Machine Design

Making Sure Bolts Keep Clamping

A bolt that loses its clamping force is next to useless, and many of today’s common methods for securing bolts fall in that same category, according to the engineers at Nord-Lock Inc., Muskegon, Mich. ( They’ve tested their bolt-securing system, a pair of special washers, and found the wedge-locking method successfully uses tension instead of friction to secure bolted joints. It is practically the only such fastener-securing technique that withstands the Junker vibration test and meets DIN 56151. (This test subjects a bolted joint to transverse movements while constantly measuring bolt tension.)

The washers come in pairs, with the mating cams or ridges on one side and a ring of serrations on the other. When the washers are properly installed and the bolt is tightened, the serrations bite into the bottom of the bolt head and mounting surface to stop the washers from turning. Thus, for the bolt to loosen, each washer must rotate against the other Radial teeth or serrations dig into the bolt or surface the bolt is being installed in. Making sure bolts keep clamping washer’s cams. This ”wedge effect” increases tension (clamp load) in the bolt and makes the joint tighter. The bolt can be loosened, however, when a technician applies torque, and the washers can be reused.

The washers have a zinc-flake coating that lets them endure 600 hr of salt spray (ISO 9227). They can be used with standard high-grade bolts, tapped holes, counterbores, through holes, stud bolts, and slotted holes. And for applications that involve temperatures up to 500°C, the company supplies Inconel 316 stainless-steel (A4) washers. Other steels, including 254SMO, Inconel 718, and Inconel/Hastelloy C-276, are also available.

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