THE ANCHOR ESCAPEMENT
by Walt Odets
When we speak today about the escapement of a wristwatch we are, almost invariably, referring to the double roller Swiss anchor escapement. It is the development and refinement of this escapement which, in the nineteenth century, vaulted the Swiss into an indisputable position of dominance in the world of watchmaking. Today, this escapement has proven itself an accurate, reliable, and durable design through literally trillions of beats. It is now found in virtually all production wristwatches.
Although it is well known that the escapement regulates the rate of the watch, the relatively simple principles by which it does so are not widely understood. While the specific geometries and details of the anchor escapement are complex and demanding, the concepts are not.
WHAT IS A WATCH?
A watch is a complicated version of an hourglass timer. The watch is more useful because it is capable of the measurement of time over longer periods than a reasonably sized hour glass would be and because the watch need not be kept upright. Instead of using sand to pass through an orifice, the watch uses a piece of resilient metal--the mainspring--that is wound up and allowed to unwind. Time is kept by indicating with a visible display, essentially, how far this mainspring has unwound, just as the hourglass measures how much sand has passed through the orifice.
All watches, regardless of other possible complications, share a mere six components to accomplish the job of "keeping time." The wound mainspring powers the mainspring barrel (off the lower right corner of Figure 1) which drives the center wheel
(#5, Figure 1). The center wheel drives the third wheel (4), which drives the fourth wheel
(3), which drives the escape wheel (1). The center wheel is geared to turn once per hour and the minute hand is attached to the extension of its pivot. The fourth wheel rotates once per minute, and the seconds hand is attached to the extension of its pivot. (The hour hand is driven off the minute hand via a 12:1 gear behind the dial--the motion works--and rotates, of course, once very 12 hours).
If the watch ended with these five components (mainspring barrel, center wheel, third wheel, fourth wheel, and escape wheel) the mainspring would unwind, but in a matter of a few seconds. It is, thus, the sixth component of all watches--the escapement, comprised of balance wheel and escape lever (and, properly speaking, the escape wheel itself)--that controls how fast the mainspring unwinds. Figure 2 shows a watch with the balance wheel removed, and the lower balance pivot at 8. The escape lever can be seen at 2, its pivot at 3. The small fork of the lever
(at 2) is engaged by the balance wheel, which rocks the lever back and forth on its bearing. At 5 and 6 the two pallet jewels of the escape lever are visible--the entry and exit pallets, respectively. It can be seen that in this photograph the lever is rocked counter-clockwise on its bearing and the entry jewel
(5) has blocked one of the teeth on the escape wheel (1). This blocking action is also shown in the drawing at right. As with the majority of escape wheels, the one illustrated has 15 teeth, in this case of the "club foot" type, indicating the flattened, foot-shaped end surface. (English escape levers normally have straight teeth.)
THE SIMPLE CONCEPT OF THE ESCAPEMENT
The power of the mainspring travels through the five-component gear train, the last component of which is the escape wheel. The escape wheel stops and starts intermittently (and thus starts and stops the entire gear train causing it to "keep time") as first one pallet jewel stops the escape wheel, releases it, and then the other pallet jewel stops it. The balance wheel (via a jewel, the impulse pin) moves the lever and its pallets to and fro at regular intervals. Thus, it is the balance that times the lever's movements, and the lever that starts and stop the escape wheel. That's all there is to it, in concept at least. Those who wish no details of this operation need read no further.
THE DETAILS OF THE MECHANISM
That the to and fro action of the escape lever is accomplished with such accuracy and regularity--despite changes in temperature, position of the watch, inertial forces applied by the wearer's movements, and shock--is all in the details of design, construction, and adjustment of the escapement.
The following description of the full escapement references Figures 3, 4, and 5, which are all consistently numbered for any given part. All pivots are drawn in yellow, all jewels in red.
(Figure 3 is from the Omega Watch Co.) The 15 tooth, club-foot escape wheel itself can be seen at 5, with its pinion at 6. This pinion is driven by the fourth wheel. The lever
(7) carries the pallets and pallet jewels at one end (9 and 10) and the small fork at the other. The two angled projections on the small fork are called horns, the area between the parallel sides of the small fork, the notch.
The balance staff
(1, 2, 3, and 4 and also Figure 5) carries three major components. The first is the balance wheel itself
(1); the second, the impulse roller (3); and the third, the safety roller
(4). The impulse roller carries the impulse pin (3A, also known as the ruby pin or the balance pin) that engages in the notch of the lever and rocks the lever to and fro as the balance wheel oscillates in each direction. The safety roller has a crescent shaped cut-out that the guard pin occupies when the balance is centered on the lever. When the balance is rotated away from center (in either direction), the space provided by the crescent cut-out is not available for the guard pin, and thus the lever cannot move laterally and accidentally release the escape wheel. With the crescent rotated away from center, the guard pin hits the edge of the safety roller, blocking the movement of the guard pin and thus the lever. This is necessary to prevent unwanted release of escape teeth while hand-setting backwards and with shock to the watch.
