"Ignoramus27965" wrote in message
news
On 2011-05-20, Ed Huntress wrote:

"john B." wrote in message
...
On Thu, 19 May 2011 09:08:16 -0400, "Ed Huntress"
wrote:


"john B." wrote in message
m...
On Wed, 18 May 2011 19:09:01 -0500, Ignoramus15557
wrote:

Guys... I am not talking to anyone specifically right now... I sense a
little bit of B/S on this topic.

Right now, I have a crane that lifts 500 lbs, mounted on a 1x1 foot
plate, with angle iron underneath the bed holding it in [place with
thru bolts. While it is slightly shaky, it works just fine.

With this Ramco crane, I am going to have a crane that would lift
about twice more weight -- 1k lbs -- supported by a 3x4 foot
plate. That is 12 times more area than the 1x1 foot plate! Even if it
is not evenly distributed across the plate, it is still a hell of a
lot more support and weight distribution than a 1x1 foot plate.

I think that I will be just fine with it without fabricating any funky
adapters and bolting them to the frame.

i
I don't know the weight of your 5,000 lb. capacity crane or your
plate, but assuming that your plate is 3/4" and the crane weighs 500
lbs then you have a maximum weight of components plus load of 5,868
lbs. divided by 1728"2 = ~3.4 psi.

I just measured the area of one foot and weighed myself. If I stand on
one foot in the back of your pickup I will exert a force of ~5.36 psi.

I haven't followed this and I may have missed this point, but I would
think
that the issue here is not transferring the load evenly to the area of
the
plate, but rather the effect of the lever arm, with the weight loaded
some
distance from the center of the vertical column. That's what I'm
picturing
as the "crane," anyway.

And if that's the case, then the specific load on the edges of the plate
can
be many, many times the force of lifting the load.


There were all kinds of arguments going on. Mash the sheetmetal, etc.

But you are correct, the limits to crane operation are usually tipping
moments, not actual maximum lifting capacity as that is usually
specified with the boom at 89 degrees.

Yeah, I just saw the photo of it. The load on bolts holding a supporting
plate under the bed itself will be reduced considerably by using a larger
plate.


So, Ed, what is the implication for what I am trying to do?

i

I've skipped many messages in this thread so I can still be missing
something. Figure out the actual loads at the bolts, Iggy. If you don't do
statics, it's worth looking it up online. For the things you do, it will
eventually be invaluable. It's essential for all kinds of custom rigging
jobs.

From that, figure out how much load you'll be putting on the bolts that pass
through the sheet metal of the bed. Unless there is some more metal there, a
plate *underneath* the bed won't help a bit in dealing with the forces
trying to pull the bolts down through the sheetmetal when you load the
crane. What you need to be concerned about is how much to distribute the
load on *top* of the bed, on the bolts that are trying to pull down through
it.

A larger plate will help, not because it distribute the load, but because
the lever arm from the base of the column to the farthest bolts in the plate
will be longer and the forces will be proportionally lower.

You're a math guy so this isn't really a concern, but for most people, a
book on statics and dynamics written for technicians (like two-year college
program students) is a lot easier to deal with than one written for
engineers. The math is all pre-calculus that way. I have such a book and I
still refer to it from time to time.

--
Ed Huntress