Carbon fibre pendulum rod

Ordinary steel varies with temperature by about 11 parts per million per degree C. Invar reduces this to about 1.2ppm/°C I guess the brass tube (between 16 and 21ppm/°C) is arranged to counter-balance the Invar.

Carbon Fibre is in the same class as Invar, or slightly better, between -0.74ppm/°C and -1.25ppm/°C depending on mix. But note that Invar has a positive coefficient of thermal expansion, while Carbon Fibre is negative. It means Carbon Fibre is also worth compensating, but the design is different. A Brass compensator built to correct Invar would make Carbon Fibre worse because it subtracts instead of adding.

At this point I feel the need for a clock expert - I know nothing about designing a real compensating pendulum! However, must be a solved problem. I'm sure one of the clock experts will know.

Dave

I think the brass tube is to allow the bob to be suspended in the middle of it's length. I'm assuming a brass bob. This allows for a rating nut to be at the bottom of the tube. The principle is that the bob expands with temperature too. If you suspend from the bottom and raise the temperature the centre of mass moves up and reduces the efective length of the pendulum. Suspension from the centre of mass ensures half the expansion is downwards and half upwards keeping the centre of mass at the suspension point.

Harrison didn't try and bring all variations to zero but used the effects of temperature, pressure and air viscosity to work in balance to acheive near perfect compensation. In particular he employed circular error to balance other effects by his adjustable suspension cheeks.

The Cambridge Trinity Clock website is a mine of information on practical compensation.

**LINK**

regards Martin

The carbon fibre will not expand downwards, it seems, like almost every other pendulum candidate. It shrinks when warm. So, I suggest that the 'compensating' needs to be the other way round too. If you add brass, say, it might be that you use a pendulum which is x mm too short and fill the space with brass. The dimension x will depend on the exact coefficients of brass and carbon fibre over the normal temperature range. Nothing magic about brass - if you chose a different material to extend the carbon, x would need to be changed too. This gives the possibility of a decorative bob on the pendulum, using material which gives the mass required in a streamlined form while adding the needed downward expansion.

I use the term compensating to mean adding a change which compensates for an undesirable (but necessary) change.

Or have I got it wrong (again) ?

PS my Vienna regulator seems to work sensibly with a wood pendulum and a brass bob resting on an adjusting screw. So, the wood expands downwards a bit and the brass expands upwards more. Once adjusted after a house move it rarely seems to get out of kilter.

NB as gravity varies from place to place, there will still need to be an adjustment in any clock, however clever the compensating devices are. And it varies with sea level, moon position etc etc ...

Cheers, Tim

Edited By Tim Stevens on 10/08/2020 11:12:23

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My clock with CF rod terminates the rod at the halfway point of the CI bob. There is a short M4 stud projecting from the rod, and the "nut" that holds the bob is an MS bar that nearly fills the lower hole to try to get equal amounts of iron above and below the bob centre. The period is slightly less than 2s, deliberately, and the displayed time is corrected by slipping it back by a second every so often (148 swings currently). The dial is an original Synchronome one without seconds hand, driven by a stepper motor, advancing the minute hand by 1/400 rev every 9s. The clock sits next to a radio controlled clock and at the moment whenever I look at the dial it seems to be within 15s or less of "real" time.

The rod is suspended by a relatively short beryllium copper spring clamped between brass chops - the expansion of the spring can't be neglected in the overall thermal compensation (though I don't know what it is!).

Pendulum period is measured by a "picPET" that counts 10 MHz pulses from an OCXO, the readings being logged by a Raspberry Pi and downloaded every few weeks. This system has a resolution of better than a microsecond. The Pi also logs temperature and pressure from a Bosch BMP280 sensor.

The clock is definitely sensitive to temperature, and analysing the results gives a rate variation of ~ +2 usec /*C. That is, the rate increases with temperature. This may be the rod having a slight positive length coefficient, or the suspension. Another issue is that atmospheric pressure varies, and air density depends on pressure and temperature. At the same pressure, if the air gets hotter its density decreases, so the bob weighs more because buoyancy is less, so the period would decrease. But also the drag decreases, amplitude increases, decreasing the period due to circular deviation! And as amplitude changes, the escapement error changes and that has another effect! So disentangling all this is rather complicated - I'm logging pressure and amplitude as well as period and so far have not reached any real conclusion. At the moment I do have an experimental brass temperature compensation sleeve fitted above the bob but I couldn't really say yet how much affect it has had.

Harrison was aiming to balance all these effects out in his RAS clock on which the Burgess "Clock B" at Greenwich is based. Certainly Clock B succeeds in this. However nether Clock B nor what survives of the RAS clock has adjustable suspension cheeks though the latter has signs of something that might have been intended to allow that. Also both use circular cheeks, which don't compensate circular error except over a very small amplitude range. Regulating Clock B required selecting-on-test the suspension spring to have the right flexibility to wrap to the cheeks a suitable amount, and adjusting the running amplitude using the remontoire to get zero sensitivity to pressure changes.

Good point Barry. My practical experience with carbon fibre is suspending an electromagnetically pulsed pendulum to test a prototype Analyser (rather like the one John describes, except his works!)

