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You Can't Build an Airplane Without Nuts and Bolts
By Tony Bingelis (originally published in EAA Sport Aviation, November 1979)
OF COURSE, YOU will use nuts and bolts. But, unless you use aircraft quality bolts and install them properly, you might as well try to build without them for you will never be quite sure that they will hold things together under the flight conditions you will encounter.
Hardware stores and discount stores don't sell aircraft quality bolts or nuts. Common steel bolts obtained from these sources must never be used in aircraft for structural purposes. Made of a low carbon (mild) steel and having a rather low tensile strength (about 55,000 psi), they lack the reliability needed in aircraft structures.
These bolts are weak! They bend easily and are very susceptible to corrosion! You can recognize these hardware store bolts by their square, or hex heads, which are usually devoid of any identification markings. Also, for the most part, these bolts have coarse threads and are ordinarily used with plain nuts having no self-locking capability.
Common steel bolts might be all right to use in making picnic benches and portable barbeque grills but not for use in making airplanes . . . not if you value your life! Even go-cart builders use aircraft hardware and they don't fly (not high, at any rate). Yes, most ultralight builders use aircraft bolts, too. And I would like to believe that all builders are informed enough to insist on using aircraft bolts and aircraft quality hardware throughout their aircraft.
Would you believe it? In this weird economy of ours you can often purchase aircraft bolts less expensively than some of the hardware store variety! So, indeed, contemplating the use of common hardware store bolts is totally without merit.
Bolt Identification and Standards
There is no easy method for a part-time builder to become familiar with the various classes and types of aircraft bolts because they are manufactured in countless shapes and with all kinds of markings.
Aircraft bolt heads are code marked for identification, with embossed symbols. These markings indicate the material from which the bolt is made . . . and often the manufacturer who made it. Sometimes the head displays only an asterisk or a simple X to identify it as a standard corrosion-resistant steel aircraft bolt. Several of the more commonly used bolt types are illustrated in Figure 1.
At one time there was only one general standard for aircraft bolts. Because the military aviation was the biggest user, the Army (Air Corps) and Navy decided to establish a series of special bolts to be known as the AN series. The AN bolt standard is still very much the one used in general aviation although other standards are making inroads and in the process, complicating things for maintenance and supply folks and the homebuilders in particular. Some of the complexity stems from the hardware manufactured under Military Standards (MS), National Aerospace Standards (NAS) and others. But fear not. The AN standard hardware still meets most homebuilt requirements and unless you want to use MS or NAS standard hardware extensively, you shouldn't experience much difficulty. Most supply sources for the homebuilder stock the better known AN standard hardware exclusively. Should they also have other types, they will most likely have it cross-referenced in their catalogs.
AN bolts are made from a nickel alloy, corrosion-resistant steel and are heat treated to a minimum of 125,000 pounds per square inch (psi). On the other hand, similar NAS or MS standard steel bolts are rated at as high (or higher) a tensile strength, with 160,000 psi being most prevelant.
AN bolts range in diameter from the AN3, which is a 3/16" or (1032) bolt, up through the AN20 (1-1/4" diameter) bolts. In supply catalogs, and on plans, aircraft bolts are normally listed by their AN number. The AN number is usually followed by a dash and other number and/or letters to denote the bolt length and whether or not the head and shank are drilled for safetying purposes. No need to memorize these dash numbers . . . simply refer to any homebuilt supply catalog and it will all be listed for you . . . including the price, of course.
Bolt Selection and Use Tips
The AN3 bolt is the smallest diameter steel bolt acceptable for use in the primary structure of your aircraft. That is any structure, the failure of which could result in disastrous consequences. Wing attachment bolts, landing gear bolts, propeller bolts, engine mount bolts, etc., are examples of primary structure. Other areas, although less visible, may be equally important.
It is poor practice to cut additional threads on a longer bolt and then to hacksaw it to the required length. For one thing, the cadmium plated protection against corrosion would be lost. In addition, workshop threading seems to weaken a bolt at the point where the thread ends . . . so, don't use these modified bolts in highly stressed locations.
There may be a time when a bolt fits just a bit too loosely to suit you. Did you know that there is a tolerance in the standard bolt diameters of the same AN number? Take as an example an AN3 bolt. It is a 3/16" bolt and its diameter will mike out to .189" plus .000" to minus .003". So you see, because of that leeway, one bolt could fit snugly while another might be a bit loose. If you have a situation like that you might sort through your bolts to find the best fitting one.
As you let your fingers sweep across your favorite supply catalog you may learn that you still have a bit of a problem in determining the exact length bolt you need. A lot of catalogs list only the total length from the bottom of the bolt head to the end of the bolt. Since the threaded portion must not bear in the structure or fitting . . . how long must the bolt be to provide the needed grip length? Maybe this will help.
