Fig. 71
For use with full-plate movements about the best plan is a spring clip or clasp to embrace the pallet staff below the pallets. We show at Fig. 71 such a device. To make it, take a rather large size of sewing needle—the kind known as a milliner's needle is about the best. The diameter of the needle should be about No. 2, so that atbwe can drill and put in a small screw. It is important that the whole affair should be very light. The length of the needle should be about 1-5/8", in order that from the notchato the end of the needleA'should be 1-1/2". The needle should be annealed and flattened a little, to give a pretty good grasp to the notchaon the pallet staff.
Good judgment is important in making this clamp, as it is nearly impossible to give exact measurements. About 1/40" in width when seen in the direction of the arrowjwill be found to be about the right width. The springBcan be made of a bit of mainspring, annealed and filed down to agree in width with the partA. In connection with the device shown at Fig. 71 we need a movement-holder to hold the movement as nearly a constant height as possibleabove the bench. The idea is, when the clampA Bis slipped on the pallet staff the index handA'will extend outward, as shown in Fig. 72, where the circleCis supposed to represent the top plate of a watch, andA'the index hand.
Fig. 72
Fig. 72 is supposed to be seen from above. It is evident that if we remove the balance from the movement shown atC, leaving power on the train, and with an oiling tool or hair broach move the lever back and forth, the index handA'will show in a magnified manner the angular motion of the lever. Now if we provide an index arc, as shown atD, we can measure the extent of such motion from bank to bank.
Fig. 73
Fig. 74
To get up such an index arc we first make a stand as shown atE F, Fig. 73. The arcDis made to 1-1/2" radius, to agree with the index handA', and is divided into twelve degree spaces, six each side of a zero, as shown at Fig. 74, which is an enlarged view of the indexDin Fig. 72. The index arc is attached to a short bit of wire extending down into the supportE, and made adjustable as to height by the set-screwl. Let us suppose the index arc is adjusted to the index handA', and we move the fork as suggested; you see the hand would show exactly the arc passed through from bank to bank, and by moving the standE Fwe can arrange so the zero mark on the scale stands in the center of such arc. This, of course, gives the angular motion from bank to bank. As an experiment, let us close the bankings so they arrest the fork at the instant the tooth drops from each pallet. If this arc is ten degrees, the pallet action is as it should be with the majority of modern watches.
Let us try another experiment: We carefully move the fork away from the bank, and if after the index hand has passed through one and a half degrees the fork flies over, we know the lock is right. We repeat the experiment from the opposite bank, and in the same manner determine if the lock is right on the other pallets.You see we have now the means of measuring not only the angular motion of the lever, but the angular extent of the lock. At first glance one would say that if now we bring the roller and fork action to coincide and act in unison with the pallet action, we would be all right; and so we would, but frequently this bringing of the roller and fork to agree is not so easily accomplished.
It is chiefly toward this end the Waltham fork is made adjustable, so it can be moved to or from the roller, and also that we can allow the pallet arms to be moved, as we will try and explain. As we set the bankings the pallets are all right; but to test matters, let us remove the hairspring and put the balance in place. Now, if the jewel pin passes in and out of the fork, it is to be supposed the fork and roller action is all right. To test the fork and roller action we close the banking a little on one side. If the fork and jewel pin are related to each other as they should be, the jewel pin will not pass out of the fork, nor will the engaged tooth drop from that pallet. This condition should obtain on both pallets, that is, if the jewel pin will not pass out of the fork on a given bank the tooth engaged on its pallet should not drop.
We have now come to the most intricate and important problems which relate to the lever escapement. However, we promise our readers that if they will take the pains to follow closely our elucidations, to make these puzzles plain. But we warn them that they are no easy problems to solve, but require good, hard thinking. The readiest way to master this matter is by means of such a model escapement as we have described. With such a model, and the pallets made to clamp with small set-screws, and roller constructed so the jewel pin could be set to or from the staff, this matter can be reduced to object lessons. But study of the due relation of the parts in good drawings will also master the situation.
In using the little instrument for determining angular motion that we have just described, care must be taken that the spring clamp which embraces the pallet staff does not slip. In order to thoroughly understand the methods of using this angle-measuring device, let us take a further lesson or two.
