16th Century Regulation
Posted: Thu Oct 24, 2019 7:25 pm
I've discussed various escapements in this thread, but I've never touched on how the balance is actually controlled. Nowadays, of course, it's controlled by the volute hairspring we're all familiar with. But that hasn't always been the case. The first idea of a (metal) spring controller was conceived by Robert Hooke in the mid-seventeenth century, several years before Christiaan Huygens's discovery of the same concept. Incidentally, Hooke also invented the anchor escapement, the simplest and most commonly used pendulum escapement. But how was the escapement controlled before his discovery?
As a case in point we'll consider a sixteenth century German watch housed in the British Museum.
It's fitted with the typical escapement controller of the day: a hog's bristle regulator. The name isn't metaphorical; inside the watch is a single hog's bristle, chosen for its convenient stiffness. It's held in place by two arms, which allow considerable adjustment of its acting length, just like a modern eccentric screw or swan neck regulator. The escapement it's controlling, by the way, is a verge. Unfortunately, there aren't any photos of the exposed regulator, so this image from Frederick Britten's "On the Springing and Adjusting of Watches" will have to do. If you're interested in diving deeper into the history of escapement controllers, here's the relevant section of the book.
We're not quite done yet, though; our British Museum watch offers another regulating curiosity. The mechanism visible in the picture below is called a "stackfreed," and it's an early constant force mechanism.
The long arm across the top presses on the snail-shaped cam, which is attached to the barrel. The arm pushes back against the force of the mainspring, retarding its force. As the cam rotates, it pushes less and less on the mainspring, which, as it unwinds, exerts less force on the movement. The effect is a relatively constant force. However, that force is pegged at close to the mainspring's minimum output, making the stackfreed a very inefficient mechanism. It was mostly used as a simpler and easier to produce version of the fusee and chain, which I wrote about here.
Note that the barrel turns through only a single rotation per wind. You can see that the watch in the picture above is completely wound down, with the winding pinion resting on a solid portion of the wheel under the snail-shaped cam. That solid section acts as a "stopwork," preventing the barrel from being wound above a certain point or running down past a certain point. This cuts off the particularly erratic top and bottom of the mainspring's range, further limiting force variation.
As a case in point we'll consider a sixteenth century German watch housed in the British Museum.
It's fitted with the typical escapement controller of the day: a hog's bristle regulator. The name isn't metaphorical; inside the watch is a single hog's bristle, chosen for its convenient stiffness. It's held in place by two arms, which allow considerable adjustment of its acting length, just like a modern eccentric screw or swan neck regulator. The escapement it's controlling, by the way, is a verge. Unfortunately, there aren't any photos of the exposed regulator, so this image from Frederick Britten's "On the Springing and Adjusting of Watches" will have to do. If you're interested in diving deeper into the history of escapement controllers, here's the relevant section of the book.
We're not quite done yet, though; our British Museum watch offers another regulating curiosity. The mechanism visible in the picture below is called a "stackfreed," and it's an early constant force mechanism.
The long arm across the top presses on the snail-shaped cam, which is attached to the barrel. The arm pushes back against the force of the mainspring, retarding its force. As the cam rotates, it pushes less and less on the mainspring, which, as it unwinds, exerts less force on the movement. The effect is a relatively constant force. However, that force is pegged at close to the mainspring's minimum output, making the stackfreed a very inefficient mechanism. It was mostly used as a simpler and easier to produce version of the fusee and chain, which I wrote about here.
Note that the barrel turns through only a single rotation per wind. You can see that the watch in the picture above is completely wound down, with the winding pinion resting on a solid portion of the wheel under the snail-shaped cam. That solid section acts as a "stopwork," preventing the barrel from being wound above a certain point or running down past a certain point. This cuts off the particularly erratic top and bottom of the mainspring's range, further limiting force variation.