Spring washers, which have axial flexibility and are used to prevent fastening or loosening due to vibrations. Locking washers, which prevent fastening or loosening by preventing unscrewing rotation of the fastening device. Locking washers are usually also spring washers. The spring washer should go under the nut, the bolt has got a lot more surface area touching the job. Which creates more friction to stop it turning than a nut which only has the threads and it’s side touching as friction.
Helical spring lock washers have been in use for well over 100 years. They are still used on many applications in the belief that they will will “lock” the nut/bolt to the joint and prevent loosening. Junker originally showed in his work published in 1969 that these washers are ineffective in preventing loosening.
In theory split washers (aka lock washers or helical spring washers) are supposed to work by squishing flat between the nut. And the mounting surface when you tighten them. At this point the sharp edges of the washer are supposed to dig into the nut. Whereas mounting surface to prevent counter-clockwise rotation. Spring Washers are also Split lock washers are helical-shaped split rings