Yes, I know, not desperately exciting, but it cropped up recently, and so possibly worth documenting. I’ve had a few Seiko watches pass through my hands fitted with the ubiquitous 7S26 movement, and have had cause to notice, on occasion, that watches fitted with this movement can require a bit more wrist time to dial in a decent amount of power reserve. I had always assumed that the gearing of the autowinding mechanism on these watches was tuned to the more active type and gave it not much more thought. Recently though, I had cause to look more closely at what might be the cause of this apparent lack of efficiency.
The 7S26 and 7S36 movements are unusual in comparison to the 6-series automatics, in requiring the winding weight, the rotor, to be fitted with the weight aligned in a particular way with respect to the reduction wheels that transmit its motion into power reserve in the mainspring. I suspect it may not be that uncommon for the weights to be refitted post-service, incorrectly, or indeed, perish the thought, in the factory (although I would hope this unlikely). Somewhat curious about the effect of rotor alignment on winding efficiency, I recently performed a rather unscientific test with a watch whose rotor was about 30 degrees out of alignment. This next photo shows the rotor of a 7S26B fitted to a mid-size dive watch, more or less in such a state:
A good solid 8 hours on my wrist was sufficient to dial in about 17 hours of power reserve, more than enough to get through the night and certainly enough not to cause any sort of concern that something might be awry. If the watch was being used regularly by someone whose level of activity was comparable to mine, then a full power reserve would build within two or three days wear. If we take off the rotor
we see the wheel on its reverse that meshes with the first reduction wheel (below) whose rotating motion causes the pawl lever, whose axis is mounted off-centre of the first reduction wheel, to move back and forth rotating the second reduction wheel which then winds in power to the barrel and mainspring:
The beauty of this arrangement is that the pawl lever rotates the second transmission wheel in the same direction regardless of the direction of the rotor or indeed the first reduction wheel. This magic lever arrangement has been used widely in Seiko automatics since the early 1960’s. Refitting the rotor, we follow the instructions given in the 7S26 technical manual:
We see that with the winding weight set so that the midpoint of its arc lies along the stem direction, the hole in the first reduction wheel should point towards the hole in the balance cock.
When the first reduction wheel is in this position, the pawl lever is precisely at its turning point, i.e. the lever is poised so that depending on the direction in which the first reduction wheel turns, either one or other of the two arms will move to engage with and rotate the second reduction wheel. My guess, and this is only a guess, is that it is supposed to be aligned thus, because wrist movement is more likely to swivel around the axis defined by the arm which itself aligns along the stem and crown axis. With the weight at its midpoint when it sits over this axis, rotating the wrist will cause it to ‘fall’ one way or t’other so winding the watch. Moving the arm up and down will rotate the watch about an axis perpendicular to this one which will be less likely to unbalance the weight.
With the rotor in its correct position, 9 hours on the wrist resulted in 37.5 hours power reserve. A bit of a result. The conclusion therefore is that a misaligned rotor is only likely to cause issue in people who aren’t especially active during the day but could result in the watch stopping overninght following a day’s wear.