. . .it's described as exclusive to BP though I suppose it could find its way into some other watch made by a Swatch group marque.  I can't recall that it has though.

Jack

Which would lend credence to MF's theory about why the Type XX used the Lemania movement as well: it was the only one available.

_john

I think I read somewhere that it takes only 2 extra parts to make FP 1185 a flyback.  So what makes it so difficult to convert the Lemania chrono in Breguet?  Is it difficult to convert ETA2894 or 7750?  Many thanks!

Hi loujo,

"Adding" flyback (it's not really an addition exactly, but...) to a chronograph movement can be remarkably easy or quite a challenge, depending on the type of engagement mechanisms employed and the details of the construction.

For the most part, vertical clutch chronographs are the easiest to convert to flyback.  Vertical clutches can actually be reset while running with no disengagement necessary, as the friction coupling in the clutch can slip without any trauma whatsoever.  You need to make the counters similarly "trauma resistant", but this is typically accomplished by making the arbors and heart cams of the counters friction coupled to their wheels as well.  In this way, the counter hands can be reset without the counting trains being affected at all.

In traditional chronographs, there's generally a little more to it however.  For one thing, the lateral clutch gear needs to disengage from the chronograph wheel so that the fine teeth are not damaged during the relatively violent reset action.  In the most traditional chronographs with flyback, the minutes counters are driven by a sliding gear that also has to be lifted out of engagement before the flyback action so that the teeth of the sliding gear and the finger on the chrono wheel are not damaged (more modern designs typically employ a fixed intermedaite minute counting wheel and a flexible chrono finger)..  This all adds up to a fair amount of gears and levers (and hammers) moving from one position, to another position and back all at the push of a single button. 

With the Lemania 1370, converting it to flyback had a host of challenges.  For one thing, the chronograph mechanism in the 1370 is actually three more-or-less independent engagements for the seconds, minutes and hours.  The seconds counter is a traditional lateral transmission mechanism on the movement side, but the minutes and hours counters are friction couplings with scissor and lever braking respectively.  That is, the minutes counter is driven by a friction coupled  gear on the cannon pinion that is held stopped by scissor-like pincers when the chronograph is off that  then release the minutes counter driving wheel when the chronograph is engaged.  The hour counter is driven directly by a small gear friction coupled to the underside of the barrel, as is common for hour counter in many chronograph movements.  It's held stopped by a brake lever and released when the chronograph is started. 

Even conceptually, you've got three counters on two sides of the movement to coordinate with a single press of a button, and one of them has to be decoupled before the reset action can take place.  The real problem though is that the hammers on the dial side have to overcome a decent amount of friction during the reset (because the heart cams they're resetting are friction coupled), and when it's a flyback reset in particular, you want it to happen all-of-an-instant with a single snapping motion.  To accomplish this, the engineers at Lemania, um, engineered a "break-over" point into the flyback mechanism that keeps anything from happening until a significant amount of pressure is placed on the reset button.  The amount of pressure needed before the break-over occurs can be more or less precisely adjusted (with a bothersome and finicky adjustment) and often must be during routine service so that a nice crisp, reliable and non-destructive flyback action can be attained.  This is without exaggeration a careful balancing of the sliding friction of more than a dozen moving parts (closer to twenty) and their associated formed springs (a cool half dozen at least)  all interacting with each other in an instant.  This particular adjustment alone can lead watchmakers to drink.

In some ways, the simple reset function is just as complicated actually, because in that "instantaneous" interaction, the brake lever must be lifted from the chronograph wheel, the scissors must be lifted from the minutes counter driver and the hour counter brake must be lifted before the three separate hammers reset the heart cams and then all the aforementioned levers go back to their braking positions.  Of course this is split across two sides of the movement also.  The pressure needed for a normal reset is also critical and can be made to more or less equal the amount of pressure needed for flyback with some effort.  With even more effort (and some luck), one can set up the start/stop pusher feel to a similar level as well, but this is going above and beyond the call of duty frankly (and outside of the manufacturer's intentions).  It is the necessarily strong flyback and reset pressures that determines the very firm pusher feel of the Type XX in general however.

It's something of a miracle that the movement works at all (I know Thomas, but it's true!), but when properly cared for, the mechanism is pretty darned reliable all things considered.

_john

The more I spend time on this forum, the more I realize how little I know about watches.  Thank you for taking time writing this reply.  It is greatly appreciated!

. . .something like this by you or Suitbert or someone else with actual bench experience without wishing that it was a full-on article with pictures so I could follow it more closely- I get the general gist of what you're saying but there are an awful lot of us out there who wouldn't know a cannon pinion if we were hit over the head with one :-) .

Jack

nt