SteveB wrote:
I got some 2" x 2" x .250" steel square receiver stock today. Sheesh. $4
per foot.

Anyway, I am making a figure 4 davit.

snip details

How much flex can I expect? Do I need the standoff in the middle? I will
probably put one on anyhow just to be sure.

Hi Steve,

I couldn't find any pictures of a "figure 4" davit online, but I think I
understand what you're talking about.

I had a bit of time this morning so I did a quick calculation. It is
about the most rudimentary calculation possible, but if I've understood
your structure correctly I believe it gives a useful answer. It ignores
all but one mode of deflection, which I believe will be the most
significant, and makes many assumptions. Naturally these assumptions are
a matter of discretion and people are welcome to discuss them. But there
wouldn't be any point in doing complex calculations without a lot more
knowledge of the project.

Here's what I did. If the diagrams don't match what you intended, let me
know. I assumed that you're going to have a pivot at the top and bottom
to allow the davit to rotate. You may not, but I doubt it will make much
difference because, either way, the anchor points are unlikely to be
highly rigid in a torsional sense.

http://www.mythic-beasts.com/~cdt22/davit_calc.jpg

The calculation suggests that the winch suspension point will be
deflected downwards about 8 inches by a 200 lb load. This is only an
order of magnitude figure, but it is way too much. It means that your
davit would be very bouncy, your top and bottom anchor points could be
damaged, and it might cause failure in an unexpected way.

You might want to rethink the need for the long, vertical column.
Presumably the davit will be supported by a wall or gantry of some kind?
Can you place the bearing carrying the vertical load just below the
horizontal member? If not, a stand-off in the middle would reduce the
deflection to about one-eighth the value I calculated (i.e., 1 inch),
but I still wouldn't be entirely happy with this. A stand-off at the
level of the horizontal member would be better.

Comments welcome!

Best wishes,

Chris