Nuts and bolts might be the stars of the fastener show, but they rely on flat washers to achieve top performance. Flat washers are stamped from metal or synthetic materials. They are circular with a central hole where a screw or bolt passes through. They can be placed beneath the head of a screw or bolt, under a nut at the end of a fastener, or in both places. Builders and mechanics use flat washers for two primary purposes, which are to protect bearing surfaces and maintain the longevity of the fastener connection.
Benefits Of Protecting The Bearing Surface
The broad surface of a flat washer distributes the tension created by a screw or bolt entering a material or being tightened on the other end with a nut. The washer effectively spreads out the force created by a fastener. As a result, the bearing surface is shielded from the direct force of the fastener assembly. Without a washer, a fastener head could scratch the bearing surface or dig into it. Tiny gaps that arise from a compromised bearing surface create opportunities for fasteners to come loose over time.
Flat washers placed underneath screws being driven into wood also protect against cracking. By distributing the tension away from the fastener entry point, the washer lessens the chance of the wood splitting around the hole. U.S.-made flat washers are commonly available from the same manufacturers and distributors that supply builders with screws and bolts.
Issues That Increase Risks Of Fasteners Coming Loose
Although flat washers improve fastener performance under any conditions, certain situations have an increased need for them. Narrow screws or bolts with small heads have a greater ability to sink into a bearing surface. A washer counteracts the focused pressure of a small fastener head that could deliver a penetrative force.
Relatively soft bearing surfaces are more vulnerable to fastener heads digging in regardless of head size. Resin surfaces and more pliable metals like aluminum will need washers beneath fastener heads.
High vibration environments also threaten the stability of fasteners. Constant jiggling, like on a vehicle or manufacturing assembly line, can eventually undermine the tension created by the threads of a screw or bolt. Once again, the presence of a flat washer diffuses these forces and helps to maintain the tension. Flat washers made from a softer synthetic material like plastic are ideal for vibration absorption.
Different Types Of Flat Washers
Operating conditions and end-use applications influence the choice of flat washers. USS flat washers represent pieces made according to the United States Standard. These washers are mostly for general purposes, like construction. However, applications that will expose materials to outdoor weather or other corrosive elements call for USS flat washers with coatings. Zinc flat washers have a zinc plating that can withstand corrosive conditions. When working with ACQ treated lumber, builders may prefer HDG flat washers. HDG stands for hot-dipped galvanized. This coating protects the washers from the outdoors and corrosive elements within the treated lumber.
The fastener industry also makes many types of specialty washers, like SAE flat washers. Designed by the Society of Automotive Engineers, these washers have narrower outside diameters and are thinner compared to USS flat washers.
Smooth and Rough Edges Of Flat Washers
The stamping process for manufacturing flat washers results in a circle of metal that has smooth edges on one side and somewhat rougher or burred edges on the other side. Which side a mechanic places against the bearing surface depends on whether marking must be avoided or if bearing surface pressure is the chief concern.