Fig. 30.
We now take 90° and, dividing it, set off 45° each side of our center line and draw radii, R, from the center to the circumference of our circle; this marks the beginnings of our pallets. Now to find our pallet center distance we draw tangents, T (at right angles), from the ends of these radii toward the line of centers. The point where they intersect on the line of centers is the pallet center.
Now we must determine how much motion we are going to give our pendulum, so that we can give the proper lift to our pallets. Four degrees of swing is usual for a seconds pendulum, so we will take four degrees and, dividing it, give two degrees of lift to each pallet. To do this we draw a line two degrees inside the tangent, T (towards the escape wheel center), from our pallet center on the entering pallet side and another line from the pallet center two degrees outside of the tangent, T, on the exit pallet side. Next, from the pallet center we draw arcs of circles cutting the tangents, T, and the radii, R, where they intersect; this gives us the locking planes on which the teeth of the escape wheel “run” (slide) during the excursions of the pendulum, if the escapement is to have unequal lockings; if the lockings are to be equidistant (if the pallet arms are to be of equal length) the arc for the entering pallet is drawn three degrees below (outside) the radius, R, while that on the exit pallet is drawn three degrees above (inside) the exit radius. Finally the lifting planes are drawn from the intersection of the arcs of circles struck from the pallet center with their tangents, T, to the lines, marking the limits of the lift, two degrees away. These lifting planes should be at an angle of 60° from the radii, R, and as a tangent is always at right angles (90°) to its radius, they are consequently at 30° to the tangents running to the pallet center. Thus we can measure these angles from either the escape wheel or the pallet center, as may be most convenient.
When making a new pallet fork, it is most convenient to mark out the lifting planes on the steel at 30° from the tangents, T, as we then do not have to bother with the escape wheel further than to get its center distance and the degrees of arc the lifting planes are to embrace. The workman who is not familiar with this rule is apt to have his ideas upset at first by the angles of inclination toward the center line which the lifting planes will take for different center distances, as owing to the fact that the tangents meet on the center line at different angles for different distances, the lifting planes assume different positions with regard to the center line and he may think that they do not “look right.” They are right, however, when drawn at 30° to their tangents. [Fig. 31] shows several pallets with different arcs arranged in line for purposes of comparison, each being drawn according to the above rule, as measurements with a protractor will show.
Fig. 31.
We have now arrived at the complete escapement, having finished our pallets. We have, however, nothing to hold them in position; they must be rigidly held in position with regard to each other and the escape wheel, consequently we will make a yoke to connect them to the pallet arbor out of the same steel, giving it any desired shape that will not interfere with the working of the clock. Two of the most usual forms are shown at Figs. [32] and [33].