We considered measuring the amount of lock on each pallet, and advised the removal of the balance, because if we left the balance in we could not readily tell exactly when the tooth passedon to the impulse plane; but if we touch the fork lightly with an oiling tool or a hair broach, moving it (the fork) carefully away from the bank and watching the arc indicated by the handA, Fig. 72, we can determine with great exactness the angular extent of lock. The diagram at Fig. 75 illustrates how this experiment is conducted. We apply the hair broach to the end of the forkM, as shown atL, and gently move the fork in the direction of the arrowi, watching the handAand note the number of degrees, or parts of degrees, indicated by the hand as passed over before the tooth is unlocked and passes on to the impulse plane and the fork flies forward to the opposite bank. Now, the quick movement of the pallet and fork may make the hand mark more or less of an arc on the index than one of ten degrees, as the grasp may slip on the pallet staff; but the arc indicated by the slow movement in unlocking will be correct.
Fig. 75
By taking a piece of sharpened pegwood and placing the point in the slot of the fork, we can test the fork to see if the drop takes place much before the lever rests against the opposite bank. As we have previously stated, the drop from the pallet should not take place until the leveralmostrests on the banking pin. What the reader should impress on his mind is that the lever should pass through about one and a half degrees arc to unlock, and the remainder (eight and a half degrees) of the ten degrees are to be devoted to impulse. But, understand, if the impulse angle is only seven and a half degrees, and the jewel pin acts in accordance with the rules previously given, do not alter the pallet until you know for certain you will gain by it. An observant workman will, after a little practice, be able to determine this matter.
We will next take up the double roller and fork action, and also consider in many ways the effect of less angles of action than ten degrees. This matter now seems of more importance, from the fact that we are desirous to impress on our readers thatthere is no valid reason for adopting ten degrees of fork and roller action with the table roller, except that about this number of degrees of action are required to secure a reliable safety action. With the double roller, as low as six degrees fork and pallet action can be safely employed. In fork and pallet actions below six degrees of angular motion, side-shake in pivot holes becomes a dangerous factor, as will be explained further on. It is perfectly comprehending the action ofthe lever escapement and then being able to remedy defects, that constitute the master workman.
Fig. 76
We can also make use of our angle-testing device for measuring our escape-wheel action, by letting the clasp embrace the arbor of the escape wheel, instead of the pallet staff. We set the index arc as in our former experiments, except we place the movable indexD, Fig. 76, so that when the engaged tooth rests on the locking face of a pallet, the index hand stands at the extreme end of our arc of twelve degrees. We next, with our pointed pegwood, start to move the fork away from the bank, as before, we look sharp and see the index hand move backward a little, indicating the "draw" on the locking face. As soon as the pallet reaches the impulse face, the handAmoves rapidly forward, and if the escapement is of the club-tooth order and closely matched, the handAwill pass over ten and a half degrees of angular motion before the drop takes place.
Fig. 77
We will warn our readers in advance, that if they make such a testing device they will be astonished at the inaccuracy which they will find in the escapements of so-called fine watches. The lock, in many instances, instead of being one and a half degrees, will oftener be found to be from two to four degrees, and the impulse derived from the escape wheel, as illustrated at Fig. 76, will often fall below eight degrees. Such watches will have a poor motion and tick loud enough to keep a policeman awake. Trials with actual watches, with such a device as we have just described, in conjunction with a careful study of the acting parts, especially if aided by a large model, such as we have described, will soon bring the student to a degree of skill unknown to the old-style workman, who, if a poor escapement bothered him, would bend back the banking pins or widen the slot in the fork.
Fig. 78
We hold that educating our repair workmen up to a high knowledge of what is required to constitute a high-grade escapement, will have a beneficial effect on manufacturers. When we wish to apply our device to the measurement of the escapement ofthree-quarter-plate watches, we will require another index hand, with the grasping end bent downward, as shown at Fig. 77. The idea with this form of index hand is, the bent-down jawsB', Fig. 77, grasp the fork as close to the pallet staff as possible, making an allowance for the acting center by so placing the index arc that the handAwill read correctly on the indexD. Suppose, for instance, we place the jawsB'inside the pallet staff, we then place the index arc so the hand reads to the arc indicated by the dotted arcm, Fig. 78, and if set outside of the pallet staff, read by the arco.
We think a majority of the fine lever escapements made abroad in this day have what is termed double-roller safety action. The chief gains to be derived from this form of safety action are: (1) Reducing the arc of fork and roller action; (2) reducing the friction of the guard point to a minimum. While it is entirely practicable to use a table roller for holding the jewel pin with a double-roller action, still a departure from that form is desirable, both for looks and because as much of the aggregate weight of a balance should be kept as far from the axis of rotation as possible.
