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Direct Tension vs Torque

Direct Tension vs Torque

Because Huck BobTail® lockbolts are not subjected to torsion during installation, they can safely be taken to higher preload values than conventional bolts. BobTail installation is under direct tension only, while conventional bolts are under a combination of tension and torsion during installation.

Conventional bolts develop torsional forces from friction and geometric factors between the mating threads, resulting in a reduction of yield and tensile strength of 10% to 20%. The torque-tension relationship is shown in the chart to the left.

Fatigue Comparison

Fatique Comparison

This chart illustrates the number of cycles required for a variety of nut and bolt types to reach fatigue under vibratory
conditions. Notice the BobTail® reaches a 2,000,000 cycle runout limit before fatigue.

Bolt Thread Comparison

Bolt thread comparison

When compared with conventional nuts and bolts, HuckBolts have a much larger cross-sectional area and larger root radius, resulting in a much stronger and more secure joint. In fact, HuckBolts have up to 30% more bolt cross-sectional area, and up to 5x more root radius than regular bolts, depending on the bolts used.

Which Bolt Will Fail

Which Bolt Will Fail First

Since these bolts are torqued on, you can expect inconsistent clamp values. The bolt with the highest clamp value will fail first as it’s doing most of the work. The bottom line? Consistent torque does not deliver consistent clamp. And we’ve proven it. In a series of tests, Huck engineers demonstrated that friction causes inconsistent clamp values in torqued-on nuts and bolts. While HuckBolts, using direct tension and swaging, are guaranteed to result in consistent clamp, each and every time. Watch the video.

Clamp Consistency

We Put it To The Test.

Despite conventional wisdom, torque control bolts do not deliver consistent clamp. And when you don’t have consistent clamp, you risk bolt failure. The tightest bolt can end up doing the work of 4 or 5 bolts, and as a result, finally fail.  Structural integrity is compromised.

HuckBolts®, applied with direct tension, are the only way to guarantee you’ll get consistent clamp, time after time. 

To prove it, Huck® performed a clamp consistency test (see the results) here comparing a torque control bolt with a Huck BobTail®. The fasteners were each installed 30 times, and the results were conclusive: the torque control bolt had a variation range of 8500 LBSF clamp scatter, while the BobTail had only 1000 LBSF clamp scatter – much more consistent clamp. The test proves that even when torque is consistent, clamp isn’t. 

So, for consistent clamp, a more secure joint, and better peace-of-mind, HuckBolts with direct tension installation are the answer.  Watch the video to see for yourself.

pdf Clamp Consistency Test Chart