Posted by Sherman in Idaho [72.47.153.112] on Sunday, February 09, 2014 at 18:46:11 :
In Reply to: Nuts/Bolts & Washers... posted by CSCameron [174.26.88.197] on Sunday, February 09, 2014 at 18:35:25 :
...with the mechanical engineers where I used to work. What I was told was this: Most people, including the people who originally designed them, think lockwashers hold by having the pointy ends dig into the metal, but it turns out that's not really the case. They hold equally well (or poorly) whether or not they dig into anything on one side or both.
What makes lockwashers work is that they act as springs, maintaining minimum tension in the bolt or screw even under vibration. The tension ensures that the threaded parts are always pressing against each other with enough force to cause enough friction to keep them from unscrewing. Otherwise, under vibration, most threaded fasteners actually unload completely during some part of the vibration cycle, and that's when they unscrew. With a variable speed vibration test table, it's often possible to "tune" the vibration frequency to just the right value to make a fastener unscrew as fast as if someone had a wrench on it. This has been proven with stroboscopes and high-speed cameras, as has the fact that a lockwasher works equally well even if the pointy corners are rounded or it's used with hard metal that it can't bite into like a grade 8 bolt.
The implication of all that, proven with tests, is that it doesn't matter whether you put the lockwasher under the head of the bolt or under the nut. You can also safely use it with a flat washer under it, for example if bolting something soft like aluminum.
What you're doing with the lockwasher is maintaining a minimum tension on the bolt, not mechanically "grabbing" the nut. With that in mind, you can put it at whichever end is most convenient for the assembly. Sometimes the bolt can only go in from one side, whereas on the other side there's just barely enough clearance for the nut with no washer. No problem -- just put the washer under the bolt head and it will be just as good.