Lock Washer

Lock washers are used to resist loosening of bolted joints. There are essentially two types: spring action and tooth.
Spring action lock washers include split (helical spring), which are also called spring washers. These washers, placed under the nut or screw head, compress as the fastener is tightened and the spring-back tension deters loosening. Also, because of their spring-like take-up action, split lock washers can compensate for slight material wear. But because spring types flatten when tightened, their "locking" action doesn't come into play until the fastener has slightly loosened. Use regular for Grade 2 hex cap screws and high strength alloy steel for Grade 5 and 8 hex cap screws.
Metric Lock Washers are a group of washers that are designed to provide a "locking" action to resist loosening. Available in steel, stainless steel and bronze; finishes include plain, zinc plated and phosphate. Some types are not recommended for new designs—DIN 128 A is the replacement for DIN 127 A, DIN 127 B and DIN 128 B.