Watch and Clock Escapements.
by Anonymous.
PREFACE
Especially notable among the achievements of The Keystone in the field of horology were the three serials devoted to the lever, cylinder and chronometer escapements. So highly valued were these serials when published that on the completion of each we were importuned to republish it in book form, but we deemed it advisable to postpone such publication until the completion of all three, in order that the volume should be a complete treatise on the several escapements in use in horology. The recent completion of the third serial gave us the opportunity to republish in book form, and the present volume is the result. We present it to the trade and students of horology happy in the knowledge that its contents have already received their approval. An interesting addition to the book is the ill.u.s.trated story of the escapements, from the first crude conceptions to their present perfection.
WATCH AND CLOCK ESCAPEMENTS
CHAPTER I.
THE DETACHED LEVER ESCAPEMENT.
In this treatise we do not propose to go into the history of this escapement and give a long dissertation on its origin and evolution, but shall confine ourselves strictly to the designing and construction as employed in our best watches. By designing, we mean giving full instructions for drawing an escapement of this kind to the best proportions. The workman will need but few drawing instruments, and a drawing-board about 15" by 18" will be quite large enough. The necessary drawing-instruments are a T-square with 15" blade; a scale of inches divided into decimal parts; two pairs dividers with pen and pencil points--one pair of these dividers to be 5" and the other 6"; one ruling pen. Other instruments can be added as the workman finds he needs them.
Those enumerated above, however, will be all that are absolutely necessary.
[Ill.u.s.tration: Fig. 1]
We shall, in addition, need an arc of degrees, which we can best make for ourselves. To construct one, we procure a piece of No. 24 bra.s.s, about 5" long by 1" wide. We show such a piece of bra.s.s at _A_, Fig. 1. On this piece of bra.s.s we sweep two arcs with a pair of dividers set at precisely 5", as shown (reduced) at _a a_ and _b b_. On these arcs we set off the s.p.a.ce held in our dividers--that is 5"--as shown at the short radial lines at each end of the two arcs. Now it is a well-known fact that the s.p.a.ce embraced by our dividers contains exactly sixty degrees of the arcs _a a_ and _b b_, or one-sixth of the entire circle; consequently, we divide the arcs _a a_ and _b b_ into sixty equal parts, to represent degrees, and at one end of these arcs we halve five s.p.a.ces so we can get at half degrees.
[Ill.u.s.tration: Fig. 2]
Before we take up the details of drawing an escapement we will say a few words about "degrees," as this seems to be something difficult to understand by most pupils in horology when learning to draw parts of watches to scale. At Fig. 2 we show several short arcs of fifteen degrees, all having the common center _g_. Most learners seem to have an idea that a degree must be a specific s.p.a.ce, like an inch or a foot. Now the first thing in learning to draw an escapement is to fix in our minds the fact that the extent of a degree depends entirely on the radius of the arc we employ. To aid in this explanation we refer to Fig. 2. Here the arcs _c_, _d_, _e_ and _f_ are all fifteen degrees, although the linear extent of the degree on the arc _c_ is twice that of the degree on the arc _f_. When we speak of a degree in connection with a circle we mean the one-three-hundred-and-sixtieth part of the periphery of such a circle. In dividing the arcs _a a_ and _b b_ we first divide them into six s.p.a.ces, as shown, and each of these s.p.a.ces into ten minor s.p.a.ces, as is also shown. We halve five of the degree s.p.a.ces, as shown at _h_. We should be very careful about making the degree arcs shown at Fig. 1, as the accuracy of our drawings depends a great deal on the perfection of the division on the scale _A_. In connection with such a fixed scale of degrees as is shown at Fig. 1, a pair of small dividers, constantly set to a degree s.p.a.ce, is very convenient.
MAKING A PAIR OF DIVIDERS.
[Ill.u.s.tration: Fig. 3]
To make such a pair of small dividers, take a piece of hard sheet bra.s.s about 1/20" thick, " wide, 1" long, and shape it as shown at Fig.
