D125A FLAT WASHER:

D125a Flat Washer also known as plain washers. Their purpose is to distribute the fastener’s load while reducing heat and friction during the tightening process. D125a Flat Washer can also be used as spacers, as is common in industrial and domestic applications. Flat washer are also called on to provide electrical insulation. Washer size refers to its nominal hole size and is based on screw size. Its outside diameter (OD) is always larger. Size and OD are usually specified in fractional inches, although decimal inches may be used instead.

General:

The primary purpose of most washers is to evenly distribute the load of the threaded fastener with which they are used. Threaded fasteners stress the material in which they are driven. Driving a screw into wood, for example, may cause the wood to crack around the surface. A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Washers often have an outer diameter (OD) about twice their inner diameter (ID). But this can vary quite widely. Plain washers, which spread a load, and prevent damage to the surface being fixed, or provide some sort of insulation such as electrical.