• A mainspring for power
• The flattened barrel it sits in
• The center wheel that the barrel rim is geared to
• The third wheel that’s turned by the center wheel
• The fourth wheel that’s turned by the third wheel
• The escape wheel
• The lever
• The balance
• The balance spring
There are other ways of breaking it down –of course, we need at minimum a click to keep the spring from unwinding inside the barrel, we need two plates to hold the gears in place, we would prefer to have a few additional gears to turn the hands so we can actually tell the time. But these nine or so parts listed above are the real heart of a watch, and in terms of basic design, watches mechanically are in fact startlingly homogenous –which is perhaps the reason we spend as much time as we do talking about dials and straps.
Of the basic parts perhaps the four most critical are the balance, the balance spring, the lever, and the escape wheel.
The rest of the power train exists merely to get energy to the escape wheel. Basically the action of a lever watch is as follows:
The rotating balance as it turns, has a small jewel (the impulse jewel) mounted vertically near the “hub” of the wheel. As the balance turns the impulse jewel enters the notch of the lever, pushing the lever from one side to the other. Two things happen: the first is that the lever as it moves unlocks the escape wheel, allowing it to advance one tooth before it’s caught by one of the two lever jewels and locked again. The second, is that as the lever unlocks the escape wheel, the flat face of the escape wheel tooth slides along the flat face of the lever jewel, pushing the lever harder in the direction it’s already going. Since the lever is being pushed by the escape wheel at one end and is in turn interacting with the impulse jewel of the balance at the other end, the result is that the escape wheel pushes, or “impulses” the balance via the lever, giving energy to the balance wheel.
So it goes, the balance wheel swings, it moves the lever, the lever unlocks the escape wheel, the escape wheel pushes the lever, the lever pushes the balance wheel, and the balance wheel keeps swinging back and forth as long as there’s energy in the mainspring to turn the balance.
The impulse jewel continues to push the lever to the left as the balance rotates clockwise. Note that the impulse face of the escape wheel tooth on the right is now sliding along the face of the entry jewel; this sliding friction imparts energy to the lever and thus to the balance through the impulse jewel.
The lever strikes the banking pin on the left and stops as the impulse jewel begins to leave the notch of the pallet fork. The escape wheel tooth has been caught by the exit jewel, and the angle of their engagement produces "draw" which pulls the lever harder against its banking. Note also that the safety dart has passed through the semicircular cutout in the safety roller, which allows the lever to move from right to left without the dart striking the roller.
The lever escapement was invented only a few years ago as horological invention goes, in 1754, by one of the most famous English watchmakers of all time, Thomas Mudge, although like most good ideas it was more or less “in the air” and if Mudge hadn’t come up with it it’s likely someone else would have in another few decades. Mudge didn’t make a finished watch with a lever escapement until 1769, for Queen Charlotte, and the watch still exists in the royal collection.
The world's first lever escapement watch, by Thos. Mudge, 1769. Note the "beetle and poker" shape of the hands and the (for the time very unusual) center seconds hand.
The other two escapements that preceded it –the verge escapement and the cylinder escapement –could both be coaxed to give good performance, and in very exceptional watches often did but aspects of the design of both meant that accuracy was an uphill battle, and the lever was significant because it was, inherently, a better design –so much so that as I write, in 2007, 253 years after Mudge is credited with inventing it, basically the same design is what’s probably inside any watch you’ve got on your wrist. If you’re a watch lover that’s a pretty staggering fact. Think about this: James Watt, the Englishman generally credited with creating the first reasonably efficient steam engine, patented his first workable design in 1769 –the same year Mudge made the first lever watch for the Queen. Flash forward 253 years and ask yourself how you’d feel if the engine inside your car were, recognizably, a Watt reciprocating steam engine?
So why? Why has the lever escapement stayed in use, with steady but incrementally small refinements, for a quarter of a millennium? The answer, basically, is because it works. It’s an escapement whose action isn’t thrown off by physical movement, for one thing. A pendulum clock can be made to run within a few seconds a year in some cases, but try a pendulum in a wristwatch and the nasty glances of your family and co-workers as you show up late yet again for another appointment or gathering will pretty rapidly convince you of the futility of the experiment (in fact, pendulum clocks were tried at sea in an attempt to design a workable marine chronometer, but as the ships pitched and rolled in heavy seas the pendulum clocks performed predictably badly.)
