Drawing of 15 Tooth Escapement
By W. H. Samelius
As we have 24 degrees space between the escape teeth, 12 degrees are used on each side for width of tooth, width of pallet and drop. The twelfe degrees may be divided thus: 5 degrees 30 minutes for width of tooth, 5 degrees for width of pallet stone and 1 1/2 degrees for drop. As this drawing is to be for a circular escapement, the escape angle passes thru the center of each pallet stone. As the pallet stones are to be 5 degrees wide, layoff two and one-half degrees on both sides of the escape angle running radial lines to the center of the wheel. Then to the left side of the receiving pallet, layoff 5 1/2 degrees for width of tooth, running a radial line to center. From the full width of the discharge stone, lay off 1 1/2 degrees on the primitive diameter, running a radial line to center. This will be the drop. From this drop line, continue laying off 5 1/2 degrees for the width of tooth. These dimensions are all made on the primitive diameter of the wheel "E." This will complete dimensions for circular motion of wheel.
As most escapements are planned to allow an angular motion to the lever of 10 degrees, these 10 degrees are divided between the lift on the pallet, the lock and lift on tooth, we will allow 1 1/2 degrees for lock from the tangent line "D". This line is below the tangent line No.2. Continue with another radial line from pallet center 4 degrees below line No.2. This will be for lift of pallet, line No.3. Above the tangent line, run a radial line thru pallet center of 4 1/2 degrees. This will be the lift, or angle for escape tooth, line No.4. By striking a circle where the inside width line of receiving pallet and the outside lift 4 intersect, scribe a circle. This circle will then represent full diameter of wheel No 5.
From center "E" or pallet center, strike two arcs, passing thru where the width of the pallet stone and primitive diameter of wheel intersect, showing path of pallet stone. The lifting angle for the escape tooth and pallet can now be inserted for the receiving side of the escapement. Between the width lines for the tooth, 5 1/2 degrees and from the primitive to full diameter of wheel, draw in top of tooth. Then from the lower lift line No.3 where the pallet crosses the arc and the lower lock line (2) will be the face of the pallet stones. From the toe of the escape tooth, layoff a clearance angle of 24 degrees, taken from the radial line passing from the toe of the tooth to the center of the escape wheel (5). From the receiving corner of the receiving pallet stone, erect a perpendicular line (6) continuing to the right of this perpendicular line, layoff 12 degrees (7). This line will be the position of the pallet stones and the draw or draft to hold the lever against the banking screws. The full width of the stone may now be determined. Where the inside arc traced from the pallet center intersects line 2 will then be the full width of the pallet stones. A parallel line to the draft angle of 12 degrees may now be drawn and the length of the stone completed. The length of the stone may be 3 to 4 times its width. The discharge stone now can be placed. Extend a line from pallet center, passing thru the outside width of the pallet stone and full diameter of the escape wheel (8). It is at this point where the escape tooth leaves the pallet. From line No. 8 extend a line from pallet center 4 degrees wide. This will be the lift of pallet stones (9). Then the face of the pallet will fall between the two arcs and the width line. Again erecting a perpendicular line from the receiving corner of the discharge stone (10) and continuing from this line 12 degrees to the right we again have a draft angle for the discharge stone (11). A parallel line to the draft angle from where the outside arc· of pallet stone and full diameter of the wheel intersect will give the full width of pallet, which can be cut off square for length or corresponding length of receiving stone. The tooth that has been passed thru the discharge stone can now be placed. The face of the tooth can now be placed which falls between the two width lines and the primitive and full diameter of the wheel (Circle Band No.5). The rest of the escape teeth can now be placed. Divide the distance on the primitive diameter of the wheel from the toe of the receiving side of the tooth to the toe of the discharge side of discharge tooth. The space between these points is divided into three equal spaces. This will locate a toe for each tooth. The length for top of tooth may be taken from the first two drawn and the same dimensions apply to the rest of the teeth. The depth of the tooth is more or less conventional. Ordinarily the depth of the tooth equals one-half the distance from tooth to tooth taken on a full diameter wheel. A construction circle is drawn from the escape wheel center and tangent to the 24 degree clearance angle. (Figure No.5). It is now easy to complete the front of remaining teeth as they will all be tangent to this construction circle.
There are several designs for back of escape tooth, however, whatever the design is, it must be such that there is no possibility of a corner of the pallet stone striking or meeting with interference when the stone enters between the teeth. The drawing shows the most common design of the back of escape tooth.
The following suggestion will give proportions that goes to make a pleasing looking wheel. The width of the rim may be equal to one-sixth of the distance from tooth to tooth. The diameter of the hub may be 1/5th full diameter of the wheel. The wheel is to have four arms and the width of the arm at the rim may be equal to the width of the rim and increasing one third in width towards center.
To design the back of the tooth, divide the space between the primitive and full diameter of wheel (Circle 5 and B), Line No. 12. Then, inside of circle B add another circle of equal space (13). Then between the two outside circles (12 and 5) run a short radial line for the heel of the tooth. On circle 13 layoff one-half the length of impulse face of tooth and connect the short radial line to this point on circle 13. This will be the underside of the club and the neck of the tooth. Divide the space from the front of each tooth on the root diameter or outside of rim and mark off all points the same amount, then span the compass to a radius equal to the distance from toe to toe of teeth and from the neck of the tooth, or circle 13 and to the point on the rim, strike a short arc for back of tooth (14). Provisions for slide are not shown. If slide is to be shown on the drawing, one-half degree may be allowed. The slide may be taken from the total lock of 1 1/2 degrees which would allow 1 degree for lock and one-half degree for slide, leaving a total lock of 1 1/2 degrees.