A flat washer is a disc-shaped metal piece used to distribute the load of a threaded bolt or screw across a larger area. It helps prevent surface damage or embedment when tightening fasteners and reduces rotational stress. A washer is also utilized as a spacer or to pre-load screws and can be made from a wide range of materials. Common types of flat washers include flat, structural and countersunk. Other types of washers are spring, Belleville, bowed or curved, finger washers and wave washers. The most commonly used type of washers are metal, but plastic or rubber can be employed when a specific application requires them.
When using a washer, it is important to match its strength to the material it will be touching. Washers that are significantly stronger than the base material could damage it. For this reason, it is generally recommended to use a washer made from a metal that matches the strength of the material it will be touching. This includes the use of a stainless steel washer when working with pressure-preservative treated wood.
Metal flat washers are manufactured from a variety of materials including aluminum, brass, steel and silicon bronze. Each of these has its own unique properties that can be beneficial in particular applications. For example, steel is a popular option when a washer must be able to withstand high temperatures and corrosion resistance. Silicon bronze, on the other hand, is an excellent choice for marine environments because it can resist pitting corrosion when exposed to salt water that lacks free oxygen.
A variety of finishes can be applied to flat washers to improve their performance. For example, a plated washer can be coated with zinc or cadmium to protect it from corrosion. This is especially useful when working with corrosive substances or harsh chemicals.
Other coatings that can be applied to flat washers include phosphating, browning and bluing. A phosphate-coating can be effective in protecting against corrosion by forming a layer of phosphorus, which acts as a sacrificial surface that will degrade before the base metal can be harmed. Browning or bluing can be accomplished by exposing the washer to an alkali salt solution, which triggers an oxidizing chemical reaction that creates a corrosion-resistant, colored surface.
A sizing system for flat washers is defined by the SAE (Society of Automotive Engineers) and USS (United States Standards). These standards dictate the thickness and diameter of a flat washer. These standard sizes are often used to create a specific set of dimensions that will work best for a given application. In addition to the common size standards, many flat washers are available in a wide array of custom sizes and shapes for specific applications.