How well can a properly adjusted lever watch run? Most sources agree that at most, for a really well made lever watch the variation in rate can be held down to perhaps three to five seconds per day and depending on your personal habits you might be lucky enough to experience a net gain or loss of only a few seconds a week.
Still, the human mind looks for improvements. The drive to improve on the lever escapement was essentially dead by the mid 20th century, and the introduction of the quartz watch –the first of which, the Seiko Astron, debuted in Christmas of 1969 –seemed to make the idea of improving on the lever watch as silly as developing a differential gearshaft for a coach and four. By the mid 1970’s the mechanical watch industry was very much on the ropes; collectors starting out today who take luxury watches for granted as a collectible are often too young to remember that 30 years ago the very idea of a fine mechanical watch was an absolutely laughable anachronism (and to our friends and relatives may very well still be.) Peculiarly enough, it was in 1975 that an obscure English watchmaker, a stubborn and plainspoken craftsman with an incurable love of mechanics, invented the first real challenge to the lever escapement’s supremacy –George Daniels developed the co-axial escapement.
Animation of the co-axial escapement. Hosted on Mark Headrick's Horology pages. To go there and look at other animations of many different types of escapements, click here.
The co-axial escapement was designed to remedy problems which limited the performance of the lever watch. The lever watch as we’ve seen keeps the balance swinging back and forth by passing energy from the escape wheel to the lever to the balance, but the escape wheel pushes the lever in a fairly inefficient way –as we have seen, a flat topped tooth on the escape wheel slides along a lever jewel (or pallet jewel to give it its proper name) and this sliding friction is not only a somewhat inefficient way to transfer energy, it’s also necessary to use a lubricating oil on the escape wheel teeth to reduce the friction as the tooth slides along the surface of the jewel, otherwise the already inefficient power transfer has even more power robbed from it and the back and forth swinging of the balance becomes anemic, feeble. Since the stability of the watch’s rate is directly dependent on even delivery of power to the balance, deterioration of oil on the escape wheel teeth –inevitable over time –means that the watch in a few years will start to show instability of rate. To the owner that means that a previously reliable timekeeper may seem to speed up or slow down inexplicably, and the only solution is to have the watch serviced –in other words, as the service indicator on the back of Urwerk watches makes plain, it’s time for an oil change.
Daniels’ invention eliminated, at least in its original form, the need for oil on the escape wheel teeth. In addition, the arrangement of the lever in his escapement meant that for at least one direction of the swing of the balance, impulse to the balance is given directly by the escape wheel rather than by a lever. Direct impulse means more efficient transfer of energy, and no oil means better stability of rate. All to the good but it was many years before a company finally adopted the co-axial in series produced watches. The risk was manifestly not worth the potential gains to watch companies in the rough years of the late 1970’s and early 1980’s and it was not until the mechanical renaissance was well along -1999 –that Omega finally introduced the first mass produced version of the co-axial in a wristwatch.
As good as the co-axial escapement undoubtedly was as a handmade craft mechanism by George Daniels, it has proven very challenging to make it a bulletproof escapement in a series produced watch, and since 1999 Omega has experimented with several different configurations, varying the rate and design until today, with the introduction this year of the in-house Omega automatic cal. 8500/8501 Omega has moved increasingly closer to Daniel’s original design. The incentive to distinguish one’s company through the demonstration of hard-core watchmaking inventiveness was on, however, and since the turn of the century we’ve seen an astonishing proliferation of new escapements as well as tantalizing patents which have surfaced here and there –Ulysse Nardin has the Dual Ulysse escapement whose ancestor is a dual impulse escapement invented by Breguet; the Triptyque Reverso by Jaeger LeCoultre introduced the ellipse isometer escapement; there are intriguing patent images from Patek Philippe, dalliances with silicon (silicium) escape wheels. . . perhaps more genuine radical horological experimentation than at any time since the early 19th century, when there was an explosion of inventiveness which gave rise to escapements like Robert Robin’s, Breguet’s echappement natural, and a whole host of others some of which lay fallow until being rediscovered and refined today –the ancestors of the modern generation of cutting edge escapements.
