The Chicago School of Watchmaking
Home Study Course
A Modern, Complete, Practical Course
PRINCIPLES OF THE LEVER ESCAPEMENT
Chicago School of Watchmaking Founded 1908 by Thomas B. Sweazy
SEC. 404 -- Unequal Roller ShakeThe, guard pin should be perfectly central with the fork slot under all conditions. It should never tilt from the perpendicular in a single roller escapement. In a double roller escapement, a line drawn from the point of the dart to the center of the pallet arbor should pass through the center of the fork slot and be parallel with its sides.
Figure 22-3 illustrates the above. In this figure an escapement is shown with the guard pin bent as in figure 22-2. The roller is assumed to have moved from the position there shown to the position shown in figure 22-3, or until the passing hollow is opposite the guard pin. In this position the fork has been pressed to the left. The safety action between the roller jewel and fork horn fails to arrest the movement of the fork until the escapement has unlocked on the discharging stone as shown.
Moving the pallet stones for the sole purpose of changing the roller shake is a method often practiced, but it is not to be advised. It necessitates more time and jeopardizes the escapment action. A pallet stone can not be moved without altering the escapement in at least four particulars: Impulse, draft, lock and drop.
The simplest and best method of making the alteration is to bend the fork. This alters no other functions of the escapement.
Bending a fork is an operation of which many students stand in awe, but there is little danger if done carefully.
To remedy a guard pin in a single roller escapement which is too far forward, proceed as follows: Push the guard pin through a small piece of tissue paper, the object being to prevent marring the polished face of the fork. With a fine file, dress the pin to a point as shown at B, figure 22-6. A is the pin before being dressed down. It may be reduced as much as necessary without injury to the safety action. In fact, a V-shaped safety pin is superior to a cylindrical one in this particular.
In the double roller escapement an alteration in the length of the guard pin is so simple that instruction is deemed unnecessary.
SEC. 406 -- Guard Pin Too Far Back
SEC. 407 -- The Escape and Pallet ActionThe escape and pallet action is the most intricate function of the escapement. A thorough knowledge of it calls for thoughtful study. Yet there is nothing about it that can not be mastered by the student. In treating it, it is deemed best to divide it into five branches: Impulse, Draft, Lock, Slide, Drop.
SEC. 408 -- ImpulseAn escape tooth, in delivering impulse to a pallet, moves in an arc of 12 degrees.
In the ratchet tooth escapement all the pulse is on the pallet; otherwise the same rules apply to it as to the club tooth. A brief description of the ratchet tooth escapement will be given later on.
The chief advantage of the club tooth escapement over the ratchet tooth is that the former can be constructed with less loss of power from drop. A minor advantage is that there is less liability of wedging the guard pin against the roller edge when the train is reversed, which sometimes occurs in setting the hands backward.
In the club tooth escapement, the circular impulse is divided between the tooth and the pallet stone. This division is in various proportion, usually the narrow limits of four--tenths to the tooth and six-tenths to the stone, and equal amounts to both.
SEC. 409 -- Draft
The dotted line g is termed the tangential line, meaning that touches the arc ii at the intersection of the radial line e. The draft angle is laid out from this tangential line. In the escapement shown at A, the broken radial line a is continued by the full line h in order to show that the tangential and the radial lines are in this case identical.
Insufficient draft is a serious error. Where it exists there is a constant liability that when the watch is subjected to some sudden motion, the fork will leave the banking and strike the safety edge of the roller, thus retarding the motion of the balance. It is a of time to attempt adjusting a watch having this fault.
A common method of testing the draft of an escapement is to lay the watch in a horizontal position, then draw the fork slightly away from the banking and see that it returns to its original position. It is advisable to try the banking under the most unfavorable conditions to which the is likely to be subjected in its owner.s possession. First, see that the watch is let down to the point it would have reached after a run of 24 hours, then hold the watch in a vertical position. If the fork is poised it will make no difference in what position the watch is held as long as it is vertical, but if the fork is unpoised it should be held with the heaviest part lying in a horizontal line with its center. If the fork is without a counterpoise the arm would then be horizontal. In this case first try the fork by drawing it slightly away from the banking while the fork is in horizontal position to the right. This will usually be on the receiving stone. Then turn the fork to the left which will be on the discharging stone. The object of using this method is to insure against insufficient draft. If the draft is not sufficient to draw the fork against the banking under the conditions stated, it should be increased.
It is rarely that an escapement is found With too much draft. Should there be a suspicion that such is the case it can be easily determined: Take a small piece of Wax and attach it to the fork by inserting it on the guard pin or by any other method that may suggest itself; then try the draft as before. Judgment will dictate the additional weight that should overcome the draft.
