The washer is placed between the bearing surface of bolts, screws, nuts, etc. and the workpiece bearing surface to prevent loosening and reduce the stress on the bearing surface.

a) Flat washer: Used to overcome the unevenness of the supporting surface of the workpiece and increase the stress area of the supporting surface, see GB848, GB95～97 and GB5287;

b) Spring washer: Spring washers rely on elasticity and oblique friction to prevent the loosening of fasteners, and are widely used in connections that are frequently disassembled. See GB93, GB859～860 and GB955;

c) Non-return washer: There are internal teeth lock washers, external teeth lock washers, single-ear lock washers, double-ear lock washers, and stop washers for round nuts. Single-ear and double-ear stop washers allow the nut to be tightened at any position to lock it, but the fasteners should be located at the edge, see GB861～862, GB854～855, GB858, etc.;

d) Taper washer: In order to adapt to the inclination of the working bearing surface, taper washers can be used. The square taper washer is used to level the inclined surfaces such as channel steel and I-steel flanges, so that the nut supporting surface is perpendicular to the nail rod to avoid bending force on the screw rod when the nut is tightened. See GB852～853, etc.