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?

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.