Draft can sometimes be changed by tilting a pallet stone in its slot. This, however, cannot be done when the stone is closely fitted. Another method by which the draft may be altered is by pushing in one stone and drawing out the other. Drawing out the receiving and pushing in the discharging stones increases the draft on both stones. Drawing out the discharging and pushing in the receiving stones decreases the draft on both stones. That is, drawing out the receiving stone causes an escaped tooth to drop farther upon the locking face of the discharging stone, which of course increases the draft on the discharging stone, but it also increases the lock, which must be remedied by pushing in the discharging stone. This causes a tooth to drop farther down on the receiving stone, increasing the draft. A slight movement of the stone in or out will change the draft to a considerable extent. In changing a draft by this method extreme care should be used to avoid introducing other errors.
It should be borne in mind that drawing a receiving stone increases the drop on the discharging stone, but does not alter the drop on the receiving, but in pushing in the discharging stone to correct the lock, the drop is decreased on the receiving stone.
It should also be borne in mind that drawing out the receiving stone increases the divergence of the impulse face of the wheel with that stone, and that pushing in the discharging stone creases the divergence on that stone in the same manner.
That is, drawing out the receiver and pushing in the discharger increases the divergence on both stones.
Pushing in the receiver and drawing out the discharger decreases it.
When the draft is altered by pushing and pulling the stones, that operation puts the fork out of angle, which must be corrected by bending. Later on in this Work, directions will be given for making a drawing of an escapement.
Figure 22-13 shows effect on the drop of moving a stone. At A the pallets and wheel are shown in two relative positions. In the first position they are shown in full lines and the teeth marked 1, 2, 3, 4. The will move forward when a tooth is released by the receiving stone. The pallets are then in the position shown in dotted lines and the wheel teeth, in dotted lines, are marked 1, 2, 3. In the drawing shown at A the drop is equal. The distance between the releasing corners of the receiving stone and tooth 1, as shown at a, is exactly the same as the distance between the releasing corners at b.
Now referring to the pallets shown at B, it will be observed that the receiving stone has been drawn out, as indicated by the black space in the pallet slot, and that the discharging stone has been pushed in, as indicated by the disappearance of the black space that was shown in the discharging stone slot in pallets A. At B the escapement is shown under the same conditions as at A: Lock on the receiver in full lines; on the discharger in dotted lines. It will be plainly seen that the distance between the releasing corners at c is greater than at d. This is to say that while the lock was equal with the stones in position, as at A, drawing out the receiving stone and pushing in the discharging made them unequal, as at B.
Figure 22-14 shows a pair of pallets with escape wheel teeth in three positions on each stone. The three positions are indicated by the wheel teeth shown in dotted lines and in broken lines. From the points of contact are drawn dotted, broken, and full lines running radially to the pallet center and at right angles to the radii. These lines are marked a, b, c on the receiving side and d, e, f on the discharging side. The inclination of the locking faces of the stones toward these lines determines the draft. The drawing is for the purpose of showing the opposite effects produced by moving a receiving stone from that produced by moving a discharging stone. Referring to figure 22-14, it must be evident to the observer that the farther up the locking takes place on the receiving stone the less will be the draft, and that the further up it takes place on the locking face of the discharging stone the greater will be the draft.
In case the foregoing is not perfectly clear to the student, let him suppose the tooth locked on the receiving stone at the junction of the lines cc and that a tooth dropped on the discharging stone at the junction of the lines ff. In this position the draft will be represented by the angle formed between the locking faces of the stones and the lines c and f, respectively. Now if -- leaving the wheel as it is -- we draw out the receiving stone until its locking corner coincides with the junction of the lines aa, we have not changed the draft; but when the stone is thus drawn out the pallets will have to swing further to release the tooth, with the result that the lock will now take place on the discharging stone at the junction of the lines dd. Thus it would be seen that the moving of a stone does not alter the draft on it, but on the opposite stone.
Referring back to figure 22-18, it will be seen that a wheel tooth is locked on the receiving stone, but the fork is not quite in contact with the left banking. Moving it into contact will make the slide on the stone. The slide is very easily changed by moving the banking, but should never be increased beyond an amount equaling the correct lock.
SEC. 412 -- DropDrop is the space that the escape wheel passes through during the interval between the release of one tooth by a pallet stone and the arrest of another tooth by the opposite stone. The drop is clearly shown in figure 22-15. At the point marked A the tooth has just been released by the discharging stone; at B a tooth has contacted the locking face of the receiving stone. The space intervening between the releasing corners of tooth and stone at A exactly equals the space between the locking corner of the tooth and its point of contact with the stone at B before it dropped. This is the drop and should be alike on both stones.
Drop may be altered by spreading the stones apart or closing them. The former increases the drop from the receiving stone to the discharger and decreases the drop from the discharger to the receiver. Closing the stones has the opposite effect; hence it is seen that change made in this manner may affect a correction with a very slight movement, as its effect is always multiplied by two. Another way in which the drop may be altered is by moving a pallet stone in or out. Moving a pallet stone does not change the drop on that stone, but on the opposite one.
