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I had always been under the impression that these and split washers were the mechanical equivalent of placebos
They are not completely ineffective, but in general if you think you need one, your design is probably wrong. Sometimes we still use "wrong" designs, either because that's what's available or because designing it "right" adds cost and complexity, or even just aesthetic negatives, which are less attractive than relying on a locking washer.
A rigorous way of making my front light stay pointing forwards would have meant having a different design for the rear, which has to have its axis at an angle to the tube it's clipped to
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Interesting, I had always been under the impression that these and split washers were the mechanical equivalent of placebos.
Your use of these suggests otherwise.