The Hoisting Cart project, part 4: The Boom.

At the end of the long end of an obtanium lever having certain dimensions, a load (a spherical elephant) is affixed, pulling vertically downward by gravity (standard issue, one each).

Affixed to the short end of the lever, an immovable object.

One-third of the way from the short end of the lever, a fulcrum (frictionless, naturally), resting upon an immovable object.

How large a spherical elephant can we hang before our obtainium lever breaks?

Thus, the design of the jib crane's boom.

 The boom should be simple right?  I mean, in my case, the electric hoist mounts over the boom with brackets, so no holes needed for that.  A threaded spud at the back to hold the diagonal straps seems pretty simple, and the middle bit just sits on top of the bearing, right?  Well yes but also no.

 In the beginning, there was just a 36in long piece of .125 wall steel tube, 1.5in x 3in.  We'll use it with the wider dimension aligned with the direction of strain/load.

 I capped the rear end with a 1/4in plate, and to this was welded the threaded spud which secures the eyes of the diagonals to the back of the boom.  So far so super-simple!

The business end was closed with a .375 plate into which I had permanently secured a real forged lifting eye good for 600lbs, just for yucks.  I sure hope nobody breaks it, as it cannot be removed easily.
Getting into why is getting into trivia, let's skip that.  I may never have a use for that eye anyhow.

 

 At the one-third point, a hole was needed in one of the 1.5in faces, to accomodate the pilot bearing housing.  In theory, I could have jacked the boom up above that (I would have had to cut the mast shorter) but we are trying to make a perfect triangle between the bearing points and the rear of the boom, jacking the boom up away from the bearing points makes things weird (less of a triangle and more of a parallelogram, and thus less strong).

 The hole was cut with a hole saw to start, and then opened using a fly cutter, to just over the OD of the pilot housing.

 "But hang on," says the annoying know-it-all in the back of the classroom, "aren't the top and bottom of a beam under the most stress?  Won't this hole dramatically weaken the boom?"

 "You are half right so I will give you a little extra credit for bringing it up, so long as you keep your mouth shut for the rest of this class."

 I worried about this, but I also considered that the dimension in the direction of strain is far more important than any other factors, and the strain is already in the wide dimension.  I considered that for hollow materials we have to worry about things like walls buckling too.  In a square tube, each of the walls can be thought of as supporting the others.

 In general, if you want to make a hole in a thin feature, yet keep the thin feature as strong as it was before, you have to put the mass of material you removed around the hole in some fashion, such as adding stiffening ribs, doublers, etc. 

 There was no room to reinforce around the hole, so I welded whacking great doublers on the long sides centered on where the hole is in the bottom.  They're oversized because they aren't where they ought to be.

They're actually a little thicker than "double" - I think I used .188 (3/16) material.  These will help prevent it from bending in either direction, frankly.

  Image shows size and welds of doubler.  Holes were made through the doublers for restraining bolts.  The heavy brackets not only keep the boom from magically floating off the top of the pilot bearing, they had a vast amount of mechanical support, including strong discouragement of buckling, in concert with the doublers.

 I'm sure it'll be fine. Don't you think it'll be fine? I think it'll be fine.  Real talk: only real commisioning tests will show how full of beans I was.  And it won't be trustworthy until then.

 

 

 

 

 

And that's pretty much all there is to say about the boom; it really was pretty simple to make.

 I haven't got anything on the outriggers yet, because I haven't done anything but cut two pieces of steel.
Apparently that is Part 5.