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

by Ward L. Goodrich

Brocot’s Visible Escapement.—[Fig. 37] represents a system of making and jeweling pallets much used by the French in their small work, especially in visible escapements. The acting parts of the pallets are simply cylinders, generally of colored stones, usually garnets, one-half of each cylinder being cut away. These cylinders extend some distance from the front of the pallet frame, and work into the escape wheel the same as the pallets of a Graham escapement—the round parts of the pallets serving as impulse planes. The neck of the brass pallet frame is cut up in the center, and the width between the pallets is sometimes adjusted by a screw, sometimes by bending the arms.

Clock movements with this escapement, of a careful construction, will frequently come for repairs, accompanied by the complaint of constant stopping and that no attempt at closely regulating can succeed with them, although they appear to have no visible disturbing cause. In such cases the depthing of the escapement is generally wrong. With proper depthing the point of the escape wheel tooth should drop on the center or a little beyond the center of the pallet stone. If it is set in this way the clock will stop when wound, especially if it has a strong spring, as the light pendulum will not then have momentum enough to unlock it against the full power of the spring. If the pallets are set shallow, in order to avoid this difficulty, then, the pendulum will take too short a swing and thus the clock will have a gaining rate. Generally the pendulum ball cannot be made enough heavier to correct the defect.

Fig. 38. Brocot’s visible escapement escaping over
90° with a small lift on the escape wheel teeth.

In these movements, in which the length of the pendulum does not exceed 4 inches, the pallet fork embraces, generally about 120°, or the one-third part of the wheel; it will be seen that unless there are stop works on the barrel of the main spring no manner of regulating is possible with these conditions, in view of the considerable influence exercised by the mainspring through the train on the very light pendulum, and by replacing this unduly high anchor by a lower one, I have always been able to produce a very satisfactory rate with movements having pendulums of three and a half to four inches. [Fig. 38] shows a 90° escapement with a small lift on the escape wheel teeth.

In spite of its incontestable qualities, the visible escapement possesses one inherent fault. I refer to the formation of its pallets, the semi-circular shape of which renders unequal the action of the train in giving impulse to the pendulum exceeding 50 centimeters (20 inches), since to make it to describe arcs of from one to two degrees only, with pendulums of from 60 centimeters to one meter in length, it became necessary to make the anchor arms extremely long, which considerably impeded the freedom of action, especially when the oil became thick, and this disposition would, therefore, stand in direct contradiction with the principles of modern horology. Both stopping and the irregularity of rate can be obviated by changing the semi-circular form of the pallets for one of an inclined plane, either by grinding a new plane or turning the stones in such manner as to offer an inclined plane to the action of the wheel, analogous to that of the Graham escapement. [See Fig. 37], the dotted lines on the pallets showing the portion to be ground away.

The importance of this transformation will readily be understood; it suffices to give to these planes a more or less large inclination in order to obtain a greater regularity of lifting, and, at desire, a lifting arc more or less considerable without being compelled to modify the proportions of the fork or to exaggerate the center distance of wheel and pallet arbor.

In adjusting an escapement, perhaps it may be advisable to mention that moving the pallets closer together, or opening them wider, will only adjust the drop on one side, while the other drop can only be affected by altering the distance between the centers of the pallets and scape wheel. This is accomplished in various ways. The French method consists of an eccentric bush, riveted in the frame just tight enough to be turned by a screwdriver. Another plan, common in America, is simply pieces of brass (cocks) fastened on the sides of the frames. The pivots of the pallet axis are hung in holes in these cocks, and an adjustment of great accuracy may be quickly obtained by loosening the clamping screws. Lock, drop and run should be of the same amount on each pallet. However, we do not approve of adjustments of any kind, except in the very highest class of clocks, where they are always likely to be under the care of skillful people, who understand how to use the adjustments to obtain nicety of action in the various parts.

In making escapements, lightness of all the parts ought to be an object always in view in the mind of the workman, and such materials should be used as will best serve that purpose. The scape wheel, and the pallets and fork, should have no more metal in them than is necessary for stiffness. The pallet arbor, and also the escape wheel arbor, should be left pretty thick when the wheel and pallets are placed in the center between the plates, to prevent their springing when giving impulse to the pendulum. We have often been puzzled to find out the necessity or the utility of placing them in the center between the plates, as they are so generally done in English clockwork. The escapement acts much more firmly when it is placed near one of the plates, and it is just as easy to make it in this way as in the other.