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The Jewel Pin Action

From The American Horologist magazine, January 1939

The Jewel Pin Action

The fork and jewel pin action involves two distinct functions: the impulse and the unlocking. In order to illustrate and make this statement clear, we will consider the different parts of the escapement in a normal position, as shown in Fig. 1. The hairspring, controlling the balance, has brought the fork, by means of the jewel pin, to the normal position of rest.

This leaves the pallet in a position where the impulse face of an escape wheel tooth will engage the impulse face of one or the other of the pallet stones, in this instance the R stone. Assuming the parts to be in relation to each other, it is evident that when power is applied to the escape wheel, the escape wheel tooth, which is engaging the R stone, will cause the pallet to turn on its pivots, and this impulse is transmitted to the balance by the fork acting on the jewel pin. The impulse being completed, the escape tooth drops off from the R stone, and the second tooth forward comes to lock on the L stone, with the fork resting against the banking, as shown in Fig 2. The fork slot is now in such a position that the jewel pin may pass out perfectly free, and this condition is necessary because the impulse which was given to the balance imparted to that member a certain momentum, causing it to continue to turn in that direction until this momentum is overcome by the tension of the hairspring. During this part of the motion, which takes place after the impulse, the jewel pin leaves the fork entirely, but the instant that the momentum in the balance is overcome by the tension in the spring, the balance will start to turn in the opposite direction, the tendency of the spring being to bring the jewel pin to the center line. Before reaching this point, however, the jewel pin has to perform the very important function of unlocking. At the completion of the impulse we left the fork resting on the banking, with the fork slot in such position that the jewel pin passed out perfectly free, and, figuring on the assistance of the draft and the safety action, which will be explained later, we are justified in expecting that the jewel pin shall pass in to the fork slot perfectly free. The instant the jewel pin has entered the slot, and comes in contact with the fork, the work of unlocking begins. And here is to be noticed, that for every tick of the watch, the pallet and fork is started from the condition of rest, by a sudden blow of the jewel pin. And not only the pallet is started, but the whole train has to be started in the reverse direction, against the power of the mainspring, to unlock the escape wheel, in order to receive another impulse. The impulse on the L stone being completed, the pallet assumes the position shown on page 6. The jewel pin passing out on an excursion, the same as on the other side, returns to unlock, receives a new impulse, and so on, at the rate of 18,000 times per hour. In view of the above, it is evident that lightness, as far as it is consistent with strength and wearing quality, is an essential feature in the construction of the several parts. It was once considered necessary to attach a counterweight to the pallet, in order to get it in poise, but with the modern light construction of pallet and fork, it has been proved beyond a doubt that the ordinary form of counterpoise was worse than useless, inasmuch as it involved an added mass of metal whose inertia must be overcome at each vibration of the balance. 
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