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Originally Posted by Archer, post: 56642518, member: 48283
The torque available from a given strength of mainspring is a finite thing. That's somewhat different from the length of the power reserve. A mainspring is a different kind of spring, so I'll explain...
The torque of the mainspring is a function of its width and it's thickness. The power reserve is a function purely of the length, all else being equal.
A typical mainspring designation goes something like this...
1.60 X .10 X 267 - all numbers are in mm.
First number is the width or height of the spring, and is related to how tall the mainspring barrel is.
The second number is the thickness, or commonly known as the strength.
The third number is the length.
The first two numbers can affect the torque delivered from the mainspring, but do in in different proportions. A change in width has a small impact, where a change in strength has a very large impact - it is cubed, so a small change in the strength measurement can have a very significant impact on the torque.
Neither of these are directly related to power reserve. That is determined by the length of the spring - simple relationship between the length of the spring determining the number of turns the mainspring barrel will make, and the gear ratio between the teeth on the barrel, and the subsequent train wheels. Again, all else being equal.
The only relationship they have to power reserve is that when you make a mainspring thinner, you can fit a longer spring in the same sized barrel. That is what Rolex has done. In an effort to catch up to other brands who have extended their power reserves, they have tried to fit more spring in the same space, rather than taking another approach such as two mainspring barrels.
So to fit more spring in the same barrel they made 2 changes:
1 - Thinner barrel wall.
2 - Thinner mainspring.
These changes have consequences. The thinner barrel wall now means that you cannot use the barrel over again, because it's too fragile to open up and close again in service. So the entire mainspring barrel must be replaced at every service. That's a servicing implication, but not necessarily a performance implication.
The thinner mainspring now reduces the torque delivered, which reduces balance amplitude. To now make up for that reduced torque, they did some very un-Rolex like things to the escapement. They made things thinner and more fragile. Smaller surface contact reduces friction, but concentrates the forces and wear that may occur. There's always a give and take in any design, and watch movements are no different in that regard.
So why is balance amplitude important? It's simple - isochronism.
A balance inside a watch is a very imperfect oscillator, so watch companies go to a lot of trouble to help it maintain its period (rate) when there are changes in the distance travelled (amplitude), but there's only so much you can do.
So this means that when amplitude drops, timing changes. How much depends on how big the drop is, but also where in the amplitude range that drop is occurring. Rolex watches don't tend to have really high amplitudes to begin with, compared to many other makers. So on say a standard ETA movement, it's not unusual for amplitudes to be in excess of 300 degrees, where on a Rolex you are more typically looking at 270's or 280's. In fact the point at which there is too much amplitude, and you experience rebanking (also known as knocking), is much lower on a Rolex than say on an ETA 2892, due to the geometry of the Rolex escapement.
When you get to lower amplitudes, such as the low 200's, then some odd things start to happen. There's an amplitude where poise errors on the balance tend to go away (these are only in vertical positions) and it's widely considered that around 220 degrees is where this happens, at least with a traditional lever escapement.
As you drop below that, there's a point where poise errors reverse, so if at full wind amplitude crown left runs faster that crown right, those reverse, and now crown right will run faster than crown left. As you drop lower, the magnitude of those poise errors gets magnified significantly. This is well known with watchmakers and is useful in the context of dynamic poising procedures, so for that I drop the balance amplitude to 160 degrees to magnify the errors, locate the heavy spot on the balance wheel, and make the appropriate adjustments.
So for example I just dynamically poised a watch last week where at full wind the positional variation was 21 seconds across 6 positions (amplitudes were from the mid 280's to around 310). When I dropped the balance amplitude to 160 degrees, those errors ballooned to 57 seconds, so they nearly tripled in magnitude.
So on these 32XX watches, Rolex wants to keep the balance amplitude after 24 hours above 200 degrees to avoid some of the more extreme effects that are seen at lower amplitudes. It is somewhat of a detriment that the amplitude starts lower on these watches to begin with, because you have less of a buffer in the amount that it can drop before you start to see those unwanted effects.
So do they know what the problem is? Yes, I'm sure they do by now. The problem that presents itself is repairing this issue without sacrificing any of the promised performance metrics that this movement is known for. So that means not changing the accuracy and power reserve goals. It's not that they don't know what to do, because they have built plenty of robust movements in the past - it's doing so while maintaining the current performance demands that is the issue.
Hope this helps those who are interested, understand the technical side of this a bit better...
Cheers, Al
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