SteveB wrote:
"Christopher Tidy" wrote in message
...

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



You are correct about the top and bottom mounting. I believe I will add a
middle standoff to keep flex down. I already have the materials for this,
and will build and test it this spring. It will either work or fail on the
first test load of 200#.

You'd be better off putting the stand-off just 1 foot down from the top
(i.e., at the same level as the horizontal member). This will mean that
the vertical column only has to support a compressive load. The
compressive load will be well within the capabilities of the column. It
is the bending of the long vertical column that is the major weakness in
your design. Fix this problem and the rest of the design will most
likely be fine.

Let us know how it goes.

Best wishes,

Chris