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.
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.
Hi Matt,
Great fill, thanks!
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.
Hi Kurt,
Wow, great you had the understanding from your professional career to fix this. And thanks for sharing the story. I have never even thought of a counter sunk rivet. My guess would be that the engineer who designed the fabrication expected machine screws to be used into a doubler plate, which is common on vang fittings.
As to the rigger, this unwillingness to at least try and better understand basic engineering is a distressingly common failing in the marine industry. I think it comes from the base line assumption of many, perhaps most, marine “professionals” that all yachties are morons and “we know best”. I long ago gave up trying to educate people like that.
Loose torque costs lives
What do you do when you can’t get a normal (long) torque wrench into the space available?
Hi Charles,
Good question. In that case a Crowfoot can help: https://www.morganscloud.com/2020/01/03/four-hand-tools-i-should-have-bought-years-ago/
Fabulous discussion.
I was an unwitting bolt man for years. Thank god for conservative engineering. This is like a weird catharsis… feel free to delete it!!
In a previous life I ran a large business maintaining rotating equipment globally. We addressed ~2 million bolted connections annually. I relied on ostensibly more technically qualified lieutenants to supervise the tasks, but after seeing every grotesque thing that could happen, I bought and read the first few chapters of the 900+ page book “introduction to bolting”. then ran screaming into the hallway, ,then as fast as i could, built the largest training center of its kind in the world to train our technicians.
I never looked at a bolt the same way again.
I debated bolts this year on my 38 year old swan 46 in a minor winter refit.
How much we took apart clean-inspect-reassemble – weight the risk it could be reassembled incorrectly. (I was not able to be present).
The well qualified old school yard did disassemble a lot and put back together with great care. Tested the keel bolt torques against original spec and they were right on. (we think of course, and we did not deliberately loosen them).
No evidence of wiggle/movement/deformation, after a few encounters with Whales and Rocks over the years. Solid backing plates, washers, steel grid etc.
Fingers crossed. Same thing with Rudder quadrant.
Anyway, the one tidbit maybe not mentioned so far is the maybe the most accurate way to tension a bolt is not to use torque at all (for all the reasons stated) but to use elongation. (Stretch of the bolt) Especially on large studs the spec was elongation. Using a large micrometer on bolts that were 3′ in diameter and 18 inches long. Tighten nuts until we got x mils of elongation and calculated clamping force.
I realize this is generally impractical. but it is interesting.
Mostly worked when we worked on a clamshell of a turbine or compressor and had easy access.
It’s the spring analogy and makes total sense.
I am gratified that there are others out there into the details of bolting.
Thanks John
Hi Rob,
And I thought I was obsessed! Interesting about measuring elongation. I can certainly see how that would be more accurate given the intrinsic inaccuracies of of the torque method: 25% either way, at best, as I understand it, and a lot worse if we don’t do everything perfectly. Sad that, as you say, in most cases on a boat that would not be practical.
We also discussed using an industrial tool that grabs the bolt head and puts a known tension on it but concluded that was not really practical either. Discussion starts here: https://www.morganscloud.com/2020/06/17/planning-and-budgeting-a-refit-keels-part-3-torquing-keel-bolts/#comment-293666