The seiko diver and 7s26 automatic movement

The Seiko Diver's 200 Meter SKX779 Featuring the 7S26 Automatic Movement Appended 1-1-2003

by John Davis (ei8htohms) Š 2-4-2002

(click on all images to view a larger version)

Introduction Watches are machines. While some of them may also be works of art, they cannot escape their machineness.

There is undoubtedly something fascinating about those examples of the watchmaker's craft, but there is also something to be learned from the droves of micro-machines that are designed and constructed with only performance and economy in mind. There is craft involved in the ability to engineer a movement for production runs in the tens of thousands that is wholly other than the craft involved in manufacturing a movement by hand. It is a skill that I respect and admire, while having even less understanding of its intricacies than I do of traditional watchmaking skills. Being a fan of Seiko's watches though, I won't let my ignorance get in the way of taking apart the 7S26 in an attempt to discover its hows and guess at its whys.

The 7S26 automatic movement is a logical step in Seiko's entry level mechanical movement line. Replacing the 7002 in their popular Diver's watches, it incorporates quickset day and date displays (the 7002 was date only), automatic bi-directional winding via Seiko's patented Magic Lever system and the lack of manual

winding capability that has become a signature of sorts in entry level automatics from Asia. The watch I will be dismantling for this exercise is the SKX779, a 200 Meter Diver's watch sold (exclusively?) in New Zealand and Australia.

The Case, Dial and Hands The SKX779 is a large watch. Its case is 41.5 millimeters across without the crown and approximately 12.5 millimeters thick including the domed crystal. It has a very pronounced, scalloped bezel with circular graining that is protected by an equally pronounced bezel-guard that extends upwards from the lugs.

While there are those that rightly question the functionality of this design (arguing that sand and dirt get into the space between them and cause the bezel to jam), it is undeniably this curious feature that attracted me to this watch. The bezel guard extends into the crown guard thanks to the location of the crown at the 4 o'clock position. This crown position is more comfortable for such a large watch and is a bit of a trademark with Seiko Diver's through the decades.

The dial of the SKX779 has an upward curving minutes chapter that gives it a wonderful depth. This effect is further enhanced by the domed crystal, which lamentably protrudes just slightly beyond the bezel (making it susceptible to scratches). A domed crystal is advantageous on a diver's watch as flat crystals can sometimes have a mirror effect under water.

Each of the three hands has a slightly different interpretation of a rocket-ship shape and when the hour and minute hand line up, the resemblance is pronounced. They are painted with an ecru color that matches the hour markers and both are filled with Lumibrite (Seiko's proprietary version of Super-Luminova) and, especially when brand new, glow as brightly as any watch I've seen. I'm always disappointed when watch companies use white day and date rings on a black dial and this is one of the few complaints I have about the classic Seiko Diver's. The color scheme of this watch's dial gets big points from me for the use of black rings with white letters/numbers. Taken altogether, the shape of the hands and case and the depth of the dial through the domed crystal have a vaguely ray-gun gothic effect that I find very appealing.

The strongest feature of this version of the Seiko Diver is the bracelet and clasp [1]. The solid link, brushed steel bracelet with polished accents is incredibly sturdy and well designed. It is heavy enough to balance the hefty case well on the wrist and has a wonderfully secure, two-button folding clasp with safety and a wet-suit extension. In addition to providing the peace-of-mind that this bracelet and clasp will in all likelihood never come off of your wrist accidentally (either by breaking or coming unhooked), it has the flexibility to be extended sufficiently to wear over a wetsuit at a moments notice. The links are held together by solid pins with sleeves that are a little tricky to remove and replace but not unduly so. Attaching this wonderful bracelet to the case are the two largest spring bars I've ever seen.

In addition to water-tightness, the ability to withstand abuse is a highly desirable feature for a utility watch of this nature. The Seiko Diver has several features that contribute significantly in this regard and a few of them are visible upon removing the solid steel back. The 7S26 uses Seiko's patented Diashock shock protection [3] on the balance pivots, has a soft, plastic spacer ring [4] closely integrated with the movement and a relatively low mass rotor [5] that is unlikely to bend or break even with very severe shocks. The plastic spacer ring, combined with the sheer massiveness of the case, provides a great deal of additional shock resistance and is a more economical solution than a metal spacer ring as well. This combination of economic and sensible engineering is a trend that persists in almost every facet of the design of the 7S26 movement.

