Before we proceed to take up the details of constructing a chronometer escapement we had better master the names for the several parts. We show at Fig. 136 a complete plan of a chronometer escapement as if seen from the back, which is in reality the front or dial side of the "top plate." The chronometer escapement consists of four chief or principal parts, viz.: The escape wheel, a portion of which is shown at A; the impulse roller B; unlocking or discharging roller C, and the detent D. These principal parts are made up of sub-parts: thus, the escape wheel is composed of arms, teeth, recess and collet, the recess being the portion of the escape wheel sunk, to enable us to get wide teeth actions on the impulse pallet. The collet is a brass bush on which the wheel is set to afford better support to the escape wheel than could be obtained by the thinned wheel if driven directly on the pinion arbor. The impulse roller is composed of a cylindrical steel collet B, the impulse pallet d (some call it the impulse stone), the safety recess b b. The diameter of the impulse collet is usually one-half that of the escape wheel. This impulse roller is staked directly on the balance staff, and its perfection of position assured by resting against the foot of the shoulder to which the balance is secured. This will be understood by inspecting Fig. 137, which is a vertical longitudinal section of a chronometer balance staff, the lower side of the impulse roller being cupped out at c with a ball grinder and finished a ball polish.
It will be seen the impulse roller is staked flat against the hub E of the balance staff. The unlocking roller, or, as it is also called, the discharging roller, C, is usually thinner than the impulse roller and has a jewel similar to the impulse jewel a shown at f. This roller is fitted by friction to the lower part of the balance staff and for additional security has a pipe or short socket e which embraces the balance staff at g. The pipe e is usually flattened on opposite sides to admit of employing a special wrench for turning the discharging roller in adjusting the jewel for opening the escapement at the proper instant to permit the escape wheel to act on the impulse jewel a. The parts which go to make up the detent D consist of the "detent foot" F, the detent spring h, the detent blade i, the jewel pipe j, the locking jewel (or stone) s, the "horn" of the detent k, the "gold spring" (also called the auxiliary and lifting spring) m. This lifting or gold spring m should be made as light and thin as possible and stand careful handling.
We cannot impress on our readers too much the importance of making a chronometer detent light. Very few detents, even from the hands of our best makers, are as light as they might be. We should in such construction have very little care for clumsy workmen who may have to repair such mechanism. This feature should not enter into consideration.
We should only be influenced by the feeling that we are working for best results, and it is acting under this influence that we devote so much time to establishing a correct idea of the underlying principles involved in a marine chronometer, instead of proceeding directly to the drawing of such an escapement and give empirical rules for the length of this or the diameter of that. As, for instance, in finishing the detent spring h, suppose we read in text books the spring should be reduced in thickness, so that a weight of one pennyweight suspended from the pipe j will deflect the detent ¼". This is a rule well enough for people employed in a chronometer factory, but for the horological student such fixed rules (even if remembered) would be of small use. What the student requires is sound knowledge of the "whys," in order that he may be able to thoroughly master this escapement.
FUNCTIONS OF THE DETENT.
We can see, after a brief analysis of the principles involved, that the functions required of the detent D are to lock the escape wheel A and hold it while the balance performs its excursion, and that the detent or recovering spring h must have sufficient strength and power to perform two functions: (1) Return the locking stone s back to the proper position to arrest and hold the escape wheel; (2) the spring h must also be able to resist, without buckling or cockling, the thrust of the escape wheel, represented by the arrows p o. Now we can readily understand that the lighter we make the parts i j k m, the weaker the spring h can be. You say, perhaps, if we make it too weak it will be liable to buckle under the pressure of the escape wheel; this, in turn, will depend in a great measure on the condition of the spring h. Suppose we have it straight when we put it in position, it will then have no stress to keep it pressed to the holding, stop or banking screw, which regulates the lock of the tooth. To obtain this stress we set the foot F of the detent around to the position indicated by the dotted lines r and n, and we get the proper tension on the detent spring to effect the lock, or rather of the detent in time to lock the escape wheel; but the spring h, instead of being perfectly straight, is bent and consequently not in a condition to stand the thrust of the escape wheel, indicated by the arrows o p.
OBTAINING THE BEST CONDITIONS.
Now the true way to obtain the best conditions is to give the spring h a set curvature before we put it in place, and then when the detent is in the proper position the spring h will have tension enough on it to bring the jewel s against the stop screw, which regulates the lock, and still be perfectly straight. This matter is of so much importance that we will give further explanation. Suppose we bend the detent spring h so it is curved to the dotted line t, Fig. 136, and then the foot F would assume the position indicated at the dotted line r. We next imagine the foot F to be put in the position shown by the full lines, the spring h will become straight again and in perfect shape to resist the thrust of the escape wheel.