We might as well consider here as elsewhere, the relation the balance bears to the train as a controlling power. Strictly speaking,the balance and hairspring are the time measurers, the train serving only two purposes: (a) To keep the balance in motion; (b) to classify and record the number of vibrations of the balance. Hence, it is of paramount importance that the vibrations of the balance should be as untrammeled as possible; this is why we urge reducing the arc of connection between the balance and fork to one as brief as is consistent with sound results. With a double-roller safety action we can easily reduce the fork action to eight degrees and the roller action to twenty-four degrees.
Inasmuch as satisfactory results in adjustment depend very much on the perfection of construction, we shall now dwell to some extent on the necessity of the several parts being made on correct principles. For instance, by reducing the arc of engagement between the fork and roller, we lessen the duration of any disturbing influence of escapement action.
To resume the explanation of why it is desirable to make the staff and all parts near the axis of the balance as light as possible,we would say it is the moving portion of the balance which controls the regularity of the intervals of vibration. To illustrate, suppose we have a balance only 3/8" in diameter, but of the same weight as one in an ordinary eighteen-size movement. We can readily see that such a balance would require but a very light hairspring to cause it to give the usual 18,000 vibrations to the hour. We can also understand, after a little thought, that such a balance would exert as much breaking force on its pivots as a balance of the same weight, but 3/4" in diameter acting against a very much stronger hairspring. There is another factor in the balance problem which deserves our attention, which factor is atmospheric resistance. This increases rapidly in proportion to the velocity.
The most careful investigators in horological mechanics have decided that a balance much above 75/100" in diameter, making 18,000 vibrations per hour, is not desirable, because of the varying atmospheric disturbances as indicated by barometric pressure. A balance with all of its weight as near the periphery as is consistent with strength, is what is to be desired for best results. It is the moving matter composing the balance, pitted against the elastic force of the hairspring, which we have to depend upon for the regularity of the timekeeping of a watch, and if we can take two grains' weight of matter from our roller table and place them in the rim or screws of the balance, so as to act to better advantage against the hairspring, we have disposed of these two grains so as to increase the efficiency of the controlling power and not increase the stress on the pivots.
Fig. 79
We have deduced from the facts set forth, two axioms: (a) That we should keep the weight of our balance as much in the periphery as possible, consistent with due strength; (b) avoid excessive size from the disturbing effect of the air. We show atA, Fig. 79, the shape of the piece which carries the jewel pin. As shown, it consists of three parts: (1) The socketA, which receives the jewel pina; (2) the partA''and holeb, which goes on the balance staff; (3) the counterpoiseA''', which makes up for the weight of the jewel socketA, neckA'and jewel pin. This counterpoise also makes up for the passing hollowCin the guard rollerB, Fig. 80. As thepieceAis always in the same relation to the rollerB, the poise of the balance must always remain the same, no matter how the roller action is placed on the staff. We once saw a double roller of nearly the shape shown at Fig. 79, which had a small gold screw placed atd, evidently for the purpose of poising the double rollers; but, to our thinking, it was a sort of hairsplitting hardly worth the extra trouble. Rollers for very fine watches should be poised on the staff before the balance is placed upon it.
Fig. 80
We shall next give detailed instructions for drawing such a double roller as will be adapted for the large model previously described, which, as the reader will remember, was for ten degrees of roller action. We will also point out the necessary changes required to make it adapted for eight degrees of fork action. We would beg to urge again the advantages to be derived from constructing such a model, even for workmen who have had a long experience in escapements, our word for it they will discover a great many new wrinkles they never dreamed of previously.
It is important that every practical watchmaker should thoroughly master the theory of the lever escapement and be able to comprehend and understand at sight the faults and errors in such escapements, which, in the every-day practice of his profession, come to his notice. In no place is such knowledge more required than in fork and roller action. We are led to say the above chiefly for the benefit of a class of workmen who think there is a certain set of rules which, if they could be obtained, would enable them to set to rights any and all escapements. It is well to understand that no such system exists and that, practically, we must make one error balance another; and it is the "know how" to make such faults and errors counteract each other that enables one workman to earn more for himself or his employer in two days than another workman, who can file and drill as well as he can, will earn in a week.
