The point on the arc n where it intersects with the line b' is termed the point b. At the intersection of the radial line a d' is established the point d. We take ten and one-half degrees in the dividers, and from the point b establish the point c, which embraces the arc of the escape wheel which is utilized by the pallet action. Through the point b the line h' h is drawn at right angles to the line a b'. The line j j' is also drawn at right angles to the line a d' through the point d. We now have an intersection of the lines just drawn in common with the line a a' at the point g, said point indicating the center of the pallet action.
The dividers are now set to embrace the space between the points b and g on the line h' h, and the arc f f is swept; which, in proof of the accuracy of the work, intersects the arc n at the point d. This arc coincides with the locking faces of both pallets. To lay out the entrance pallet, the dividers are set to five inches, and from g as a center the short arc o o is swept. On this arc one degree is laid off below the line h' h, and the line g i drawn. The space embraced between the lines h and i on the arc f represents the locking face of the entrance pallet, and the point formed at the intersection of the line g i with the arc f is called the point p. To give the proper lock to the face of the pallet, from the point p as a center is swept the short arc r r, and from its intersection with the line a b' twelve degrees are laid off and the line b s drawn, which defines the locking face of the entrance pallet. From g as a center is swept the arc c' c', intersecting the arc n n at c. On this arc (c) is located the inner angle of the entrance pallet. The dividers are set to embrace the space on the arc c' between the lines g h' and g k. With this space in the dividers one leg is set at the point c, measuring down on the arc c' and establishing the point t. The points p and t are then connected, and thus the impulse face of the entrance pallet B is defined. From the point t is drawn the line t t', parallel to the line b s, thus defining the inner face of the entrance pallet.
DELINEATING THE EXIT PALLET.
To delineate the exit pallet, sweep the short arc u u (from g as a center) with the dividers set at five inches, and from the intersection of this arc with the line g j' set off eight and one-half degrees and draw the line g l. At one degree below this line is drawn the line g m. The space on the arc f between these lines defines the locking face of the exit pallet. The point where the line g m intersects the arc f is named the point x. From the point x is erected the line x w, perpendicular to the line g m. From x as a center, and with the dividers set at five inches, the short arc y y is swept, and on this arc are laid off twelve degrees, and the line x z is drawn, which line defines the locking face of the exit pallet.
Next is taken ten and one-half degrees from the brass degree-scale, and from the point d on the arc n the space named is laid off, and thus is established the point v; and from g as a center is swept the arc v' v' through the point v. It will be evident on a little thought, that if the tooth A' impelled the exit pallet to the position shown, the outer angle of the pallet must extend down to the point v, on the arc v' v'; consequently, we define the impulse face of this pallet by drawing a line from point x to v. To define the outer face of the exit pallet, we draw the line v e parallel to the line x z.
There are no set rules for drawing the general form of the pallet arms, only to be governed by and conforming to about what we would deem appropriate, and to accord with a sense of proportion and mechanical elegance. Ratchet-tooth pallets are usually made in what is termed "close pallets"; that is, the pallet jewel is set in a slot sawed in the steel pallet arm, which is undoubtedly the strongest and most serviceable form of pallet made. We shall next consider the ratchet-tooth lever escapement with circular pallets and ten degrees of pallet action.
DELINEATING CIRCULAR PALLETS.
To delineate "circular pallets" for a ratchet-tooth lever escapement, we proceed very much as in the former drawing, by locating the point A, which represents the center of the escape wheel, at some convenient point, and with the dividers set at five inches, sweep the arc m, to represent the periphery of the escape wheel, and then draw the vertical line A B', Fig. 19. We (as before) lay off thirty degrees on the arc m each side of the intersection of said arc with the line A B', and thus establish on the arc m the points a b, and from A as a center draw through the points so established the radial lines A a' and A b'.
We erect from the point a a perpendicular to the line A a, and, as previously explained, establish the pallet center at B. Inasmuch as we are to employ circular pallets, we lay off to the left on the arc m, from the point a, five degrees, said five degrees being half of the angular motion of the escape wheel utilized in the present drawing, and thus establish the point c, and from A as a center draw through this point the radial line A c'. To the right of the point a we lay off five degrees and establish the point d. To illustrate the underlying principle of our circular pallets: with one leg of the dividers set at B we sweep through the points c a d the arcs c'' a'' d''.
From B as a center, we continue the line B a to f, and with the dividers set at five inches, sweep the short arc e e. From the intersection of this arc with the line B f we lay off one and a half degrees and draw the line B g, which establishes the extent of the lock on the entrance pallet. It will be noticed the linear extent of the locking face of the entrance pallet is greater than that of the exit, although both represent an angle of one and a half degrees. Really, in practice, this discrepancy is of little importance, as the same side-shake in banking would secure safety in either case.