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Carbon fibre pendulum rod

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John Haine17/07/2020 09:58:41
3334 forum posts
177 photos

Is the bottom glazed or open?

Alan Crawley17/07/2020 10:09:20
19 forum posts

It's solid wood, so weight don't fall below case.

Martin Kyte17/07/2020 11:02:37
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2057 forum posts
37 photos
Posted by Alan Crawley on 17/07/2020 09:33:14:

As a newcomer to clock making I read lots of information about it, and one thing i learned is that the weight of the pendulum is not important.

Until you start to consider sensitivity to disturbances. Changing the mass of the bob does not alter the period but does increase the stored energy.

regards Martin

Andrew Johnston17/07/2020 11:17:13
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5660 forum posts
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Posted by Alan Crawley on 17/07/2020 09:33:14:

.............one thing i learned is that the weight of the pendulum is not important.

Surely that's only true for an idealised simple gravity pendulum. It's not true for a compound pendulum, aka a physical pendulum.

Andrew

Alan Crawley18/07/2020 14:39:08
19 forum posts

Regarding the distance the weights descend and the height of the case, i have managed to calculate that it does not need sixteen turns on the barrels but about fourteen for a month going, and that there is probably just enough room if the weights are wound up to their limit.

I am completely out of my depth now with the information about pendulums that I don't understand regarding the comment I quoted about bob weight being unimportant. I read this in a couple of publications from respected clock makers, and during my process of crude lash-ups to see if my creation would run (an 'expert' told me it had no chance of running because I used involute tooth form) I used various objects on the pendulum rod ranging between around 2 kilo to the correct 4kilo as designed weight for no difference in timekeeping over periods of 36-48 hours. With the correct weight it has run several days spot on.

duncan webster18/07/2020 15:10:02
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2795 forum posts
41 photos

For a definitive view on cycloid vs involute see **LINK**

If you change the mass of a pendulum bob without changing the radius of gyration (RoG) it doesn't affect the rate, but as Andrew points out the bob isn't all of the story, the rod has mass which moves the effective centre up from the centre of the bob, and so making the bob heavier also increases the RoG. Setting Big Ben by adding pennies on top of the pendulum works by reducing the RoG

Radius of gyration is the distance from the pivot to the centre of oscillation see

**LINK**

SillyOldDuffer18/07/2020 15:53:46
Moderator
6324 forum posts
1387 photos
Posted by Alan Crawley on 18/07/2020 14:39:08:

...

I am completely out of my depth now with the information about pendulums that I don't understand regarding the comment I quoted about bob weight being unimportant. I read this in a couple of publications from respected clock makers, and during my process of crude lash-ups to see if my creation would run (an 'expert' told me it had no chance of running because I used involute tooth form) I used various objects on the pendulum rod ranging between around 2 kilo to the correct 4kilo as designed weight for no difference in timekeeping over periods of 36-48 hours. With the correct weight it has run several days spot on.

Everybody is right.

The formula for period of a pendulum making small swings is:

pendulum.jpg

Where l is the length of the pendulum, and g is the acceleration due to gravity. No mention of mass in the equation, therefore confirmed the weight of the bob doesn't alter the period.

But in a practical clock the bob has another vital purpose - it stores energy, keeping the pendulum going between ticks despite air resistance. It also helps keep the path straight rather than an ellipse, while its inertia reduces the effect of vibration.

At the extreme, a pendulum clock fitted without a bob tends to stop because the impulse can't store enough energy in the rod alone.  Conversely, the impulse may not be strong enough to keep an excessively heavy bob going, or demand too much power of the clock for good timekeeping. So, although any bob between not too small and not too big will work, for any particular clock there'll be an optimum bob weight, and it's worth keeping close to it because it improves running.

My only working clock so far was made of Meccano and it only had crude involute gears. All self-respecting clock experts should know that clocks can be made from a construction toy. But they're not wrong to be critical of them! Meccano clocks don't work well: difficult to adjust, poor timekeepers, and hard to keep going. A successful mechanical clock is a fight with friction, and a well-made gear train makes life much easier. Clock-makers believe cycloidal gears to be lower friction than involute. Not convinced myself; I think it would be hard to tell the difference between equally well-made clock wheels of either tooth form. I expect an expert will explain shortly why I'm wrong. Again!

