Dual axis pivot?

I've seen such a thing in minature. 18" pendulum that swings over a bed of sand tracing out lissajou patterns. It used 2 loops of hard wire inside each other.

If people are going to be playing under it, better go for safe rather than buck-rogers-ultra-low-friction in the design.

Short length of chain?

How about an inverted pin resting on a plate with the pin in the center of a yoke?

Earle Rich Mont Vernon, NH

Easiest, cheapest would be a spherical rod end or bearing, but perhaps too much friction.

A hard conical pin in a divot on a hard plate would be great, but I can't picture how to support everything without potential interferences.

How aboout this...

The pivots could be small ball bearings, or for lower friction(but more work), knife edges.

Ned Simmons

No flashes of inspiration here, but I hope you'll be able to inspire us all with a write up when you're done.

Steve Smith

Glenn Ashmore wrote:

Ah! Ha! The light turned on! I can do that. Maybe I can sacrefice some old thickness planer blades for the pivots and some rings of heavy walled oil hardening machine tube.

Thanks.

Mach> Typically these type of units use a knife edge setup to minimize

The ones I know of don't have a pivot at all. The wire itself is the pivot. What's critical for a Foucault Pendulum is that it be equally free to swing in all directions and the simplest way of ensuring that is to simply allow the thinnest wire that will carry the weight of the bob to flex. When pumping the pendulum by lifting the pivot it is essential that the pumping motion be purely vertical. Assuming a stainless wire with a yield strength of 400MPa and a four-fold safety factor I come up with a

0.053 wire (if I did the math right). And if I didn't I'm sure someone will gleefully point out the error.

Or some tapered roller bearing races for the rings...

On Wed, 07 Jul 2004 17:11:48 -0400, Glenn Ashmore calmly ranted:

That sounds terribly inefficient and bumpy, Glenn. Why does it have to jump like that, isn't that very visible, etc.?

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You are about right on with what I figured for the wire size. I was planning on 250# stainless big game fishing leader.

What has me worried about letting the wire flex is that this thing is probably going to be swinging long after I am gone sailing and I am worried about wire fatigue. The diameter of the pin circle is 12' so it will swing through about 26º. Also I am concerned that if the pumping solenoid is fixed it will tend to amplify drift caused by drafts in the room. The idea is to shorten the radius on the upswing so I think it should be in line with the wire therefore I am thinking it needs to pivot too.

All this because I menti> The ones I know of don't have a pivot at all. The wire itself is the

In that case you'll want to incorporate a safety into the design. That can be accomplished by having the wire pass through a hole a short distance below the pivot point just above the hole you need a disk larger in diameter than the hole that the wire passes through (and is firmly attached to). As long as all flexing occurs above the disk you don't have to worry about breakage below the disk. In the event the wire were to break the disk will catch the pendulum. And as long as the wire and the safety disk don't touch the catch plate it won't affect the pendulum.

There are some descriptions of lift pumped pendulums on the net. As I recall all use a straight lift for the pivot point. That's a simpler mechanism to build and reduces the chance of the pumping action disturbing the pendulum's track of the earth's rotation.

With a 40lb weight my gut feeling is that minor drafts aren't going to significantly affect the swing. That affect can be computed, but I don't feel like investing that much time it. I'll leave it as an "exercise for the reader".

BTW: How long will your pendulum arm be? I think the most impressive ones are the long period types, like the one at the Smithsonian.

How about a small drive line universal joint? Local auto parts store should have a wide selection.

Take Care, James Lerch

(My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance. Talent will not; nothing is more common than unsuccessful men with talent. Genius will not; unrewarded genius is almost a proverb. Education will not; the world is full of educated derelicts. Persistence and determination alone are omnipotent. Calvin Coolidge

Oooh, I like it!

Ned Simmons

Small ones are often found in model radio-control boats - they have some powerful motors now, should be one strong enough to hold your 50 pounds.

How about a small U-joint using a cross and some small ball bearings? Ken.

Oooooohhhhh, I LIKE that..... :-) Ken

Traditional is to let the support wire flex. It is typical to bring the wire out through a fitting that looks like the bell of a trumpet so that the flexing is limited and forced to happen along a length of the wire.

BTW I think that 25' length and 12' swing is going to be impractical. They usually have more like a

20:1 ratio than a 2:1 ratio of length to swing. I have only ever seen them set up where there is a tower or dome with a very high ceiling to swing it from. Before expending a lot of work, I think I would have the customer do some serious research on the subject.

-- --Pete "Peter W. Meek"

There are two potential places it can be mounted. One is from an overhanging balcony about 25' above the floor and the other is from the center of the main hall which is about 50'. It would be a lot easier to install on the balcony but the ceiling sure would be a lot more impressive. That 50' length would give it a nice graceful 9 or 10 second period.

Personally, I like the 50' swing better. And it has another advantage if you use a flexing wire in that the flex angle can be much less for the same footprint.