On delineating our chronometer escapement shown at Fig. 141 we have followed no text-book authority, but have drawn it according to such requirements as are essential to obtain the best results. An escapement of any kind is only a machine, and merely requires in its construction a combination of sound mechanical principles. Neither Saunier nor Britten, in their works, give instructions for drawing this escapement which will bear close analysis. It is not our intention, however, to criticise these authors, except we can present better methods and give correct systems.
It has been a matter of great contention with makers of chronometer and also lever escapements as to the advantages of “tangential lockings.” By this term is meant a locking the same as is shown at C, Fig. 141, and means a detent planted at right angles to a line radial to the escape-wheel axis, said radial line passing through the point of the escape-wheel tooth resting on the locking jewel. In escapements not set tangential, the detent is pushed forward in the direction of the arrow x about half a tooth space. Britten, in his “Hand-Book,” gives a drawing of such an escapement. We claim the chief advantage of tangential locking to lie in the action of the escape-wheel teeth, both on the impulse stone and also on the locking stone of the detent. Saunier, in his “Modern Horology,” gives the inclination of the front fan of the escape-wheel teeth as being at an angle of twenty-seven degrees to a radial line. Britten says twenty degrees, and also employs a non-tangential locking.
Our drawing is on an angle of twenty-eight degrees, which is as low as is safe, as we shall proceed to demonstrate. For establishing the angle of an escape-wheel tooth we draw the line C d, from the point of the escape-wheel tooth resting on the locking stone shown at C at an angle of twenty-eight degrees to radial line C k. We have already discussed how to locate and plant the center of the balance staff.
We shall not show in this drawing the angular motion of the escape wheel, but delineate at the radial lines c e and c f of the arc of the balance during the extent of its implication with the periphery of the escape wheel, which arc is one of about forty-eight degrees. Of this angle but forty-three degrees is attempted to be utilized for the purpose of impulse, five degrees being allowed for the impulse jewel to pass inside of the arc of periphery of the escape wheel before the locking jewel releases the tooth of the escape wheel resting upon it. At this point it is supposed the escape wheel attacks the impulse jewel, because, as we just explained, the locking jewel has released the tooth engaging it. Now, if the train had no weight, no inertia to overcome, the escape wheel tooth A^2 would move forward and attack the impulse pallet instantly; but, in fact, as we have already explained, there will be an appreciable time elapse before the tooth overtakes the rapidly-moving impulse jewel. It will, of course, be understood that the reference letters used herein refer to the illustrations that have appeared on preceding pages.