Finally, in our detailed description of the escapement, are the banking pins
(Figure 6). They stand on either side of the lever to limit the lever's travel. The banking pins thus help determine the exact engagement of the pallet jewels with the escape teeth. Instead of banking pins, a solid banking may be used
(shown in Figure 6). While banking pins are easier to adjust, some feel that solid bankings are more stable. One of the requirements of the Geneva Seal (awarded by the Canton of Geneva to watches produced in Geneva and meeting the standards of the Seal), is solid bankings. These may be adjusted only by shaving metal in the plate or creating notches which allow the contacting surface to be bent.
WHY THIS ALL WORKS: THE ANGLES
Figure 7 is included, not so much for the details, but to suggest the real complexity of the anchor escapement if it is to work properly. The angles of escape wheel tooth surfaces, pallets, pallet jewels, and the lever must be exact in the excellent watch. Pallet jewel engagement and release must be precise, smooth, and almost absolutely consistent. A difference as small as .05 mm in the dimensions or alignment of a pallet jewel makes the difference between a pallet properly engaging an escape tooth and failing to engage it consistently. Should a pallet fail to engage properly on only one out of 1,000 cycles, the error in time-keeping would amount to hours per day.
Furthermore, pallet jewels do much more than simply block and release the teeth of the escape wheel. On release, the pallet jewel actually propels the balance wheel on its travel in the opposite direction. Each pallet jewel has both a locking face and an impulse face as shown in the figure at left. As the tip of the pallet stone begins to engage the tooth of the escape wheel, the geometries of all components insure that the rotation of the escape wheel (and pressure of the tooth on the locking face of the jewel) draws the pallet down and into firmer contact with the tooth. This is termed the draw. The slight downward motion of the draw causes the run to the banking, in which the lever contacts its banking pin or solid banking. As the impulse pin on the impulse roller comes around on the return swing of the balance and contacts the notch in the lever, the pallet stone is freed from the lock and the escape wheel continues its rotation. As illustrated in the figure at right, the tip of the escape tooth contacts the impulse face of the releasing pallet stone, and the angle of stone and tooth creates an upward push on the stone. This is known as the impulse. Via the lever, lever notch, and impulse pin, the impulse propels the balance wheel in the opposite direction swing. The flattened end surface of the club-foot escape tooth is thought to distribute the impulse force more evenly on the impulse face of the jewel. Note that both the entry and exit pallet stones go through a cycle of engagement, draw, run to the banking, and release. As one stone is releasing, the other is coming into position for engagement. To see the entire escapement cycle in a set of six detailed drawings click here.
THE BALANCE SPRING
I have left the balance spring (also known as the "hairspring") for last because, in some senses, it is the heart of the escapement and its most
subtle component. The illustration at left shows a balance cock (a cock is a bridge attached to the movement at only one end) with spring and balance attached. The assembly is inverted (upside down) from its position in the watch . Item 1 is the impulse roller carrying the impulse pin
(4). The safety roller (2) with its crescent lies just below (above, in the watch) the lower balance pivot
(3). The three locating pins at 6 simply position the balance cock accurately on the movement plate. The large hole between the locating pins is for the single screw holding the balance cock to the movement. The slot cut to the left of the screw hole allows the insertion of a 1.0 mm screwdriver to pry the cock loose of the movement plate after the screw has been removed. The balance spring
(5) lies between the balance and the cock.
At its inner end, the balance spring is attached in a small slot in the collet (French for collar) on the balance staff as illustrated at right (spring in red). The large collet slot is used to release the collet from the balance staff. As both the balance wheel and collet are rigidly attached to the balance shaft, this end of the spring rotates back and forth with the balance.
The outer end of the spring is attached to the spring stud on the balance cock. The stud is a screwed clamp rigidly attached to the cock, often indicated with a small engraved triangle on the cock. Alternatively, the stud may be part of a movable assembly mounted on the balance cock known as a movable or adjustable stud. In either case, this end of the spring is stationary with regard to operation of the balance wheel. As illustrated in Figure 8, the stud is seen at 1, and the last outer turn of the spring at 2. The spring attaches on the underside of the stud at 3. The collet for the inner attachment is at 4.
Some watches are said to have flat hairsprings, which means that the last outer coil of the spring (before it attaches to the stud on the balance cock) is at the same level as the other coils. (This has nothing to do with the cross-section of the spring itself, as all hairsprings are of a flat or rectangular cross-section.) When the hairspring is not "flat," this indicates the use of an overcoil hairspring as illustrated below right. The overcoil hairspring is often referred to as a Breguet overcoil. The Breguet overcoil may be configured in any of hundreds of shapes, the best known of which are the Lossier curve and the Phillips curve. The illustration below right shows a Phillips curve, with its characteristic "flat" spot. The Lossier curve is fully radiused. This illustration also shows the outer stud attachment of the spring at the blue triangle (which is actually part of the balance cock, not shown in this illustration); and the inner attachment at the collet in green. The spring is seen entering the collet at the green arrow.
