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Builders guide to safe aircraft materials
AN, MS hardware — rivets, bolts
In addition to the fittings discussed in module 11 the term 'aircraft hardware' also includes the fasteners used in aircraft assembly. In this and the next module of the Guide we look at rivets, threaded fasteners and locking devices.
Airframe fasteners are designed and manufactured to conform with long-established and proven standards for a system of fasteners. The standards most widely recognised are those originally defined by committees associated with the US military prior to and during World War 2 — the Army–Navy [AN] standards. The Military Standards [MS] followed — which superseded some AN standards and complemented others — and later came the National Aerospace Standards [NAS]; thus the common terms AN, MS and NAS. Those older US standards are in inches and fractions of inches only so there are few references to SI units on this page.
Some of the following material is noted as an extract from the FAA advisory circular AC 43.13-1B Chapter 7. The complete volume 'Acceptable methods, techniques, and practices — aircraft inspection and repair' (~ 650 pages and incorporating the 2001 changes) is available from the RA-Aus online shop for a reasonable price. It is bound together with the FAA advisory circular AC 43.13-2A 'Acceptable methods, techniques, and practices — aircraft alterations' (~ 100 pages).
There are quite a number of solid rivet types each made from a variety of materials — aluminium alloys, steel, corrosion-resistant steel, Inconel and Monel. However, apart from engine bay applications, there is generally only one solid rivet type of interest to light aircraft builders. That is the general structural use aluminium alloy 2117-T4 rivet generally with a 'universal' head (the upper rivet in the diagram) or possibly a 100° countersunk head (the lower rivet) — note the differing methods for measuring rivet length. The 2117-T4 material is galvanically compatible with the 6061 alloy and although such rivets have only about 80% of the strength of 2024-T3 rivets, the handling process is much simpler. Another type occasionally used in non-structural applications is the softer, low-strength 99% aluminium alloy 1100 rivet with the same head options.
The 100° countersink is the aircraft standard and all AN, MS standard countersunk head rivets and screws will conform. However, some brand name pulled rivets may have 120° or other taper.
Countersunk head rivets are used to attach skins to the substructure so that the heads are flush with the surface, thus reducing drag. However, the skin material in very light aircraft is so thin that metal removal to produce a countersunk hole is not possible; consequently 'dimpling', as shown at left, is commonly used. Skin thicknesses of 1mm (0.040") are on the border between countersinking and dimpling; greater than 1 mm the metal becomes difficult to dimple, below 1 mm countersinking will probably produce a weaker joint. When the skin is being riveted to thicker structures such as longerons then the drilled hole in that structure would be countersunk and only the thin skin dimpled – with a dimple die set. However, protruding head riveting is not a significant drag problem for low-speed aircraft and universal head riveted joints will be easier, probably stronger and certainly less time-consuming to use.
Hole clamps inserted at intervals and generally known as clecos — the trade name of the first such clamp — are used to hold the metal parts tightly together while riveting.
Identification code: airframe fasteners are generally specified and catalogued by their hardware identification code (or dash number), which consists of the AN/MS/NAS basic specification followed by a series of numbers/letters. Rivets manufactured in accordance with the AN/MS standards are identified by a four-part code:
Use: solid rivets are generally used in single-lap (two layers of material) or double-lap (three layers) joints where parts formed from sheet metal are clamped together by rivets, and the riveter is able to access both sides of the work, with the manufactured head on the exterior. The diameter of the drilled holes is made slightly greater than the rivet shank so that when the (roughly) 0.5 gram rivet is repetitively driven against the inertia of a 0.4–0.6 kg bucking bar (shown at left), or a dolly, the compressive strain causes the shank to expand to completely fill the hole. The required bucking bar mass varies with rivet diameter.
At the same time the shank end or tail is plastically deformed to produce the 'driven' head, which clamps the parts together tightly. The driven head should have a thickness around 0.5 times and a width around 1.5 times the diameter of the shank. The dimensions achieved primarily result from the choice of rivet length compared to the thickness of the materials; if the length of the chosen rivet exceeds the combined thickness of the parts being joined (the 'grip length') by about 1.5 × rivet diameter then a satisfactory driven head should result.
More information is contained in the US Military Specification MIL-R-47196A 'Rivets, buck type, preparation for and installation of'.
If correctly prepared and riveted, the material being joined is under a compressive load and the elastic reaction places a tensile load on the rivet; these loads ensure a good seal between the undersides of the rivet heads and the material, providing a waterproof and corrosion-resistant joint — particularly so if driven when wet with zinc chromate primer that fills the small chamfers formed in the barrel of the hole where the edges have been deburred. Also the expansion of the shank to completely fill the drilled hole precludes any relative movement between rivet and material. The repetitive cold-working of the rivet strain hardens the material so the driven T4 rivet gains strength equivalent to the T3 temper.
The military specification MIL-R-47196A states: "When the rivet material is dissimilar to the material being riveted [see galvanic corrosion], the rivet hole, countersink, and rivet shall be coated with zinc chromate primer in accordance with TT-P-1757 prior to installation. The rivet shall be installed while the primer is in the wet condition." There are other corrosion-inhibiting zinc chromate or barium chromate pastes (e.g. JC5A, Duralac) are also used under rivet/bolt heads but there appear to be conditions where such compounds break up when in high pressure contact with water; e.g. in the proximity of the landing gear.
