Bruce, what you offer is a well written generic safety message. "be
careful modifying things and also, when lifted, things might fall, and
if you stand under things that fall, they will fall on you, and you
need a lot of expensive doodads and well paid engineers to be OSHA
compliant".

It is hard to argue with this.

What Roger is trying, though, is NOT overhead lifting, and he is just
trying to get stuff out of his pickup truck.

What I personally think is a more specific opinion, that Roger's
modification will end up bending the vertical post if used at full
capacity.

In any case, I think, if he wants to go ahead, he should, it is his
right, but he should test the crane at his planned capacity, very
carefully.

i


On 2012-02-09, Bruce L. Bergman (munged human readable) wrote:
On Sun, 5 Feb 2012 19:03:33 -0600, "RogerN" wrote:

"Ignoramus15251" wrote in message
om...


I think that he was trying to find a solution that does not cost as
much money.

Also, lifting stuff out of a truck is not overhead lifting.

i

I'm kind of wanting to experiment around a bit too. I'm thinking of making
steel sockets that will fit over a pair of 2 X 4's for a boom. By hooking
up the winch cables to the steel ends I should be able to keep all bending
forces off the 2 X 4's so they are only in compression. I can test with
various weights and figure it's save at about 2/3's the maximum test weight.
The idea would be to make a truck mount crane that can have interchangeable
booms, shorter for heavy loads and longer for light loads with long reach,
or taller lifts.

I've looked at some manuals on commercial manual cranes, pdf files. The
weight rating seems to be with the distance horizontal, not vertical. For
example one with a maximum 2000 with the boom in horizontal shortest length
has a rating of 1200 lbs with the boom extended to the maximum. However
with the boom angled up to the maximum, the lift rating is 2000 lbs at the
maximum extended position. This seems to agree with the calculations in the
rigging manual that the steeper the boom is, the less the amount of force is
on the guy wires and the greater the force is in compression on the boom.

All in all the cherry picker is pretty much just a C frame with the load in
the opposite direction of a C clamp. I think the force on the frame of a C
clamp is more related to throat depth than it is opening distance. That's
why I don't think my rearrangement would destroy the vertical post as long
as I don't exceed the load for the horizontal distance of the original boom.
I do think t would be much easier to tip over though.

The main message sent by the crowd is "Be CAREFUL with this!"

You're messing with something that could kill you if you screw it up
just right, and can easily mess up your load and anything that was
underneath or nearby, you gotta approach this with that in mind.

That said, you could make a longer main-post and rear braces, change
the angle a little bit so the upper arm hinge point is in the same
plane, and make some side-facing outriggers to reduce the tipping
chances.

But up the steel gauge on that main post another notch, as you are
increasing the stresses on it. And no shortcuts - nice multiple pass
welds with good penetration. And if you don't trust your welding or
don't have a big enough machine to do it right, just tack it together
and have someone who lays down big beads every day do the real
welding.

I wouldn't mess with the lifting arm ratings or geometry at all, or
moving the fulcrum point for the hoist cylinder.

-- Bruce --