Washers
PLAIN WASHERS
Their purpose is to distribute the fasteners load while reducing heat and friction during the tightening process. They can also be used as spacers, as is common in industrial and domestic applications.
Flat spring washers
It operates more like a spring, holding the threaded fastener in place with friction.
Square spring washer
The spring washer should go under the nut, the bolt has got a lot more surface area touching the job, which creates more friction to stop it turning than a nut which only has the threads and it & is side touching as friction.
Internal star washers
They are designed to increase locking force by resisting torque applied by the nut, preventing it from unscrewing or separating.
TAPER WASHERS
Taper washers are designed to be used on channel sections, underneath nuts with tapered flanges to enable the bolt assembly to fit square when tightened.
