The Hardlock Nut controls the preload with the convex nut, and the primary function of the concave nut is to lock the convex nut.
With ordinary double nut tightening, two nuts are merely fastened together, and when the outer nut is tightened, the preload increases, making control difficult.
In the tightening preload variance tests below, there is no large change in the preload of the Hardlock product when the upper concave nut is tightened as shown in the first graph.
As can be seen in the last graph, when the upper nut of a double-nut system is tightened, the preload changes to the degree the upper nut is tightened.
HLN convex nut tightening
HLN concave nut tightening
Double nut tightening – lower nut (type 1) tightening
Double nut tightening – upper nut (type 3)