Welcome!

Classic watches, watchmaking, antique tools, history, vintage ephemera and more!

Learn about mechanical timepieces and how they work, the history of the American watch industry and especially all about the Elgin National Watch Company! Check back for new content daily.

Although this is technically a blog, the content is not generally in a time-based sequence. You can find interesting items throughout. Down the page some is an alphabetical word cloud of keywords used here. A great way to dig in is to look through those topics and click anything you find interesting. You'll see all the relevant content.

Here are a few of my favorites!

There are some large images on some posts, so that might impact your load times, bit I think you will find it worth the wait. Thanks for visiting!

Laying Out and Drafting an Escapement

From The American Horologist magazine, September 1938

Laying Out and Drafting an Escapement 


IN LAYING out and drafting an escapement we may take the center distance between the escape wheel and pallet arbors as a starting point, then all the other measurements can be calculated in parts of the center distance. Escapement measurements consist of both linear and angular distances and are mostly trigonometric functions. Angular measurements are made in degrees; a degree being divided into 60 minutes, and a minute into 60 seconds. A circle has an angular measurement or division of 360 degrees.
In this article we shall give only the results of the calculations necessary as they are applicable to all escapements of the design we shall consider. In Fig. 1 is given the preliminary layout of an escapement. AB is the center distance. From B, as a center, and with a radius of one-half the center distance, the circle aa is drawn. Lines BC and BD are laid off at an angle of 600 to the right and to the left of AB and drawn as shown. From A, as a center, and with a radius of AD, a circle bb is drawn through the points C and D. 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.
If we use an angle of 3° for lift of the tooth we will find that at no time do the inclines of tooth and pallet stand parallel to each other, so we have selected this lift for our drawing. Referring to Fig. 1 again, from B, as a center, an angle of 30 is marked off, as shown, and the line BE drawn. With A, as a center, and with a radius of AE, the circle cc is drawn which is the outer diameter of the escape wheel. The radius AD is found mathematically to be .866 of the center distance AB, and the dis tance DE is found to be .0262 (for an angle of 30) of AB. Then AE, the radius of the circle cc, is equal to .866 plus .0262 which is .892 of the center distance AB, and twice this radius gives the total diameter of the wheel which is 1.78 of AB.


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.
The angle of 12° above referred to, through which the tooth has passed to the right of line AD, is the angle of inclination given to the pallets in order to produce the draw which is necessary to hold them in locked position. This angle is drawn as follows: From points I and K draw perpendicular lines 1M and KN to the lines BI and BK. These will form angles of 900, as shown. From I and K, as centers, layoff an angle of 12° to the right of lines 1M and KN for each pallet for draw. This gives the inclination of the locking sides of the pallets. Draw the rear sides of the pallets parallel and complete the rectangles which outline them. Lines projected from the pallet impulse faces to the impulse tangent circles and lines projected from the tooth impulse faces to a tangent circle will give the means of representing the relative positions of the inclines of tooth and pallet at any stage of their passage across each other. In Figs. 3 and 4 we have shown the relative positions of tooth and pallet, by this means, using the same tooth lift as in Fig. 2. In position 1 the tooth and pallet are shown in full lock. In position 2 the let-off corner of the pallet is shown on the primitive circle and it will be seen that the locking corner of the tooth has acted as would a ratchet pointed tooth and that the impulse plane of the pallet has performed its full duty of moving the lever through an angle of 5° and 30'. In position 3 the let-off corner of the pallet is shown on the outside diameter circle and this shows that the impulse plane of the tooth has completed its portion (30) of the total tooth-pallet impulse of 8 ~ o. We can see that at no time do the inclines of tooth and pallet stand exactly parallel to each other but that the locking corner of the tooth first glides along the entire length of the incline of both the entrance and the exit pallets, and then the lift of the tooth is applied wholly at the let off corner of the pallets. The equidistant pallet escapement, which we have drawn, is used in the construction of the highest grade watches. The circular pallet ecsapement is a design that is not often met with but if the reader wishes to draw one there will be a change in the position of the pallets as follows: The width apportioned to the pallets is divided equally to the right and left of lines AC and AD, these lines passing through the center of the pallet impulse faces. The locking corners of the pallets will not, therefore, be equidistant from the pallet center and there will be two pallet path circles offset from each other the width of the pallet. The inclines of lift of both the entrance and exit pallets will, when projected as are the lines IJ and LK in Fig. 2, form tangents to a common impulse tangent circle.


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.
Post a Comment

Click "Older Posts" just above for more, or use the archive links right here.

Blog Archive