Aircraft bolts have a relatively short threaded portion which makes up approximately 3/8" of an AN3 bolt . . . 7/16" of an AN4 or AN5 bolt and about 9/16" of the larger bolts. This severely limits the use of any bolt lengths to a very narrow range because only the shank (grip) portion is designed to carry shear loads. When you install a bolt, its shank must be long enough that the threads are not used in bearing. To put that another way . . . not more than one thread of the bolt should be permitted to bear inside a fitting. If it does, use the next size longer bolt. You may have noticed from catalogs that bolts are obtainable in graduated lengths of 1/8" increments. Sometimes the next size longer bolt will be a bit too long but you can use up to two standard washers to fill in.
It has already been pointed out that AN bolts, for any particular diameter, vary by approximately .003". If you need a very snug fit in a metal-to-metal bolt installation you should drill the hole slightly undersized and ream it to size. When such a tight fit is required you should use close tolerance bolts. These bolts are identified by an embossed triangle on the bolt head. Close tolerance bolts are accurately machined after cadmium plating. This results in very uniform diameters between bolts, The manufactured tolerance is listed as .0005" or less.
Aluminum Alloy Bolts
With builders becoming more weight conscious and with more and more ultralight aircraft making the scene, I expect the use of aluminum alloy bolts will increase. The rule for conventional aircraft has been to never use aluminum bolts in the primary structure in diameters of less than 1/4". Aluminum bolts are normally used in shear applications - never in tension. Install aluminum bolts with aluminum nuts. Although there is no prohibition against the use of an aluminum nut on a steel bolt, I don't like that practice. Mixing aluminum and steel nuts and bolts is an undesirable practice particularly when the assembly is one in a seaplane. The potential for serious corrosion is great any time unprotected dissimilar metals, in close contact, are exposed to salt water or spray.
Aluminum alloy bolts and nuts are not suitable for use in locations where they must be frequently removed for maintenance or inspection because they do not tolerate repeated removal and reinstallation.
You can identify an aluminum bolt from its head markings, two raised dashes, and a comparatively light weight. They are quite strong for their weight and are rated at a tensile strength of approximately 65,000 psi.
Always Do It This Way . . . Sometimes
There is a lot of what you might call folklore passed around about aircraft bolts and how and where they should be used. Most of these revelations are based on logic, certainly, but in making their rounds, the truth tends to become a bit bent. For example:
"It is mandatory for bolts to be installed with their heads up or forward facing the slipstream."
Not so really. It is a good customary practice but that is far from being a governmental edict. The origin stems from the premise that if, in the most unlikely event a nut comes off, the bolt will not fall out and the assembly will not come apart. Logical enough but it will not always work. Sometimes the bolt can only be inserted from one direction and if it happens to be from the bottom . . . well, so be it. In other instances, should the nut come off some assemblies would still come apart (fail) even though the bolt was installed in the classical head-up position. Indeed, if you can, do insert the bolt head up and/or forward if at all practical. If not, you will not be violating regulations or inviting dire consequences.
Here is another one most builders accept as gospel:
"Never use an elastic stop nut with a drilled bolt."
The edges of the bolt hole, supposedly, will tear the fiber locking feature and destroy its self-locking provision. Again, this is not so. Of course, if there is a burr around the cotter pin hole (most unlikely), it could do some damage . . . but to destroy the elastic stop nut's self-locking capability, most unlikely!
How about this one?
"Never, never re-use an elastic self-locking nut."
Wrong again. These nuts can be re-used any number of times . . . perhaps as many as 50 times without reducing the self-locking feature. The obvious check for most any self-locking nut is that it should not be possible to spin on the nut completely without your fingers. If it goes on that easily, the self-locking feature is "kaput", However, here I am falling into the same trap of making a sweeping generalization. There are self-locking nuts which spin onto the bolt all the way with the fingers and the self-locking feature does not engage until the nut is torqued.
And speaking of torque. How about this one:
"All nuts and bolts in aircraft work must be torqued with a calibrated torque wrench."
Not bad advice at all, really, but in real life it is often impractical and sometimes ineffective. Most builders and mechanics don't use a torque wrench except for engine work. To reduce the risk of overtorquing use a short handled wrench. This is important because I doubt that any homebuilder will ever be guilty of undertorquing a nut . . . if he does, it will most likely be because he forgot to tighten it in the first place. Most builders apply too much torque to nuts and bolts . . . especially to the small AN3 bolts. AN3 bolts can be snapped off because the nuts are designed not to strip.
Bolts in wood structures can rarely be torqued to the ideal chart value without crushing some of the soft wood fibers. Here's a useful tip for torquing nuts and bolts used in wood structures. Tighten the nut until you see the washer slightly emboss itself into the wood. Stop right there. Do not continue to tighten the nut until the washer sinks all the way into the wood surface!
