11.1 The lever the check got wrong
This is the best story in v6 (11), and it's about a check that was wrong.
The screen hangs out past the shelf's end, so it tries to turn the clamp about the shelf's top end edge. The first check counted the top jaw as holding that turn.
It can't. Only the pad, pushed up by the bolt, holds it, and only through the bolt's preload. The check had the lever the wrong way round.
Claude drew three options:
| Option | Preload needed |
|---|---|
| Bolt where it was, 13 mm in | 100 N |
| Bolt just before the leg | 65 N |
| Bottom jaw runs on as a tail inside the leg's loop, to a nose past it | Not relied on |
I chose the tail. If the bolt's grip falls, the C turns until the nose bears on the underside of the shelf, and the nose carries the screen.
Two more things the checks found
- A bigger pad doesn't help much. A pad 24 to 28 mm wide spreads the load and protects the oak, but it adds little grip. Friction depends on the force, not the area. What counts is the distance in from the end.
- The clamp couldn't go on. Sliding along the shelf onto its end, the nose hit the tip of the leg's front flange by 2.8 mm. The check had only looked at the part in its final place. Now it moves the part along its path as well.
And one about access. With the bolt head up, the 44.6 mm bolt had to go in between jaws 31 mm apart. Head down, it goes in from below. Check that every fastener can actually go in.