On 2009-04-29, R T Smith wrote:

"Andre Majorel" wrote in message
...

Note that I suspect that the original posting is a troll. Look
at the e-mail address, not the user name in quotes.

| Howdy. I'm looking into building a frame made of equal leg angle
| to put a tall 19" rack on wheels.
|
| Do you know of a formula that yields the minimum size of the
| angle for a given load and span ? Not the full blown one for
| structural engineers, I wouldn't understand it ; just a basic
| rule of thumb.

I suggest you explain more in details what you are really trying to do.
Many shapes could still possibly form equal leg angles. What is a 19" rack
on wheels???

A '19" rack' is a frame for supporting electronic equipment with
front panels 19" wide, and several possible panel heights in integer
multiples of 1-3/4".

The front rails are drilled and tapped (or designed to accept
clip-in nuts) at varying spacings which all work out for the standard
rack panel definitions. If drilled and tapped, the most common thread
in my experience is #10-32, but #12-24 are also sometimes used for
heavier loads. It is this variation which makes the clip-in nuts in
plain holes more attractive to manufacturers.

The equipment *may* be supported purely by the front panel ears
(the space between the front rails is 17-1/2" IIRC), or may be supported
by side slides which are attached at both the front and the back to the
vertical rails.

The simplest ones are simply a base plate and a frame providing
the front rails only (open frame rack). The better ones have side
panels, rear doors, and some even have front doors.

The weight of an individual item could vary from perhaps a
couple of pounds for a 1-3/4" high module which extends only a short
distance back, to something which might weigh as much as 300 pounds for
a single 10-1/2" high module. And this module can be anywhere from near
the bottom (which is what would make sense for that heavy a module) to
near the top. And the taller racks have 72" of panel height, so you
could easily have 1800 pounds in a single rack.

If 1800 pounds, this means a bit more care in selecting the
casters as well.

I *think* that what he is asking for is a frame made of angle
into which to drop an existing rack, instead of using the usual hard
points on the bottom designed for attaching casters.

A good rack will have jack screws at the corners which can be
lowered into contact with the floor once it is in the right position,
thus taking the load off the casters, and also have slide-out legs at
floor level to keep the rack from tipping forward when one or more of
the modules is pulled forward on its slides for access. Both my 72"
high Sun rack and my 40" high DEC rack have such braces and jack screws.

They typically don't have diagonals at the sides, so equal-leg
angle would not suffice with a load such as that unless it had very long
legs. The side panels are sometimes attached by screws so they can
offer some resistance to parallelogram buckling, but they also are
sometimes left off, or just rested in place on hooks for easy access.

Front-to-back dimensions also vary, My tall one full of various
vintage Sun computers is 36" front to back, and a shorter one (made
by/for DEC) is 24" front to back.

A good specific question could normally generate good specific answers.
Conversely, vague situations could lead one into vague assumptions.

Even knowing what a 19" rack is does not give anywhere near
sufficient information to answer his question. Some clues as to what
size modules (panel height, front-to-back length, weight, whether they
are supported only at the front or on slide rails supporting at both
front and back) will give some improved chances. How many of these
modules, etc, etc.

Good Luck,
DoN.

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