I was struck by how accurate my ultra-simple pendulum was. Though the impulse was adjusted to minimise disturbance it had no temperature compensation and swung in the open on a wobbly Dining Table, far from high-tech. Basically a steel cylinder on the end of a 400mm long carbon fibre kept time as well as a good wristwatch. Not as good as a precision timepiece, but noticeably better than all my cheap Digital Clocks.

I ought to try again. Measuring thermal expansion would be a start. Bought off ebay with no technical details whatever. Although it must be good, I've no numbers from which to derive temperature compensation. And who's to know if another length bought from ebay would be equally good. At least buying Invar there's a trustworthy specification.

Dave

There ought to be when you see the price! CF is cheap as chips by comparison. I did dream up a scheme to measure the CTE by mounting a length on the bench with a DTI at one end in an insulated tube, passing a current through the rod to heat it up. Actually I suspect that CF has such a low CTE it will be insignificant compared to other effects.

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The Invar pendulum rod on my regulator expands when cold and contracts when warm. I was told by a BHI member

that it should have been heat treated. The suggestion was two days in a butchers freezer then two days at room temperature. Repeated until it settled down. I didn't bother and by adding or subtracting two 10BA brass washers to the tray it keeps time within a couple of seconds a week. I was also advised to use the same BBC time signal every time I check.

Brilliant thread that has demonstrated the strength of this site........

Some points that might help in no particular order....

1) Invar has to be 'aged' to get the full CTE...it's not straightforward.

2)western red cedar has a reputed zero CTE and is not hydroscopic

3)bamboo has zero CTE but IS very hydroscopic....can be sealed with strong shellac etc

4) the best way of interpreting pendulum 'questions' is by evaluating their 'Q'...which is a measured calculation of their efficiency

Edited By Bob Stevenson on 10/08/2020 16:52:59

Recommended Reading

Matthys : Accurate Clock Pendulums

**LINK**

https://books.google.co.uk/books/about/Accurate_Clock_Pendulums.html?id=cJZBbsuCZRQC&printsec=frontcover&source=kp_read_button&redir_esc=y#v=onepage&q&f=false

MichaelG.

The supplier of the pultruded CF tube I used for my regulator quoted -0.1 to -0.3 ppm/C. I used a threaded bottom fitting for the rod for use with a regulator nut at the bottom of the bob. I calculated that making the transition from CF to steel 30mm below the centre of the bob would compensate for 0.2 ppm/C for the rod with a small positive contribution from the suspension.

Seems to work fine.

Russell

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My first clock was a regulater which as I had no tools was built with a home made turns and hand cut teeth.The pendulem was a length of straight grained deal with a heavy lead bob.When the clock was running I rated the pendulem with lead fishing weights and coins.The best I achieved was 2 seconds a day for a fortnight.I know that the post is about carbon fibre about which I know nothing,but the killer of all manual timepieces is friction ,reduce that and the difference between Invar and carbon fibre will hardly matter.

Frank

Hmm, well, up to a point! High (or low) Q doesn't help with CTE for example. High Q pendulums are more sensitive to horizontal motions of the whole clock, but less sensitive to internally generated noise (e.g. impulse variations). Very accurate clocks - Shortt and Fedchenko for example - have their master pendulums in vacuum for high Q (~100,000) and isolation from barometric variations; but Burgess' Clock B works in air, and has a Q of about 5,000 or less, but is probably the most accurate purely mechanical clock ever. On the whole I think high Q is to be preferred but it depends...and the Q topic is immensely divisive amongst horologists.

I did measure the Q factor of my pendulum with all fittings. I can't find my notes just now but I think it came out at about 6000. Easy enough - set it going and count the number of cycles it takes to decay to 37% of it's starting amplitude. Multiply that number by 2 x Pi to get the Q. The most important factor is the air resistance so best to use lead or even uranium for the bob and an aerodynamic shape. I stuck with steel not having uranium to hand!

Russell

I’m beginning to realise my IQ is too low for this conversation!!

Let’s try to simplify my original question- do I need any compensation for the Carbon Fibre rod?

My main interest is going into my little workshop and making nice bits. To be honest to me this engineer- designed regulator is running with amazing accuracy with common BMS rod.

This is not to say I am not grateful for the interest shown.

For your purposes, and most others probably not.

regards Martin

Edit:- comma after "to be honest"

Accuracy is the really big challenge in clockmaking. Every right to be pleased with a clock that runs and even more delighted if looks well-made. Unfortunately visitors will only be really impressed if it keeps time correctly. Experts know what to look for and a mechanical clock accurate to within a minute per year will enthuse anyone who knows about clocks.

By horological standards your clock can't be running with amazing accuracy because it's rate must vary with season, weather and the central heating due to the pendulum. Easy to do better than BMS by substituting a material with a low temperature coefficient of expansion: wood, Invar or Carbon Fibre. And maybe even better by also fitting a compensator. Probably not worth compensating a CF pendulum because other mechanical  shortcomings are likely to swamp temperature errors.

They way to find out is to monitor the clock's timekeeping over a long period. Does the rate of gain or loss vary with temperature? Worth fixing if the link is obvious.

Dave

Edited By SillyOldDuffer on 11/08/2020 21:20:27