Sliding bearings for telescoping square tubing?

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.
--
Regards, Terry King ...In The Woods In Vermont
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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:

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..snip
Grant, That's an interesting idea; I wondered how those booms were spaced internally.
How / what lubrication is used on those booms??
--
Regards, Terry King ...In The Woods In Vermont
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The lower edge runs on a wheel - that part gets complicated, actually. But they have big hydraulic cylinders extending and retracting them, so even if they are a little sticky, it doesn't mean much. - GWE
Terry King wrote:

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Terry King wrote:

YOu want it cheap. wrap a piece of waxed paper around the inside tube and pour the gap with epoxy compound. Use a couple layers of paper to get the clearances you need. Use a piece of styrofoam to hold it in place and for the bottom dam on the tubes. For the same process in bigger dollars use Moglice.
John
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I like that idea, too! I do a lot of boat stuff with West Epoxy...
What IS Moglice??
--
Regards, Terry King ...In The Woods In Vermont
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Terry King wrote:

Moglice is a bearing material used to repair ways in machine tools. You can get it in either liquid two part or putty two parts composition. It has wear material in the epoxy compound. The bad part is that it is expensive but not so if you consider the time it saves in a repair. Here is the site for more info:
http://www.moglice.com /
John
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I have seen some pretty slick arrangements using small 6000 series ball bearings on bolts. The tubing in the one case sat corners vertical and four bolts with bearings supported the tube running horizontally. The bolts were set at 45 degrees so that the bearings ran on the sides of the tube. For your vertical arrangement you would need eight bearings to completely capture the tube post. Sealed bearings that fit on a half inch bolt don't run much more than five dollars Canadian from a bearing house. You might get them as cheap as a couple of dollars each. Randy
The one who Dies With The Most Parts LOSES!! What do you need?
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Randy, I'm having trouble visualizing this interesting arrangement. Can you explain more, or point to any sketch etc??? Thanks!
--
Regards, Terry King ...In The Woods In Vermont
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Tell your metal supplier that you want "Telescopic Tubing". It may cast a little more but it will save you a load of work. -Steve
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UHMW would work well for bearing material. Look here http://woodworker.com/cgi-bin/FULLPRES.exe?PARTNUM 3-784
Frank

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THANKS! For the many ideas and perspectives.
-- I will check with my supplier about 'telescoping' tubing.
-- The different high-density bearing materials look very interesting. And some can be 'machined like wood' making adjustable-clearance approaches more reasonable.
-- I will put a dial indicator inside some 4 inch tubing I have to see the variation in size over a couple of feet.
What if: UHMW or similar was used as sliding bearing material, grooved to fit over the 'flash' on one internal side.
What if: that side and another side adjacent to it had fixed-to-the-outer-tube UHMW, and the other two sides had large areas of UHMW backed up with, um, 1/8" plate, with clearance adjustment screws thru tapped holes on those two adjacent sides. The total raw clearance would have to be reasonable for all that UHMW and backing plates.
What if: 'almost all' the clearance was adjusted out, pushing the inner tube 'into the corner' with the two fixed UHMW bearings. I don't need really low friction: probably 20 pounds would be 'pulling down' and a strong leadscrew jack pushing up. I could 'antibacklash' like I always do on my radial arm saw, if necessary.
This is not intended to smoothly move a cutting tool, but rather to position and reposition the cutting tool which has it's own degree of freedom for the cut. I need 'woodworking' accuracy here, not machine- tool accuracy. Maybe say .016 to .032 repeatability of position would be workable, .005 Would Be Nice.
I would appreciate your comments or suggestions on the above.
OK, this is starting to make more sense. I know some pitfalls I might have missed. I know some new materials and ideas. I have some stuff to research more. I hope to get closer to an approach I'll try Real Soon Now. I'll keep in touch here.
Everyone's suggestions are appreciated.
--
Regards, Terry King ...In The Woods In Vermont
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