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.
[Illustration: Fig. 17.]
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.
[Illustration: Fig. 18.]
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.
[Illustration: Fig. 19.]
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.
[Illustration: Fig. 20.]
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.