DaveI

 

Edited By SillyOldDuffer on 18/07/2020 15:57:00

Andrew Johnston18/07/2020 16:35:26
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5660 forum posts
653 photos

I'm afraid SoD is incorrect. The formula he quotes is for a simple pendulum, the model of which explicitly assumes a weightless cord/rod and zero friction, neither of which is realistic.

Once the pedulum rod has mass then, as Duncan says, the equations get more involved and include mass. That's why a compound pendulum is also known as a physical pendulum.

Alan states that an expert said his clock wouldn't run due to involute gearing, which is plainly wrong. So why believe other experts when they say pendulum mass doesn't matter?

Andrew

John Haine18/07/2020 16:38:14
3334 forum posts
177 photos

Dave's formula is correct for a pendulum where all the mass can be assumed to be concentrated at the CoG of the bob. Real pendulums have distributed mass, some of which is in the rod above the bob; and the whole thing has a moment of inertia around its CoG (which is not the same as the CoG of the rod). The complete formula is:

T = 2 \pi \sqrt{\frac{I} {mgL}}

where I is the MoI of the whole thing about the pivot, L is the distance from the pivot to the CoG of the whole, and m the total mass. If you add mass to the pendulum above the CoG of the bob, the CoG moves up and the pendulum will speed up slightly.

If you make a pendulum to a design that used a steel rod but using CF instead, it is very much lighter than the steel, so will contribute much less to the MoI and because its mass above the bob is much less the clock will run slow for the same bob mass and rod length, so you may have to make the rod shorter.

For accurate calculation you ideally have to include each component of the pendulum and compute the "radius of gyration" and mass. This is easily done with a spreadsheet though it's tedious to make sure you have all the components. Then Excel can use its solver to find the exact length to give the period you want. For most purposes the answer is the same as the simple formula, but for example the pendulum on the replica "RAS" clocks being built by a BHI team, which uses a gridiron pendulum with a relatively small bob, the spreadsheet is quite elaborate.

Russell Eberhardt18/07/2020 17:20:37
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2600 forum posts
85 photos

I used a pultruded carbon fibre tube for the pendulum of my regulator. I used 7 mm O.D. and 1 mm wall thickness. It has supported a 5 kg bob for the last five years without problems. I used slow cure Araldite to glue steel fittings to the ends. The fibre tube manufacturer quoted -0.1 to -0.3 ppm/°C and I have no reason to doubt that. You have to remember that the bob, suspension, and rod end fittings will all have a positive expansion coefficient but the threaded end of the rod for the regulating nut, combined with the bob itself can be made to approximately balance the expansion of the suspension.

The only unexpected effect I noticed was that the clock gradually slowed down over the first six months or so as the rod crept under load but it then settled down. I guess that was a result of the stress being redistributed from the epoxy to the carbon.

Hope that helps.

Russell

Alan Crawley18/07/2020 17:41:45
19 forum posts

This has produced some interesting replies and I particularly liked Duncan's link about gearing. That's life-we all enjoy the situation of "tell us what we want to hear" I wish I had read it earlier and I would not have worried so much about many hours of work being in vain.

Lockdown has at least given me the time to spend on this project and after over sixty years in engineering/toolmaking, and nearly fifty with my own small specialist business, I can still get immense enjoyment from something new and different.

You won't be able to imagine my excitement after a few little setbacks, how excited I was when this clock settled down and ran on its initial test.

I have one other small query about the CF rod, will it be ok where it is subjected to the friction of the crutch? This consists of two 3mm silver steel pins in a brass disc that can be rotated to adjust clearance to the pendulum rod. I would like to thank everyone who has taken the trouble to help me with my project.

