May 2012
Fasteners keep our machines together. They’re simple, strong—and often neglected, overlooked and misunderstood. They’re also critical to our machines’ condition, and ultimately to our own longevity. It’s worth looking at them and knowing what we’re looking at.
Bolts hold things together, or keep things from shifting. Tension pulls on the bolt; shear forces try to bend it or cut it off. (The best example of a fastener designed for use in shear is a pin.)
A bolt is usually stronger in shear than in tension (and it is stronger yet in “double-shear,” where it is supported in two places, with a center load), but it is in shear that we see most failures—precisely because engineers know how to load bolts optimally, and engineers try to put fasteners into shear rather than tension, when there is a choice.
A bolt in a tension application will sometimes have a reduced-diameter shank to prevent other things from rubbing against it; a bolt in shear is usually snug around its circumference.
Bolts are graded. Higher grades are stronger, but a bolt’s tensile strength should not be the only consideration an aircraft designer examines. Brittleness (or its opposite, malleability) can be more important that ultimate strength. SAE Grade 8 bolts, for example, have 150,000 psi tensile strength. A typical AN bolt is rated at 125,000 psi, but a Grade 8 bolt won’t bend much before it breaks.
In an aircraft, you may want a bolt to partially fail (stretch or bend a little), rather than simply hold on a tiny bit longer… and then snap. This characteristic also demonstrates why, when we stress an aircraft, we need to log the incident. These fasteners need to be inspected, and we need to let our mechanics know where to look.
Threaded Fasteners: Three Parts
Let’s examine the three typical components of a threaded fastener system: the washer, the nut and the bolt.
Unless the washer is designed to afford a large surface for clamping (such as a washer on a soft aluminum engine head), the washer will be the softest of the three components. It takes up the tension of the assembly, and deforms just a bit to allow the bolt and nut to achieve their proper clamping force. Washers can be—and often are supposed to be—ruined with one use.
In tension applications, use new washers every time. In shear, you may use washers repeatedly, as long as they pass a visual inspection for damage and corrosion. The full-thickness AN960 washers can be used in either tension or shear; the thinner AN960L washers are for shear applications.
Nuts are softer than bolts, because one or the other needs to have a little “give,” and nuts are nearly always easier and cheaper to replace than their mated bolts. In a perfect world, we would not reuse nuts in a tension application, and we would not reuse self-locking nuts anywhere. Certainly, the world isn’t perfect, but use your head when considering reuse of nuts in tension.
Used Fasteners
Because of galling, work-hardening of nuts, bolts and washers, microscopic deformation of the threads, and other factors, used fasteners require a little extra torque to achieve the same clamping force. Using a thread lube or a substitute alters this balance dramatically.
Some fastener manufacturers offer guidelines for torque requirements under “clean and dry” or various “lubricated” scenarios. Lubes vary greatly—WD-40, for example, doesn’t have the same effect as a graphite-based anti-seize—but all manufacturers’ specs are based starting with clean and dry components. Unfortunately, we all know that isn’t possible, every time. That’s another reason why experience matters.
When assembling lubricated versus dry threads, the torque values can vary by 100 percent or more. Don’t waste the bolt’s safety factor by tensioning it more than it needs, and don’t waste its strength by making it too loose.
Friction and Yield
Dave Miller at RS Technologies reminded me, “The effect of friction on the torque-clamp load relationship cannot be overstated. Torque is simply a measure of the energy that is used to tighten the fastener. It can sometimes function as an indicator of the amount of clamp load that will hold the assembly together.
“However, variations in the state of friction can produce significant changes in the amount of clamp load developed by applying the same torque, and clamp load is what is really holding the assembly together.
“Less friction can produce more clamp load and too much clamp load can lead to fastener failure; more friction can produce less clamp load which can lead to assembly failure due to loosening.”
The strength of a bolt is usually listed as both yield strength and tensile strength. An experienced mechanic feels when the fastener is “just right” as he approaches its yield strength. If he continues to apply torque, the bolt will stretch ever-quicker until it fails.
A properly installed bolt will hold to its tensile strength, which is the force it takes to literally pull the bolt apart.
Nuts are at the other critical end of the assembly. Whether the nut is castellated and kept in its relation by a cotter key, or held with a self-locking feature (typically Nylock—a nylon insert lock nut—as in the AN365 series; or deliberately distorted threads in either an eccentric or conical pattern), it’s there to keep things together.
