From _A_ as a center and with our dividers set at 5", we sweep the arc _f_. From the scale of degrees we just used we lay off fifteen degrees on each side of the line _A B_ on the arc _f_, and establish the points _g h_. From _A_ as a center, through the points just established we draw the radial lines _A g"_ and _A h"_. The angular extent between these lines defines the limit of our roller action.
Now if we lay off on the arc _f_ six degrees each side of its intersection with the line _A B_, we define the extent of the jewel pin; that is, on the arc _f_ we establish the points _l m_ at six degrees from the line _A B_, and through the points _l m_ draw, from _A_ as a center, the radial lines _A l"_ and _A m"_. The extent of the s.p.a.ce between the lines _A l"_ and _A m"_ on the circle _a_ defines the size of our jewel pin.
TO DETERMINE THE SIZE OF A JEWEL PIN.
[Ill.u.s.tration: Fig. 56]
To make the situation better understood, we make an enlarged drawing of the lines defining the jewel pin at Fig. 56. At the intersection of the line _A B_ with the arc _a_ we locate the point _k_, and from it as a center we sweep the circle _i_ so it pa.s.ses through the intersection of the lines _A l"_ and _A m"_ with the arc _a_. We divide the radius of the circle _i_ on the line _A B_ into five equal parts, as shown by the vertical lines _j_. Of these five s.p.a.ces we a.s.sume three as the extent of the jewel pin, cutting away that portion to the right of the heavy vertical line at _k_.
[Ill.u.s.tration: Fig. 57]
We will now proceed to delineate a fork and roller as the parts are related on first contact of jewel pin with fork and initial with the commencing of the act of unlocking a pallet. The position and relations are also the same as at the close of the act of impulse. We commence the drawing at Fig. 57, as before, by drawing the line _A B_ and the arcs _a_ and _b_ to represent the pitch circles. We also sweep the arc _f_ to enable us to delineate the line _A g"_. Next in order we draw our jewel pin as shown at _D_. In drawing the jewel pin we proceed as at Fig. 56, except we let the line _A g"_, Fig. 57, a.s.sume the same relations to the jewel pin as _A B_ in Fig. 56; that is, we delineate the jewel pin as if extending on the arc _a_ six degrees on each side of the line _A g"_, Fig. 57.
THE THEORY OF THE FORK ACTION.
To aid us in reasoning, we establish the point _m_, as in Fig. 55, at _m_, Fig. 57, and proceed to delineate another and imaginary jewel pin at _D"_ (as we show in dotted outline). A brief reasoning will show that in allowing thirty degrees of contact of the fork with the jewel pin, the center of the jewel pin will pa.s.s through an arc of thirty degrees, as shown on the arcs _a_ and _f_. Now here is an excellent opportunity to impress on our minds the true value of angular motion, inasmuch as thirty degrees on the arc _f_ is of more than twice the linear extent as on the arc _a_.
Before we commence to draw the horn of the fork engaging the jewel pin _D_, shown at full line in Fig. 57, we will come to perfectly understand what mechanical relations are required. As previously stated, we a.s.sume the jewel pin, as shown at _D_, Fig. 57, is in the act of encountering the inner face of the horn of the fork for the end or purpose of unlocking the engaged pallet. Now if the inner face of the horn of the fork was on a radial line, such radial line would be _p B_, Fig. 57. We repeat this line at _p_, Fig. 56, where the parts are drawn on a larger scale.
To delineate a fork at the instant the last effort of impulse has been imparted to the jewel pin, and said jewel pin is in the act of separating from the inner face of the p.r.o.ng of the fork--we would also call attention to the fact that relations of parts are precisely the same as if the jewel pin had just returned from an excursion of vibration and was in the act of encountering the inner face of the p.r.o.ng of the fork in the act of unlocking the escapement.
We mentioned this matter previously, but venture on the repet.i.tion to make everything clear and easily understood. We commence by drawing the line _A B_ and dividing it in four equal parts, as on previous occasions, and from _A_ and _B_ as centers draw the pitch circles _c d_.
By methods previously described, we draw the lines _A a_ and _A a"_, also _B b_ and _B b"_ to represent the angular motion of the two mobiles, viz., fork and roller action. As already shown, the roller occupies twelve degrees of angular extent. To get at this conveniently, we lay off on the arc by which we located the lines _A a_ and _A a"_ six degrees above the line _A a_ and draw the line _A h_.
Now the angular extent on the arc _c_ between the lines _A a_ and _A h_ represents the radius of the circle defining the jewel pin. From the intersection of the line _A a_ with the arc _c_ as a center, and with the radius just named, we sweep the small circle _D_, Fig. 58, which represents our jewel pin; we afterward cut away two-fifths and draw the full line _D_, as shown. We show at Fig. 59 a portion of Fig. 58, enlarged four times, to show certain portions of our delineations more distinctly. If we give the subject a moment"s consideration we will see that the length of the p.r.o.ng _E_ of the lever fork is limited to such a length as will allow the jewel pin _D_ to pa.s.s it.