Tim Stevens18/07/2020 17:52:21
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1268 forum posts

You ask about flexibility of carbon fibre in this application. The forces on a clock pendulum tend to lengthen it (very slightly) each mid-tick, but not to bend or twist it, once the clock is going, so flexing is not an issue to worry about. Setting it up might cause twisting but only minor and it will spring back and overshoot a few degrees each side for a few seconds only. And a tube will resist twisting better than a rod of the same mass (but its air resistance will be greater).

The 'best' shape is a squashed oval in section, and Victorian wood pendulums are often this shape - but finding a metre of carbon fibre tube of this section might be impossible. The object of this shape is to reduce the air resistance to a minimum.

Hope this helps

Tim

Tim Stevens18/07/2020 17:52:22
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1268 forum posts

Sorry - came up twice ... !

You ask about flexibility of carbon fibre in this application. The forces on a clock pendulum tend to lengthen it (very slightly) each mid-tick, but not to bend or twist it, once the clock is going, so flexing is not an issue to worry about. Setting it up might cause twisting but only minor and it will spring back and overshoot a few degrees each side for a few seconds only. And a tube will resist twisting better than a rod of the same mass (but its air resistance will be greater).

The 'best' shape is a squashed oval in section, and Victorian wood pendulums are often this shape - but finding a metre of carbon fibre tube of this section might be impossible. The object of this shape is to reduce the air resistance to a minimum.

Hope this helps

Tim

Edited By Tim Stevens on 18/07/2020 17:52:59

John Haine18/07/2020 20:54:21
3334 forum posts
177 photos

I have seen elliptical cross section CF tube somewhere actually.

My pendulum rod is 10mm dia x 8mm bore CF tube. Being in an "Arduino" style clock it is impulsed every minute, and it is definitely observable that there is a small lateral vibration of the rod just after impulse. You can just see this by looking carefully, and it also shows up in the detailed electronic timing measurements though it doesn't seem to have any effect on the long term timekeeping. It isn't clear whether it is the rod flexing or caused by the impulse not being through the "centre of percussion", which for a very light rod is much closer to the bob.

Andrew Johnston18/07/2020 22:37:01
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5660 forum posts
653 photos

Elliptical, oval and rectangular carbon fibre tube is available from some model boat and aircraft suppliers.

Andrew

Emgee18/07/2020 23:26:52
1714 forum posts
231 photos

I'm sure that if anyone on here needs any specific shape or size of carbon rod/tube it could be produced by some of the current membership, at reasonable cost of course.

Emgee

Sam Longley 119/07/2020 07:55:25
786 forum posts
27 photos

In model RC planes I use arrow shaft blanks. Stiff, cheap & 1 metre long

Edited By Sam Longley 1 on 19/07/2020 07:57:33

Russell Eberhardt19/07/2020 11:40:27
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2600 forum posts
85 photos
Posted by Alan Crawley on 18/07/2020 17:41:45:

I have one other small query about the CF rod, will it be ok where it is subjected to the friction of the crutch? This consists of two 3mm silver steel pins in a brass disc that can be rotated to adjust clearance to the pendulum rod. I would like to thank everyone who has taken the trouble to help me with my project.

I had the same worry at first but my regulator has the same type of crutch and, after five years of running, I can't see any mark on the CF tube.

Russell

Russell Eberhardt19/07/2020 11:41:51
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2600 forum posts
85 photos
Posted by Emgee on 18/07/2020 23:26:52:

I'm sure that if anyone on here needs any specific shape or size of carbon rod/tube it could be produced by some of the current membership, at reasonable cost of course.

Emgee

That is if anyone has the equipment to do pulltruding. A normal lamination will not be stable enough.

Russell

Alan Crawley10/08/2020 10:04:11
19 forum posts

i have now made up a carbon fibre pendulum rod with glued and pinned threaded ends.

The original design with Invar rod has a brass tube inside the bob as a compensating device. I guess that this is no longer needed with carbon fibre, has anyone any information or thoughts on this, please?

The clock is still running with an ordinary mild steel rod and it is obvious with the current temperatures how much it affects the time. It is almost spot-on at 6-30 am but probably about 10-15 seconds behind in the early evening, and then is accurate next morning.

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