Air Bearing Clock
A clock pendulum is normally supported by hanging it from a suspension spring or knife edge. Both solutions work well when carefully designed and constructed. Some problems still exist (note: these are minor for most clocks):
The location of the "pivot point" of a suspension spring has been a question of debate in horological circles. (references later)
The properties of the spring may change with temperature and effect the performance of the clock.
The shape (initial and after wear) of a knife edge effects the performance of the clock. (references later)
This experimental clock was constructed using an alternative technology in an attempt to resolve the issues mentioned above. Air bearings have the advantage of providing very low friction, no wear and very low dependence on temperature. They are not a practical solution for every day clocks because they are expensive and require a noisy air compressor to function but they may be useful as a test stand for experiments.
Early air bearings were very expensive because they were custom made and required very close tolerances. They were very easy to damage became unstable air pressure and load were carefully controlled. The advent of porous carbon air bearings made this clock possible. Porous carbon bearings are much less fragile - you can score them with a knife and they will still work. Porous carbon bearings are available as "off the shelf" design components from New Way Bearings (www.newwaybearings.com), The air bearing and the "pillow block" that supports it for this clock cost approximately $200.
A pair of pillow blocks could have been used to construct a clock with a single pendulum rod, but because of alignment issues and cost, I chose to use a single bearing and a pair of rods in the configuration shown below. The dual rod configuration provides some additional flexibility in where the impulse can be delivered but also adds additional aerodynamic loses. The only other dual rod pendulum I have observed is the Robertson clock at Bristol University.
An overview picture of the clock (without the impulsing components) installed on a 10" thick poured concrete wall in my basement is shown at the right. The aluminum channel and the upper cross bar are attached to the wall with 3/8" expansion bolts that pass through isodamp (no bounce) rubber pads. The air bearing (bushing) is attached to the black anodized aluminum beam at the top of the channel. Axial motion of the shaft through the bushing is controlled by opposing magnets at each end.
The pendulum bob is a 3" dia by 8" tall steel cylinder from a Synchronome clock. The bob weighs approximately 13.5 lbs. The bob is currently supported by a pair of 3/8" threaded rods and a pair of Lexan plates. A MicroSet sensor is mounted on an adjustable support in order to make measurements of period and amplitude.
