OT: mechanical spindle+wire/cord properties

Firstly what is it called , friction drive of a few turns around a spindle and kept under tension? the traditional radio tuner frequency change mechanism, linking dial and pointer to a spindle. Then how to evaluate slip/not-slip for various drum surfaces,diameters, wire/cord types and tension

Reply to
N_Cook
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Well, it's the principle of a yacht winch. We put up to four turns of the jib sheet round the winch to get a really good pull, though with modern "self-tailing" winches we aren't so aware of the tailing tension. The texture of the barrel surface is another factor; different manufacturers use different patterns but I don't know if they are patented.

When I built my own valve oscilloscope decades ago I used a similar mechanism to move a flexible scale past a small aperture in the front panel, but I don't remember how I designed that aspect!

Mike.

Reply to
Mike Coon

Capstan. The hold force is an exponential function of the total angle wound around the capstan, so if it slips, add another turn.

Reply to
Clifford Heath

Google was getting me nowhere near Eytelwein's formula or even mention of capstan, thanks. A historical investigation into how an extreme tide height got erroneously into the records via a type of tide gauge made by Cary Porter 100 years ago. Looks like a design flaw relating to this effect.

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shows one of those recorders. It looks as though the wire to the pen carrier traverse goes around a

1.5 inch drum, behind the brass flange , on same axis as the 120 tooth cog. The 2 weight carriers ,under the table, providing the friction . Rather than the wire fixed to the drum, relying on this capstan friction. So in an extreme tide, one of the endstops, on the wire at the top, jams against the small end pulley, friction on the 1.5 inch capstan is loovercome and the wire slips on the drum and bang goes the calibration. A permanent offset until recalibrated
Reply to
N_Cook

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** This device, once common in radio tuners is a friction drive.

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I have one in my bench audio oscillator driving a large, dual gang WW pot.

.... Phil

Reply to
Phil Allison

I decided to convince myself of the formula. A 1 inch diameter rod of brass and some ex-draughtsmans drawing board multi-strand steel wire

0.88mm diameter. Clamped the rod and suspended the weight from the wire. 1 turn , onset of slip one way 2Kg 2 turns , one way 6Kg. So coefficient of friction about 0.12
Reply to
N_Cook

I enjoyed reading the wiki formula, and noted that the diameter of the capstan/winch is not significant. Except that the cordage has to be flexible to get full friction, and the flexibility must be relative to the diameter.

Mike.

Reply to
Mike Coon

Intuitively you'd think diameter was in there. The hidden subtlety behind using multifilament steel wire on drawing tables, to retain parallelism . Once prestretched, the wire is flexible to go round half inch pulleys ,without work-hardening and breaking, 1mm solid steel wire would not. For anyone interested

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is a 1949 compilation of a few anomalous Southampton tide gauge recordings. The one of interest is the 1924 one. The original charts were lost in WW2 bombings. For the 1924 event, around midnight , not noon, as the compilation plot would have it The red overlaid trace is the predicted astronomic tide for that time. The legend "highest recorded level" is true ie highest recorded level before the mechanism jammed, end-stop hitting a pulley. Then someone added the nipple bit and gave it as the height the tide reached. But in actuality (cross-referenced from comtemporaneous newspaper reports) is something like the yellow trace, 2 inches above the ebb slope , 2 inches of slippage, meaning the tid ewas about 1.5 feet higher than what got in the records. The meteorology record of the time ,also supports something more like the yellow overlaid trace The same make of recorder at Dover also crapped out in the same storm event.

Reply to
N_Cook

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