In my post “Come Together, Right Now” I glossed over some of the gravity escapement. This caused a comment/question to be submitted …
There?s a fan-looking like assembly to the left of the escapement assembly.. Is that an air governor to slow the speed of the strike train or is its purpose to cushion the action of the escapement itself?
Well, it is an air governor of sorts but the clock is a “single train” or “time only” so it’s not for a strike.
There is a drawing of the double three-legged escapement on page 91 of “A Rudimentary Treatise on Clocks, Watches and Bells” by Edmund Beckett. (You can download the eBook and the link is elsewhere in this blog.) The description starts on page 85, with the four-legged escapement and by the time you get to page 87, the “fly” is finally referenced.
The great feature of them is the regulation of the velocity and the avoidance of the banging on the pallets and the risk of tripping, either actual or approximate, by putting a common fan-fly on the scapewheel arbor.
So, it is, for all intents and purposes, an air brake. It is help in place by friction so when the escapement arbor is arrested on a stop, the fan continues to rotate forward by inertia and there by preventing a bounce. Further, it applies a small amount of resistance, at the start of the release, to prevent a trip.
The picture is an enlargement of the center of the fly. The fly is loose on the horizontal arbor. If you click on the picture and look closely, you will see there is a second “plate” that is slightly bowed. The second plate is trapped along with the fly between a fixed brass disk (on the right) and a pinned brass disk (on the left). The distance between the two disks is enough to allow the fly to rotate but close enough to compress the spring plate slightly – creating the necessary friction.
Early in the clock project I tried to explain the escapement. The description leaves a bit to be desired but I make reference to a video clip (from Mark Frank‘s website) that does a great job of showing the escapement and the fly in action.