re:RE: LARGE Gantry table

Just a thought about the weight of a 20 ft gantry,
If you are really going to build a 20 ft gantry, shouldn't it be built like
large structural spans usually are: as a space frame? Bridges can usually
do without much torsional stiffness, so they aren't space frames, but you
see lots of space frames holding up the roof in stadiums. Race cars use
these a lot too.

If welded steel tube doesn't do it for you, then graphite/epoxy is the next
step up. Aluminum has about the same stiffness/weight as steel, so it has
no advantage in this sort of structure.

A space frame is one of the two most efficient structures, and is great for
connecting concentrated loads over large distances. Welded steel tubing
does it. All triangles -> all members are stressed in tension or
compression, no bending.

Like Les said, a FEA analysis is a good idea. The tubes can be modeled as
beams of appropriate moment of intertia. This simplifies the model/reduces
number of nodes considerably.

regards-
Elliot


Chris, Les, and others wrote about a 20 ft gantry:

Heh. Confused Chris? My friend Bob Campbell said a Gecko would
handle it. I said it would need something bigger.

To put things in perspective my 1000 lb gantry IS run by a Copley
412 which is about the same as a Gecko in power. So Bob is absolutely
correct for the weight you asked about.

I just think that a 20 ft gantry will be much more massive than my
system. Things tend to go up in mass as the cube of length for equivalent
stiffness. Sometimes even more. A finite element structure analysis
can define this in the design stage.

Les
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