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Inside the Seiko Caliber 5606

May 25 2003 at 11:50 PM
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Inside the Seiko Caliber 5606.

According to The Seiko Book, commercial production of the cal. 5606, based on the 56 caliber stream, started in 1968.

The 5606 had a 21600 beat rate, offered quick set day and date, hand winding from the crown, and the ability to precisely set the seconds though a hacking lever.

The 56 stream seams to have been developed between two other caliber streams, the 51(starting in 1967), and the 52 (starting in 1970). When the 51 was developed, it had a thickness of 4.9mm. The Cal. 56 steam had a reduced thickness of 4.3mm, and the 52 stream further reduced the movements thickness to 3.9mm.

In this continuing effort to reduce the thickness of its mid to high grade movements, many novel design elements were incorporated into the 5606. The following essay is designed to highlight some of these features and design elements, and to allow the reader to appreciate what Seiko was doing at the time.

Below, we see the top plate, or back of the movement, as it is after being decased from a one piece case.

In the photo above, we can see a high degree of consideration in the movement. From the colimaconnage on the winding weight, crown, and ratchet wheels. The high polished angles on the plates. The cap jewels on the escape and third wheels, to the micro regulator used on the balance cock, this movement is no slouch in the visual appreciation department.

With the winding weight (commonly referred to as the rotor) removed, we can see the major deciding design factor in the reduction of thickness in these three movements (51,56,52). Placing (integrating) the autowinding train wheels at the same level of the time keeping train was the key to reducing the thickness of the automatic watch. With the autowinding and time trains under the same bridge, valuable real-estate was removed from the top plate. Prior to this integration, most automatic watches relied on a separate autowinding system that was attached to an already existing hand winding caliber. This system of adding a autowind train to an existing caliber can be best remembered by the Rolex Bubble Back, a term that was used to describe the fact that if you wanted an automatic, you paid for it in the thickness of the watch. Seiko was a major developer of this "integrated" thinking.

In the photo above, I`ve removed the winding rotor, and have exposed the entire top plate of the movement. The one thing that tickles me the most about the design of this movement is its watchmaker friendliness. In both the calibers 51 and 52, the watchmaker has to remove two very tiny screws, a autowinding cover, and a wheel in order to let down the mainspring. This is because with most automatic winding systems, the winding train blocks the releasing or reversal of the ratchet wheel. In the ETA 2892, an entire bridge must be removed prior to letting down the mainspring. In the Seiko cal. 56, one simple screw needs to be removed, a locking bolt, and a wheel, and the mainspring can be let down. After the mainspring is let down, removing the ratchet wheel allows the removal of the entire top plate. I can not think of any other movement that allows the watchmaker access to the inner most parts of the movement, in so few a number of steps.

In the photo above, the top plate has been removed, and many of the design elements of the watch can be appreciated. In the bottom left corner, we can see the only two components of the autowind system that are completely under the top plate, the differential (or reverser) wheel, and the secondary idler. Note that the idler wheel is made from a dissimilar mettle, and is running on a large ruby jewel. This helps eliminate some of the wear that might other wise be caused if both wheels where made from steel. Not counting the rotor or the ratchet wheel, the system only accounts for 4 wheels. Most autowind systems of this limited size are reduced to a single winding direction, but because the differential wheel is actually two wheels stacked on top of each other, it does offer bi-directional winding. It can also be seen that the differential wheel is using a simple ratcheting spring loaded pawl system, not un-like the magic lever system of today's 7s series. From what I`ve seen, these pawls are just about bullet proof. Of the several dozen 56 caliber watches I`ve seen, none of them suffered from any type of wear in the autowind system, all of them working and winding perfectly, even after years of neglect. Moving up and clock wise from the autowind parts, I`ve noted that Seiko further economized the space available by using an angled pallet fork, allowing for a compound angle of the escapement. This means that Seiko did not need to use a straight line from the escape wheel to the balance, instead they where able to curve the arm, moving the entire escapement closer to the outside edge of the movement. The hacking lever has been fixed to a pivot, fulcrumning to and fro by a extended pin at the end of the stem. The far left fingerer of the lever rests on and arrests the balance when the stem is pulled out. Unlike the fairly complicated hacking lever found in the cal 51 series, this simple system was further retained and economized in the caliber 52 series, which is still in production as the 4s series today. Another notable design element of this movement is the direct drive of the fourth, or seconds hand wheel. Both the 51 series and the 52 series use a in-direct drive pinion, allowing for a certain amount of slop in the second hand when hacked for precise setting. No such slop, or jitter is found in the caliber 56 series.

There is, unfortunately, an Achilles Heal to this movement. It saddens me that so much effort was put into this caliber, and yet one probably unforeseeable element was left to chance. The quick setting of the day and date were compromised by the very materials used in the design.

In the photo above, we`re looking at the dial side of the movement, and I`ve removed the day ring for clarity. The part circled called the day date rocker, is the reason for this distress. This part engages the date ring on the right, and the intermediate day switching wheel on the left, when the crown is pulled out to its second position. This four spoked wheel is made out of plastic. It is held friction tight to the steel wheel below it by the spring washer that can be seen above it. I believe that this wheel used a friction fit, in hopes that if the wheel was engaged during the forbidden zone, it would slip, and the possibility of damage was reduced. Unfortunately the plastic used in the construction of these wheels has a propensity to shrink with age, causing the wheel to split, thereby causing it to slip between the spring washer and the wheel below. I`ve seen many examples of these wheels, and approximately 95% of them are split in such a manor. I even found a stash of almost a dozen of these rocker assemblies, supposable NOS, and in NOS packaging, that where all split, and would not function. This is how I postulate that the plastics used in the design have failed due to age.

Lets take a look at the forbidden zone, and why you where advised not to quickset the day or date between 8pm and 1am. Just incase your 5606 is still QSing the day and date.

In the photo above, with the day wheel removed for clarity, it can be seen that both the date changing finger, and the day changing finger are solidly fixed to their wheels. In most watches, these fingers are equipped with springs that allow the fingers to slip out of the way if the time is reversed, or the quick setting mechanism is brought into engagement. In the photo above, the date setting finger is in the middle of turning the date over, I set it up like this for the photo. Because the finger is solidly fixed to the wheel, there is no give here if the quick set mechanism is brought to bare on the date ring on the right hand side (also set up to show its position when being engaged). As you can see, if these two wheels are forced to turn against each other, something is going to give. While I believe the construction of the day/date switching wheel was supposed to allow for this inevitability, because of the materials used, it has not stood the test of time. Sadly, roughly 90-95% of the watches with the caliber 56 series, no longer quicksets the day or date. This is something that should be made clear of when looking for one of these otherwise fantastic watches. Needless to say, I am working on some sort of a replacement for this part, if and when it becomes available, will be immediately posted.

Although the 56 series was latter upgraded and used in some chronometer grade King Seiko's, I find it sad that it was not chosen for the task when Seiko made the decision to resurrect the 52 series as the 4s15, with all of its siblings. I really think Seiko was approaching the epitome of automatic movement design in this caliber, and the 4s series is a compromise of some excellent ideas.

Thanks for reading, all comments are welcome.

Best regards,
Randall Benson

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