Which washer has two ends that dig into the nut and material to secure the screw?

The correct answer is split-lock washers. Split-lock washers are designed with a split or gap that allows them to be slightly compressed when a nut is tightened onto a screw. This design creates a spring effect, which helps to prevent the nut from loosening due to vibrations or movement. When installed, the ends of the washer dig into both the nut and the material being fastened, effectively locking the nut in place and improving the overall security of the connection.

In contrast, flat lock washers are designed to distribute the load of a nut or bolt over a larger surface area but do not create the locking mechanism found in split-lock washers. Shake-proof washers serve a similar purpose as they have an irregular shape that helps resist loosening but do not feature the split design. Spring washers are used primarily to provide tension or to absorb shock, but they do not specifically dig into the materials like split-lock washers do. Understanding these distinctions helps clarify why split-lock washers are the most effective choice for securing nuts against loosening.