Fig. 54.
Then to set out the escapement, if a lift of say 30° be decided on, a circle on which the points of the teeth will fall is drawn within one representing the extreme diameter of the escape wheel, at a distance from it equal to 30° of the circumference of the cylinder. Midway between these two circles the cylinder is planted ([see Fig. 54]). If the point of one tooth is shown resting on the cylinder, a space of half a degree should be allowed for freedom between the opposite side of the cylinder and the heel of the next tooth. From the heel of one tooth to the heel of the next equal 24° of the circumference of the wheel, 360 ÷ 15 = 24°, and from the point of one tooth to the point of the next also equals 24° so that the teeth may now be drawn. They are extended within the innermost dotted circle to give them a little more strength, and their tips are rounded a little, having the points of the impulse planes on the inner or basing circle. The backs of the teeth diverge from a radial line from 12° to 30°, in order to give the cylinder clearance, a high angled tooth requiring to be cut back more than a low one. A curve whose radius is about two-thirds that of the wheel is suitable for rounding the impulse planes of the teeth. The internal diameter of the cylinder should be such as to allow a little freedom for the tooth. The rule in fitting cylinders is to have equal clearance inside and outside, so as to equalize the drop. The acting part of the shell of the cylinder (where the lips are placed) should be a trifle less than seven-twelfths of a whole circle, with the entering and exit lips which are really the pallets, rounded as shown in the enlarged plan, [Fig. 55], the entering lip or pallet rounded both ways and the exit pallet rounded from the inside only. This rounding of the lips of the cylinder adds a little to the impulse beyond what would be given by the angle on the wheel teeth alone. The diameter of the escape wheel is usually half that of the balance, rather under than over.
Size of Cylinder Pivot.—To establish the size of the pivot with relation to its hole is apparently an easy thing to do correctly, but to an inexperienced workman it is not so. The side shake in cylinder pivot holes should be greater than that for ordinary train holes; one-sixth is the amount prescribed by Saunier; the size of the pivot relatively to the cylinder about one-eighth the diameter of the body of the cylinder. It is very necessary that this amount of side shake should be correctly recognized; if less than the amount stated, the escapement, though performing well while the oil is fresh, fails to do so when it commences to thicken.
Fig. 55.