"Pete C." wrote in message
...
SNIP
| How about doing the rolling gantry style, but using the adjustable
| height design and using the hydraulics to telescope both sides of the
| gantry. That way you don't need to use any of the headroom to
| accommodate a chain hoist. Just lower the gantry beam to an inch above
| the item, chain with as many points as needed and then telescope the
| whole thing up.
|
| The air-hydraulic jacks would sync you on the up stroke, but down would
| be a bit tricky. Probably work best with two basic cylinders and a
| separate manual or air-over pump so you have a single lowering valve.
| You could probably dig a cable/chain leveling mechanism if needed.
|
| Pete C.

There is a gadget for making sure that the flow to two cylinders is
synchronized. Don't know the name of it, but fluid goes in one end, passes
around two gerotors which keeps the split flow matched. Can't find my
catalogs, but the usual surplus sources sell them.

One of my upcoming projects is a gantry crane, made with bases similar
to the uni-dolly at http://www.uni-dolly.com but with a few changes to make
it more interchangeable with assorted other projects. Two inch square
tubing is the common links, with the one size up being fabbed together for
connectors. I don't know if I'll hang an I beam from the crossbar or not
yet, but that depends on how much load I expect to be working with.