Re: Spinnaker winch

William wrote:
"Paul Davenport" <davers00@xxxxxxxxxxxxxx> wrote in message news:dnfil1$72c$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I have designed a large spinnaker for an 8ft long cat'. This is to be housed
in an tube below the centre section. The problem is to power it up to
operational height and this means using a winch of some description - the
run will be 3ft 4ins.

Any ideas gratefully received. Perhaps it is possible to do something with a
geared pot'. Other than that I have no idea!

As always a virtual beer for the first plausible answer! 


I think the geared pot might be the easiest to engineer, although, if all
you need is all up or all down, limit switches are also a possibility.

Modifying a server for continuous rotation is easy enough. (Check
sites devoted to robot building for instructions, but it's basically a
matter of breaking off any stops and/or removing the internal pot's
wiper.) Wire an external pot of the same value as the internal pot
and connect it to your drum with enough gearing so your drum turns
often enough to raise/lower the sail. (Say, ~7:1 for a 2" drum.)

Another idea is to use a simple gear motor to turn your winch and
arrange physical stops on the cable. Use a friction clutch somewhere
in the gear train to let the drum slip if it encounters enough force. If
the cable's stops also trip limit switches, bingo. (Use diodes of
the right current-carrying capability to isolate the limit switches based
on motor polarity - that way, the one that stops it at the top, will not
keep you from reversing it to lower the sail.)

A trickier method is to sense the current spike in the motor when the
cable hits a stop and use that to trigger a limit switch circuit or relay.
Again, diodes can be used to make it sensitive to direction. (Nice
side effect is that if the rig jams, it stops safely and doesn't try to
push on to the real stop.) -Wm


There also are 10 turn small pots available, check DigiKey, Mouser Newark etc. that will fit in large servos.

I found some surplus 10 turn pots with 1/8 inch shafts that I was able to fit in a 1/4 scale servo. Removed the stops and I had about a 9 - 9 turn proportational servo. Machined a Al 2 groove drum and mounted it to the servo output wheel.

Works great for a Star 45

Have also built a heavy duty servo amp for the old Dumas sail control unit. Used a salvaged servo amp from an old servo and added optical isolators and a 4 transistor bridge to drive the Dumas motor. Just replaced the 1/4 inch main shaft with a 5 K pot that had a steel 1/4 inch shaft

Also learned that the Traxxas Eazy Start replacement motot for the Traxxas 2.5 engine is a good replacement for the origonal Dumas electric motor even comes with the gear already installed.

Hugh







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