Sunday, 23 February 2014

An "In Beat" Design by W. Ehrhardt.

W. Ehrhardt Full Plate Keyless. 1920
At first glance this is a fairly standard full plate keyless watch, albeit a very late one from 1920. However things  are rather different under the covers, instead of a normal balance cock this design has the balance cock mounted on a ring that fits into the top plate of the watch.

This design makes the watch rather more robust, slightly slimmer (although it is still considerably thicker than a three quarter plate) and makes it very easy to get the watch "in beat".

Being in beat means that the balance action is symmetrical swinging an equal distance in each direction and that the balance staff, impulse jewel and pallet arbour are in perfect alignment. If you think in terms of a long case clock the "tick" equals the "tock" and it has a steady rhythm.

This can normally be a tricky operation involving quite a lot of work and often much fiddling with the hairspring which always has some risk attached to it. With this design the impulse jewel can be lined up simply by rotating the balance cock to the correct position and locked there with the two screws that hold it to the movement.


Three views of the movement, left before fitting the balance cock, the lever pallet can just be seen in the "well" where the balance fits, centre is the balance cock and regulator in place and right with the balance assembly in place (click on the image for a larger view).
After initial adjustment this example was within beat to 0.14%,  better than my  modern Rolex which is still excellent at 0.45%.

Unfortunately it did little for Ehrhardt who was out of business shortly after, or for watch design as the full plate movement was already obsolete by the time this was made, replaced by the three-quarter and split plate movements.

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