Moving the receiver out increases the drop on the discharger; moving the discharger out increases it on the receiver; moving it in has the opposite effect. Drop is sometimes termed inside and outside shake, meaning that when a tooth has been released by the discharging stone and another locked on the receiving, if the pallets are then swung so as to almost -- not quite -- lock, the wheel may be moved to and fro. The locking corner is arrested by the receiving stone and the releasing corner of another tooth is arrested by the discharging stone. This is called the outside shake. With a tooth just locked on the discharging stone there will be three teeth embraced by the stones and the play between them is called the inside shake.
Figure 22-13 will illustrate what is meant by inside and outside shake. In both A and B the escapement in full lines shows the outside shake; that in dotted lines the inside shake. At A the lock is equal and the inside and outside shakes are equal, While at B the locks are unequal, making the outside shake close. In speaking of shake as applied to pallets, it is technically termed close inside and close outside, the former meaning that the drop is less on the discharging stone, the latter that it is less on the receiver.
The effect upon the drop -- shake -- of moving a pallet stone is clearly demonstrated by figures 22-16 and 22-17. In figure 22-16 the full line a gives the distance that embraces three escape wheel teeth when the escapement is properly locked on the discharging stone. The broken line b gives the distance when the receiving stone has been drawn out to the position shown in dotted lines. This proves that drawing out the receiving stone increases the inside shake. Figure 22-17 shows the effect of drawing out the discharging stone, which decreases the outside shake, but not to so great an extent as the effect of moving the receiving stone.
SEC. 413 -- Equi-distant Locking and Equi-distant Center EscapementIt is well that the student should learn the precise meaning of the above terms and the different conditions they produce. They apply solely to the pallet.
Figure 22-18 shows both forms. At A is shown equi-distant locking. The locking corners of both stones are at equal distances from the center of the pallet as indicated by the full circular line. This feature gives the escapement its name. The releasing corner of the receiving stone is nearer the center by the distance between the broken and full circle. The releasing corner of the discharging stone is farther from the center by the distance between the dotted and the full line circle; thus it will be seen that, while the locking corners are equi-distant, the releasing corners differ in distance by double the width of the stone.
At B is shown the equi-distant center escapement. In this drawing two full circular lines are drawn from the pallet center. The locking corner of the receiving stone and the releasing corner of the discharging stone are at equal distances from the pallet center. The same is the case with the locking corner of the discharging stone and the releasing corner of the receiving. This brings the centers of the impulse faces equi-distant from the pallet center, which gives this escapement its name.
As has been explained, the lift -- the angular impulse -- is measured from the pallet center. It is shown at A as indicated by lines embracing the angles 1 and 2. It will be noticed that the impulse face of the discharging stone forms a greater angle with its locking face than the impulse face of the receiving stone does with its locking face. At B the angles 3 and 4, determining the impulse angles of the stone, are the same as 1 and 2, but in this case the impulse faces form equal angles with the locking faces.
Another feature in connection with these escapements is that in the equi-distant locking the discharging stone must embrace a greater angle than the receiving. This peculiar feature will be demonstrated in the latter part of this Work when the subject of drafting an escapement is taken up.
Supplement to Section 409Students often have trouble understanding what is meant by draft (or draw, as it is more commonly called nowadays) and how draw differs from slide. Draw is a force which cannot be seen. It is created when the teeth of the escape wheel and the pallet stones are set at such an angle as to cause the tooth to pull (or draw) the fork against the banking.
The result of draw is slide (Sec. 411), which can be seen. A simple experiment will make this relationship clear.
You will need two unsharpened pencils and two pins (push pins preferred). Drill a small hole through the center of one pencil and just before the eraser cap of the other. Pin the two pencils at right angles on a piece of flat board or heavy cardboard as shown below:
If you now try to push Pencil A towards Pencil B, as indicated by the arrow, nothing happens, because the two opposing forces are equal and cancel each other.
If you new turn both pencils slightly, so they form less than right angle as shown above at the right and again put pressure on A, both pencils will move, with B sliding down A. What you have seen is slide. The force which caused it is draw.
The angle which causes draw is set by the manufacturer of the watch, The repairman is seldom justified in changing it and normally doesn.t. What appears sometimes as a lack of draw is usually the result of improper cleaning and oiling rather than an improper angle of draw. When you replace a pallet stone, therefore, you should fit it snugly in the slot Without sideplay in order to keep the correct amount of draw.
You can test the draw with the balance out of the movement by pushing the fork away from the banking with a pointed pegwood but not to the point of unlocking. If you now lift the pegwood and release the fork, it should return promptly to the banking pin.
When the balance is in the movement, you can test draw by turning the balance wheel so the jewel is outside the fork horns. While it is in this position, push the fork away from the banking with a pointed piece of pegwood or small broach until the guard dart contacts the roller. Now take away the pegwood. If draw is present, will at once return to the banking.
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