On Wednesday, July 29, 2015 at 7:51:48 AM UTC-7, dpb wrote:
Just guessing, but a first shot approximation...

From the size of the tractor, and giving benefit that the 3-pt is Cat
1, not Cat 0, I'm guesstimating by measuring from the pictures that the
arm is 1", 1/8"T tubing and roughly 16" long from the gusset to the lift
point. If so, from the following calculator, the bending stress
approach 30k psi for a load of 200 lb. That is, of course, for a
fully-supported end and has no compensation for the point loading effect
of the narrow gusset at the center of the tubing as the load
concentrator. One thing I'd suggest would be to add outside plates
there so there isn't just the one point in the middle but support at the
edges of the tubing instead. And, of course, if the length is longer
it's directly proportional if the tubing dimensions are heavier/lighter,
they have a effect by the fourth power of the difference between
inner/outer dimensions from the geometric moment of inertia of hollow tube.
I think it's somewhere between Cat 0 and Cat 1; something Kubota dreamed up.. The square tubing is 1-1/4 in. square and it's all 1/8" wall. The jib is 30 in from root to tip. The tee piece that supports the whole thing on the lower arms of the 3-point hitch is 2 in. square tube, also 1/8 in. wall.


I didn't try to estimate the lifting moment on the tractor to the rear
axle to see how much weight you need minimum to hold it down but the
seat is directly over the rear axle so your weight isn't helping much at
all in that regards.

That may be enough capacity, altho as another mentioned without any
lateral bracing whatever I'd worry quite a lot about it collapsing
sideways when it's subjected to some side loading from a bump or a
slight tilt from vertical (owing to ground slope, perhaps). I also
didn't do any calculation on the compression of the lower extension arm
against buckling but I suspect it's better than the above (but that's
purely speculative just based on the visuals).

https://www.easycalculation.com/engineering/mechanical/deflection-hollow-rectangular-beams.php

It is cute, though; I've thought of doing something similar for the
utility tractor here altho it's a fair amount larger (~30 hp green).

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