OT: mechanical spindle+wire/cord properties

[N_Cook]

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

 

[Mike Coon]

In article <omaihl$753$1@dont-email.me>, diverse@tcp.co.uk says...

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

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.

 

[Clifford Heath]

On 08/08/17 06:32, N_Cook wrote:

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

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

<https://en.wikipedia.org/wiki/Capstan_equation>

 

[N_Cook]

On 08/08/2017 02:20, Clifford Heath wrote:

On 08/08/17 06:32, N_Cook wrote:
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

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

https://en.wikipedia.org/wiki/Capstan_equation

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.
http://eprints.nottingham.ac.uk/33859/1/resubmitted_pdf%20%28Green%20Version%29.pdf
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

 

[Phil Allison]

Nutcase_Kook wrote:

-----------------------

Firstly what is it called, friction drive ......

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

https://upload.wikimedia.org/wikipedia/commons/a/af/Planetary_reduction_drive.JPG

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



..... Phil

 

[N_Cook]

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 <0.5kg on a spring balance, other way >2Kg
2 turns , one way <0.2Kg , other way >6Kg.
So coefficient of friction about 0.12

 

[Mike Coon]

In article <omfnk8$8lp$1@dont-email.me>, diverse@tcp.co.uk says...

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 <0.5kg on a spring balance, other way >2Kg
2 turns , one way <0.2Kg , other way >6Kg.
So coefficient of friction about 0.12

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.

 

[N_Cook]

On 09/08/2017 23:04, Mike Coon wrote:

In article <omfnk8$8lp$1@dont-email.me>, diverse@tcp.co.uk says...

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 <0.5kg on a spring balance, other way >2Kg
2 turns , one way <0.2Kg , other way >6Kg.
So coefficient of friction about 0.12

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.

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
http://diverse.4mg.com/1924+EZ.jpg
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.