"Commander Kinsey" wrote in message
news
On Sat, 18 May 2019 21:48:51 +0100, Rod Speed
wrote:



"Commander Kinsey" wrote in message
news
On Sat, 18 May 2019 19:22:33 +0100, dpb wrote:

On 5/18/2019 9:17 AM, Commander Kinsey wrote:
On Sat, 18 May 2019 03:24:06 +0100, dpb wrote:

On 5/17/2019 4:56 PM, Commander Kinsey wrote:
On Fri, 17 May 2019 21:42:59 +0100, dpb wrote:

On 5/17/2019 3:23 PM, Commander Kinsey wrote:
...

I can imagine the actual screw taking a fair sideways load, but
it'll
gradually work it's way loose in the wood.

What do you mean by "sideways"? The screw is in tension, not
shear.

It's in shear. The force is downwards, the screw is horizontal.
...

Absolutely not. The screw is in tension.

The shelf isn't hanging on the screw, it's hung on the rod which
transfers the load to the plate

What plate?

The mounting plate the rods are welded to that the screws go thru,
silly...

I thought the whole idea was it was invisible.

It is, that plate is covered by the shelf.

So the plate is nor bigger than the thickness of the shelf,

Correct.

nowhere near big enough to prevent leverage.

Plenty to see that the force on the screws that hold it to
the wall are in tension, pull the screws out of the wall.

Jut a rod terminating in a single screw, all inline.

That's much harder to install, getting the rod in the
right place to fit into the hole in the shelf and getting
it at right angles to the wall in both directions.

If there's a plate, surely that's visible?

Nope, its covered by the shelf.

by the rigid mounting which tries to
rotate and thus puts the screw in tension.

NB: both screws are position _above_ the rod to resist that
transferred
moment.

Weight on the shelf tries to make the screw become lower than
horizontal. Just like if you stretch your arms out and someone hangs
heavy objects on your hands. Your arms are pulled down.

Look at the geometry of the device. The load is _NOT_ placed directly
downward on the screw at the wall; it's placed on the rod welded to the
plate which is screwed to the wall. That load is transferred as a
moment arm on the rod to the plat which is pressed against the wall at
the bottom below the rod and tried to be pulled away from the wall by
the plate at the top--which is where the screws are. Ergo, the screws
are in tension resisting that force; the wall is exerting the
equivalent
opposing force on the bottom half of the plate in the opposite
direction
to prevent the rotation.

Your arm analogy is flawed in that your shoulder joint is not rigidly
affixed to a surface which is supported against a solid resisting
object
below with a tension member above resisting that moment. You feel the
moment, yes; but you don't have the rest of the mechanical device to
help.

If you can't see it, go study a text in engineering statics; I've done
all I can do, no point in just repeating the same basic physics lesson
over and over and over...

What you described sounds like a normal shelf bracket. So how are these
invisible?

The plate is much smaller and is covered by the shelf.
http://www.leevalley.com/en/Hardware...648,43649&ap=1

Consider how deep the shelf is.

A floating shelf has to be reasonably deep so that there is still plenty of
shelf above and below the rod which cant be all that thin or it will bend.

Consider the small distance from the rod to the screw along the tiny
plate.

Its not tiny, just smaller than the ****ing great brackets you use.

Consider the huge leverage ensuing from this. The screw would be easily
pulled straight out of the wall.

In practice that one works fine with 100lb on the shelf.