The proportion in size between the two rollers in a double-roller escapement is an open question, or, at least, makers seldom agree on it. Grossmann shows, in his work on the lever escapement, two sizes: (1) Half the diameter of the acting roller; (2) two-thirds of the size of the acting roller. The chief fault urged against a smaller safety roller is, that it necessitates longer hornsto the fork to carry out the safety action. Longer horns mean more metal in the lever, and it is the conceded policy of all recent makers to have the fork and pallets as light as possible. Another fault pertaining to long horns is, when the horn does have to act as safety action, a greater friction ensues.
In all soundly-constructed lever escapements the safety action is only called into use in exceptional cases, and if the watch was lying still would theoretically never be required. Where fork and pallets are poised on their arbor, pocket motion (except torsional) should but very little affect the fork and pallet action of a watch, and torsional motion is something seldom brought to act on a watch to an extent to make it worthy of much consideration. In the double-roller action which we shall consider, we shall adopt three-fifths of the pitch diameter of the jewel-pin action as the proper size. Not but what the proportions given by Grossmann will do good service; but we adopt the proportions named because it enables us to use a light fork, and still the friction of the guard point on the roller is but little more than where a guard roller of half the diameter of the acting roller is employed.
The fork action we shall consider at present is ten degrees, but subsequently we shall consider a double-roller action in which the fork and pallet action is reduced to eight degrees. We shall conceive the play between the guard point and the safety roller as one degree, which will leave half a degree of lock remaining in action on the engaged pallet.
In the drawing at Fig. 81 we show a diagram of the action of the double-roller escapement. The small circle atArepresents the center of the pallet staff, and the one atBthe center of the balance staff. The radial linesA dandA d'represent the arc of angular motion of fork action. The circleb brepresents the pitch circle of the jewel pin, and the circle atc cthe periphery of the guard or safety roller. The points established on the circlec cby intersection of the radial linesA dandA d'we will denominate the pointshandh'. It is at these points the end of the guard point of the fork will terminate. In construction, or in delineating for construction, we show the guard enough short of the pointsh h'to allow the fork an angular motion of one degree, fromAas a center, before said point would come in contact with the safety roller.
Fig. 81
We draw through the pointsh h', fromBas a center, the radial linesB gandB g'. We measure this angle by sweeping the short arciwith any of the radii we have used for arc measurement in former delineations, and find it to be a trifle over sixty degrees. To give ourselves a practical object lesson, let us imagine that a real guard point rests on the circlecath. Suppose we make a notch in the guard roller represented by the circlec, to admit such imaginary guard point, and then commence to revolve the circlecin the direction of the arrowj, letting the guard point rest constantly in such notch. When the notchninchas been carried through thirty degrees of arc, counting fromBas a center, the guard point, as relates toAas a center, would only have passed through an arc of five degrees. We show such a guard point and notch ato n. In fact, if a jewel pin was set to engage the fork on the pitch circleb a, the escapement would lock. To obviate such lock we widen the notchnto the extent indicated by the dotted linesn', allowing the guard point to fall back, so to speak, into the notchn, which really represents the passing hollow. It is not to be understood that the extended notch atnis correctly drawn as regards position, because when the guard point was on the lineA fthe pointowould be in the center of the extended notch, or passing hollow. We shall next give the details of drawing the double roller, but before doing so we deemed it important to explain the action of such guard points more fully than has been done heretofore.
We have already given very desirable forms for the parts of a double-roller escapement, consequently we shall now deal chiefly with acting principles as regards the rollers, but will give, at Fig. 82, a very well proportioned and practical form of fork. The pitch circle of the jewel pin is indicated by the dotted circlea, and the jewel pin of the usual cylindrical form, with two-fifths cut away. The safety roller is three-fifths of the diameter of the pitch diameter of the jewel-pin action, as indicated by the dotted circlea.
The safety roller is shown in full outline atB', and the passing hollow atE. It will be seen that the arc of intersection embraced between the radial linesB candB dis about sixty-one and a half degrees for the roller, but the angular extent of the passing hollow is only a little over thirty-two degrees. The passing hollowEis located and defined by drawing the radial lineB cfrom the centerBthrough the intersection of radial lineA iwith the dotted arcb, which represents the pitch circle of the safety roller. We will name this intersection the pointl. Now the end of the guard pointCterminates at the pointl, and the passing hollowEextends onbsixteen degrees on each side of the radial lineB c.
