A plain washer is a thin, flat, circular piece of metal or other material with a hole in the center. It is primarily used to distribute the load of a threaded fastener, such as a screw or bolt, to reduce surface damage and increase the grip of the fastener on the material. Plain washers also prevent loosening due to vibrations and protect the surface beneath the fastener.

Plain washers work by spreading the pressure applied by a fastener over a larger surface area. This reduces the likelihood of the fastener sinking into or damaging the material. Additionally, the washer acts as a barrier between the fastener and the material, protecting the surface and ensuring a more secure fit.

Plain washers are made from a variety of materials, including steel, stainless steel, brass, aluminum, plastic, and even rubber. The choice of material depends on the application and environmental factors, such as the need for corrosion resistance, electrical insulation, or tolerance to extreme temperatures.

Plain washers come in a wide range of sizes to accommodate various fastener dimensions. They are typically specified by their inner diameter (to match the fastener) and outer diameter (to determine the load distribution area). Standard sizes range from M1 (1 mm inner diameter) to M100 or larger, depending on industrial requirements.

Plain washers should be used when a fastener is tightened against soft or brittle materials, such as wood, plastic, or thin metal, to prevent surface damage. They are also helpful in applications where load distribution, alignment, or vibration resistance is required for a secure fastening.