How do bonded washers work?
The elastic inner ring of the bonded washer acts as a kind of gasket between the head and body of the screw or bolt. When the ring is compressed as the bolt or screw is tightened, the pressure seals the ring to the surfaces either side of it.
Why are bonded washers useful?
Their simple but effective sealing action is useful when you need the fixture to be watertight, for example to prevent leakages. They provide a better seal than a simple washer. In addition, because the seal is an inbuilt part of the fixture, the process of using them is easier than using another sealing fixture such as an O-ring.