Fig. 82
The roller action is supposed to continue through thirty degrees of angular motion of the balance staff, and is embraced on the circleabetween the radial lineB kandB o. To delineate the inner face of the hornpof the forkFwe draw the short arcg, fromAas a center, and on said arc locate at two degrees from the center atBthe pointf. We will designate the upper angle of the outer face of the jewel pinDas the pointsand, fromAas a center, sweep through this pointsthe short arcn n. Parallel with the lineA iand at the distance of half the diameter of the jewel pinD, we draw the short linest t', which define the inner faces of the fork.
The intersection of the short linetwith the arcnwe will designate the pointr. With our dividers set to embrace the space between the pointrand the pointf, we sweep the arc which defines the inner face of the prong of the fork. The space we just made use of is practically the same as the radius of the circlea, and consequently of the same curvature. Practically, the length of the guard pointC'is made as long as will, with certainty, clearthe safety rollerBin all positions. While we set the pointfat two degrees from the centerB, still, in a well-constructed escapement, one and a half degrees should be sufficient, but the extra half degree will do no harm. If the rollerB'is accurately made and the guard pointC'properly fitted, the fork will not have half a degree of play.
The reader will remember that in the escapement model we described we cut down the drop to one degree, being less by half a degree than advised by Grossmann and Saunier. We also advised only one degree of lock. In the perfected lever escapement, which we shall describe and give working drawings for the construction of, we shall describe a detached lever escapement with only eight degrees fork and pallet action, with only three-fourths of a degree drop and three-fourths of a degree lock, which we can assure our readers is easily within the limits of practical construction by modern machinery.
Fig. 83
The guard pointC', as shown at Fig. 82, is of extremely simple construction. Back of the slot of the fork, which is three-fifths of the diameter of the jewel pin in depth, is made a square hole, as shown atu, and the back end of the guard pointCis fitted to this hole so that it is rigid in position. This manner of fastening the guard point is equally efficient as that of attaching it with a screw, and much lighter—a matter of the highest importance in escapement construction, as we have already urged. About the best material for such guard points is either aluminum or phosphor bronze, as such material is lighter than gold and very rigid and strong. At Fig. 83 we show a side view of the essential parts depicted in Fig. 82, as if seen in the direction of the arrowv, but we have added the piece which holds the jewel pinD. A careful study of the cut shown at Fig. 82 will soon give the horological student an excellent idea of the double-roller action.
We will now take up and consider at length why Saunier draws his entrance pallet with fifteen degrees draw and his exit pallet with only twelve degrees draw. To make ourselves more conversant with Saunier's method of delineating the lever escapement,we reproduce the essential features of his drawing, Fig. 1, plate VIII, of his "Modern Horology," in which he makes the draw of the locking face of the entrance pallet fifteen degrees and his exit pallet twelve degrees. In the cut shown at Fig. 84 we use the same letters of reference as he employs. We do not quote his description or directions for delineation because he refers to so much matter which he has previously given in the book just referred to. Besides we cannot entirely endorse his methods of delineations for many reasons, one of which appears in the drawing at Fig. 84.
Fig. 84
Most writers endorse the idea of tangential lockings, and Saunier speaks of the escapement as shown at Fig. 84 as having such tangential lockings, which is not the case. He defines the position of the pallet staff from the circlet, which represents the extreme length of the teeth; drawing the radial linesA DandA Eto embrace an arc of sixty degrees, and establishing the center of his pallet staffCat the intersection of the linesD CandE C, which are drawn at right angles to the radial linesA DandA E, and tangential to the circlet.
Here is an error; the lines defining the center of the pallet staff should have been drawn tangent to the circles, which represents the locking angle of the teeth. This would have placed thecenter of the pallet staff farther in, or closer to the wheel. Any person can see at a glance that the pallets as delineated are not tangential in a true sense.
