The Offshore Voyaging Reference Site

Under-Torquing Is Dangerous, Too

Seems like it’s torque week here at AAC; actually, it’s torque month since we have a fascinating two-part series coming from Eric on understanding torque as it relates to engines and transmissions.

Anyway, in the last tip I mentioned the dangers of over-torquing: wring off a fastening or weaken it so it fails later under load.

But here’s something that I only relatively recently really wrapped my aging brain around: the danger of under-torquing fastenings, other than the obvious one of them coming loose.

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Matt Marsh

Nicely summarized, John.

Bolts, machine screws, studs, rivets, and most other fasteners used in metal and composites are best thought of as *springs*. Very stiff springs, with very little movement, but they are springs in tension – not rigid objects.

Imagine trying to fight a fully relaxed spring. Easy.

Now imagine trying to fight a spring that’s been pre-tensioned to 500 pounds. It’s not going to budge a single micron until you apply more than 500 pounds of axial force.

And the thing it’s tensioned against is not going to slide sideways, at all, until you apply more than 500 * μ pounds of lateral force (μ being the coefficient of static friction between the surfaces in contact).

Side note – this is the exact same principle behind pre-stressed and post-tensioned reinforced concrete. Tighten the steel in the tension flange of the girder’s still-wet concrete to a few thousand tons before you hoist the girder onto the bridge or skyscraper, and the concrete around it will remain safely in compression as the live load changes.

Kurt Kachler

Real life experience here. Several years ago, I had a new mast installed by a professional rigger. The vang attachment point was a wrought aluminum fabrication that was fastened to the mast with eight 1/4″ stainless steel rivets. Over the first 2 years that fitting sheared all 8 rivets during a controlled heavy air gybes. As an aerospace engineer, I took a close look and noted that the rivet holes in the fabrication were countersunk. Unfortunately, the rigger used plain rivets. I had to personally purchase countersunk rivets for the rigger to install and have never had another incident even in much heavier weather.

This is the same thing as the under-torqued fasteners you mention. Normally a rivet is loaded in plain shear across the fastener axis. By installing a plain rivet in a countersunk holes each rivet was partially loaded in tension. Under extreme load such as a heavy air gybe individual rivets were loaded well past their capacity and the eight rivets would shear lice a zipper. When the countersunk rivet was installed the joint was pure metal supporting the load in shear. To this day the rigger does not understand this concept. Scary.

Charlie Armor

Loose torque costs lives

Charles Ethridge

What do you do when you can’t get a normal (long) torque wrench into the space available?