Fastener Hardness versus Strength: Is there a Difference?

July 25 2017

It's back to basics today as the fastener hardness versus strength question occupies our full attention. Material hardness, its resistance to physical deformation, also includes thread rigidity and an overall ability to withstand the kind of indentations that would weaken the head or rod of the fastener. Strength, on the other hand, is gauged as how well the material copes with a force, but what kind of forces are we talking about?

Gauging Turning Forces

Long wrench handles and wide screwdriver grips turn screws and bolts until they're locked down. The torque continues another half turn until the underlying surfaces are absolutely anchored. The strength of the fastener ensures that last half turn takes place on every bolt head so that no material shearing ever occurs. Meanwhile, the hardness of that same material prevents the well-defined threads from deforming. Similarly, the head of the bolt doesn't round and lose its shape when the fastening tool really applies the torque.

High-Quality Fastener Characteristics

There's nothing worse than a low strength screw or bolt, is there? The soft metal on the screw head becomes a crushed mess when any real twisting force is applied. Worse still, the threads give way. The fastener has fully engaged against its surface, yet it's still turning because the failing threads have lost their helical profile. Tensile loading requirements have a similar impact on the quality metric discussed here, with the strength of the fastener determining just how well the anchoring effect is executed. That optimally strengthened bolt, driven deep into a loaded structure, carries the necessary clamping force, but the weaker metal doesn't. No, because of that insufficient strength rating, the bolt shears. Either the rod or head, the weakest point on the fastener, breaks. Imagine the work that now must be done to correct the fracture problem. That broken section isn't about to come out, not easily. Of course, the matter becomes conceivably worse if the fasteners just support the assigned load. The surfaces, structures, or objects are fixed in place, but that lower strength rating suggests a future flaw in the design. A small impact, a vibration or two, these extra loads, plus time, will be enough to fracture the strained-to-their-limits fasteners.

A strengthened fastener won't shear when it's installed, nor will it fracture when the load applies extra stress. That's because the suitably specified bolts have enough tensile loading overhead to safely carry that load. As for the hardness parameter, hard alloys don't deform, which means the helical threads and their fastening heads maintain their shape, no matter how much torque is applied.

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TCI Fasteners - Topcope
13 Slater Parade, Keilor East VIC 3033 Australia

Telephone: (03) 9336 0155