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| Paxolin rod/bar anything similar? |
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| Lew_Merrick_PE:
--- Quote from: craynerd on May 22, 2011, 02:29:48 PM ---That is a good question Lew, I am pretty sure it was m/m/oC. So tufnol at 1.9 x10e-5 m/m/oC which would make it a little less than aluminium! --- End quote --- Chris, The basic design of a pendulum is to concentrate the mass at the bob and setting your period as t = 2*pi*(l/g)^0.5 where 'l' is the length of the pendulum and 'g' is the (appropriately unit-based) acceleration due to gravity. If we assume that "indoors" is the environment, then somewhere in the 20°F is going to be your maximum delta-temperature. At 6 inches nominal length and 20°F your nominal delta T, then aluminum (12 X 10^-6 in/in/°F) would have a delta-l of .00144 inches. Steel would give you a value half that. Steel will be (nearly) 3X as stiff (27 X 10^6 psi tensile modulus as opposed to 20 X 10^6 psi for aluminum), so the only question is which you could turn to a thinner wall tube to minimize mass? The answer is almost certainly aluminum -- though you can get very thin-wall steel tubing that might be a better choice. I don't know the numbers in metric, but most steels are about .284 lb/in³ while most aluminum are about .099 lb/in³ and most plastics run in the .05 lb/in³ range. The general factor of stiffness for a symmetric column or beam is EI where E is the tensile (or compression) modulus and I is the area moment of inertia. I would assume that you would want to maximize the stiffness while minimizing the mass of the pendulum's connector. Right? |
| raynerd:
Lew, Would stiffness really be such a consideration with any of the materials we are discussing in the situation that it will be in? Don`t get me wrong, it needs to be stiff but would the period really vary due to this factor whether aluminium, steel or plastic was used? I do understand most of what you have written but I`m curious why such a prolific clock builder, John Wilding, would have suggested Paxolin as the most suitable material if it was not the case - aluminium, brass and steel are used elsewhere in the clock build so he clearly could have choosen this during the build - yet he specifies Paxolin. Regards Chris |
| andyf:
FWIW, most Foucault's pendulums you see in museums etc (one is shown here http://en.wikipedia.org/wiki/Foucault_pendulum ) have their bobs suspended on flexible wire. Andy |
| Boz:
--- Quote from: craynerd on May 23, 2011, 04:50:26 AM ---I`m curious why such a prolific clock builder, John Wilding, would have suggested Paxolin as the most suitable material --- End quote --- One suggestion is that Paxolin is lower density than any metals, so you'll end with a larger pendulum, same mass. Does larger look better for the design? ie is it a purely aesthetic choice? Paxolin won't corrode either, although I suspect that won't help the clock's accuracy significantly :palm:. Boz |
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