Montblanc

Jack Forster
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Well, it's just my take but. . .

. . . this was a point about which I asked during the press presentation and I think MB may not have realized it was one that needed to be clarified.  I'm of course not in any position to say with any authority as to whether this particular tourbillon configuration has any practical advantages --and as Suitbert mentions below I don't know if or how it is different from the so-called "half flying" tourbillon that BLU produced last year --the BLU tourbillon has a sort of cock for the upper balance pivot which acts, I believe, as a minute hand, whereas the MB ExoTourbillon has the upper pivot running in a fixed bridge so there are differences in configuration but I'm well outside my competence commenting on the differences/similarities between them.

It does seem conceptually sound, of course, insofar as if there is no cage, well, there's no cage.  There is a so called cage-less tourbillon by Benoit which Daniels mentions (he doesn't give any dates unfortunately) but that tourbillon is different yet again from either the BLU tourbillon or the MB ExoTourbillon. 

It's a very clever device and very simple --the lower pivot of the balance staff runs in a jewel that sits in the top of a bearing attached to escape wheel pivot, and spherical balance spring has one end attached to the collet of the balance staff and the other end attached to the escape wheel pivot, so the escape wheel and balance are actually mechanically connected by the balance spring. 

The action is almost impossible to visualize without a diagram but basically the tension in the balance spring keeps a lever (with two pallets) locked against an escape wheel tooth when it swings in one direction, and when the balance swings in the other direction it unlocks the lever, allowing the escape wheel to advance.  Impulse is given not by the lever, but directly by the balance spring via its attachment to the advancing escape wheel.  It's a very odd animal, and although it is theoretically beautiful apparently there are some serious practical problems with it and I certainly would be surprised to hear of anyone adapting it to a wristwatch as to run at all it seems to require an extremely delicate and perfect symmetry in the power of the escape wheel vs. the tension in the balance spring.

To return to the ExoTourbillon Montblanc does say in their press material that there is about a 30% savings in power without the extra inertia of a full cage enclosing the balance.  Assuming that's true eliminating the cage and leaving only a carriage below for the escape wheel and lever (and I think an additional reduction wheel to make it a four minute rather than one minute tourbillon) does make for a more efficient tourbillon.

Jack