Under the Dial

While there is often much disdain amongst watch enthusiasts for plastic components in mechanical wristwatches, I propose that there are instances where it is acceptable and possibly even preferable. One particular area in which plastic is a perfectly logical solution is the calendar mechanism. These are parts that rotate at very slow speeds (or sometimes intermittently) and with very little torque for the majority of their rotation. This combination of features makes them controversial with regards to lubrication. While lubricating them significantly will increase the drag on the movement and possibly ultimately stop the watch, leaving them sparsely lubricated or dry will ultimately result in wear. Plastic is an ideal solution for these components because it is light and self-lubricating. I won't pretend that Seiko's primary concern here is not one of economics, but it is combined with intelligent engineering as well.

The plastic parts in question are the quickset wheels, the intermediate calendar wheel and the calendar advance wheel [left]. The calendar advance wheel [8] has two plastic fingers to advance the date and day disks that will easily slip out of the way if the quickset is activated while the calendar is advancing. The calendar mechanism is secured under a very thin but nicely polished metal plate that is held in place with three standard screws and one Phillips head [7]. The presence of this one tiny Phillips screw in the movement is something of a mystery and along with the molded plastic and thin metal plates lends the bottom plate the appearance of a very well made calculator.

The first wheel in the quickset mechanism [9] is permanently attached to the underside of the calendar plate.

It's only after removing this thin plate that the date ring can be removed and, subsequently, the spacer ring.

Such close integration of the spacer ring with the rest of the movement is unique in my experience and sharply contrasts with the usual method of securing the spacer ring to the case. This novel arrangement conceivably contributes to the shock resistance by separating the movement and ring, as one unit, from the case.

After removing the calendar plate we can also observe the oddly shaped teeth of the clutch and quickset pinion [10], all the more visible because of the utter lack of keyless works on the bottom plate. Because there is no winding pinion (no manual winding capability), in its place is a quickset pinion. The square teeth of this pinion mesh with identical teeth on the clutch when the stem is in the second position and allow the quickset pinion to turn in either direction: clockwise to advance the date and counterclockwise to advance the day indicator. The second quickset intermediate wheel (the white plastic wheel with traditional teeth) then slides into engagement with either the date ring or the third intermediate wheel (with the wolf teeth) which advances the day disk. This is a very functional and robust quickset and calendar mechanism and, being

largely made of plastic components, requires no lubrication. Another thin plate holds the intermediate calendar wheel, c! alendar advance wheel and hour wheel in place and after removing them we can contemplate the top plate of the movement.

The Automatic System One of my favorite features of Seiko automatics is the Magic Lever winding system. Earlier versions of this winding system involved only three moving parts: the rotor, the Magic Lever and the pawl wheel. Current implementations use one extra wheel for a total of four moving parts. This simplicity of design adds to its robustness while maintaining a high level of functionality. Along with the lack of manual winding, it makes the 7S26 one of the simplest automatics around. The basic functioning of the Magic Lever system can be understood from these diagrams [11,12] from a Seiko Credor catalog. The coupling between the lever and the intermediate wheel functions on the same principle as a locomotive (or a choo-choo as shown in the diagram). The two arms of the Magic Lever [13] then drive the pawl wheel. They alternately pull and push the pawl wheel in the counterclockwise direction as the intermediate wheel rotates in conjunction with the rotor. The intermediate whee! l and pawl lever cannot be removed until the ž bridge is removed as the intermediate wheel is held onto the bridge with a semi-circular clip on the underside of the bridge [14].

For the sake of comparison, an ETA 2892 winds the mainspring arbor one rotation with 155 turns of the rotor. The current implementation of the Magic Lever winds the mainspring arbor one rotation for 166 turns of the rotor. Another factor to consider when contemplating automatic system efficiency is the dead angle.

