Thread thread
Hello,
I once conducted a come-along test with a padeye in a section of scrap spar, once with sheet metal screws, and once with machine screws. We tore the former out fairly easily, and couldn't budge the thing with the latter, even when we got down to just two screws.
Sheet metal screws have such big, deep threads because the threads are meant to act as backing -- there's enough space between threads so that they can sandwich the sheet metal. When dealing with thicker stuff, like, oh, a mast, one must drill a bigger hole, just to get the screw in. So rather than having the thread projecting beyond the hole, as with sheet metal, they merely scrape against the sides of the hole, with very little "bite". That's why they can pull out.
Now of course given enough screws and a light enough load, you can have hardware stay put. But with how much reserve strength? Clearly, the right tool for the job, for anything heavier than a winch handle pocket, is a tapped screw, with a good rivet a distant second.
Fair leads,
Brion Toss
|