Typically consisting of a flattened, circular- or disc-shaped piece of metal, commonly steel or stainless steel, with a hole in the middle, washers are often used in conjunction with threaded fasteners. Before a threaded fastener like a screw is driven into a surface, a washer may be placed through the end. The actual washer, however, isn’t driven into the surface, leaving many people to wonder what purpose they serve.
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. Washers reduce the risk of such damage by evenly distributing the fastener’s load across the surface of the material. Not all materials require the use of washers. But for wood and other relatively soft materials, washers are useful to protect against stress-related damage when the threaded fastener is driven into the material.
Washers can also be used as spacers. Why would you even need a spacer when driving a threaded fastener into an object? If the threaded fastener is longer than the depth of the object, you won’t be able to drive it all the way into the object — not without having some of the fastener stick out the back of the object, at least. Driving a 4-inch screw into an object that’s 3 inches deep will result in 1 inch of the screw’s tip protruding out of the back of the object. A simple solution to this problem is to use washers. Placing washers through the threaded fastener before driving it into the object creates padding so that the fastener doesn’t go too deep.
Certain types of washers are designed to absorb vibrations. Known as vibration damping or vibration isolating washers, they usually aren’t made of metal. Instead, they are made of a softer material like plastic, rubber or urethane. Softer materials such as these are more effective at absorbing vibrations than hard materials, including metal. If a threaded fastener is being used to connect two objects, and one of those objects producing vibrates aggressively, using vibration damping washers can protect the other object from damage.
Other types of washers prevent the ingress of water and liquids. They are often used in water pipes and connections to create a waterproof seal. Like vibration damping washers, liquid-sealing washers are made of a soft material that’s able to press completely against the surface of the object.