3. It should be explained, the part cut from the sheet bra.s.s is shown below the dotted line _k_, the portion above (_C_) being a round handle turned from hard wood or ivory. The slot _l_ is sawn in, and two holes drilled in the end to insert the needle points _i i_. In making the slot _l_ we arrange to have the needle points come a little too close together to agree with the degree s.p.a.ces on the arcs _a a_ and _b b_. We then put the small screw _j_ through one of the legs _D""_, and by turning _j_, set the needle points _i i_ to exactly agree with the degree s.p.a.ces. As soon as the points _i i_ are set correctly, _j_ should be soft soldered fast.
The degree s.p.a.ces on _A_ are set off with these dividers and the s.p.a.ces on _A_ very carefully marked. The upper and outer arc _a a_ should have the s.p.a.ces cut with a graver line, while the lower one, _b b_ is best permanently marked with a carefully-made p.r.i.c.k punch. After the arc _a a_ is divided, the bra.s.s plate _A_ is cut back to this arc so the divisions we have just made are on the edge. The object of having two arcs on the plate _A_ is, if we desire to get at the number of degrees contained in any arc of a 5" radius we lay the scale _A_ so the edge agrees with the arc _a a_, and read off the number of degrees from the scale. In setting dividers we employ the dotted s.p.a.ces on the arc _b b_.
DELINEATING AN ESCAPE WHEEL.
[Ill.u.s.tration: Fig. 4]
We will now proceed to delineate an escape wheel for a detached lever.
We place a piece of good drawing-paper on our drawing-board and provide ourselves with a very hard (HHH) drawing-pencil and a bottle of liquid India ink. After placing our paper on the board, we draw, with the aid of our T-square, a line through the center of the paper, as shown at _m m_, Fig. 4. At 5" from the lower margin of the paper we establish the point _p_ and sweep the circle _n n_ with a radius of 5". We have said nothing about stretching our paper on the drawing-board; still, carefully-stretched paper is an important part of nice and correct drawing. We shall subsequently give directions for properly stretching paper, but for the present we will suppose the paper we are using is nicely tacked to the face of the drawing-board with the smallest tacks we can procure. The paper should not come quite to the edge of the drawing-board, so as to interfere with the head of the T-square. We are now ready to commence delineating our escape wheel and a set of pallets to match.
The simplest form of the detached lever escapement in use is the one known as the "ratchet-tooth lever escapement," and generally found in English lever watches. This form of escapement gives excellent results when well made; and we can only account for it not being in more general use from the fact that the escape-wheel teeth are not so strong and capable of resisting careless usage as the club-tooth escape wheel.
It will be our aim to convey broad ideas and inculcate general principles, rather than to give specific instructions for doing "one thing one way." The ratchet-tooth lever escapements of later dates have almost invariably been constructed on the ten-degree lever-and-pallet-action plan; that is, the fork and pallets were intended to act through this arc. Some of the other specimens of this escapement have larger arcs--some as high as twelve degrees.
PALLET-AND-FORK ACTION.
[Ill.u.s.tration: Fig. 5]
We ill.u.s.trate at Fig. 5 what we mean by ten degrees of pallet-and-fork action. If we draw a line through the center of the pallet staff, and also through the center of the fork slot, as shown at _a b_, Fig. 5, and allow the fork to vibrate five degrees each side of said lines _a b_, to the lines _a c_ and _a c"_, the fork has what we term ten-degree pallet action. If the fork and pallets vibrate six degrees on each side of the line _a b_--that is, to the lines _a d_ and _a d"_--we have twelve degrees pallet action. If we cut the arc down so the oscillation is only four and one-quarter degrees on each side of _a b_, as indicated by the lines _a s_ and _a s"_, we have a pallet-and-fork action of eight and one-half degrees; which, by the way, is a very desirable arc for a carefully-constructed escapement.
The controlling idea which would seem to rule in constructing a detached lever escapement, would be to make it so the balance is free of the fork; that is, detached, during as much of the arc of the vibration of the balance as possible, and yet have the action thoroughly sound and secure. Where a ratchet-tooth escapement is thoroughly well-made of eight and one-half degrees of pallet-and-fork action, ten and one-half degrees of escape-wheel action can be utilized, as will be explained later on.
