Laying Out and Drafting an Escapement
The circle aa is called the locking circle and is the path followed by the pallet locking corners. The circle bb is called the primitive circle and is the path followed by the locking corners of the escape wheel teeth. Lines AC and AD are drawn from A, through the points C and D and form tangents to the locking circle aa. These lines form, also, the angles of 30° to the right and left of AB and the angles of 90° at C and D. C and D are the locking points of tooth and pallets and it will be noticed that the pressure of the tooth against the entrance pallet is directly towards the pallet arbor and on the exit pallet directly away from the pallet arbor, there being no tendency for the pallet to turn in either direction. The pallets are set, however, at an angle which forms an inclined plane in relation to the direction of pressure and this causes the drawing in action of the pallet towards the center of the wheel, to be explained later.
The lift that we can conveniently and practically give to the teeth will have to be considered before the outer diameter of the escape wheel can be determined. The lift may vary from, say, 2 1/2 to 4° and the choice of lift is determined from the following conditions: The more lift a tooth is given the higher it becomes, i.e., the greater the wheel diameter, and therefore the less clearance it has for passing the lever at its arbor; with a short lift on the tooth the steeper the incline on the pallet will have to be in order to give the required pallet arc of movement, and this requires a stronger mainspring to give and maintain a good motion of the balance; with a 4° lift there is a moment when there is a parallel contact of the tooth and the impulse face of the exit pallet which allows the oil between them to cause a slight cohesion of them. (This effect may be seen by putting a drop of oil between two flat pieces of glass and noticing the effort required to pull or pry them directl y apart.) With the 4° lift tooth there is first a sliding action of the locking corner of the tooth down the exit pallet incline until the tooth locking corner arives at or about the center of the pallet, there is now a momentary parallel contact and immediately following this the turning of the wheel presents the heel of the tooth to the pallet which completes the rest of the impulse. As this action takes place 150 times per minute, any slight hesitancy in breaking the cohesive effect will have a bearing on the rate of the watch as it runs down. The cohesive effect is indeed small but, as you know, high-grade watches are made by eliminating such small defects.
In laying out and drawing an escapement we shall not have to make any calculations but need only a set of drawing instruments, including a protractor, which is a metal or celluloid half-circle divided into degrees, and a drawing board. The drawing equipment may be purchased for as little as a dollar, for pencil drawings only, and there are various grades of instruments to be had for ink drawings, which are not expensive.
A drawing, to show details clearly, should always be made on a fairly large scale. If the reader will follow the instructions given herein he will have no difficulty in making drawings and will have a well defined idea of escapement construction.
Fig. 2 is a development of the preliminary layout of Fig. 1. For the center distance AB we may choose, for our purpose, any distance convenient to our drawing board. We would suggest that this distance be made not less than 6 inches and somewhat longer if board space permits. The development of the drawing is as follows: From B, as a center, and with a radius of 0 the center distance you have chosen, draw the locking circle aa. From A, draw lines AC and AD tangent to the circle aa. These lines will form angles of 30° to the right and left of center line AB. From B, draw lines BC and BD. These lines will form angles of 60° to the right and left of AB, and will form angles of 90° at C and D. From A, as a center, and with AD as a radius, draw circle bb through points C and D. This is called the primitive circle. The points C and D are also the points of intersection of circles aa and bb and are the locking points of tooth and pallets. Next measure the angular widths of tooth and pallet on the primitive circle bb. They together, including the angle necessary for drop, can occupy only 12° which is 1/2 the angular distance between two teeth. Deducting the 10 ° allowance for drop there is left 10 1/2° to be apportioned between pallet and tooth. Of these the pallet is usually given about 1° more than the tooth. In this drawing the pallets have been given 5° and 45' and the tooth 4° and 45/. The width apportioned to the pallets is laid off to the right of the lines AC and AD as shown by lines AF and AG. The width apportioned to the tooth is laid off to the left of the line AD and is shown by the line AH. From B, as a center, lay off on the inside of the line BD the locking angle of 10 ° and draw line BI; on the inside of line BI layoff the angle of 5 ° and 30', for the lift of the pallet, and draw line B J; then layoff the angle of 3° outside the line BD, for the lift of the tooth, and draw line BE. From A, as a center, draw circle cc through the intersection of lines AE and BE, which gives the circle of the outer 'diameter of the escape wheel. From B, as a center, draw circle dd through the intersection of line AF and the outer diameter circle cc, and then the circle ee through the intersection of line AG and the circle cc. Circles dd and ee are the paths in which the let-off corners of the pallets move, while the locking corners move in the circle aa. When a tooth is in locking on the entrance pallet, the locking corner D will be at the intersection of line BI and the locking circle aa, while its let-off corner will be at the intersection of line B J and the circle dd. Draw the line IJ and this will form the incline for lift of the entering pallet.
There is another design of escapement which is used in the majority of our modern watches. This is a design which takes advantage of the practical qualities of both the equidistant pallet construction and the circular pallet one and gives a good action with less exactness of construction. This design is known as the semi-equidistant pallet construction, the pallet width being divided 1/3 to the left and 2/3 to the right of lines AC and AD, these lines passing through the pallet impulse faces and dividing them in that manner. The locking corners of the pallets will not be equidistant from the pallet center and there will be two pallet path circles offset from each other 2/3 the width of the pallet. The inclines of lift of the pallets will form tangents to individual impulse tangent circles but these circles will be closer together as compared with those of the equidistant pallet construction.
The other details entering into the drawing of an escapement should require no explanation and if the reader will get the fundamental principles well in mind the balance of their construction follows of its own accord.
We would suggest that those interested in making escapement drawings make one of each of the designs we have discussed and, also, make drawings representing the relative positions of the inclines of tooth and pallets, as in Figs. 3 and 4, for each design and using tooth lifts of 2 1/2°, 3° , and 4° . You will have, then, a visual means of comparing these designs.