The dead angle is the angle of back and forth movements that the rotor can experience without any winding energy being transmitted to the barrel. The dead angle of the Magic Lever is slightly larger than in the 2892 (by five degrees or so) although I haven't precisely calculated either. There are many other subtle factors that effect the efficiency of an automatic system but I feel safe in assuming that Seiko's system is slightly less efficient than ETA's (at least the 2892, which differs from the 2824 and 7750). ETA's automatic systems are remarkably more complex and expensive to manufacture though and I've yet to hear of a Seiko automatic that does not w! ind sufficiently in use. It is not at all uncommon to find some wear around the lever arms and intermediate wheel coupling in older versions of the Magic Lever system. This example showed some wear [15] underneath the pawl wheel. This amount of wear is fairly significant for a watch that is less than two years old. On the whole, the automatic system is a triumph of simplicity that comes with some apparent sacrifices to longevity as well as efficiency.

The Power Train Upon removing the ž bridge, we can get at the barrel, the power train and the oddly placed keyless works [left]. It is quite rare to see keyless works on the bridge side of the movement and is facilitated in this case by the absence of manual winding capability. By placing the clutch lever, set lever and detent where the crown wheel would usually be, the space on the dial side can be used for the quickset mechanism alone. This allows all of these components to be quite a bit beefier than they would be otherwise. The increased size of these levers and wheels adds to their robustness as well as their ease of manufacture, owing to the looser tolerances required. The set lever has a three-position detent and pivots on a post, rather than a screw. It's tail end serves as a push-button to release the stem from the movement and can only be operated when the stem is in the neutral (1st) position. Most keyless arrangements have one position in which it is most convenient to! replace the stem, it is nice that the 7S26 dictates this by only allowing the stem to be removed in one position. The set lever acts as its own return spring by way of its extended tail. This does away with the need for a wire spring, none of which are found in the 7S26.

The click [16] on the 7S26 is also remarkably simple. It consists of a long steel spring held in place by sheer geometry, without the use of screws. It lies under the click wheel and is held in place by a brass pin on one side, a raised portion of the mainplate on the other and the ž plate bridge on top. The barrel [17] is circular grained on the top and bottom and is replaceable as a whole unit. Although it is possible to open the barrel, it

is not designed with this in mind. These pictures [18,19] are of a faulty barrel my friend Randall Bensen encountered. Inside we can see the insufficiently applied black graphite-laden grease [18]. He replaced it with a whole new barrel (an appropriate precaution given the permanently sealed nature of these barrels) and proceeded to open up the bad one. After he took the mainspring out and cleaned it, we can see that the inner surfaces of the barrel are unfinished [19]. While this will undeniably effect the consistency of the ! power flow from the mainspring, a host of other refinements throughout the train and escapement would be necessary before evidence of this effect could be noticed.

The power train of the 7S26 is a direct-seconds layout of the simplest configuration [right]. Quite remarkable for a low-cost movement such as this, are the Diafix cap jewels on the third wheel and escape wheel [20]. These jewels bring the total number to 21, quite sufficient for an automatic watch, especially with only two jewels in the automatic system (for the intermediate wheel). The power train wheels are crudely finished and appear to be made of nickel. This is the first time I've seen nickel train wheels. Traditionally, train wheels are made of brass because it is relatively strong, cheap, easily machined and wears well in contact with steel. Nickel has these same features while being slightly stronger and more difficult to machine. It's use for train wheels does not seem to be dictated by cost so I can only surmise that its greater strength is desired.

All of the train wheels have properly shaped teeth although their faces are not well polished with the exception of the escape wheel. The center wheel [22] is actually a solid disk, having no spokes or even holes drilled through it (as do some train wheels in earlier movements). While this is not ideal from the standpoint of inertia, the second wheel is the slowest moving of the four train wheels, making its inertia the least critical. Disconcertingly, the lower pivot on the third wheel was completely flooded with oil [21]. There was so much oil present that it had contaminated the teeth of the center wheel [22] and would have resulted in serious wear issues, over time capturing the inevitable micro-metallic dust and turning it into an abrasive paste. Apparently one of Seiko's robots was asleep at the wheel when applying lubrication to this pivot.

