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

by Ward L. Goodrich

Fig. 27.

Another thing which will help us in understanding these drawings is that the effective length of a lever is its distance from the center to the working point, measured in a straight line. Thus in a pallet of a clock the distance of the pallets from the center of the pallet arbor is the effective length of that arm of the lever, no matter how it may be curved for ornament or for other reasons.

The lines and circles drawn to enable us to take the necessary measurements of angles and center distances are called “construction lines” and are generally dotted on the paper to enable us to distinguish them as lines for measurement only, while the lines which are intended to define the actual shapes of the pieces thus drawn are solid lines. By observing this distinction we are enabled to show the actual shapes of the objects and all their angular measurements clearly on the one drawing.

With these explanations the student should be able to read clearly and correctly the many drawings which follow, and we will now turn our attention to the escapements. In doing this we shall meet with a constant use of certain terms which have a peculiar and special meaning when applied to escapements.

The Lift is the amount of angular motion imparted to the verge or anchor by the teeth of the escape wheel pressing against the pallets and pushing first one and then the other out of the way, so that the escape wheel teeth may pass. According as the angular motion is more or less the “lift” is said to be greater or less; as this motion is circular, it must be expressed in degrees. The lifting planes are those surfaces which produce this motion; in clocks with pendulums the lifting planes are generally on the pallets, being those hard and smoothly polished surfaces over which the points of the escape wheel teeth slide in escaping. In lever escapements the lifting planes are frequently on the escape wheel, the pallets being merely round pins. Such an escape wheel is said to have club teeth, as distinguished from the pointed teeth used when the lifting planes are on the pallets. In the cylinder escapement the lifting planes are on the escape wheel; they are curved instead of being straight; and there is but one pallet, which is on the lip of the cylinder. In the forms of lever escapement used in watches and some clocks the lift is divided, part of the lifting planes being also on the pallets; in this case both sets of planes are shorter than if they were entirely on one or the other, but they must be long enough so that combined they will produce the requisite amount of angular motion of the pallets, so as to give the requisite impulse to the pendulum or balance.

The Drop is the amount of circular motion, measured in degrees, which the escape wheel has from the instant the tooth escapes from one pallet to that point at which it is stopped by the other pallet catching another tooth. During this period the train is running down without imparting any power to the pendulum or balance, hence the drop is entirely lost motion. We must have it, however, as it requires some time for the other pallet to move far enough within the pitch circle of the escape wheel to safely catch and stop the next tooth under all circumstances. It is the freedom and safety of the working plan of our escapement, but it is advisable to keep the drop as small as is possible with safe locking.

The Lock is also angular motion and is measured in degrees from the center of the pallet arbor. It is the distance which the pallet has moved inside of the pitch circle of the escape wheel before being struck by the escape wheel tooth. It is measured from the edge of the lifting plane to the point of the tooth where it rests on the locking face of the pallet. A safe lock is necessary in order to prevent the points of the escape wheel teeth butting against the lifting planes, stopping the clock and injuring the teeth. We want to point out that from the instant of escaping to the instant of locking we have the two parts of our escapement propelled by different and entirely separate forces and moving at different speeds. The pallets, after having given impulse to the pendulum, are controlled by the pendulum and moved by it; in the case of a heavy pendulum ball at the end of a 40-inch lever, this control is very steady, powerful and quite slow. The escape wheel, the lightest and fastest in the train, is driven by the weight or spring and moves independently of the pallets during the drop, so that safe locking is important. It should never be too deep, as it would increase the swing of the pendulum too much; this is especially true with short and light pendulums and strong mainsprings.

The Run.—After locking the pallet continues to move inward towards the escape wheel center as the pendulum continues its course, and the amount of this motion, measured in degrees from the center of the pallet arbor, is called the run.