https://www.superiorwasher.com/washers/8822/Belleville-Washers.html
This video brings to light these unique cone-shaped washers that provide deflection and shock absorption in specific applications. They are called Belleville washers — a useful piece of hardware with spring action and which can be used as locking device. This type of washer may be of high grade industrial stainless steel, and other high strength materials such as spring steel.
Aside from high load capacity and better utilization of space, Belleville washers can be stacked on top of each other because of its damping or high hysteresis property. For reliable performance in specific applications, this type of washer is used.