Re: Steel Channel flex ?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "leslie watts" <leswatts@a...>
wrote:

>
> Dave I'll take a stab at this one...
>
> OPEN sections (I beams, channel, angle, etc) are very soft in torsional
> or twisting.
>
> CLOSED sections (box, tubing, etc) are dramatically more resistant to
> twist.
>
> A classical experiment is with two thin walled pipes....one closed
> and the other with a narrow longitudinal slit...like a "c" cross

section.

>
> Both will be stiff in bending, but the c cross section will be very
> compliant
> in torsion. You can try this by cutting a cardboard paper roll.
>
> This begs the question as to why open sections are even used.

Well,they have

> some advantages:
>
> 1) they can easily be twisted to conform to another member without undue
> stress
> 2) They are easier to fabricate
> 3) they can be joined without blind fasteners
>
> In your case an offset load will create torsion in a single open member.
> This will directly reduce the stiffness at the spindle. The lightest and
> stiffest structure for this will be a closed section. In general open
> sections
> should not be used for torsional loading. The material will not be
> efficiently used.
>
> Does that help?
>
> Les

Yes, but in the wrong way.....

Mounting a set or rails under a table, then running one channel across
to have the flat to mount the bearings on.

then, bolting two veticals, one on each side. then two runners across
the top to complete the square. the verticles would seem to be bent
along the web, which I assume would be very strong against such a load.

the two runners would be used to mount rails. haveing two would offer
more distance between the rails, so would present bowing forces.
again, probably not a bad use.

But.... now that bottom rail... the ends of that will see mostly those
torsional forces to which you speak.

Closed square tubing had a large radius so things can only be bolted
in the center. That makes channel more attractive. Espically when
one can drill and tap into the thicker sections at the corners.

Thinking about the gantry, the lower member would be driven from the
center, and that means that if the cutter were all the way to one
side, the verticals would twist....

Now I have to decide if I want to add a plate to box frame so I can
mount the parts, or if welding a plate to the channel to close the
side would be a workable solution.

Dave