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Recommended Fasteners for Metal to Metal Construction

November 7, 2018

It's ironic sometimes, you know, just how well a material benefit can turn back upon itself. Hardened metals, for example, are rigid and capable of supporting heavy loads. Yet somehow, perhaps because their surfaces are so rigid, that hardness feature makes a mockery of fastener anchoring. For wood it's different. Wood and other soft materials work with bolt and nut tightening forces. Metal-to-metal constructs aren't quite as accommodating.

Soft Material Advantages

Granted, softer materials can't support heavy loads, not unless there's plenty of support elements underneath the load. Even then, the approach has its limitations. When it comes to fastening capability, however, those materials accept the gripping, abrasive, rough-edged forms of locking nuts or ridge-surfaced jam-nuts. The bolt tightens because the nut, mated on the opposing side, digs into the softer surface material. That's a beneficial feature, one of the only ones that can be assigned to semi-malleable materials, but it's not a feature we can ever associate with hardened metals. For these heat treated alloys, another approach is preferable.

Hardened Metal-To-Metal Anchoring

Taking the time to drill a few pilot holes, the first recommended fastening method makes itself known. These are the self-tapping screws that use specially sharpened threads to facilitate their passage. Susceptible to stripping, though, their load-bearing capabilities are not reliable, plus they cannot be recommended as a reusable fastening solution. Next, geometrically similar self-drilling screws perform much the same duty, except their tips have a drill-like profile and fluted threads, which are designed to clear any cut metal debris. Again, thread stripping/slippage and reuse issues may discourage a construction project manager from selecting this product range.

High Tensile Strength Nuts and Bolts

Equipped with at least as much load-bearing strength and material rigidity as the metal-to-metal contact points, large diameter nuts and bolts are the most obvious solution, except for in those instances where the opposing surface is inaccessible. In the construction industry, such "blind" access instances are common, especially when the fastening project requires work on large area surfaces. To cope with this problem, a number of developments have entered the construction industry as viable solutions. For example, threaded rivet nuts install in places where only one surface is accessible. Their studs bulge and grip when the rivet thread accepts a matching bolt.

Beyond conventional heavy-duty nuts and bolts and their grip washers, there are many available methods of making metal-to-metal bonds. There are weldable nuts, which offer clearance for weld seams. Metal adhesives come to the rescue when all other options are exhausted. At the end of the day, though, there's nothing more reliable than a plain nut and bolt, except perhaps a fully covered weld seam.

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