washer
a) Flat washer: to overcome the unevenness of the support surface of the workpiece and increase the stress area of the support surface, see GB848, GB95-97 and GB5287;
b) Spring (elastic) washers: Spring washers rely on elastic and oblique friction to prevent loosening of the fasteners and are widely used for frequently disassembled connections. The inner tooth elastic washer and the outer tooth elastic washer have a plurality of sharp elastic warping teeth on the circumference, and are pressed against the support surface to prevent the fastener from loosening. The internal toothed elastic washer is used under the screw head with a small head size; the external tooth elastic washer is mostly used under the bolt head and nut. The toothed elastic washer is smaller than the ordinary spring washer, and the fastener is evenly stressed and prevented from loosening, but it is not suitable for the frequent disassembly. See GB93, GB859~860 and GB955;