I think this is normally done by using a threaded drum instead of a 
wheel so the windings on the cable can move axially while the drum 
turns.

Another way to get more grip could be to use two, or ever three 
(driven) wheels in series and loop the cable through them (more or less 
like the gear-shifter in a racing bike) . With proper placement of the 
wheels you should be able to get 3/4 turn on each wheel.

Ever consider using timing belt instead of wire rope? You can get that 
steel-reinforced and cut to any length. A lot more expensive though.

Michiel


On 07 apr 2005, at 13:31, Lee McLaren wrote:

> Hi,
>
> I am building a electric gate and are trying to use 4mm wire rope to 
> pull the 4m gate in and out, my first attempt was a failure as I 
> (mistakenly) thought that if I cut a groove in a pulley the right 
> width for the rope that the rope would pay in and out in the same 
> spot.
> I know how to get half a turn around the drive wheel by cutting a 
> parallel groove but would like to get three if possible so it doesn't 
> slip.
> I am using a PIC and rotary encoder to know where the gate is so I can 
> accelerate and decelerate as quickly as possible, as the rotary 
> encoder is on the drive wheel I can only tolerate a small amount of 
> slippage.
> I have spent hours on the net trying to find the answer but don't seem 
> to have hit on the magic search terms, I am sure I have seen this done 
> with some sort of thread on the drive drum but might be dreaming it!

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