Since reading Dava Sobel's "Longitude", I'm obsessed by John Harrison's clocks. I liked the idea of building precision clocks almost entirely from wood, and the grasshopper escapement, seeming like a kinetic sculpture.
The fundamental principles of my clock project were:
- I wanted to make all parts by hand only with hand-held tools, such as a jigsaw and files. For machined parts, a mill and a lathe with manual feed were used, but strictly no CNC machining, no 3D printing, no laser cutting etc.
- The movement should be built mainly from wood. The only metal parts, made from brass and copper, are the arbors, pivots, and the Pendulum. The dials are from nickel silver, etched and laid out with black shellac.
- As little friction as possible and strictly no lubrication. This avoids the influence of oil getting resinous over time. The escapement wheel arbor uses anti-friction wheels, and the anchor knife-edge bearing on agate stones. All other bearings consist of beryllium bronze pivots in lignum vitae bushings.
- Harrison's unique Grasshopper escapement, which works withoutNo sliding friction and therefore needs no lubrication.
This is one of my first sketches. To minimise friction, the drivetrain is as simple as it can be, and consists of only one wheel and one single pinion. Both the great wheel and the escapement wheel have 120 teeth each. With a second pendulum, the escapement wheel rotates once every four minutes.
Only the great wheel with the minute hand, which is the most important to read intuitively, rotes clockwise. The escapement wheel and the hour dial rotate anti-clockwise, but as I wanted to keep the familiar direction of reading from left to right, the seconds and hours are read from a rotating dial against a fixed hand.
For the wheels, I have used fine-layered beechwood multiplex.
For the bearings, the pinion rollers and for the anchor pallets, I've used lignum vitae, a rare tropical hardwood that also Harrison had used. Lignum vitae, latin for wood of life, or officially guaiacum officinale, is onle of the densest hardwoords on earth, heavier than water, and it is saturated with an oil that does not change over time that makes it self lubricating and and gives it better friction properties than PTFE. It is an absolutely amazing stuff with a wonderful smell like gingerbread.
I have put some effort in optimizing the shape to the teeth of the great wheel. It is an modified cycloidal toothing on the great wheel and the lantern pinion with rollers on the escapement wheel, to reduce sliding friction.
I have created and assessed a dozen of variants to find a geometry with an absolutely even torque ratio and a definite transition of the contact point from one tooth to the next. For cutting the wheel, I have scratchbuilt a pantograph router to transfer the tooth shape from a template in 10:1 scale to the wheel.
Apart from the that, all wooden parts were sawn by hand using a hand-held jigsw, varnished with boiled linseed oil and glued with bone glue.
The escapement is the twin pivot type of Harrison's grasshopper, which I've calculated for a 120 teeth escapement wheel. The articulated pallets are carved from snakewood.
The so called Gridiron pendulum is maybe Harrison's most famous invention: It consists of alternating brass and iron wires. Since the thermal coefficient of expansion of iron is only two thirds of brass, the expansions cancel each other so that the pendulum keeps its effective length unchanged independent from temperature changes.
For barometric compensation, the suspension spring has to be suspended between circular cheeks that make the effective length of the pendulum shorter when the amplitude rises.
Conventional precison pendulum clocks, such as Graham's, require a very heavy pendulum with only a small amplitude. In total contrary, Harrison clocks require a comparatetively light pendulum with a considerably high air resistance and a high amplitude for the complex compensation to work: If set up correctly, the escapement error, the circular error, barometric changes and temperature changes cancel each other.
Because I wanted the clock to keep on running even while I'm away for a few weeks, I'v integrated an electrical windup. It is a Huygens-type endless rope remontoir. The windup mechanism is powered by a synchronous motor and is installed on a separate plate to decouple it dynamically from the rest of the clock. The windup is triggered every hour by a reed contact, and another reed contact switches it of again when the weight has risen to it's top position. This is an emergency cutoff if one of the reed contacts should jam for any reason.
The remontoir plate is suspended in felt pads for insulation of noise and vibrations.
The escapement wheel whereas floats freely on anti-friction-wheels that also Harrison had used extensively. The pivots of the escapement wheel have only rolling friction on the circumference of the anti-friction-wheels. The hubs of the friction wheels are turned from lignum vitae and run on beryllium copper pins. The anchor has an arbor from beryllium copper with knives edges and rocks in vee-shaped grooves manually cut into agate stones.
This youtube video explains the concept of the clock and its assembly in detail:
Regards, Norbert
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