23-11-2019, 11:00 PM
It's called a "belleville" washer - the design shown being one of several types that carry the name of its French inventor. Besides its use as a locking washer, they can also be positioned between two surfaces able to move relative to each - the idea being to impose a degree of (often adjustable) friction that allows one half to be moved easily, yet stay in its new position when the whole assembly rotates e.g. they are sometimes found on machine tools to allow a micrometer dial on a feed screw to be zeroed or its position altered