WM9v2 Winding Efficiency

[iceberg888]

Question for all you lucky WM9v2 Sub owners: does your watch get sufficiently wound from normal daily activity?

I've read that you need to be really active to keep a ladies automatic movement fully wound. The reason is detailed in the post below:

Micro Rotors in Automatic Watches

Posted by Walt A. on August 19, 1998:

The physical measurements of the rotor have a profound effect on the winding efficiency of a watch. Many women complain about their Rolexes stopping at unpredictable times and frequently take the watches to jewelers with the hope of correcting an assumed manufacturing problem. In actual fact, it takes a very active woman to keep a small-sized ladies' automatic running reliably day after day. No mystery about it -- it's all in the physics.

Consider this: Your wrist motion transfers kinetic energy from your hand to the rotor. The rotor temporarily stores this energy as rotational kinetic energy, then in turn transfers all its kinetic energy (via the winder gears) to potential energy stored in the form of mainspring tension. When the rotor stops, you know that ALL the energy from your wrist's last motion has been sent to the mainspring -- excepting a little loss to friction in the intervening gears.

The kinetic energy of a rotor is given by:

E = 1/2 I(w^2)

where I is the MOMENT OF INTERTIA of the rotor and w is the rotor's angular velocity in radians per second. This is 2 pi (about 6.28) times the revolutions per second.

The big issue here is the moment of inertia. Every tiny particle of mass comprising the rotor contributes to the rotor's moment of inertia about its pivot in proportion to the particle's mass (i.e., what it's made of) times THE SQUARE OF ITS DISTANCE from the pivot. So you can see how important physical size is to the rotor's ability to store and transfer kinetic

energy.

To get a good idea of this importance, consider that if we start with a 15mm rotor of a ladies' watch, say, scale it up perfectly by a factor of 2 in all dimensions, so that it is now a 30mm rotor, still made of the same material, the larger rotor will have a moment of inertia 32 TIMES (!) that of the smaller one. This also means that it can store up 32 times as much energy as the small one when it is spun up to a certain angular velocity.

I believe Patek, Lange and a few other high-end manufacturers get away with using their mini-rotors because they have exemplary winding gear trains, almost free of friction and also use very dense (hence also expensive) materials, as Mycroft indicated. As for the rest of us 'peasants', we are better off with Krugerrand-sized rotors winding our hockey-puck-sized watches.