The balance spring has a single function. Once the impulse from the exiting pallet jewel has propelled the balance in one direction, the spring reverses the direction of the balance at the end of its swing. As the balance swings counter-clockwise, it unwinds the spring (in most watches); as the balance swings clockwise, it winds the spring. The tension produced in the spring (in either direction) reverses the travel of the balance. In performing this operation, the balance spring is responsible for the arc of the swing (amplitude) and thus for the entire timing and accuracy (i.e. consistency) of the movement. The swing of the balance times the action of the escape lever, and thus the rotation of the escape wheel and the rate at which the mainspring unwinds. Normal amplitude in a contemporary watch is about one and one-half turns, or about 270 degrees (a "turn" is 180 degrees in discussing balance wheels). If the amplitude is too small, the watch will run weakly and irregularly; if too large, knocking is risked. Knocking occurs when the impulse pin rotates completely around and hits the outside of the lever.
ADJUSTING THE BALANCE SPRING
Adjustments to the balance spring are a very specialized task usually performed by specially-skilled watchmakers known as timers or vibrators. These adjustments are done after basic adjustments to the balance wheel itself. While I have discussed the balance wheel in a previous Horologium article, suffice it to say here that the wheel must be perfectly round and poised (i.e., in balance, without heavy points). Unavoidable residual errors in the poise of the balance, however, may be partly compensated for in adjustment of the balance spring.
Adjustment of the spring--often known as timing--has four primary objectives, all related to obtaining either accuracy or absolute rate:
(1) To have the spring extend and contract symmetrically from the exact center of its length (i.e. the center of the center coil) in order to compensate for the effects of gravity on the spring, which "sags" under its own weight and thus introduces running differences in different positions of the watch. This is a matter of accuracy.
(2) To maximize the isochronism of the movement so that balance swings all take (as nearly as possible) the same amount of time regardless of the arc traveled. This is also a matter of accuracy.
(3) To adjust the spring so that the watch is "in beat," meaning that the time between the impulse pin hitting the small fork notch traveling in one direction is equal to the interval when the impulse pin is traveling in the other direction. (This is approximated by having the impulse pin centered on the small fork with the balance at rest). This is also a matter of accuracy. And
(4) adjustment of the spring to assure that the absolute rate of the watch is correct (i.e. the watch maintains correlated to a time standard). Note that accuracy is, far and away, the complex issue. Without accuracy the matter of absolute rate is, in any case, moot.
All of these objectives are accomplished through relatively simple, if sometimes elusive adjustments of the balance spring. These include rotating the collet to change the position of the inner spring attachment; lengthening or shortening the spring at the outer stud , either by changing the position of a movable stud or by loosening the stud clamp of a stationary stud and moving the spring slightly
(8, Figure 9); and by changing the radius and shape of the overcoil
(9) or, in the case of a flat hairspring, the radius and shape of the outmost coil. These are the only spring adjustments available for centering the spring, compensating it for gravity and positional effects, achieving isochronism, and adjusting beat.
The last adjustment available on many watches is accomplished with the regulator,
(1, 5, and 6, Figure 9) which is used to adjust daily rate after all other adjustments have been made. The regulator index
(1) points to markings on a scale engraved on the balance cock for visual reference during adjustment. The index is attached to the regulator ring
(5) which carries the curb pins
(6) which slide along the length of the spring as the regulator is moved
(illustrated right). Sometimes the adjustment is made simply by moving the index, which can be difficult to do with precision, particularly for very small adjustments.
In the case of the swans-neck precision regulator illustrated, the regulator is moved by turning the finely-threaded screw
(4 and 4A), against which the regulator is held by the swan's-neck spring
(2). Such an arrangement provides a minimum of "backlash" in the system. The movement of the curb pins changes the effective or resonant length of the spring. "Shortening" the spring speeds up the rate; lengthening slows it. Unlike the plain curb pins illustrated, some carry guards which prevent other spring coils from becoming engaged in the regulator. Regulation may also slightly disturb beat, and this will often require some adjustment after a rate adjustment. However, some regulator designs such as the Triovis
(illustrated left), automatically compensate for beat during regulation. The precision adjustment screw is seen at 1, and the beat compensation block at 3. The curb pin carrier is seen at 2.
As simple as the concepts are, the modern double-roller anchor escapement is a remarkable achievement. In its most common current form, running at 28,800 beats per hour, a pallet jewel engages and releases an escape tooth 691,200 times per day. Any disturbance is any part of the mechanism, even of miniscule proportions, can have dramatic effects on accuracy. For example, any change in the balance spring, balance pivots, lever, or impulse action of the pallets equivalent to only a one percent change in the inertia of the balance wheel will produce about a seven and one half minute error over 24 hours. This is equivalent to a change in the effective length of the balance spring of only 1.2 mm.
Perhaps as remarkable as the accuracy of the anchor escapement is its reliability and durability. In the four years between service, the 28,800 beat-per-hour watch will have completed about 505 million complete cycles of the escapement: more than one billion engagements and impulses from each pallet and an equivalent number of swings of the balance wheel.