The Escapement

The escape wheel teeth [23], while appropriately polished, are not beveled at all, leaving a rather wide face to impulse the pallet jewels. This means more friction and less power. One possible reason for these wide impulse surfaces is to prolong the life of the pallet stones. The wide faces of the escape wheel teeth are less likely to groove the pallet stones even after years of use.

Seiko escapements also seem to have an unusually large locking depth (the extent to which the pallet jewels lock the escape wheel teeth). In all likelihood this is a concession to looser tolerances in the manufacturing process and in my experience often results in a lower balance amplitude.

The pallet bridge [24] is nicely shaped and supports a very homely pallet lever [25]. The topside of the pallet appears to have had its insides scooped out and is utterly unfinished. Interestingly, the underside of the pallet fork is fairly well polished, contrasting with the Swiss tendency to finish the top of the pallet fork and not the bottom. At least I can say they weren't trying to hide anything. I can only guess that the weird, semi-hollow form of the pallet fork is an attempt to make the pallet fork lighter, something that is highly desirable in this critical component.

The balance itself [27] has two arms and is of unknown composition. If I had to guess, I'd say it was made of a nickel alloy and the hairspring is made of some form of elinvar. It is not as critical for the balance to be made of a material that is stable over different temperatures as the hairspring, but without a variant of elinvar for the hairspring, it would be impossible for the movement to perform well in daily use. The hairspring is flat and is attached to the collet [28] in a manner that avoids the problems inherent in traditional pinning methods. A pinned hairspring has its elasticity compromised in close proximity to the pinning point as the spring's cross-section curves around the pin. In addition, the hairspring must be bent profoundly from its even, concentric spiral shape to where it enters the collet. Seiko's arrangement does away with both of these problems as the inner coil of the hairspring is crimped in a groove in the collet without disturbing its! shape.

The outer coil of the hairspring is similarly crimped in a slot in the stud [29]. Although this is a perfectly functional solution, it negates the possibility of altering the length of the hairspring in the future, resulting in a balance that is effectively disposable.

The regulator uses a buckle [30] (as opposed to simple pins) to keep the hairspring from slipping out when subjected to shocks. Like all of the components of the movement, it is crudely made but well designed and functional. A watch is said to be "in beat" if the tick and the tock are equally distributed around the resting position of the balance and this is controlled by either rotating the hairspring collet on the balance staff or changing the position of the stud. For ease of beat adjustment, the balance cock of the 7S26 features a movable stud carrier that I fault only for being a little too large. Its excessive, unbalanced weight can cause it

to shift during shipping or when dropped, resulting in a significant rate change as well as throwing the escapement out of beat. If the escapement goes too far out of beat, its ability to self-start after running down will be hampered in addition to positional performance problems.

Conclusion There are many positive things I can say about this movement, but the most relevant is that it is exceedingly honest, almost to a fault. It is an economic design and construction that is created for maximum performance and robustness at minimal cost. It has no pretenses of decoration or "fineness". In fact, it uses this very freedom from tradition to incorporate engineering solutions that would otherwise be shunned, like the plastic calendar wheels and spacer ring. It is heartily true unto itself as a low cost, low maintenance, long lasting and well functioning automatic movement. It is not pretty and it is not glamorous. It is in all likelihood untouched by human hands in its manufacture. There is, however, significant accomplishment in its conception and implementation. Without the Seiko 7S26 and comparable movements from Miyota (Citizen), Orient, and Swatch, many people would not be able to experience the joy of owning a mechanical watch at all.