Fig. 85
We have previously considered engaging friction and also repeatedly have spoken of tangential lockings, but will repeat the idea of tangential lockings at Fig. 85. A tangential locking is neutral, or nearly so, as regards engaging friction. For illustration we refer to Fig. 85, whereArepresents the center of an escape wheel. We draw the radial linesA yandA zso that they embrace sixty degrees of the arcssort, which correspond to similar circles in Fig. 84, and represent the extreme extent of the teeth and likewise the locking angle of such teeth. In fact, with the club-tooth escapement all that part of a tooth which extends beyond the linesshould be considered the same as the addendum in gear wheels. Consequently, a tangential locking made to coincide with the center of the impulse plane, as recommended by Saunier, would require the pallet staff to be located atC'instead ofC, as he draws it. If the anglek'of the toothkin Fig. 84 was extended outward from the centerAso it would engage or rest on the locking face of the entrance pallet as shown at Fig. 84, then the draw of the locking angle would not be quite fifteen degrees; but it is evident no lock can take place until the angleaof the entrance pallet has passed inside the circles. We would say here that we have added the letterssandtto the original drawings, as we have frequently torefer to these circles, and without letters had no means of designation. Before the locking anglek'of the tooth can engage the pallet, as shown in Fig. 84, the pallet must turn on the centerCthrough an angular movement of at least four degrees. We show the situation in the diagram at Fig. 86, using the same letters of reference for similar parts as in Fig. 84.
Fig. 86
As drawn in Fig. 84 the angle of draftG a Iis equal to fifteen degrees, but when brought in a position to act as shown atG a' I', Fig. 86, the draw is less even than twelve degrees. The angleC a Iremains constant, as shown atC a' I', but the relation to the radialA Gchanges when the pallet moves through the anglew C w', as it must when locked. A tangential locking in the true sense of the meaning of the phrase is a locking set so that a pallet with its face coinciding with a radial line likeA Gwould be neutral, and the thrust of the tooth would be tangent to the circle described by the locking angle of the tooth. Thus the centerC, Fig. 86, is placed on the linew'which is tangent to the circles; said linew'also being at right angles to the radial lineA G.
The facts are, the problems relating to the club-tooth lever escapement are very intricate and require very careful analysis, and without such care the horological student can very readily be misled. Faulty drawings, when studying such problems, lead to no end of errors, and practical men who make imperfect drawings lead to the popular phrase, "Oh, such a matter may be all right in theory, but will not work in practice." We should always bear in mind thattheory, if right, must lead practice.
If we delineate our entrance pallet to have a draw of twelve degrees when in actual contact with the tooth, and then construct in exact conformity with such drawings, we will find our lever to "hug the banks" in every instance. It is inattention to such details which produces the errors of makers complained of by Saunier in section 696 of his "Modern Horology," and whichhe attempts to correct by drawing the locking face at fifteen degrees draw.
We shall show that neitherCnorC', Fig. 85, is the theoretically correct position for the pallet center for a tangential locking.
We will now take up the consideration of a club-tooth lever escapement with circular pallets and tangential lockings; but previous to making the drawings we must decide several points, among which are the thickness of the pallet arms, which establishes the angular motion of the escape wheel utilized by such pallet arms, and also the angular motion imparted to the pallets by the impulse faces of the teeth. We will, for the present, accept the thickness of the arms as being equivalent to five degrees of angular extent of the pitch circle of the escape wheel.
Fig. 87-88
In making our drawings we commence, as on former occasions, by establishing the center of our escape wheel atA, Fig. 87, and sweeping the arca ato represent the pitch circle of such wheel. Through the centerAwe draw the vertical lineA B, which is supposed to also pass through the center of the pallet staff. The intersection of the lineA Bwith the arcawe term the pointd, and from this point we lay off on said arcathirty degrees each side of said intersection, and thus establish the pointsc b. FromA, through the pointc, we draw the lineA c c'. On the arca aand two and a half degrees to the left of the pointcwe establish the pointf, which space represents half of the thickness of the entrance pallet. FromAwe draw through the pointfthe lineA f f'. Fromf, and at right angles to said lineA f, we draw the linef euntil it crosses the lineA B.
Now this linef eis tangent to the arcafrom the pointf, and consequently a locking placed at the pointfis a true tangential locking; and if the resting or locking face of a pallet was made to coincide with the lineA f', such locking face would be strictly "dead" or neutral. The intersection of the linef ewith the lineA Bwe call the pointC, and locate at this point the center of our pallet staff. According to the method of delineating the lever escapement by Moritz Grossmann the tangent line for locating the center of the pallet staff is drawn from the pointc, which would locate the center of the pallet staff at the pointhon the lineA B.