We will now resume the drawing of our escape wheel, as ill.u.s.trated at Fig. 4. In the drawing at Fig. 6 we show the circle _n n_, which represents the periphery of our escape wheel; and in the drawing we are supposed to be drawing it ten inches in diameter.
We produce the vertical line _m_ pa.s.sing through the center _p_ of the circle _n_. From the intersection of the circle _n_ with the line _m_ at _i_ we lay off thirty degrees on each side, and establish the points _e f_; and from the center _p_, through these points, draw the radial lines _p e"_ and _p f"_. The points _f e_, Fig. 6, are, of course, just sixty degrees apart and represent the extent of two and one-half teeth of the escape wheel. There are two systems on which pallets for lever escapements are made, viz., equidistant lockings and circular pallets.
The advantages claimed for each system will be discussed subsequently.
For the first and present ill.u.s.tration we will a.s.sume we are to employ circular pallets and one of the teeth of the escape wheel resting on the pallet at the point _f_; and the escape wheel turning in the direction of the arrow _j_. If we imagine a tooth as indicated at the dotted outline at _D_, Fig. 6, pressing against a surface which coincides with the radial line _p f_, the action would be in the direction of the line _f h_ and at right angles to _p f_. If we reason on the action of the tooth _D_, as it presses against a pallet placed at _f_, we see the action is neutral.
[Ill.u.s.tration: Fig. 6]
ESTABLISHING THE CENTER OF PALLET STAFF.
[Ill.u.s.tration: Fig. 7]
With a fifteen-tooth escape wheel each tooth occupies twenty-four degrees, and from the point _f_ to _e_ would be two and one-half tooth-s.p.a.ces. We show the dotted points of four teeth at _D D" D""D"""_.
To establish the center of the pallet staff we draw a line at right angles to the line _p e"_ from the point _e_ so it intersects the line _f h_ at _k_. For drawing a line at right angles to another line, as we have just done, a hard-rubber triangle, shaped as shown at _C_, Fig. 7, can be employed. To use such a triangle, we place it so the right, or ninety-degrees angle, rests at _e_, as shown at the dotted triangle _C_, Fig. 6, and the long side coincides with the radial line _p e"_. If the short side of the hard-rubber triangle is too short, as indicated, we place a short ruler so it rests against the edge, as shown at the dotted line _g e_, Fig. 7, and while holding it securely down on the drawing we remove the triangle, and with a fine-pointed pencil draw the line _e g_, Fig. 6, by the short rule. Let us imagine a flat surface placed at _e_ so its face was at right angles to the line _g e_, which would arrest the tooth _D""_ after the tooth _D_ resting on _f_ had been released and pa.s.sed through an arc of twelve degrees. A tooth resting on a flat surface, as imagined above, would also rest dead. As stated previously, the pallets we are considering have equidistant locking faces and correspond to the arc _l l_, Fig. 6.
In order to realize any power from our escape-wheel tooth, we must provide an impulse face to the pallets faced at _f e_; and the problem before us is to delineate these pallets so that the lever will be propelled through an arc of eight and one-half degrees, while the escape wheel is moving through an arc of ten and one-half degrees. We make the arc of fork action eight and one-half degrees for two reasons--(1) because most text-books have selected ten degrees of fork-and-pallet action; (2) because most of the finer lever escapements of recent construction have a lever action of less than ten degrees.
LAYING OUT ESCAPE-WHEEL TEETH.
To "lay out" or delineate our escape-wheel teeth, we continue our drawing shown at Fig. 6, and reproduce this cut very nearly at Fig. 8.
With our dividers set at five inches, we sweep the short arc _a a"_ from _f_ as a center. It is to be borne in mind that at the point _f_ is located the extreme point of an escape-wheel tooth. On the arc _a a_ we lay off from _p_ twenty-four degrees, and establish the point _b_; at twelve degrees beyond _b_ we establish the point _c_. From _f_ we draw the lines _f b_ and _f c_; these lines establishing the form and thickness of the tooth _D_. To get the length of the tooth, we take in our dividers one-half a tooth s.p.a.ce, and on the radial line _p f_ establish the point _d_ and draw circle _d" d"_.
