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Munger's Improved Chronometer Escapement

In this escapement, which is illustrated in Fig. 64, the detent and staff are in one piece, with a notch in the head to hold the locking jewel. The detent arm has a hole through it to receive the detent staff, fitting friction tight, so that the detent and arm can be set at the proper angle to each other, and then the staff driven down to the shoulder. These should not fit so tightly that they cannot be separated without danger of breaking, but so firmly as not to be moved out of proper position to each other by use or handling, etc. Figure 64 shows the escapement and impulse pallet of the usual construction. The diameter of the pallet should not be less than six-tenths (.6) the diameter of the escapement, or larger than two-thirds the size of the wheel. A a shows the line of centers of the escape wheel and balance; b b the circle of depthing of the escapement and detent from the balance holes; cc extends from the intersection of the periphery of the wheel with the line a a, and the point of the third tooth of the wheel across the circle b b. At the intersection of the lines b b and cc is the point of location of the detent pivots (a slight variation from this point is not important). The discharge, or unlocking pallet, shown by the dotted lines, is a light bar or arm with a notch at one end to hold its jewel, and is the same length or diameter as the impulse pallet (or nearly so). In planting the escapement, first mount the wheel on its pinion and top it just enough to have it round. Then use a temporary brass or steel disc, the size required for the impulse pallet, place it on the balance staff, and adjust the depth of it and the wheel in the depth tool, and mark the circle b b. Locate the escape wheel and detent pivots the distance apart indicated by the drawing, and pivot them in, as also the balance, in their proper positions. In the brass disc cut a notch large enough to allow the escape-wheel tooth to pass through as in escaping. As the detent jewel is too long to allow of the proper locking of the wheel, let a tooth of the wheel rest on the locking face of the detent (the notch in the disc allows this), and the distance of the entrance tooth from the disc readily shows about how much of the detent jewel has to be ground off before beginning to polish its locking face. Two or three trails will bring it so near that the final polishing will make the locking correct. All this really takes but a few minutes to do. To obtain the adjustment of the detent use a special tool. The spindle is hollow, and the front end reamed out to take in a small brass taper that also fits a chuck in your lathe-spindle; this small taper is drilled out, with as large a hole as convenient, from the back end to within about one-eighth of an inch of the front. It is then put in the lathe-spindle and the front end turned small enough to be out of the way of the lap, and carefully centered and drilled through into the large hole, and broached out just enough to hold the detent staff firmly by friction, while grinding and polishing the locking surface; and it is readily removed from the taper to try the detent for the correct locking.

In making this adjustment, the arbor carrying the polishing lap should be held firmly, (by means of a loose button on the front end of the spindle) against the adjusting screw r, and this screw turned a little at a time, as the length of the detent jewel is shortened, so that the locking face will be a true circle from the pivots; for if the locking corner is much rounded off, or beveled, the pressure of the wheel against it would push the detent out, and cause it to trip or unlock at the wrong time. If the circle is true from the pivots, no amount of pressure of the wheel against the detent can push it out or release it.

With the size of discharge pallet used, the unlocking action can begin, if desired, from 1/4 to 1/3 of the whole motion of the detent arm before the line of centers, and is adjusted by the eccentric banking screw d. The relative position or angle of the detent to the arm is then so adjusted as to give the right amount of locking to the escape wheel, (which can be very shallow). The gold spring is then shortened, so that it will be released from the discharge pallet when the escape wheel tooth and impulse jewel are about on the line of centers; if the gold spring is too short, the detent will return too soon, so that the inside face of the locking jewel will strike the point of the tooth of the escape wheel, and the wheel drag along the face of the jewel until the detent arm rests against the banking screw d. This must be avoided. The discharge pallet, of course, must be set so as to unlock at the right position of the impulse pallet, but less drop is required in this escapement than in the usual construction.

The spiral return spring on the detent staff, under the arm, should have tension enough to return the detent to its banking, when it is moved a trifle, with all the pressure on it from the train that the mainspring can give. Use five or six coils for the return spring, and put on the spring so that it open when the detent is moved to unlock, as this gives a trifle quicker action to start the detent on its return. The detent, all complete, is so very light that there is no recoil as it strikes the banking, and no jarring or outside motion will cause it to trip, as all others are liable to do. The extreme lightness of the detent with its locking so near the pivot, lessens the friction so greatly that the discharge pallet can be of the same diameter as the impulse pallet, and thus greatly lessen the angle or extent of motion required to start it going; and it also admits of very shallow locking, without the least danger of tripping, and requires much less strength of mainspring than any other detached escapement.