Consider two pieces of square steel tubing, one 5", one 4",
both 1/4" wall thickness (these are hypothetical, not real).
If you put the 4" piece inside the 5" piece it will have 1/2"
clearance, or 1/4" on each side.

What if you welded 1/4" flat bar just inside the end of the 5"
piece, all around? Then the 4" piece would fit it closely. If
there were negative clearance and you did a little grinding,
you could soon achieve a good fit. Call this end (the end of
the 5" piece with the flat bar welded inside it) the "outer
end".

Now what if you welded 1/4" flat bar all around the outside
of the other end of the 4" piece, and similarly ground it to
fit? Suppose the 4" piece fit entirely inside the 5" piece
except for a small bit.

This is the principle of how the telescoping boom on my knuckle
boom truck works. The inner piece cannot exit the outer piece
because the 2 weldments interfere. On my boom, the inner piece
cannot exit the outer piece backwards either, because there is
a pin over the opening.

Grant Erwin
Kirkland, Washington

Terry King wrote:
Ok, I'm designing another machine. This is for making
accurate, repeatable cuts on / into large wood beams,
therefore posted to both groups...

I need to build a telescoping vertical elevation section,
much like some Radial Arm Saws have. I want to use standard
steel-supplier square tubing, like 4 to 5 inches, probably
.125 wall or, so. There will be a threaded rod actuator,
possibly based on a trailer jack.

This is a one-off at this point, although I'd like to be
able to document how to make one. IF it works well. So I need
easy-to-get parts. The question:

** What is a good approach to providing bearings for a
sliding, telescoping mechanism with square steel tubing??**

I don't THINK I need antifriction (ball or roller) bearings.
There won't be high-speed or continuous motion.

So, I see something like this:

Two sizes of square steel tubing about 5 inches or so
that telescope with a clearance of, um, .25 inches to .5 inches.
The wall of the outer tube has cutouts of some size and
shape which hold 'things' that reach through and 'bear'
against the inner tube. There probably need to be 'lots'
of these, say 2 per side x 4 = 8 at each 'level' of
contact. Maybe there are only two levels of contact.

What are your thoughts? What is used in existing commercial
designs?

My only thought so far is nylon (other plastic?) bolts in
threaded holes in the outer tube, hex heads inside, jamb
nut on the outside. Adjust for just a little clearance.

Any suggestions or pointers appreciated.