The Grand Synthesis: The Lever and Chronometer Detent Escapements
The Daniels escapement is intended to combine the advantages of a chronometer detent escapement and a lever escapement. The lever escapement offers a secure action –secure in the sense that it is relatively impervious to being disrupted by physical shocks. A strong jolt has the potential to shift the lever enough that it might actually unlock the escape wheel at the wrong moment. The lever escapement is prevented from unlocking at the wrong time by two factors that keep the lever firmly against its banking pin until it is moved by the only thing that should move it: the impulse jewel on the balance wheel entering the notch of the pallet fork.
These two factors are the “draw” of the escapement, and the “safety roller.”
“Draw” is the force created by the angle of the escape wheel’s tooth with the locking surface of the pallet stone. When the escape wheel is locked by the lever, this angle actually pulls, or draws, the lever firmly against its banking pin, and since the draw is maintained by the rotational force of the escape wheel the strength of the mainspring is directly related to the amount of draw (the other factor is the angle of the escape wheel tooth to the pallet stone.) A sharp angle and stronger mainspring will produce more draw and a safer action, but will also make it harder for the balance to shift the lever as the impulse jewel enters the notch of the lever as the balance, when unlocking, has to actually move the escape wheel backwards against the force of the mainspring, albeit by a very tiny distance. This does however represent some wasted energy and as a result constructing lever escapements with draw was originally resisted by watchmakers, though it is now a feature of all lever escapements.
The second factor preventing unlocking of the lever at the wrong time is the safety roller. The safety roller is a metal disk mounted on the balance wheel pivot which has a small cutout in it. On the lever, just under the notch of the pallet fork which receives the impulse jewel, there is a small prong of metal called a safety dart. If the balance unlocks the lever at the right moment the lever, as it swings from one banking pin to the other, (delivering impulse to the balance as it goes) swings the safety dart into the cutout on the safety roller, which receives it, allowing the lever to keep moving. If on the other hand a strong jolt unlocks the lever (against the energy of the draw) the safety dart will impact the safety roller before the lever has a chance to move far enough to unlock the escape wheel.
The chronometer escapement in its simplest form consists of an escape wheel which is “pushed” or given impulse, not by the intermediary of a lever, but rather by the escape wheel itself. The escape wheel is held in position and kept from turning until exactly the right moment, by a small “detent,” which is basically a blade of metal with a small flat jewel on it that blocks an escape wheel tooth. As the balance turns, it periodically shifts the metal blade slightly out of the way of the escape wheel teeth. This unlocks the escape wheel and allows it to advance one tooth, just as the impulse jewel on the balance is passing in front of the moving tooth, and so, impulse is delivered to the balance. As the escape wheel continues to turn, the metal blade flicks back into its original position and locks the escape wheel again.
Diagram of the chronometer detent escapement, Britten's Clock and Watchmaker's Handbook, 9th Edition, 1896
The chronometer escapement has two distinct advantages. The first is that it is a direct impulse escapement, in which impulse is given to the balance directly by the escape wheel and not through a lever. The advantage to this is that no energy is lost by moving a lever (which represents both inertia to be overcome, as well as a mechanical disadvantage.) The second is that since the contact between the escape wheel tooth and the impulse jewel is radial (perpendicular to the radius of a circle swept out by the impulse jewel) rather than sliding, no oil is necessary at the impulse surface.
The great problem with the chronometer escapement is that it is very vulnerable to shocks. Any strong jolt will tend to shift the detent and cause the escape wheel to accidentally unlock.
An ideal escapement would combine the secure action of the lever, with the radial direct impulse and lubrication free properties of the chronometer escapement. It is the synthesis of these two elements which the Daniels escapement was intended to produce.
Robert Robin’s Big Idea
The idea of synthesizing the two escapements is not a new one. In 1791 one Robert Robin, former horloger du roi (watchmaker to the king, a title which had at one time been one of tremendous prestige –an horloger du roi under Louis XIV could ignore guild rules prohibiting watchmakers from working on gold cases, for instance) developed an escapement which was intended to combine the best attributes of the lever and the chronometer detent escapement.
Late form of the Robin escapement. Note the circular cutout to allow the lever, which lies on the radius of the balance, to oscillate around the pinion of the escape wheel. Also note the very large counterbalances and generally large size of the lever itself. From Clutton and Daniels, Watches, 1965.
Known then and now simply as the Robin escapement, it enjoyed a brief vogue –even being adopted occasionally by Abraham Louis Breguet himself –before certain problems with the design caused it to fade into obscurity.
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