Description Of A Watch Movement
( Originally Published 1918 )
IT is not proposed in this book to enter into the question of the history of the watch, nor to discuss who invented this or that portion of it, but simply to take the modern watch as it is, and describe as dearly as possible how it works and how to repair and keep it in order.
It will, perhaps, be well first to describe, in general terms, the mechanism of a watch, and for this purpose a Geneva. " bar" movement will be used as an illustration. Fig. I shows such a movement. The term " movement," it may be explained, is applied to the works of a watch as distinguished from the case.
This particular movement is chosen, as its " bar " construction enables all the wheelwork to be seen. The mechanism of this movement may be divided into four portions. First, the motive power ; second, a train of wheels to transmit the power; third, an escapement and balance to control the power; and fourth, motion work and hands to record the revolutions of the train wheels upon the dial.
The Motive Power. This, in all watches, is a main-spring. A mainspring is a thin and flat strip of steel, hardened and tempered to give the maximum of strength and elasticity. It is coiled up around a steel centre arbor, to which its eye is hooked, and enclosed in a box or " barrel," to the inside of which its outer end is attached. If a barrel, containing such a spring, be held firmly, while the centre arbor is turned round, coiling up the spring tightly around it, until the Outer end pulls hard at its attachment, as at A (Fig. 2), the barrel, when released, will revolve in the direction that the spring pulls it, until the spring has unwound itself and is prevented by the containing barrel from unwinding further, as at B (Fig. 2). The number of complete revolutions thus made by a watch barrel with an average mainspring is five, and the number of revolutions used in driving the watch for twenty-four hours is generally three, thus leaving two to spare.
There are three principal methods of making a mainspring drive a watch. The first method, and the one adopted in the movement illustrated in Fig. r, is to make the barrel into a toothed wheel by cutting teeth around its circumference. The barrel then becomes the first or "great wheel " of the watch train. This is termed a " going barrel." In a watch with this arrangement the barrel arbor is squared, and to wind the watch a key is placed upon it, and it is turned round three or four complete revolutions, being held by " clickwork." During the going of the watch, the barrel arbor is stationary and the barrel turns round, hence the term "going barrel." "Clickwork" is the name given by watchmakers to an arrangement of a ratchet and pawl, the latter, in watches, being termed a "click." Fig. 3 shows a clickwork arrangement. In the figure A is the ratchet, B the click, and C the click- spring. In many watches the click and spring are in one piece, as in Fig. I, but the action remains the same. - In the second method the barrel is stationary while the arbor revolves, carrying with it a separate toothed wheel. In some watches on this plan the barrel is turned clickwork. round in the act of winding the watch, and in others it is merely a sink recessed out in the solid watch plate, and, of course, a fixture. This method of driving is used mainly in American watches. The third method is for the barrel to be merely a drum, driving the watch by means of a chain wound upon it. This indirect and some-what unsatisfactory method was adopted in order to equalize the force or pull of the spring. When a mainspring is fully wound up it exerts its maximum force. As it unwinds the force becomes gradually less and less, until it is zero. In the old watches the force of the mainspring directly affected the timekeeping of the watch, hence it was necessary to introduce some arrangement to equalize it. The arrangement adopted is shown in Fig. 4. A is the barrel containing the mainspring, B is the chain, C is the " fusee." The fusee is a cone-shaped pulley having a continuous spiral groove cut upon it. The chain runs in this groove, When the spring is wound up the chain is on the fusee and pulls at its smallest diameter, thus exerting but a small leverage upon the fusee, to which is attached the first or main wheel of the watch train. Fig. 4 shows the arrangement when wound up. As it unwinds the barrel revolves and unwinds the chain from the fusee, coiling it up on itself. During this process the chain gets lower and lower upon the fusee body until, when nearly run down, it pulls upon the largest diameter of the cone, thus giving the diminished force of the mainspring an advantage in leverage. If the proportions of the cone are suited to the mainspring, it is possible by this means to have a constant force driving the watch throughout the twenty-four hours. With verge watches the fusee was a necessity. Though foreign makers quickly found that with all other kinds of watches the fusee was unnecessary, English makers, almost without exception, continued using it with lever watches for many years, and some use it now. In marine chronometers it is still in use.
