AP Escapement - for tech geeks

The Jules Audemars Chronometer from AP which came out a few years ago features an interesting movement (cal. 2908) with some resemblance to the Omega co-axial. Here are some characteristics:

- Unlike to the typical Swiss Lever escapement, the locking function and the impulse function are done by separate jewels in the 2908. This is more akin to the Co-axial escapement or a marine chronometer's Spring Detent Escapement. The Swiss Lever escapement design is a very good compromise between accuracy and reliability but has the disadvantage of escapement wheel teeth sliding on jewel surfaces which in turns necessitates lubrication for proper operation and reliability. The lubrication's characteristics changes over time, compromising timekeeping accuracy. In addition, the Swiss Lever escapement has some loss of power in operation. AP novel escapement configuration sidesteps these disadvantages by separating the locking and impulse functions and having little slide.
- No lubrication of the escapement jewels. This feature improves -at least in theory- the long term time keeping accuracy of the watch compared to a conventional watch. This was George Daniels' motivation for the creation of his escapement design but Omega uses lubrication on its implementation.
- High beat operation: 43 200 beats per hour, instead of the usual 18 000, 28 800, etc. High beat operation reduces the time for a watch to react to a change of position of the watch.
- The 2908 uses two hairsprings, operating co-axially on the balance wheel axis, but wound in opposite directions. This is said to improve isochronocity. I see two effects of adding a second hairspring on the performance of the watch. 1) It would reduce positional variances because hairsprings are inherently physically asymetrical and by having a pair cancels out some the effects caused by this asymmetry on timekeeping performance. 2) my guess is that the escapement configuration of the 2908 reduces escapement error and the movement designers found themselves with a movement with worse isochronicity when using a single typical flat hairspring. Indeed a modern Swiss Lever escapement has some escapement error but it is compensated by the hairspring design. The typical flat hairspring with no overcoil "develops" asymmetrically and causes it to run anisochronaly, but this can be used to compensate anichronicity caused by escapement error. This is speculation on my part of course but I'm curious to know the design reasons to add a second hairspring.

Yes, this is AP's version of the co-axial escapement. It's based on the Robin escapement.

13 years after it's introduction, you still don't hear all that much about it. From what I can tell looking at the movements on the AP web site (which is terrible) I don't see that it has been rolled out across their entire line like the co-axial has been with Omega.

I find it telling that in that video, with all the talk of precision, no timekeeping tolerances are mentioned. Unless they have numbers to back up the claims, then its all marketing in my view. People often cited improved accuracy as the driver behind the Omega co-axial - in fact the timekeeping specs didn't change one bit due to the escapement, and Omega told us specifically in the training that accuracy was not the driver.

The whole thing about 2 balance springs - first the high frequency would require a stiffer balance spring if you still want a reasonably large balance, so having two springs would help. In terms of cancelling out errors created by having a single spring, that's what dynamic poising does. Again without some actual numbers I don't really put much faith in that reasoning, in particular when it's common to get low single digits in positional variation with a normal balance system...and that a silicon balance spring already adds such a high degree of potential accuracy.

Seems like a less successful marketing move by AP...not much more.

The tried and true Swiss Lever escapement can be made nowadays remarkably accurate: 1 s/day rate corresponds to 0.001% error. It sets a very high bar for alternative escapement schemes.

Yes, more new and improved buggy whips...😉

Yes, this was the specific reason for the co-axial cited by Omega. However just because you use longer lasting tires on your car, doesn't mean you don't need to change the oil at all. As much as these companies talk about the lubrication in the escapement failing in their marketing materials, keep in mind that many parts of the movement have lubrication that fails, and it all tends to fail around the same time (this is by design since they don't want particular weak spots leading to more frequent than necessary service intervals), so an escapement that lasts a very long time between servicing is great, but only if the rest of the watch is improved to meet the same specs.

On the power reserve, ALS made a watch than runs for a month using a traditional escapement...

Asymmetrical doesn't necessarily mean inconsistent or unpredictable. Asymmetrical motion happens in traditional lever escapement watches as well, when the beat error is large. The beat error is expressed in ms and is the time difference between the rotation of the balance in one direction, compared to the rotation in the other. A watch with a large beat error can still keep time well.

Yes, I know what you were referring to - I was drawing a comparison and giving you a real world example of why it's not an issue. Not sure why you think this is a problem in the AP escapement though - the impulse given to the balance is done at very regular intervals, so what happens in the escapement in that regard is moot. Again, asymmetrical doesn't necessarily mean inconsistent or unpredictable.

Asymmetrical Random

You might want to check to see if the beat rate and dial markings on existing chronograph watches always match up...😉