HOW TO DELINEATE THE p.r.o.nGS OF A LEVER FORK.
[Ill.u.s.tration: Fig. 58]
[Ill.u.s.tration: Fig. 59]
To delineate this length, from _B_ as a center we sweep the short arc _f_ so it pa.s.ses through the outer angle _n_, Fig. 59, of the jewel pin.
This arc, carried across the jewel pin _D_, limits the length of the opposite p.r.o.ng of the fork. The outer face of the p.r.o.ng of the fork can be drawn as a line tangent to a circle drawn from _A_ as a center through the angle _n_ of the jewel pin. Such a circle or arc is shown at _o_, Figs. 58 and 59. There has been a good deal said as to whether the outer edge of the p.r.o.ng of a fork should be straight or curved.
To the writer"s mind, a straight-faced p.r.o.ng, like from _s_ to _m_, is what is required for a fork with a single roller, while a fork with a curved p.r.o.ng will be best adapted for a double roller. This subject will be taken up again when we consider double-roller action. The extent or length of the outer face of the p.r.o.ng is also an open subject, but as there is but one factor of the problem of lever escapement construction depending on it, when we name this and see this requirement satisfied we have made an end of this question. The function performed by the outer face of the p.r.o.ng of a fork is to prevent the engaged pallet from unlocking while the guard pin is opposite to the pa.s.sing hollow.
The inner angle _s_ of the horn of the fork must be so shaped and located that the jewel pin will just clear it as it pa.s.ses out of the fork, or when it pa.s.ses into the fork in the act of unlocking the escapement. In escapements with solid bankings a trifle is allowed, that is, the fork is made enough shorter than the absolute theoretical length to allow for safety in this respect.
THE PROPER LENGTH OF A LEVER.
We will now see how long a lever must be to perform its functions perfectly. Now let us determine at what point on the inner face of the p.r.o.ng _E"_ the jewel pin parts from the fork, or engages on its return.
To do this we draw a line from the center _r_ (Fig. 59) of the jewel pin, so as to meet the line _e_ at right angles, and the point _t_ so established on the line _e_ is where contact will take place between the jewel pin and fork.
It will be seen this point (_t_) of contact is some distance back of the angle _u_ which terminates the inner face of the p.r.o.ng _E"_; consequently, it will be seen the p.r.o.ngs _E E"_ of the fork can with safety be shortened enough to afford a safe ingress or egress to the jewel pin to the slot in the fork. As regards the length of the outer face of the p.r.o.ng of the fork, a good rule is to make it one and a half times the diameter of the jewel pin. The depth of the slot need be no more than to free the jewel in its pa.s.sage across the ten degrees of fork action. A convenient rule as to the depth of the slot in a fork is to draw the line _k_, which, it will be seen, coincides with the circle which defines the jewel pin.
HOW TO DELINEATE THE SAFETY ACTION.
[Ill.u.s.tration: Fig. 60]
We will next consider a safety action of the single roller type. The active or necessary parts of such safety action consist of a roller or disk of metal, usually steel, shaped as shown in plan at _A_, Fig. 60.
In the edge of this disk is cut in front of the jewel pin a circular recess shown at _a_ called the pa.s.sing hollow. The remaining part of the safety action is the guard pin shown at _N_ Figs. 61 and 62, which is placed in the lever. Now it is to be understood that the sole function performed by the guard pin is to strike the edge of the roller _A_ at any time when the fork starts to unlock the engaged pallet, except when the jewel pin is in the slot of the fork. To avoid extreme care in fitting up the pa.s.sing hollow, the horns of the fork are arranged to strike the jewel pin and prevent unlocking in case the pa.s.sing hollow is made too wide. To delineate the safety action we first draw the fork and jewel pin as previously directed and as shown at Fig. 63. The position of the guard pin should be as close to the bottom of the slot of the fork as possible and be safe. As to the size of the guard pin, it is usual to make it about one-third or half the diameter of the jewel pin.
The size and position of the guard pin decided on and the small circle _N_ drawn, to define the size and position of the roller we set our dividers so that a circle drawn from the center _A_ will just touch the edge of the small circle _N_, and thus define the outer boundary of our roller, or roller table, as it is frequently called.
