Improved Experimental Pendulum

How does a vacuum affect the thermal conductivity of a metal to metal joint?

Joy of CAD shows there is room for a dummy pendulum, so both methods are do-able. There's enough space for the fake bob to fit between the swinging real bob and the pipe. Just a simple mock-up:

Dave

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Temperature surely needs to stable [not necessarily without some gradient] throughout the pendulum assemblage before any of this becomes useable.

I fear that we are both starting to adopt a ‘Not Invented Here’ posture, Duncan … so I will say no more.

Dave will do as he sees fit, and we will be suitably impressed.

MichaelG.

I blame the forum format because it makes it hard to follow developments, and sometimes there's a long gap between the problem being discussed (haphazardly), and me declaring what I'm doing about it.

Going back a couple of years, my original idea was to use a thin carbon-fibre as a spring - no suspension at all. Not satisfactory because:

• Carbon fibre rod is humidity sensitive, because (I think), it affects the springiness of the plastic matrix.
• When a conventional spring is inserted, the fibre-rod twangs, which I think explains some of the variation in my pendulum's period.

The fibre is bendy!

So I've switched to steel, assuming that the existing software temperature compensation would cope with it. In true time-nut fashion, that led to the realisation that it's the temperature of the rod that matters, and how to measure it. Plus the rod is well-insulated and will lag behind ambient by some time because it's heated by a circuitous route.

I don't intend to tackle the problem until running the clock in a vacuum shows this to be worth fixing. It's possible the clock and pendulum have so many other faults that this one will be undetectable! An Invar or much thicker fibre rod is also on the cards. It's all experimental!

Dave

I think you need to read my post slowly...

I did not say 'use a carbon fiber rod' - I suggested a single fibre, pure fibre, no plastic matrix involved to absorb moisture( which would nominally not be present in a vacuum anyway) - no twanging possible, much less temp coef than your steel, in fact nominally negative when made hot...

Yikes, I didn't know such a thing existed. Who sells it?

Dave

Joe, and Dave

Please see Bazyle’s post, here [in 2020] **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=167568

MichaelG.

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a Google search for

carbon fibre tow

will find plenty for sale, Dave.

Edited By Michael Gilligan on 20/04/2023 16:01:54

Normally metal to metal joints will have a ‘filler’ of air between the surfaces except at the points where the surfaces are actually in contact ( where the hills of surface roughness coincide). Remove the air film and you loose the conductivity provided by the air film.

Thats my interpretation anyhow.

You really need the rod temperature not the bob which is going to be hard to measure with the IR device. It’s maybe possible with a tiny thermocouple but fiddly and you would probably have to make it coaxial within the rod. The ideal way is the thermal duplicate rod but with a sensor to measure the expansion rather than the temperature but for a first approximation temperature would do.

regards Martin

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I disagree, but presumably [having read my recent post] you know that already.

MichaelG.

Why, string under tension, make a decent harp

Not quite sure which bit you were referring to. However I’m assuming that Dave’s bob is suspended in the middle so cancelling any thermal expansion. The bit you have to worry about is then the expansion of the rod itself. Hence my remark about needing the rod temperature not the bob. You also need the temperature along the length of the rod in order to have a factor that tracks with expansion. That’s also why I suggest measuring the expansion of a dummy rod directly essentially giving you a single number which will track to period.

regards Martin

Edited By Martin Kyte on 20/04/2023 16:59:27

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The bit that I quoted

MichaelG.

It does not have the flex springiness of a carbon fibre rod, ie a matrix of fibres in resin. It's mass is nill and I would think would be superior to a 'solid' rod - unless the rod was of significant dimension and mass so that it's inherent resonance is far lower than the pendulum rate.

Joe's other post was right - I hadn't taken his point properly!

Based on what I found with a single rod, there's a tendency to twist when the bob is impulsed, perhaps because the magnetic field isn't quite aligned with the bob's flight path. Although better engineering might fix this, in a two string swing there's very little to stop oscillation around the Z-axis. Gut feel is strings have more degrees of freedom than a stuff rod and that a flat suspension spring at the top is better at resisting and correcting Z rotation. No numbers backing me up though!

Tradition doesn't help. Most pendulum clocks have rod supported bobs, but maybe that's because they're usually impulsed at the top, and top impulsing a string hung bob wouldn't work. As I'm impulsing the bob at the bottom, maybe a child-swing type pendulum would be OK. Got to be worth trying - more work!

Dave

Zero mass? that's an interesting concept. The mass might be low, but a string under tension will still go twang. It could well be that it will have no discernable effect.

However, a very thin string will have little or no stiffness in the plane normal to oscillation, so could it start acting as a Foucault pendulum, is precess due to earth's rotation? 2 strings forming a very shallow vee with the bob at the bottom sounds more sensible then it has resistance to swinging sideways.

You could make impulsing at the top work with a string pendulum if you used something similar to the Reifler method, ie shift the suspension point sideways at the centre of swing (or just before even better)

Edited By duncan webster on 20/04/2023 18:40:28

Edited By duncan webster on 20/04/2023 18:50:49

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Save that they didn’t have the luxury of carbon fibre, this was the system used for some of the earliest serious experiments.

MichaelG

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Ref. __ https://www.alamy.com/illustration-showing-bifilar-pendulum-vll-invented-by-the-accademia-del-cimento-from-saggi-di-naturali-esnerienze-fatte-nell-accademia-del-cimento-florence-1691-2nd-edition-image235248347.html?imageid=24D008B9-F3C8-42AD-83C6-88F8891622CB&p=75935&pn=1&searchId=8ecdbc311ab8b5397f21163e8311f44e&searchtype=0

Edited By Michael Gilligan on 20/04/2023 19:22:19

I thought that is what I typed...

is it not practical to use a fibre to support the bob - each end of the fibre fixed say 100mm apart at the top of the pendulum support structure and the bob fitted at the apex of the fibre down below. That would eliminate lateral swing.

regarding the twang...The string twangs on a string instrument since it is stretched between two FIXED ( grounded) points.

In the pendulum, one point moves, and the string tension is due to the bob's weight only. If the bob is pulsed in the direction of the swing, there is nothing to twang the string - for practical discussion, that single fibre has NO mass - it is maybe 10-15um diameter. It will also not drag behind the bob position as it is in a vacuum...

I just hung a 4 kg vice from 1.4 m of fabric tape (couldn't find any string) top hooked to my lifting beam. If plucked in the middle it vibrates. Not for very long, too much damping. Agreed it won't twang if you're careful about impulsing, but Michael's post just said it won't twang.

I must read other's posts more carefully re 2 strings! Again you'd need to be careful with impulsing to make sure the bob didn't rock around the string/bob interface

You almost got to rock around the clock there Duncan

😋

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You do me [or perhaps Joe] a dis-service, Duncan !

Michael said no such thing.

MichaelG.

And so does a steel rule held flat on a table top with the other end in mid air.

I seem to remember a school physics experiment with a rope that demonstrated standing waves between two fixed ends and travelling waves when one end was free? It was a long time ago, and I wasn't paying attention! But I'm pretty sure my thin carbon fibre pendulum rod vibrated when it was impulsed, obviously so when impulsed hard.

Dave