Now the angular extent on the arc c between the lines A a and A h represents the radius of the circle defining the jewel pin. From the intersection of the line A a with the arc c as a center, and with the radius just named, we sweep the small circle D, Fig. 58, which represents our jewel pin; we afterward cut away two-fifths and draw the full line D, as shown. We show at Fig. 59 a portion of Fig. 58, enlarged four times, to show certain portions of our delineations more distinctly. If we give the subject a moment’s consideration we will see that the length of the prong E of the lever fork is limited to such a length as will allow the jewel pin D to pass it.
[Illustration: Fig. 58]
[Illustration: Fig. 59]
To delineate this length, from B as a center we sweep the short arc f so it passes through the outer angle n, Fig. 59, of the jewel pin. This arc, carried across the jewel pin D, limits the length of the opposite prong of the fork. The outer face of the prong of the fork can be drawn as a line tangent to a circle drawn from A as a center through the angle n of the jewel pin. Such a circle or arc is shown at o, Figs. 58 and 59. There has been a good deal said as to whether the outer edge of the prong of a fork should be straight or curved.
To the writer’s mind, a straight-faced prong, like from s to m, is what is required for a fork with a single roller, while a fork with a curved prong will be best adapted for a double roller. This subject will be taken up again when we consider double-roller action. The extent or length of the outer face of the prong is also an open subject, but as there is but one factor of the problem of lever escapement construction depending on it, when we name this and see this requirement satisfied we have made an end of this question. The function performed by the outer face of the prong of a fork is to prevent the engaged pallet from unlocking while the guard pin is opposite to the passing hollow.
The inner angle s of the horn of the fork must be so shaped and located that the jewel pin will just clear it as it passes out of the fork, or when it passes into the fork in the act of unlocking the escapement. In escapements with solid bankings a trifle is allowed, that is, the fork is made enough shorter than the absolute theoretical length to allow for safety in this respect.
We will now see how long a lever must be to perform its functions perfectly. Now let us determine at what point on the inner face of the prong E’ the jewel pin parts from the fork, or engages on its return. To do this we draw a line from the center r (Fig. 59) of the jewel pin, so as to meet the line e at right angles, and the point t so established on the line e is where contact will take place between the jewel pin and fork.