I'm sure you heard this one before:
"Never turn the bolt head . . . always torque the nut."
Again, a dreamer's ideal. Well, if you can't do it any other way, obviously, the bolt head will have to be turned with the wrench. The need to do this arises in locations where nutplates are installed and you may even find it necessary to do the same thing in other assemblies. But logic will tell you that turning the bolt should be avoided normally as there is a tendency for the cadmium to rub off the bolt and for the hole to become enlarged, however slightly.
The standard washer is 1/16" thick and is available in two types for each of the most commonly used bolt diameters. The smaller diameter washer is known as the AN960, and the larger diameter washer, the AN970. The larger washer is often called a wood washer because it is used most frequently against wood surfaces to provide a larger bearing surface.
Plain washers are used in bolt installations to present a smooth level bearing surface for either the bolt head or nut . . . sometimes both. Use a washer under the part to be torqued, be it the nut or bolt head or under both if you deem it necessary. Any place a bolt is not perpendicular to the surface may require the use of a tapered shim or specially made tapered washer.
Since bolt lengths are made in 1/8" increments, washers may sometimes be necessary to obtain the correct grip length for a bolt that is a little too long. A maximum of two standard 1/16" thick washers are sufficient to correct for a slightly long bolt. If not, use the next size shorter bolt.
Steel lock washers may be used as a locking feature for plain nuts but do not place a steel lock washer against an aluminum alloy nut or aluminum fitting.
If you are a newcomer to the realm of homebuilt aircraft and aircraft maintenance, you may be surprised to learn that aircraft bolts do not come with nuts. Each nut and bolt is bought separately. Primarily, I suppose, because of the very large variety of nuts which are available for specific aircraft requirements.
Aircraft nuts have no identifying markings or lettering on them but they are made of the same material as the bolts.
All nuts can be started with the fingers. If a nut gives you trouble in this regard, better recheck the threads . . . they may be coarse instead of fine. Aircraft nuts and bolts do sometimes have coarse threads although they are rarely used by homebuilders.
Self-Locking Nuts and Nut Safetying Features
All nut/bolt installations in your aircraft should have some safety features to prevent their loosening in service. Methods employed include the self-locking nuts, cotter-key combinations, jam nuts, lockwashers, palnuts, and still found in primitive areas, peening of the bolt end with a hammer.
Self-locking nuts are, by far, the most commonly used type of nuts as they require no external help for safetying. The nuts feature an integral safetying provision as part of their construction.
Self-locking nuts are used because they provide tight shake-proof installations which can withstand continued severe vibration. They must not, however, be used in any joint or connection where either the nut or bolt is subject to rotation. Self-locking nuts may be used with any antifriction bearings (rod ends) or control pulleys but only if the inner race of the bearing is clamped to a fitting (or part of the structure) by a bolt and nut.
The two types of self-locking nuts (all-metal type and fiber-lock type) are interchangeable except that the fiber-locking type may not be used in locations where the temperatures exceed 250°F. Metal self-locking nuts are used in places where temperatures exceed 250°F and may run as high as 400°F.
Elastic stop nuts may be reused many times, certainly until the fiber has lost its locking friction or has become brittle with age. If you can turn the nut up all the way with your fingers, replace it.
Shear nuts, either the self-locking or castle types, are easily recognized. They are those thin looking nuts and they are used most often with clevis bolts. Never use shear nuts where the installation is subjected to tension loading.
These nuts have no self-locking provision so they must be secured to the bolt mechanically with a cotter pin or safety wire or some sort of clip, otherwise they might loosen under vibration.
The castle nut is slotted and requires a bolt with a drilled hole to accept a cotter pin. Regular castle nuts are rugged and capable of withstanding high tension loads.
Be careful when using a castle nut on a clevis bolt in the control system where rudder pedals or other linkage might interfere with other parts. Cotter pins have a way of snagging.
Except as a last resort, do not torque a bolt by turning it.
Tighten or torque castellated nuts as you would any other standard nut. If the slots and cotter pin hole in the bolt will not line up for safetying, tighten the nut to the next slot even though the recommended torque values will be exceeded. This overtorquing leeway applies only to castle nuts and not others .
Cotter pins when used should fit snugly but not so tight you have to hammer them into the bolt hole. Bend the ends so that one end is over the end of the bolt reaching to its center or slightly beyond. The other leg is bent snugly against the nut.
Coarse threads require slightly less torque and so do shear nuts. Actually, about 1/2 the amount of torque is sufficient for them.
And, finally, even if you don't use a torque wrench . . . no self-respecting homebuilder will ever allow himself to be caught using a pair of pliers to tighten a nut. (Oh, the shame of it all!)