To facilitate the drawing of the other teeth, we draw the circles _d" c"_, to which the lines _f b_ and _f c_ are tangent, as shown. We divide the circle _n n_, representing the periphery of our escape wheel, into fifteen s.p.a.ces, to represent teeth, commencing at _f_ and continued as shown at _o o_ until the entire wheel is divided. We only show four teeth complete, but the same methods as produced these will produce them all. To briefly recapitulate the instructions for drawing the teeth for the ratchet-tooth lever escapement: We draw the face of the teeth at an angle of twenty-four degrees to a radial line; the back of the tooth at an angle of thirty-six degrees to the same radial line; and make teeth half a tooth-s.p.a.ce deep or long.
[Ill.u.s.tration: Fig. 8]
We now come to the consideration of the pallets and how to delineate them. To this we shall add a careful a.n.a.lysis of their action. Let us, before proceeding further, "think a little" over some of the factors involved. To aid in this thinking or reasoning on the matter, let us draw the heavy arc _l_ extending from a little inside of the circle _n_ at _f_ to the circle _n_ at _e_. If now we imagine our escape wheel to be pressed forward in the direction of the arrow _j_, the tooth _D_ would press on the arc _l_ and be held. If, however, we should revolve the arc _l_ on the center _k_ in the direction of the arrow _i_, the tooth _D_ would _escape_ from the edge of _l_ and the tooth _D""_ would pa.s.s through an arc (reckoning from the center _p_) of twelve degrees, and be arrested by the inside of the arc _l_ at _e_. If we now should reverse the motion and turn the arc _l_ backward, the tooth at _e_ would, in turn, be released and the tooth following after _D_ (but not shown) would engage _l_ at _f_. By supplying motive to revolve the escape wheel (_E_) represented by the circle _n_, and causing the arc _l_ to oscillate back and forth in exact intervals of time, we should have, in effect, a perfect escapement. To accomplish automatically such oscillations is the problem we have now on hand.
HOW MOTION IS OBTAINED.
In clocks, the back-and-forth movement, or oscillating motion, is obtained by employing a pendulum; in a movable timepiece we make use of an equally-poised wheel of some weight on a pivoted axle, which device we term a balance; the vibrations or oscillations being obtained by applying a coiled spring, which was first called a "pendulum spring,"
then a "balance spring," and finally, from its diminutive size and coil form, a "hairspring." We are all aware that for the motive power for keeping up the oscillations of the escaping circle _l_ we must contrive to employ power derived from the teeth _D_ of the escape wheel. About the most available means of conveying power from the escape wheel to the oscillating arc _l_ is to provide the lip of said arc with an inclined plane, along which the tooth which is disengaged from _l_ at _f_ to slide and move said arc _l_ through--in the present instance an arc of eight and one-half degrees, during the time the tooth _D_ is pa.s.sing through ten and one-half degrees. This angular motion of the arc _l_ is represented by the radial lines _k f"_ and _k r_, Fig. 8. We desire to impress on the reader"s mind the idea that each of these angular motions is not only required to be made, but the motion of one mobile must convey power to another mobile.
In this case the power conveyed from the mainspring to the escape wheel is to be conveyed to the lever, and by the lever transmitted to the balance. We know it is the usual plan adopted by text-books to lay down a certain formula for drawing an escapement, leaving the pupil to work and reason out the principles involved in the action. In the plan we have adopted we propose to induct the reader into the why and how, and point out to him the rules and methods of a.n.a.lysis of the problem, so that he can, if required, calculate mathematically exactly how many grains of force the fork exerts on the jewel pin, and also how much (or, rather, what percentage) of the motive power is lost in various "power leaks," like "drop" and lost motion. In the present case the mechanical result we desire to obtain is to cause our lever pivoted at _k_ to vibrate back and forth through an arc of eight and one-half degrees; this lever not only to vibrate back and forth, but also to lock and hold the escape wheel during a certain period of time; that is, through the period of time the balance is performing its excursion and the jewel pin free and detached from the fork.