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

by Ward L. Goodrich

So, also, the details of construction are of little consequence, so long as the main points are well looked after—the perfect solidity of all parts, with very few of them, and the free movement of all working surfaces without play, so that the compensating action may be constantly maintained at all times. Where this is not the case the sticking, rattling, binding or cramping of certain parts will give different rates at different times under the same variations of temperature, according as the parts work smoothly and evenly or move only by jerks.

The necessary and useful parts of a pendulum are all that are really admissible in thoroughly good construction. Any and all pieces attached by way of ornament merely are apt to act to the prejudice of the necessary parts and should be avoided. In this chapter we shall give measurements and details of construction for a number of compensated pendulums of various kinds, as that will be the best means of arriving at a thorough understanding of the subject, even if the reader does not desire to construct such a pendulum for his own use.

Principles of Construction.—Compensation pendulums are constructed upon two distinct principles. First, those in which the bob is supported by the bottom, resting on the adjusting screw with its entire height free to expand upward as the rod expands downward from its fixed point of suspension. In this class of pendulums the error of the bob is used to counteract that of the rod and if the bob is made of sufficiently expansible metal it only remains to make the bob of sufficient height in proportion to its expansibility for one error to offset the other. In the second class the attempt is made to leave out of consideration any errors caused by expansion of the bob, by suspending it from the center, so that its expansion downward will exactly balance its expansion upward and hence they will balance each other and may be neglected. Having eliminated the bob from consideration by this means we must necessarily confine our attempt at compensation to the rod in the second method.

The wood rod and lead bob and the mercurial pendulums are examples of the first-class and the wood rod with brass sleeve having a nut at the bottom and reaching to the center of the iron bob and the common gridiron, or compound tubular rod, or compound bar of steel and brass, or steel and zinc, are examples of the second class.

Wood Rod and Zinc Bob.—We will suppose that we have one of the Swiss imitation gridiron pendulums which we want to discard, while retaining the case and movement. As these cases are wide and generally fitted with twelve-inch dials, we shall have about twenty inches inside our case and we may therefore use a large bob, lens-shaped, made of cast zinc, polished and lacquered to look like brass.

The bobs in such imitation gridiron pendulums are generally about thirteen inches in diameter and swing about five inches (two and a half inches each side). The pendulums are generally light, convex in front and flattened at the rear, and the entire pendulum measures about 56 inches from the point of suspension to the lower end of the adjusting screw. We will also suppose that we desire to change the appearance of the clock as little as possible, while improving its rate. This will mean that we desire to retain a lens-shaped bob of about the same size as the one we are going to remove.

We shall first need to know the total length of our pendulum, so that we can calculate the expansion of the rod. A seconds pendulum measures 39.2 inches from the point in the suspension spring at the lower edge of the chops to the center of oscillation. With a lens-shaped bob the center of gravity will be practically at the center of the bob, if we use a light wooden rod and a steel adjusting screw and brass nut, as these metal parts, although short, will be heavy enough to nearly balance the suspension spring and that portion of the rod which is above the center. We shall also gain a little in balance if we leave the steel screw long enough to act as an index over the degree plate, in the case, at the bottom of the pendulum, by stripping the thread and turning the end to a taper an inch or so in length.

We shall only be able to use one-half of the expansion upwards of our bob, because the centers of gravity and oscillation will be practically together at the center of the bob. We shall find the center of gravity easily by balancing the pendulum on a knife edge and thus we will be able to make an exceedingly close guess at the center of oscillation.

Now, looking over our data, we find that we have a suspension spring of steel, then some wood and steel again at the other end. We shall need about one inch of suspension spring. The spring will, of course, be longer than one inch, but we shall hold it in iron chops and the expansion of the chops will equal that of the spring between them, so that only the free part of the spring need be considered. Now from the adjusting screw, where it leaves the last pin through the wood, to the middle position of the rating nut will be about one inch, so we shall have two inches of steel to consider in our figures of expansion.

Now to get the length of the rod. We want to keep our bob about the size of the other, so we will try 14 inches diameter, as half of this is an even number and makes easy figuring in our trials. 39.2 inches, plus 7 (half the diameter of the bob) gives us 46.2 inches; now we have an inch of adjustment in our screw, so we can discard the .2; this leaves us 46 inches of wood and steel for which we must get the expansion.