A plain washer is a thin plate, which is most commonly has a disk-shape with a hole in middle. It is widely used to distribute the load of a threaded fastener. Plain washers are used to provide a larger bearing surface where screws, bolts and nuts come in contact with the bearing surfaces.
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.
Incidentally, since flat washers are basically manufactured by pressing, one side has rounded corners and one side has burrs, as shown in the figure.
There is no fixed rule about which side goes up and which goes down, but given the issue of bearing surface pressure mentioned above, it’s more effective to put the burred side facing down.
DIN 125,ISO 7089,DIN 6916, ISO7092, IS 2016 Type A,IS 2016 Type B,DIN 433, ASTM F436-023
Metric: M5 to M64
Inches: 3/16 to 2 ½”