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

by Ward L. Goodrich

Fig. 47.

Although the gravity escapement will not give a superior performance under the most favorable conditions for timekeeping, it is distinctly superior when these conditions are unfavorable and therefore fully merits its high place in the estimation of the horological fraternity. We have instanced its value in tower clock work; it has another advantage in running cheap and poorly made (home made) regulators with rough and poor trains; therefore, it is a favorite escapement with watchmakers who build their own regulators while they are still working at the bench, before entering into business for themselves. As the price of a first-class clock for this purpose is about $300 and the cheapest that is at all reliable is about $75, it will be seen that the temptation to build a clock is very strong and many of them are built annually.

Regulators with the gravity escapement are built by the Seth Thomas Clock Co., the Howard, and one or two others in this country, but they are furnished simply to supply the demand and sales are never pushed for the reasons given previously. Clocks with this escapement are quite common in England and many of them have found their way to America. It is one of the anomalies of trade that our clockmakers are supplying Europe with cheap clocks, while we are importing practically all the high priced clocks sold in the United States and among them are a few having the three-legged and four-legged gravity escapements, therefore the chances are that when a repairer finds such a clock it is likely to be either of English origin or homemade, unless it be a German regulator.

Figs. [47] and [48] show plans and side views of the three-legged escapement. [Fig. 48] also shows an enlarged view of the escape wheel, showing how the three-leaved pinion between the two escape wheels, is made where it is worked out of the solid. A, B and C and a, b and c show the escape wheel which is made up of two three-armed wheels, one on each side of a three-leaved pinion marked D¹ and D² in the enlarged view of [Fig. 48]. The pallets in this escapement consist of the two arms of metal suspended from points opposite the point of bending of the pendulum spring and the lifting planes are found on the ends of the center arms in these pallets, which press against the three leaves of the pinion, while the impulse pins e¹ and e², [Fig. 47] and [48] act directly upon the pendulum in place of the verge wire. The pallets act between the wheels in the same plane as each other. The lifting pins or pinion leaves act on the lifting planes after the line of centers when the long teeth or legs of the escape wheels have been released from the stops, F and G, Figs. [47] and [48], which are placed one on each side of the pallets and act alternately on the wheels. These pallets are pivoted one on each side of the bending point of the suspension spring. To lay out the escapement, draw a circle representing the escape wheel diameter, then draw the line of centers and set off on the diameter of the escape wheel from each side of the line of centers 60° of its circumference, thus marking the positions for the pallet stops 120° apart. Draw radii from the center of the escape wheel to these positions and draw tangents from the ends of these radii toward the center line. The point where these meet will be the bending point of the pendulum spring.

Fig. 48.

This is clearly shown at H, [Fig. 47]. The points of suspension for the pallets are planted on the line of these tangents and a little below the point H, where the tangents meet on the line of centers. This is done to avoid the mechanical difficulty of having the studs for the two pallets occupy the same place at the same time. The arms of the pallets below the stops may be of any length, but they are generally constructed of the same angle as the upper arms and will be all right if drawn parallel to these upper arms. They are in some instances continued further down, but this is largely a matter of taste and the lower portion of the escapement is generally drawn so as to be symmetrical.

The impulse of the pendulum is given by having pins projecting from the pallet arms and bearing upon the pendulum rod, which pins may be of brass, steel or ivory. In the heavier escapements they are made of ivory in order to avoid any chatter from contact with the pendulum rod of a heavy pendulum. These pallets should be as light as it is possible to make them without having them chatter under the impact of the escape wheel arms on the stops. They have only to counteract the force of the pendulum spring and the resistance of the air and for light pendulums this force is much less than is generally understood. Two ounces of impulse will maintain a 250-pound pendulum, but two pennyweights is more than sufficient for a fifty-pound pendulum. The reader can see that in the case of a pendulum weighing but eight to fourteen pounds, there will be a still greater proportionate drop, as the spring itself is thinner, the rod is thinner, the pendulum ball offers little resistance to the air and the consequence is that it is difficult to get the pallet arms light enough for an ordinary clock.

Watchmakers who make this escapement for themselves, to drive an eight to fourteen pound pendulum, generally make the escape wheel three inches diameter and make the escape wheel and pallet arms all from the steel obtained by buying an ordinary carpenter’s saw. The lifting planes should not be more than one-eighth its diameter from the center of the escape wheel, as where this is the case the circular motion of the center pins will be so great that the pallet in action will be thrown out too rapidly and will chatter when striking the pendulum rod. On the other hand it should not be less than one-twelfth of the diameter of the escape wheel, or the pendulum will not be given sufficiently free swing and the motion will be so slow that while such a clock will work under favorable conditions, jarring, shaking in wind storms, etc., will have a tendency to make the pendulum wabble and stop the clock. From what has been said above, it will also be seen that the necessity for slow motion of the pallet arms unfits this escapement for use with short pendulums.