball O with screws; the ball makes about one and one-half revolutions each beat, which causes the spring to twist. It twists more at the point S than it does at L; as it twists at L it carries the fork with it, so that the latter vibrates from one side to the other, similar to a fork in a watch. This fork H carries the pin D, which is fast to the pallet staff E, far enough to allow the teeth to escape.

In the most common form of this escapement, see Fig. 18, the fork does not allow the pin D to leave the slot H, and the beat pins are absent, the pendulum not being as highly detached as in the form previously mentioned. In this case great care must be taken to have the edges of the slot, which slide on the pallet pin, smooth, parallel and properly beveled, so as not to bind on the pin. The pendulum ball makes from eight to sixteen vibrations a minute. Of course the number depends upon the train of the clock.

In suspending the pendulum it is necessary to verify the drop of the teeth of the escape wheel as follows: The pendulum is suspended and the locking position of the pallets marked, taking as a guiding point the long, regulating screw, which, fixed transversely in the support, serves for adjusting the small suspension block. An impulse of about a third of a turn is given to the pendulum while observing the escapement. If the oscillations of the pendulum, measured on the two sides, taking the locking point as the base, are symmetrical, the drop is also equal, and the rate of the clock regular and exact; but if the teeth of the escape wheel are unlocked sooner on one side than on the other, so that the pendulum in its swing passes beyond the symmetrical point on one of the pallets and does not reach it on the other, it is necessary to correct the unequal drop.

The suspension block B, Fig. 18, between the jaws of which the steel ribbon is pressed by two screws, has a lower cylindrical portion, which is fitted in a hole made in the seat, and is kept immovable by the screw A. If the vibration of the pendulum passes beyond the proper point on the left side, it is necessary to loosen A and turn the suspension block slightly to the right. If the deviation is produced in the opposite direction, it is necessary to turn it to the left. These corrections should be repeated until the drop of the escape wheel teeth on the pallets is exactly equal on the two sides. As the drop is often disturbed by the fact that the long thin steel ribbon has been twisted in cleaning, taking apart or handling by unskilled persons before coming to the watchmaker, it is desirable to test the escapement again, when the clock is put into position on the premises of the buyer.

The timing adjustment of the pendulum is effected with the aid of regulating weights, placed on the ball. By moving these away from the center by means of a right and left hand screw on the center of the disk (see Fig. 19), the centrifugal force is augmented, the oscillations of the pendulum slackened, and the clock goes slower. The contrary effect is produced if the weights are brought nearer the center. In one form of ball the shifting of the regulating weights is accomplished by a compensating spring of steel and brass like the rim of a watch balance, Fig. 20.

If necessary to replace the pendulum spring, the adjustment is commenced by shortening or lengthening the steel ribbon to a certain extent. For this purpose the end of the spring is allowed to project above the suspension block as a reserve until adjustment has been completed, when it may be cut off. If the space between the ball and the bottom of the case, or the bottom of the movement plates, does not allow of attaining this end, it is necessary to increase or decrease the weight of the disk, adding one or several plates of metal in a depression made in the under side of the ball, and removing the plates screwed to it, which are too light.

There are some peculiarities of the trains of these clocks. The cannon pinion is provided with a re-enforcing spring, serving as guide to the dial work, on which it exercises a sufficient pressure to assure precise working. The pressure of this spring is important, because if the dial work presses too hard on the pinion of the minute wheel, the latter engaging directly with the escape wheel, would transmit to the latter all the force employed in setting the hands. The teeth of the escape wheel would incur damage and the consequent irregularity or even stopping of the clock would naturally follow.

In order that it may run for so long a time, the motive force is transmitted through the train by the intervention of three supplementary wheels between the minute wheel and the barrel, in order to avoid the employment of too large a barrel; the third wheel is omitted; the motion work is geared immediately with the arbor of the escape wheel. It is evident that the system of the three intermediate wheels, of which we have spoken, requires for the motive force a barrel spring much stronger than that of ordinary clocks.

The points which we have noticed are of the most importance with reference to the repair and keeping in order of an annual clock. It very often happens that when the repairer does not understand these clocks, irregularities are sought for where they do not exist. The pivot holes are bushed and the depthings altered, when a more intelligent examination would show that the stopping, or the irregular rate of the clock, proceeds only from the condition of the escapement. Unless, however, they are perfectly adjusted, a variation of five minutes a week is a close rate for them, and most of those in use will vary still more.

Annual clocks are enjoying an increased favor with the public; their good qualities allow confidence, the rate being quite regular when in proper order. They are suitable for offices; their silent running recommends them for the sick chamber, and the subdued elegance of their decoration causes the best of them to be valued ornaments in the home.