Loose nuts in shear can enhance wear; in tension, they can quickly precipitate disaster. Although our cars are threaded together with few obvious “keepers” in place, our cars typically operate at 20 percent power settings, have the benefit of modern vibration-reducing design… and don’t have to fly. If the drain plug falls out of your Plymouth, you chug to a stop, even in the mountains, even at night. If it falls out of your Lycoming… well, you understand.
Added Security
Sometimes (but not yet in aviation), bolts will have a “lock” built into them: a spot of nylon or an insert works in some other applications. Some assemblies are held together with a thread-locking compound, exemplified by various Loctite products.
In certain applications, like the rod-end bearings that may be found in master cylinder installations, it’s impractical to rely on a locking thread, so a jam nut is run against the thread of the first component. Since these are seldom in heavy tension (usually they’re in compression) mode, jam nuts stay put; however, when there are tension loads—even mild ones—the jam nuts can come loose. Check these assemblies on every preflight.
Safety wiring, as mentioned earlier, is effective at keeping bolts from turning. Safety wire does not add any torque to the assembly, and isn’t supposed to. It serves its purpose, but don’t assume it’s magic. (It’s also good to have a look—safety wire permits a quick indicator of what’s gone wrong!)
Cotter pins, too, keep the nut/bolt relation honest. In shear, they keep the nuts from falling off; in tension, they ensure that the assembly torque hasn’t changed. (If you have brought your castellated assembly up to its prescribed torque and the nut isn’t yet lined up to a hole, continue tightening until the next available opportunity, and insert the cotter pin.) And just as you would never think of reusing safety wire, don’t reuse cotter pins.
It’s Your Responsibility
Remember, don’t assume anything. The supply house buys from only reputable sources, and they check their incoming hardware now and then, but in years past a few unethical suppliers have made inferior, “look-alike” aviation hardware. You may be able to spot a sharp radius under a bolt head, poor grade stampings and marks, ugly threads, or suspicious platings, but if you have any doubt, test the fasteners yourself* or at least get a local house to do a Rockwell test of the new fasteners against your known good ones.
Designers always build in a safety factor; still, it’s up to us and our mechanics to ensure that as little of that “extra” is required at any time. A good, calibrated torque wrench, proper lubrication (if any), new washers (and hopefully nuts, if used in tension), and a clean assembly go a long way toward keeping our aircraft in one piece.
*Put the bolt in a fixture you can make of scrap and torque it until it breaks, noting that value. Then break one of your known, new (and clean!) bolts. If you see any significant difference, you can also “spark” the bolts against a clean carborundum wheel; different steels give off different-color sparks, and in different patterns. It doesn’t take too long to get proficient enough at this to consider your eyes reliable. Of course, if you are in any doubt about the quality of any fastener, find out for certain—or don’t use it.
Know your FAR/AIM and check with your mechanic before starting any work.
Tim Kern, CAM, MBA, has authored features in over 40 aviation publications. He writes technical, publicity and expository pieces for several companies in the aviation industry. Kern is a private pilot with a seaplane rating, and is listed as the manufacturer (“with a lot of help!”) of an experimental aircraft. Send questions or comments to editor@www.piperflyer.com.
RESOURCES >>>>>
Professional Fastener Testing
RS Technologies, a division of PCB Load & Torque, Inc.
24350 Indoplex Circle
Farmington Hills, MI 48335
Toll-Free in the USA: (888) 684-2894
Email: rsinfo@pcbloadtorque.com
Grade Markings and Mechanical Properties for Steel Fasteners
industrialhardware.com/skin1/pages/en/Markinggrade.htm
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GENERAL RULES
• Always use the proper grade and type of fastener.
• Never bottom a bolt’s thread in the female thread; have two or more complete turns exposed on both sides of the nut.
• Use new washers in every tension application.
• Use new nuts in every critical tension assembly.
• Never use “shear” (low-profile) nuts in a tension application.
• Cotter pins should be snug, not loose, in their holes.
• Assemble threads clean and dry unless otherwise specified. Corrosion protection may usually be applied after assembly. If lube is used, check its recommendation with the manufacturer, and follow the manufacturer’s recommendations for that particular lube.
• Use a calibrated torque wrench. Remember: “checking the torque” does not mean “tightening it some more.”
• Use safety wire, properly, wherever possible.
• Periodically remove bolts that are hidden: inside struts, through tubing. I’ve seen these rusted nearly through, while their heads look just fine, on an airplane that just passed an annual.
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