It sounds like your alternative has to be to just use the lowest friction bearing type. That lead me down the rabbit hole from simple ceramic bearings through magnet bearings to air foil bearings….presumably you can also do something to the shape of the pendulums to reduce their air resistance… which you keep them runnng longer but may reduce 'chaos-icity' (I think that's a new word)..

pgk

Don't both of those work in one direction using gravity to keep the parts in place? would have thought the arms flailing about on on this pendulum would see it flying off or at the very least not keep bearing contact through the whole 360 deg rotation?

I would have thought that as the pendulum gets vertical at very slow speeds then gravity will try to push the contacts apart, just think of the direction of the arrows being reversed on your unipivot link

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Very likely, Jason … but the Unipivot concept can be extended to involve a pair of opposed bearings, acting like the cones on the ‘scape wheel of a traditional Alarm Clock. … It was really the question of bearing friction that I was trying to discuss.

MichaelG.

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P.S. ___ Likewise, Harrison’s pair of wheels could be increased to a trio … it’s his use of ‘leverage’ that interests me.

Edited By Michael Gilligan on 02/10/2020 08:25:12

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Sounds a very good idea … but given the unpredictable motion, I suppose there is a risk that they would ‘wind-up’

[ or do you have a devious work-around for that ? ]

MichaelG.

But the bright bit was thinking about alternative low-friction bearing configurations

… Even if they are impractical for this particular device, all variants are useful waypoints on Robin’s road to enlightenment.

MichaelG.

I think the same general rules apply to chaotic pendulums as those for normal clock pendulums in that air and bearing frictional losses determine the system Q. Given the degree of freedom needed ball races are probably the the simplest bearing arrangement. However to reduce the effects of air friction, like the Shortt clock, the pendulum needs to operate in a low air pressure or vacuum environment which has its implications. So why not apply the same method of maintaining oscillation as a pendulum clock by giving the pendulum an occasional impulse. This can be done electro-magnetically, in a similar way to that being devised by SOD for his pendulum clock. For a conventional pendulum clock the impulse is usually provided on a regular basis with a defined amount of energy added to maintain pendulum amplitude and precise timing. However for a chaotic pendulum the impulse energy can be varied to give different levels of excitement to its movement.

Clive

As we are no longer treating this as an academic problem … The most practical and cost-effective ‘production engineering’ solution would probably be to base the design on a cheap bought-in bearing assembly

… a ‘Fidget Spinner’

MichaelG.

I think the coefficient of friction will be much the same whatever the size of the bearing (of the same design, materials and tolerances)

What is different is the amount of energy needed to overcome inertia and the amount of metal in contact. A big bearing is sized to manage bigger loads as much as it is to reduce friction. As a percentage of the total energy in the system, the friction of a car wheel bearing is low, but the bearing would be unsuitable in a fine clock because it's far too heavy. Conversely jewelled spindle bearings are far too light for rough work.

The cheap bearings I initially fitted to my Ridder's Coffee Cup Stirling engine were significantly stiffer than expensive ones of the same size. Removing the seals to reduce friction helped, but allowed dirt into the bearing – they soon stiffened up agaoin. On a low-power engine, better bearings are essential. Probably the same applies to Chaotic Pendulum bearings. The smallest good bearing that will do the job.

Taking a cheap bearing apart didn't reveal obvious defects, so I think the difference is due to better tolerances all round in the 3x more expensive version. I bet Ketan knows!

Dave

Rex Swensen in Australia is the expert on this.

https://www.bocabearings.com/general/ball-bearings-in-clocks

**LINK**

What about something like this…