Movement Stories: Gruen N 710 SS
Welcome to my Blog! Among other things, I plan to blog about some of the interesting watch movements that I encounter and, as Doug DeMuro would say, “their quirks and features.”
Kicking off the series, let’s take a look at the Gruen N 710 SS. This 1960s automatic movement was manufactured for Gruen by… someone. It’s a bit of a mystery, as, beyond a stamp indicating Swiss manufacture, the movement only has Gruen markings.
I was actually somewhat surprised by the Swiss stamp, because, from a design perspective, this movement has features that I tend to associate with German-made movements, and it bears a strong resemblence to contemporary automatic movements by German manufacturer PUW. Some of the telltale signs include the full plate design, with a single large bridge covering both train wheels and mainspring, as well as a two-piece barrel wherein the arbor, barrel cap, and ratchet wheel are combined into a single component.
Two-piece mainspring barrel of the N710SS
In any event, underneath the big ‘ol bridge (who we will henceforth call Bob) lies a fairly conventional 17-jewel movement with a direct-drive center second hand. The center wheel sits underneath the barrel and is held by a secondary bridge. The 17 jewel count is acheived by capping the escape wheel on both sides of the movement, so the center wheel has a jewel on the movement side (which is shares with the 4th wheel), but is unjeweled on the dial side. This version of the movement actually has 25 jewels total, but the additional 8 jewels reside entirely in the automatic winding works.
The train of wheels for the N710SS, before Bob covers everything.
Bob the bridge and the escapement installed and running.
Full plate bridges like Bob have a potential to be a royal pain to install, but fortunately the N710SS seems to be pretty well-made and all the pivots fell into place easily other than the escape wheel, which is fortunately located at the edge of the bridge and therefore is easy to access and manipulate. There are a couple of additional quirks to note in this movement. The winding components are completely concealed, with the crown wheel and click screwing to the underside of the bridge (the click in this movement engages the crown wheel rather than the ratchet wheel). Fortunately there is a small cutout in the bridge which provides access to the click in order to let the power down if needed.
More unusual is the design of the upper cap jewel setting for the escape wheel. This cap jewel is contained in a brass holder with two retention tabs that rotate in the setting, allowing the jewel to be uninstalled/installed. This is not, as it may first appear, a shock setting, and I was confused as to why this design was different from the much more conventional screw-down cap jewel setting on the dial side of the movement. However, if you look closely you will notice the jewel setting resides in a cutout that holds a spring for the automatic winding mechanism. The unusual cap jewel design is screwless and thus very low-profile, providing a flush surface that won’t interfere with the spring while keeping the overall thickness of the bridge under control. It’s a clever design, but a bit fiddly to work on.
The automatic winding works design is interesting, unusual, and also quite clever. Instead of complex multi-part reversing wheels, it uses a small intermediate pinion in a wig-wag setting to allow the rotor to spin in either direction. A rachet mechamism ensures small rotor movements are captured. There is an additional wig-wag gear, with a small jumper spring, that fully decouples most of the mechamism when the watch is hand wound, and this is one of the better hand-winding automatic designs that I have seen. Given its relative simplicity and excellent hand-winding performance, I’m actually not sure why this design isn’t more common. It may lack a bit of efficiency when compared to more common designs with racheting reversing wheels, since there is a small amount of wasted motion needed to re-engage the wig-wag pinion when the rotor changes direction.
I should also note that the winding works in this watch is integrated with the full plate bridge and cannot be removed as a separate unit. This was probably done to keep the movement as thin as possible.
The automatic winding works of the N710SS with the rotor and top plate removed. Note the small wig-wag reversing pinion, immediately below the jewel in the center of the movement. When the watch is hand-wound, and additional wig-wag pinion, hidden in this photo, decouples everything but the final gear (top left) that is directly engaged with mainspring ratchet wheel.
The fully assembled movment, with the automatic winding rotor attached. Note how the final drive wheel from the automatic winding works overlaps the balance cock. Unfortunately this makes the balance impossible to install or remove with the automatic mechamism in place :( .
All in all, working on the Gruen N 710 SS was an interesting watchmaking experience. Despite its quirks, it wasn’t a particularly difficult movement to work on. However, the case that goes with this movement is definitely in need of some TLC. Look for a future post that covers the case restoration process for this watch.