Grossmann, in delineating his locking face for the draw, shows such face at an angle of twelve degrees to the radial lineA f', when he should have drawn it twelve degrees to an imaginary line shown atf i, which is at right angles to the linef h. To the writer's mind this is not just as it should be, and may lead to misunderstanding and bad construction. We should always bear in mind the fact that the basis of a locking face is a neutral plane placed at right angles to the line of thrust, and the "draw" comes from a locking face placed at an angle to such neutral plane. A careful study of the diagram at Fig. 88 will give the reader correct ideas. If a tooth locks at the pointc, the tangential thrust would be on the linec h', and a neutral locking face would be on the lineA c.
To aid in explanation, let us remove the pallet center toD; then the line of thrust would bec Dand a neutral locking face would coincide with the linem m, which is at right angles to the linec D. If we should now make a locking face with a "draw" and at an angle to the linec D, say, for illustration, to correspond to the linec c'(leaving the pallet center atD), we would have a strong draw and also a cruel engaging friction.
If, however, we removed the engaging tooth, which we have just conceived to be atc, to the pointkon the arca' a', Fig. 88, the pallet centerDwould then represent a tangential locking, and a neutral pallet face would coincide with the radial lineA k'; and a locking face with twelve degrees draw would coincide nearly withthe linel. Let us next analyze what the effect would be if we changed the pallet center toh', Fig. 88, leaving the engaging tooth still atk. In this instance the linel lwould then coincide with a neutral locking face, and to obtain the proper draw we should delineate the locking face to correspond to the linek n, which we assume to be twelve degrees fromk l.
It is not to be understood that we insist on precisely twelve degrees draw from a neutral plane for locking faces for lever pallets. What we do insist upon, however, is a "safe and sure draw" for a lever pallet which will hold a fork to the banks and will also return it to such banks if by accident the fork is moved away. We are well aware that it takes lots of patient, hard study to master the complications of the club-tooth lever escapement, but it is every watchmaker's duty to conquer the problem. The definition of "lock," in the detached lever escapement, is the stoppage or arrest of the escape wheel of a watch while the balance is left free or detached to perform the greater portion of its arc of vibration. "Draw" is a function of the locking parts to preserve the fork in the proper position to receive and act on the jewel pin of the balance.
It should be borne in mind in connection with "lock" and "draw," that the line of thrust as projected from the locked tooth of the escape wheel should be as near tangential as practicable. This maxim applies particularly to the entrance pallet. We would beg to add that practically it will make but little odds whether we plant the center of our pallet staff atCorh, Fig. 87, provided we modify the locking and impulse angles of our pallets to conform to such pallet center. But it will not do to arrange the parts for one center and then change to another.
Apparently there seems to be a belief with very many watchmakers that there is a set of shorthand rules for setting an escapement, especially in American watches, which, if once acquired, conquers all imperfections. Now we wish to disabuse the minds of our readers of any such notions. Although the lever escapement, as adopted by our American factories, is constructed on certain "lines," still these lines are subject to modifications, such as may be demanded for certain defects of construction. If we could duplicate every part of a watch movement perfectly, then we could havecertain rules to go by, and fixed templets could be used for setting pallet stones and correcting other escapement faults.
Let us now make an analysis of the action of a lever escapement. We show at Fig. 89 an ordinary eighteen-size full-plate lever with fork and pallets. The dotted linesa bare supposed to represent an angular movement of ten degrees. Now, it is the function of the fork to carry the power of the train to the balance. How well the fork performs its office we will consider subsequently; for the present we are dealing with the power as conveyed to the fork by the pallets as shown at Fig. 89.
Fig. 89
The angular motion between the linesa c(which represents the lock) is not only absolutely lost—wasted—but during this movement the train has to retrograde; that is, the dynamic force stored in the momentum of the balance has to actually turn the train backward and against the force of the mainspring. True, it is only through a very short arc, but the necessary force to effect this has to be discounted from the power stored in the balance from a former impulse. For this reason we should make the angular motion of unlocking as brief as possible. Grossmann, in his essay, endorses one and a half degrees as the proper lock.
In the description which we employed in describing the large model for illustrating the action of the detached lever escapement, we cut the lock to one degree, and in the description of the up-to-date lever escapement, which we shall hereafter give, we shall cut the lock down to three-quarters of a degree, a perfection easily to be attained by modern tools and appliances. We shall also cut the drop down to three-quarters of a degree. By these two economies we more than make up for the power lost in unlocking. With highly polished ruby or sapphire pallets ten degrees of draw is ample. But such draw must positively be ten degrees from aneutral locking face, not an escapement drawn on paper and called ten degrees, but when actually measured would only show eight and a half or nine degrees.
