Zenith Defy Zero-G Engineering Analysis

Different comments have already been posted in various forums about the polarizing exterior design of the Zenith Defy Zero-G (ZZG). I would like to add some thoughts from a pure engineering point of view.

An ‘exploded view’ of the ZZG movement was presented in the Aril/May issue of the German ‘Chronos’. The picture seems to be taken out of a press package by Zenith. I didn’t manage to find it on the web yet. If someone has access to the picture, it would be nice to have it posted here.

First some appreciation. The layout of the ZZG movement is absolutely unique. The escapement mechanism is fitted on top of a freely rotating gimbal-mounted platform integral to the base movement. The concept is supposed to eliminate the impact of gravitation on the isochronism of the escapement mechanism by keeping the balance wheel in a horizontal position independent of the orientation of the base movement in space. From an engineering point of view the most amazing feature of the ZZG movement is the gear train between the spring drive and the escapement mechanism The design of the gear train fulfills two major requirements: 1. the rotation of the second wheel shall be transmitted from the platform across the two gimbal hinges to an intermediate wheel at the base movement. 2. the free rotation of the platform itself around the 1st and 2nd hinge must have no effect on the transmission from the second wheel to the intermediate wheel (assuming that the rotational speed of the second wheel is fully determined by the escapement mechanism). The first requirement is easily fulfilled by a set of three bevel gears surrounding the gimbal at a 90 degree angle. The second requirement however can be considered as a real challenge in mechanical engineering. The solution is intriguing: a second transmission across the two gimbal hinges is put in parallel to the first one. The gear wheel at the starting point of this second transmission does not rotate, but is fixed to the base movement. As a result the gear wheel at the end point of the second transmission exactly reflects the angular deviation which would occur in the first transmission by the free rotation of the platform. This deviation is then fully compensated at the platform with the help of a tricky twin planetary gear set. It is obvious that this compensation could also be achieved with the twin planetary gear set positioned at the base movement end of the two transmissions, but the perfect integration into the 2nd hinge of the gimbal-mounted platform is by far the more elegant solution.

And now some concerns. One of the biggest problems in watch making is friction. Friction opposes accuracy because it is not deterministic. In short timeframes it changes with load, velocity and lubricant temperature, in larger timeframes additionally with wear and lubricant aging. The worst however, is the stick slip effect. Whenever the shafts and wheels are supposed to start moving (and they stop and start about 4 times a second) the spring force has to overcome the stiction forces within the bearings and between the teeth of the gear wheels. The distribution of roughness and lubricant on the surfaces inside the bearings and on the tooth flanks is stochastic from a microscopic point of view. As a result the impulse force transmitted to the escapement mechanism varies slightly depending on the momentary position of each contact surface in the whole movement. The varying impulse force has a bad impact on the isochronism or the accuracy of the movement. The figure below was taken out of a patent granted to the Hamilton Watch Company in 1950. It shows a single planetary gear in the gear train between the escapement wheel and the intermediate wheel of a marine chronometer. The planetary gear arrangement is supposed to allow an adjustment of the second hand position without stopping the movement. I have never seen such a feature in reality. I believe that the idea was given up due to the bad impact of the additional friction on the accuracy of the chronometer.

Looking at the movement of the ZZG again it is obvious that there is a much higher number of contact surfaces involved in the transmission between the intermediate wheel and the escapement wheel compared to a conventional watch movement. It does not require much explanation to see that the number of contact surfaces in this transmission correlates with the variation of the impulse force applied to the balance wheel resulting in a bad impact on the isochronism or the accuracy of the movement. I don’t expect any test reports about the accuracy of the ZZG to be published, but my guess would be that the advantage of eliminating the gravitation impact on the isochronism is completely offset by the friction impact.

My conclusions must not be considered as a disregard of the ZZG concept. The opposite is true. The unique layout of the ZZG movement, the sophisticated details of the mechanical design and the superior craftsmanship required to manufacture such a time piece are truly admirable. However, I consider it more as a micro mechanical work of art rather than a technical breakthrough in horology.

Best regards from Germany,

Heinrich 

P.S.: I wouldn’t be too disappointed, if I had to correct my conclusions regarding the ZZG concept one day.