The main role of flat washers is to increase the size of a screw’s bearing surface area and reduce the surface pressure applied on the fastened object. Looseness can result when the bearing surface sinks under the surface pressure, so using a flat washer to reduce the surface pressure is very effective.
Flat Washer M42 Black Cut MS EG 126
Flat washers are used to increase the bearing surface of a nut or fastener's head thus spreading the clamping force over a larger area. They can be useful when working with soft materials and oversized or irregular shaped holes. Washer size refers to its nominal hole size and is based on screw size.