Hi Lee

If you use a single turn around a pulley, then there is always slip as it
enters on one side of the pulley, and exits 4mm stepped over. So the wire
is always rubbing against itself. Works, but the wire will soon get scuffed.

If you have more than one turn, the situation becomes impossible, as the
the wire will 'walk' to one side of the pulley, as you've found out. In
order to work, the wire approaching the pulley has to shift the other turns
sideways to make space for itself. This situation is fine if the width of
the pulley exceeds the travel. In other words, four turns around a pulley,
but travel is only three revs. You've probably seen this on older printer
mechanisms, or some floppy drives that use a flat strip.

I would suggest using a bicycle type chain. It's positive, easy to install,
and if you put a keeper above the chain sprocket, it means someone cannot
lift the gate off the rack as in most situations.

Regards
Roland

At 09:31 PM 07/04/2005 +1000, you 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!
>
>regards
>
>Lee McLaren
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