A. Lange & Söhne

  

Long story short, the left barrel does five revolutions from full to empty, while the right barrel does six revolutions. The next paragraph is a mathematical explanation, which can be skipped if needed.

  

The old movement has a central cam of six teeth, while the orbital cam has five recesses. Mathematically, their lowest common multiple (LCM) is 6x5=30, i.e. after 30 teeth engagements the long tooth engages the shallow recess again. 30 teeth engagements is five revolutions of the barrel around the central cam. Meanwhile, the new movement has 7 central teeth and 6 recesses, which is a LCM of 42, i.e. after 42 teeth engagements the long tooth hits the shallow recess - this is six revolutions of the barrel. The difference of number of teeth and recess has to be 1, so that the two numbers are mutually coprime - in other words, N and N+1 both cannot be cleanly divided by any number larger than one. This results in the LCM simply being the product of the two numbers. If the two numbers are not mutually coprime, the orbital cam will repeat the cycle quicker - for example, a central cam having 6 teeth with the orbital cam having 4 recesses has a LCM of only 12, which means only after two revolutions of the barrel the stopwork happens. This of course, is too little as a mainspring barrel usually unwinds in 4-6 full turns.

 

Thus, it appears that the modern production Zeitwerks have a barrel that unwinds 20% faster than the old one - the first Zeitwerk barrel empties after five turns, while modern Zeitwerk barrels empties after six turns. Yet, both still maintain the same power reserve of 36 hours. This means that the modern Zeitwerk has a longer mainspring (to ensure it can unwind for that extra 20%), and the gear ratio between the barrel and second wheel of the geartrain has a different gear ratio (20% slower, since the barrel is now faster).

 

This is not attributed to cost-related matters - I believe this is all part of the optimisation of the design. This appears to be the decision for a longer mainspring with more spread out torque, rather than a shorter mainspring needing to supply higher torque. Note also that the Zeitwerk movement, especially the remontoir portion, has underwent numerous revisions/changes since its inception - the initial release remontoir functions and behaves differently compared to the later releases!

 

Interestingly, another major difference is the crown wheel which is further evidence of this - since the new movement's barrel unwinds in more turns than the old one, conversely this means that you need more turns of the crown to fully wind it. Thus to preserve the number of winds, a crown wheel with more teeth is needed (Refer images below, comparing Peter Chong's original article versus Lange's stock image. I count 30 here vs old one of 20) such that the gear ratio is stepped down less to the barrel cover. This results in overall, similar number of turns of the crown to fully wind both barrels despite the difference in mainspring lengths. Another interesting implication - newer Zeitwerks will have more "clicks" as you turn the crown, as the gear ratio is now quicker and thus you'll hear the clickspring ratcheting at a higher frequency!

 

Quite fascinating how much information and implied consequence of the design change one can infer, just from a simple difference of number of teeth on a couple of wheels! (and, knowing both movements preserve the same length of power reserve)

 

Regards,
skyeriding

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