Jonathan,
If all else fails, with all the photos available on the web, it seems
like it would be a fairly easy task(famous last words) to build a full
size working prototype scissors mechanism from scrap wood or even
corrugated cardboard. You could use dowels for the pins.
From what I've seen from looking at mine the things to watch for are
that the cylinder and rod have clearance from the mechanism at full
lift and all intermediate positions, and of course that there is
enough leverage available to lift the dump bed at full capacity.
Once you get it figured out, then you can transfer the dimensions to
steel and cut and weld away.
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's catalog has
probably half a dozen 12V hydraulic power packs. I think that
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has the best source of good quality hydraulic
cylinders. It seems that single acting cylinders are scarce, and you
can use a double acting cylinder as a single acting one by putting a
breather valve on the unused side of the cylinder.
I paid about $30 for a set of dump trailer plans, but the hydraulic
system just used a straight ram, no scissors.
Bob