M18 Spring Washer Square Section DIN7980 Self Colour

Spring washers are used a a preventative measure to prevent a fixing nut from coming loose as it grips into the bottom of the nut and the washer when it is tightened.  So the spring washer must be used in-between the flat washer and the nut. These washers are DIN 7980 which are used to prevent the nut loosening wnen movement strain or vibration may occur.  It is always advisable to use a locking washer when a plain nut is used along with a flat washer.  Always replace the spring washer after undoing the nut to provide the best reduction in the nut disingaging. 

These are self colour fasteners which has no preventative measure from corrosion so a coating with paint or plating will provide an element of protection.  Also bear in mind using different materials can cause fusing through corrosion especially in damp conditions when mixed with aluminium or stainless steel materials.

More information is available from Central Fasteners regarding fastener details, but its always worth checking with a manufacturer's specifications or drawings if an alternative size, tensile strength or types of washers are used.