|The Little Engine that Could
An indepth look at the ETA 2892
Hans asked me a while ago to do a write up on the ETA 2892. Both because of its enormous popularity and also the fact that it is deemed accurate and reliable enough to be used as a base movement for many high-end manufacturers complications.
There’s not much I can add to Walt Odets’ history and evolution of the movement itself, except to say that most changes and updates were done in order to improve the efficiency of the automatic winding.
The first problem arose because originally the basic movement beat at a leisurely 18,000 beats per hour (BPH) and had a fairly large diameter balance wheel. When ETA upgraded the movement by increasing the beat to 28,800 BPH, this required a much stronger mainspring than what was used previously. This required upgrading the automatic unit by improving its winding efficiency in order to deal with the extra resistance due to the stronger mainspring.
No sooner had the above problem been adequately dealt with, when ETA gave themselves more winding efficiency headaches by reducing the diameter of the movement from 28mm to 25.6mm so that it could be used in a wider range of cases. Hence the change in model number from 2890 to 2892. Thickness remained unchanged, i.e. 3.6mm for both models. This had the effect of reducing both the diameter and mass of the oscillating weight. A problem that was finally put to rest with modifications that resulted in the latest model, 2892/A2.
For the more technically minded, the final modifications to the automatic winding was to reduce the chamfer around the circumference of the oscillating weight – thereby increasing its mass. And to switch the stud and the jewel which supported the intermediate reduction wheel. This wheel, which drives the ratchet driving wheel, originally had a stud, but now has a jewel. The stud has now replaced the jewel in the upper part of the automatic winding bridge. This resulted in less winding friction due to this wheels much greater stability. Omega (and Girard Perregaux) have increased the winding efficiency further by reducing the diameter of the ball bearing support.
I have ‘dissected’ a Bulgari labeled Girard Perregaux
modified 2892 for you. So let’s take a closer look at it starting with
its source of power, the mainspring. While not being very high, this is
quite a strong mainspring and provides the movement with a more than adequate
supply of torque. The barrel arbor has a small diameter relative to the
size of the barrel. This is in keeping with most modern movements that
use fairly long mainsprings. This results in the mainspring taking up about
75% of the available space, as opposed to older movements whose mainsprings
only utilized about 50% of it.
Picture 2 shows the barrel complete. Like the rest of the watch train, it utilizes micro gear toothing. Also known as ETA style teeth. This particular shape, invented by ETA, was chosen for both the wheels and pinion leaves, in order to improve the efficiency of energy transmission by keeping friction to a minimum. Most modern movements, regardless of the factory producing them, have switched over to these style teeth. I think it was Curtis, after much research, who gave us a pretty accurate description of these teeth quite a while back.
What amazes me as a watchmaker, is how well these teeth work. One can easily check this out by assembling the going train of any high grade watch that has the epicycloidal teeth. Without oiling any of the pivots, give the great wheel a push with a thin oiler. Now do the same with an ETA 2892, or even a Rolex 3135 for that matter. The wheels of the latter turn much more freely.
Picture 3 shows the complete train and the escapement. Nothing fancy there, just overall good basic engineering. Note that the wheels are made of glucydur and the pinion leaves are made of steel. Dissimilar metals rolling together produce considerably less friction than if they were made of identical material. All the wheels in quartz watches, high grade ones included, are made of brass. Due to the fact that there is much less torque in a quartz movement, there is no need for the greater strength of the glucydur wheels.
The main plate, like almost all of the rest of the bridges and plates, is made of stamped brass. Most manufacturers rhodium plate these, both for aesthetic reasons and corrosion resistance.
Here is the going train and barrel sans bridges. The brass colored wheel on the barrel is the ratchet wheel.
The inner workings of the wheels that fit under the barrel bridge. The ones on the left facilitate hand winding, while the one on the right, the ratchet driving wheel, is the final working wheel of the automatic unit. The spring, together with the smaller diameter steel wheel, serves a double function. The part engaged with the brass colored wheel functions as a click. While the part resting on the steel wheel, functions like a clutch. Forcing the wheel to be engaged during hand winding, and allowing it slip and move out of the way when the auto winding is engaged. The football (rugby for our European readers) shaped post that it turns on determines its free play in both directions.
Picture 7 shows the barrel bridge fully assembled and ready to be installed.