The lock washer prevents nuts and bolts from turning, slipping and coming loose because of vibration and torque.

The washer has sharp edges that dig into the nut as well as mounting surface in order to prevent rotation in the counter clockwise direction.

Spring Lock Washers are of many types, most common one being the helical split washer. A ring split at one point and bent into a helical shape. This causes the washer to exert a spring force between the fastener’s head and the substrate, which maintains the washer hard against the substrate and the bolt thread hard against the nut or substrate thread, creating more friction and resistance to rotation.

ASME B18.21.1 (EXTRA DUTY) is standard which is
used in American society and covers the dimensional requirements, physical properties, and related test methods for helical spring-lock washers (# 0 through 3 in.)
These have different dimensions and are heavier than Heavy once.
Usage and applications of Extra duty is very limited to some certain applications only.