[Ill.u.s.tration: Fig. 61]
[Ill.u.s.tration: Fig. 62]
For deciding the angular extent of the pa.s.sing hollow we have no fixed rule, but if we make it to occupy about half more angular extent on the circle _y_ than will coincide with the angular extent of the jewel pin, it will be perfectly safe and effectual. We previously stated that the jewel pin should occupy about twelve degrees of angular extent on the circle _c_, and if we make the pa.s.sing hollow occupy eighteen degrees (which is one and a half the angular extent of the jewel pin) it will do nicely. But if we should extend the width of the pa.s.sing hollow to twenty-four degrees it would do no harm, as the jewel pin would be well inside the horn of the fork before the guard pin could enter the pa.s.sing hollow.
[Ill.u.s.tration: Fig. 63]
We show in Fig. 61 the fork as separated from the roller, but in Fig.
62, which is a side view, we show the fork and jewel pin as engaged.
When drawing a fork and roller action it is safe to show the guard pin as if in actual contact with the roller. Then in actual construction, if the parts are made to measure and agree with the drawing in the gray, that is, before polishing, the process of polishing will reduce the convex edge of the roller enough to free it.
It is evident if thought is given to the matter, that if the guard pin is entirely free and does not touch the roller in any position, a condition and relation of parts exist which is all we can desire. We are aware that it is usual to give a considerable lat.i.tude in this respect even by makers, and allow a good bit of side shake to the lever, but our judgment would condemn the practice, especially in high-grade watches.
RESTRICT THE FRICTIONAL SURFACES.
Grossmann, in his essay on the detached lever escapement, adopts one and a half degrees lock. Now, we think that one degree is ample; and we are sure that every workman experienced in the construction of the finer watches will agree with us in the a.s.sertion that we should in all instances seek to reduce the extent of all frictional surfaces, no matter how well jeweled. Acting under such advice, if we can reduce the surface friction on the lock from one and a half degrees to one degree or, better, to three-fourths of a degree, it is surely wise policy to do so. And as regards the extent of angular motion of the lever, if we reduce this to six degrees, exclusive of the lock, we would undoubtedly obtain better results in timing.
We shall next consider the effects of opening the bankings too wide, and follow with various conditions which are sure to come in the experience of the practical watch repairer. It is to be supposed in this problem that the fork and roller action is all right. The reader may say to this, why not close the banking? In reply we would offer the supposition that some workman had bent the guard pin forward or set a pallet stone too far out.
We have now instructed our readers how to draw and construct a lever escapement complete, of the correct proportions, and will next take up defective construction and consider faults existing to a lesser or greater degree in almost every watch. Faults may also be those arising from repairs by some workman not fully posted in the correct form and relation of the several parts which go to make up a lever escapement. It makes no difference to the artisan called upon to put a watch in perfect order as to whom he is to attribute the imperfection, maker or former repairer; all the workman having the job in hand has to do is to know positively that such a fault actually exists, and that it devolves upon him to correct it properly.
BE FEARLESS IN REPAIRS, IF SURE YOU ARE RIGHT.
Hence the importance of the workman being perfectly posted on such matters and, knowing that he is right, can go ahead and make the watch as it should be. The writer had an experience of this kind years ago in Chicago. A Jules Jurgensen watch had been in the hands of several good workmen in that city, but it would stop. It was then brought to him with a statement of facts given above. He knew there must be a fault somewhere and searched for it, and found it in the exit pallet--a certain tooth of the escape wheel under the right conditions would sometimes not escape. It might go through a great many thousand times and yet it might, and did sometimes, hold enough to stop the watch.
Now probably most of my fellow-workmen in this instance would have been afraid to alter a "Jurgensen," or even hint to the owner that such a thing could exist as a fault in construction in a watch of this justly-celebrated maker. The writer removed the stone, ground a little from the base of the offending pallet stone, replaced it, and all trouble ended--no stops from that on.
STUDY OF AN ESCAPEMENT ERROR.
[Ill.u.s.tration: Fig. 64]
Now let us suppose a case, and imagine a full-plate American movement in which the ingress or entrance pallet extends out too far, and in order to have it escape, the banking on that side is opened too wide. We show at Fig. 64 a drawing of the parts in their proper relations under the conditions named. It will be seen by careful inspection that the jewel pin _D_ will not enter the fork, which is absolutely necessary. This condition very frequently exists in watches where a new pallet stone has been put in by an inexperienced workman. Now this is one of the instances in which workmen complain of hearing a "sc.r.a.ping" sound when the watch is placed to the ear. The remedy, of course, lies in warming up the pallet arms and pushing the stone in a trifle, "But how much?"
say some of our readers. There is no definite rule, but we will tell such querists how they can test the matter.
Remove the hairspring, and after putting the train in place and securing the plates together, give the winding arbor a turn or two to put power on the train; close the bankings well in so the watch cannot escape on either pallet. Put the balance in place and screw down the c.o.c.k.