So...
What exactly is going on?
Let's do a thought experiment.
Would you rather Jamo stepped on your foot with his shoes?
or
Would your rather have a tiny 100 pound woman step on your foot with a stiletto heel?
This, is called psi, or pounds/square inch.
Mild steel (1018) has a yield strength of around 50,000 psi. No matter how hard you jump on it, you probably won't do anything to it.
Now, take a simple nail in your hand and scratch the steel. Can you do it? Of course you can. If you push hard enough, you can probably even pull up a burr on the steel. Now, you can only pull up a burr if you YIELD the steel or surpass 50,000 psi. That means you somehow put OVER 50,000 pounds/square inch on the steel. So, you lean on the steel (maybe 100-150 pounds) and you drag a SMALL nail across the surface...WALLLA, you get a scratch.
Now lets talk about your nuts (pins).
People on this thread have been talking about pins loosening up, using Locktite, galling, and all sorts of other nasty things.
Why would, could the pin loosen up? I mean your torqued it to 80 foot pounds, right?
Well, if you have a small area of material (think the small shiny ring in the above pictures) and you put a lot of force on it (think 60-80 foot pounds of torque multiplied by the mechanical advantage you get due to the incline of the threads on your nuts) then you have put a lot of force (pounds) on a very small amount of area (square inches). (Kinda like that woman who stomped on your foot with high heels or that nail you scratched the mild steel plate with.)
Now it gets worse. The very, very small end of taper on your nuts is rather sharp. There is very little material there...or there is a small amount of square inches of material there. As the square inches go down, the pounds required to yield that particular area goes down as well. Think it is way easier to bend a paper clip than it is to bend your shift lever. Why? There is more material in your shift lever to bend EVEN THOUGH the yield strengths of the shift handle and the paper clip are roughly the same.
Well, it gets even worse.
Take the pin and load the contact area further by bending the pin with your wheel as you accelerate or brake. The pin will smash even further into the small contact area until it...yields.
Yield means it moves. Now there are two types of yielding-- elastic and plastic. Elastic yeilding is what happens in a typical spring. You apply the force the spring deflects and then when you remove the force the spring moves back to its original position (or shape). Plastic yielding (or more correctly stated plastic deformation) occurs when you go beyond the elastic range and go to where you are permanently bending the material. A good example of this is when you play with a slinky. In the beginning, you play nicely with the slinky it will always return back to the original position. Then you get a little naughty and pull it too far you bend the coils. You ALL KNOW what I am talking about. What has happened is you have plastically deformed your slinky. In other words you bent it. Now back to the your nuts.
If the material under the nut (or heaven forbid, your nuts themselves) yields into the plastic region the preload that you had on your nuts is now gone. In other words, your nuts are now loose.
David