With ten degrees angular motion of the lever and one and a half degrees lock, we should have eight and a half degrees impulse. The pith of the problem, as regards pallet action, for the practical workman can be embodied in the following question: What proportion of the power derived from the twelve degrees of angular motion of the escape wheel is really conveyed to the fork? The great leak of power as transmitted by the lever escapement to the balance is to be found in the pallet action, and we shall devote special attention to finding and stopping such leaks.
If we use a ratchet-tooth escape wheel we must allow at least one and a half degrees drop to free the back of the tooth; but with a club-tooth escape wheel made as can be constructed by proper skill and care, the drop can be cut down to three-quarters of a degree, or one-half of the loss with the ratchet tooth. We do not wish our readers to imagine that such a condition exists in most of the so-called fine watches, because if we take the trouble to measure the actual drop with one of the little instruments we have described, it will be found that the drop is seldom less than two, or even three degrees.
If we measure the angular movement of the fork while locked, it will seldom be found less than two or three degrees. Now, we can all understand that the friction of the locking surface has to be counted as well as the recoil of the draw. Locking friction is seldom looked after as carefully as the situation demands. Our factories make the impulse face of the pallets rounded, but leave the locking face flat. We are aware this condition is, in a degree, necessary from the use of exposed pallets. In many of the English lever watches with ratchet teeth, the locking faces are made cylindrical, but with such watches the pallet stones, as far as the writer has seen, are set "close"; that is, with steel pallet arms extending above and below the stone.
There is another feature of the club-tooth lever escapement that next demands our attention which we have never seen discussed.We refer to arranging and disposing of the impulse of the escape wheel to meet the resistance of the hairspring. Let us imagine the dotted lineA d, Fig. 89, to represent the center of action of the fork. We can readily see that the fork in a state of rest would stand half way between the two banks from the action of the hairspring, and in the pallet action the force of the escape wheel, one tooth of which rests on the impulse face of a pallet, would be exerted against the elastic force of the hairspring. If the force of the mainspring, as represented by the escape-wheel tooth, is superior to the power of the hairspring, the watch starts itself. The phases of this important part of the detached lever escapement will be fully discussed.
We will now take up a study of the detached lever escapement as relates to pallet action, with the point specially in view of constructing an escapement which cannot "set" in the pocket, or, in other words, an escapement which will start after winding (if run down) without shaking or any force other than that supplied by the train as impelled by the mainspring. In the drawing at Fig. 90 we propose to utilize eleven degrees of escape-wheel action, against ten and a half, as laid down by Grossmann. Of this eleven degrees we propose to divide the impulse arc of the escape wheel in six and five degrees, six to be derived from the impulse face of the club tooth and five from the impulse plane of the pallet.
The pallet action we divide into five and four, with one degree of lock. Five degrees of pallet action is derived from the impulse face of the tooth and four from the impulse face of the pallet. The reader will please bear in mind that we do not give these proportions as imperative, because we propose to give the fullest evidence into the reader's hands and enable him to judge for himself, as we do not believe in laying down imperious laws that the reader must accept on our assertion as being correct. Our idea is rather to furnish the proper facts and put him in a situation to know for himself.
The reader is urged to make the drawings for himself on a large scale, say, an escape wheel 10" pitch diameter. Such drawings will enable him to realize small errors which have been tolerated too much in drawings of this kind. The drawings, as they appear in the cut, are one-fourth the size recommended, and many of thelines fail to show points we desire to call attention to. As for instance, the pallet center atBis tangential to the pitch circleafrom the point of tooth contact atf. To establish this point we draw the radial linesA candA dfrom the escape-wheel centerA, as shown, by laying off thirty degrees on each side of the intersection of the vertical linei(passing through the centersA B) with the arca, and then laying off two and a half degrees onaand establishing the pointf, and throughffrom the centerAdraw the radial lineA f'. Through the pointfwe draw the tangent lineb' b b'', and at the intersection of the linebwithiwe establish the center of our pallet staff atB. At two and a half degrees from the pointcwe lay off two and a half degrees to the right of said point and establish the pointn, and draw the radial lineA n n', which establishes the extent of the arc of angular motion of the escape wheel utilized by the pallet arm.