Given the reliability and robustness of the 7S26 movement, the quality of execution and general massiveness of the SKX779's bracelet and case, and it's striking looks, this is a lot of watch for the money. I would not hesitate to recommend it to someone interested in a watch that will perform well under any circumstances (ok, maybe not with a tux) while requiring no special attention. I will not be shocked in the least if this watch runs for twenty years without service (maybe fifteen now that I've replaced the Seiko lubricants with Moebius greases and oils) and fully expect to read extreme stories in the future regarding 7S26 based Seiko Diver's. It is a fitting addition to the illustrious Seiko Diver line. _john

Addendum In commenting on the wear I found in the automatic system, I believe I over-emphasized its significance and the likeliness of long term repercussions. While the amount of wear present under the pawl wheel was more than I would expect to see in such a young movement, the Magic Lever system remains only slightly modified from earlier designs, many examples of which have been observed to perform well for 10, 20 or more years without service. I think the amount of cosmetic wear is significant but I don't believe that it will adversely affect the longevity of the automatic system or movement overall.

Since publishing this review, I have come to better understand the action of the Magic Lever and how its dead angle relates to winding efficiency. In the discussion of the Magic Lever in the review, I said "The dead angle of the Magic Lever is slightly larger than in the 2892" but I now realize that is not exactly true. While most automatic systems have a dead angle that is more or less consistent regardless of the position of the rotor, the geometry of the eccentrically mounted Magic Lever results in a dead angle of varying size, depending on the position of the rotor.

In the 7S26, a gear on the underside of the rotor is coupled with a drive wheel. The Magic Lever is in turn driven by an eccentrically mounted post on the drive wheel from a point I have called the "pawl lever axis" in the diagram. The key to understanding the action of the Magic Lever lies in understanding that the "entrance pawl" and "exit pawl" both provide winding (or maintaining) action and slipping (or releasing) action, sometimes alternately and at other times together, depending on the position of the lever at the time.

Considering only the entrance pawl first, it can be seen that if the pawl axis is moving from A to B (in either the counterclockwise or clockwise direction), the entrance pawl will be pulling on the teeth of the pawl wheel and providing winding action. Conversely, if the pawl axis is moving from B to A, it is either slipping over the pawl wheel teeth or allowing it to recoil somewhat.

As regards the exit pawl, when the pawl axis is moving from D to C, it is providing winding action by pushing on the pawl wheel teeth and is slipping or allowing recoil when the pawl axis is moving from C to D.

At first glance, it might seem that the range of motion of the pawl axis between A and C in the clockwise direction and between D and B in the counterclockwise direction would be the most productive portions of the motion of the drive wheel as the winding action of both pawls is being utilized. In fact though, since there is no slipping action, there is nothing to maintain the winding action if the direction of the drive wheel is reversed anywhere along that section of its rotation. This translates to a very large dead angle in the short portions of the drive wheel rotation between these four points. That is to say, if the Magic Lever wobbles back and forth in this region for a while, no winding action will be transferred to the mainspring.

Conversely though, the larger portion of the pawl axis' motion between these four points results in a very small dead angle. When the pawl axis is moving between A and D (in either direction) or between B and C (in either direction), the action of one pawl is complimented by the slipping action of the other, translating

the maximum amount of winding action to the pawl wheel (keeping in mind that the pawl wheel teeth in the diagram are greatly exaggerated and are in actuality very fine).

The dead angle is at its smallest when the pawl axis is halfway between A and D or halfway between B and C (approximately where it is in the diagram). By attaching the rotor so that the minimum dead angle of the rotor corresponds to its natural, relaxed state in the crown down position, the winding efficiency is maximized. In this way, the normal movements of the arm (particularly in activites like walking, where the watch is crown down most of the time) will result in the greatest winding action.

While it is true that in certain rotor positions the dead angle is quite large, in other positions it is incredibly small (relative to other automatic systems). If the rotor is positioned properly when installed in the movement, the dead angle is minimized when the rotor is in the crown down position (approximately). This ensures that the minimum dead angle zone is capitalized on during wear (when the majority of the time is spent in the crown down position) to maximize winding efficiency. To assist the watchmaker in positioning the rotor correctly, there is a small hole in the drive wheel that lines up with a hole in the balance bridge when the rotor is in line with the crown. This capitalization on the idiosyncracies of the Magic Lever geometery likely accounts for the efficient winding action most Seiko wearers experience in actual use.

Unfortunately for us it also makes it impossible to compare its efficiency with that of other automatic systems without exhaustive real world testing. _john January 1, 2003

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