The Train. The mainspring thus, either directly or in-directly, drives the main wheel, which is the first wheel of the watch train. This wheel, in an average watch, turns once in eight hours. It gears into the centre pinion of the watch, causing the latter to revolve eight times to once of the main wheel, and thus turn once in one hour. This is effected by the main wheel having eight times as many teeth as the centre pinion has leaves. The centre pinion, as its name implies, occupies the centre of the watch, and its axis or "arbor" projects through the dial, and has the minute hand affixed to it. Upon the same arbor, with the centre pinion, is the centre wheel, the second wheel of the train, centre wheel and pinion forming one and revolving together. In the same way, the centre wheel drives the third wheel and pinion, and causes the latter to revolve eight times in one hour, or one revolution in seven and a half minutes, the centre wheel having eight times as many teeth as the third pinion has leaves. The third wheel, again, drives the fourth wheel, and has seven and a half times as many teeth as the fourth pinion has leaves. The fourth wheel and pinion therefore perform one revolution in one minute. A prolongation of one pivot of the fourth pinion projects through the dial and carries the seconds hand. The fourth wheel, in its turn, drives the scape pinion and wheel, causing the latter to perform ten revolutions in one minute. This completes the watch train and brings us to the escapement. In Fig. I all these wheels are visible, and the above explanation can be followed by reference to them. The wheels in most watches are of hard brass; a few have German silver or nickel wheels, and a few have wheels of a special alloy combining lightness and strength, such as aluminium-bronze. The pinions which they drive are of fine quality steel, hardened and tempered. The axis of a wheel is, in watchwork, called its " arbor." The pinions of the train wheels are in one piece with their arbors. Upon the ends of the arbors fine pivots are turned. These run either in pivot holes drilled in the plates or bars, or else in jewel holes, to diminish friction and reduce wear. A jewel hole is a small circular plate of garnet, sapphire, or ruby let into the brass of the watch frame. It is perforated in its centre with a fine, true, and polished hole, in which the pivot runs. Fig. 5 shows a wheel and pinion running in jewel, holes. A is the wheel, B the pinion, C C the jewel holes.
The teeth of the wheels ana the leaves of the pinions are cut to very exact curves, so as to ensure a smooth and even motion when they are running together.
The Escapement and Balance. It is obvious that, given a mainspring and a train of wheels such as that just described, if the mainspring were wound up the train would run at full speed, the spring unwinding itself in a few moments. Some arrangement is therefore necessary to check it. In a watch this checking mechanism is termed an "escapement," the duty of the escapement being to allow only one tooth of the scape wheel to pass at a time, and that at perfectly regular intervals.
The duty of measuring and regulating the intervals is per-formed by the balance and hairspring. The balance is a fly-wheel, mounted upon an axis having extremely fine pivots running in jewel holes. It is controlled by a hairspring. The hairspring is a flat spiral of thin steel wire. Its inner end is affixed to a collet upon the axis of the balance. Its outer end is fixed to a stud rigidly fastened to some part of the watch frame. If a balance, mounted and fitted in this way, be given a turn round in one direction and then let go, it will return under the influence of the hairspring and go nearly as far again in the reverse direction until its force is spent. The spring, then, causes it to return again, and it will be kept vibrating for some time before it finally comes to rest. It, in fact, acts in much the same way as a pendulum, which, when set swinging, continues to swing, traversing a smaller and smaller arc each time, until it is brought to rest by friction at its point of suspension and the resistance of the air.
The short intervals of time (generally one-fifth of a second) thus measured by the balance and its spring are always very nearly equal, and, under some conditions, exactly equal, whatever the distance traversed by the balance may be.
The escapement divides up the power of the mainspring into small portions, and delivers these portions to the balance at each of its vibrations, giving it, as it were, a little helping push an " impulse " as it comes round each time.
Thus, through the medium of the escapement, the main-spring keeps the balance vibrating, and the balance regulates the running of the train.
The balance and hairspring are well seen in Fig. I.
The Motion Work. All the mechanism before de-scribed would be of little practical use unless the revolutions of the various wheels could be recorded in some way. The centre arbor, it has been seen, revolves once in one hour. Therefore a hand affixed to it will travel round the dial and serve to show the minutes. By gearing down from this arbor with a pair of wheels and pinions whose combined ratios are as 1 to 12, one pair being usually r to 4 and the other pair r to 3, and placing another hand upon the arbor of the last wheel, the hours r to 12 can be also shown in the usual way. These reducing wheels are termed the " motion work," and are not visible in Fig. r, being hidden between the dial and the watch plate.
This is a brief description of a simple form of watch movement, and it will easily be believed that a great many different materials are used in its construction and in the processes of repairing it ; also that it takes but a very little to upset its action or stop it altogether. Merely to enumerate all the possible faults and inaccuracies to which a watch is subject would fill many pages, and it is the purpose of this book to describe them all in detail and the way to overcome them.