Riveted joints in light aircraft are usually designed so that the rivets will give way before the structural parts fail; the premise being that it is easier to replace a few failed rivets than repair/replace torn metal.
A pulled rivet, being hollow, does not have the same tensile strength of a solid rivet of the same diameter, so stronger material or larger diameter rivets — or more of them — are necessary; and of course the retained steel stem makes them perhaps 20–50% heavier than a solid aluminium rivet performing the same function. However, a filled core improves the shear strength of the aluminium rivet.
The diameter of the manufactured head of a blind rivet is usually twice the shank diameter and will have a greater surface area than the blind side head. The joint should be designed and the rivet installed so the blind side head doesn't apply its more concentrated load to the thinner of the materials being joined.
Good quality blind riveting requires precise hole drilling. Generally the shanks of the less costly pulled rivets do not swell when installed and the joint should not be considered watertight. Although the steel stems of aluminium rivets are phosphated or otherwise coated, the bare metal exposed after the stem is broken off must be considered a corrosion potential and sealed in some way.
Some manufacturers of sheet metal kit aircraft recommend extensive use of particular brands of aluminium pulled rivets and, though more costly than solid rivets, they are certainly easier to install. There is no reason not to use pulled rivets instead of solid rivets in a homebuilt aircraft as long as the type and manufacturer are carefully selected.
The POP® rivet is the brand name of a range of blind rivets originally manufactured by the United Shoe Machinery Co. [USM]. These rivets have been used in home and industrial applications for over 80 years. Sometimes the generic pop rivet is used when referring to blind rivets or the manual installation tools for blind rivets are referred to as pop riveters but, just to make it clear, POP® rivets are blind rivets but most blind rivet types are not POP® rivets.
The common POP® rivets are not suitable for use in the primary structure of an aircraft though they might be used in other areas where there are no loads requiring structural grade riveting. However, if the engine/propeller installation is of the pusher type you might want to consider the consequences of part, or all, of a POP rivet being ingested into the engine or passing through the propeller disc.
There are two commonly used structural bolted joint designs, one type where the high tensile strength of the bolt shank is used to clamp members together and the joint functionality relies on the surface friction between the members rather than the bolt shank; the joint will hold as long as the friction force is greater than any shear force applied. The other joint type is where the joint relies primarily on the shear strength of the bolt shank — such as seen in aluminium tubular truss structures — and there is only sufficient tensile load applied to the bolt/nut to prevent movement after locking.
Torque. If a turning force or torque is applied with a wrench to the nut of a bolt and nut pair already 'snugged up' (i.e. holding all joint interfaces in intimate contact but with little or no tension in the bolt) the under-surface of the bolt head and the inner surface of the nut (or intermediate washers if fitted) will apply a compressive force to the members, clamping them together. Depending on the stiffness of the joint members, the periphery of that compressive effect extends to around 4–5 times the diameter of the bolt shank. The greater the torque applied to the nut, the greater the tension in the bolt and the greater the compression in the members (or the crushing force applied to the member(s) and any intermediate sealing gasket). 'Hard' joints may only require the nut to be rotated through a 30° angle from the snugged position to achieve the full torque. A 'soft' gasketed joint may require a rotation of two full turns from the snugged position.
Pre-loading. Referring to the stress-strain diagram in the module 'Properties of metals' it can be seen that as long as the tensile stress in the bolt is less than the yield strength, the resulting bolt stretch (the strain) will stay within the elastic region. While that tension continues, the bolt elasticity (the potential energy) will apply the clamping force holding the joint together. This clamping force is called the pre-load or pre-tension which, for a high-stress joint (such as a propeller hub/crankshaft flange joint), might be set at 70% or more of the bolt yield strength — the position indicated by the small green cross in that stress-strain diagram.
(Because bolt threads act as stress concentrators, permanent deformation will occur at loads a little below yield strength — maybe around the 95% level. This is termed the bolt proof strength, proof stress or proof load.)
The compressive force in the members is equal to the tensile force in the bolt(s) but if the members are stiffer than the bolts, the amount of compressive movement would be less than the amount of bolt elongation.
The stretch in a pre-tensioned bolt is probably less than 0.25% of its initial length. But of course a 0.25% strain in a bolt 100 mm long is 10 times the physical stretch of a 0.25% strain in a bolt 10 mm long.
Note on turning force: only about 10–15% of the torque applied increases bolt tension; i.e. stretches it. Perhaps 40–50% of the turning force is needed to overcome the friction between the male and female threads; the balance is needed to overcome the turning friction between the under-surface of the nut and the material being clamped. Thus if some form of thread lubricant is used, the torque required to produce the same pre-load is perhaps 25% less. The cadmium plating on the bolt and nut for corrosion protection also acts as a lubricant, so the torque required is reduced.
The turning force to be applied to a nut (or the angle through which it is to be turned from the snugged position) to achieve a particular pre-load will be specified in torque charts or by the designer. If any coating, corrosion inhibiting compound/paste or lubricant is used that is not specified by the designer, then there is a very good chance that applying the specified torque will stress the bolt beyond its yield point and lead to joint failure. Also, torque wrenches may have only a plus/minus 25% accuracy.
There is more information below.
Clamped joints. Having calculated the in-service loads that will be applied to a structural joint the aircraft designer will determine the number of bolts required and their spacing plus tensile strength, physical dimensions, thread type, thread pitch, corrosion protection and then the pre-load to be applied. Most of the resistance to shear within the joint comes from the friction between the clamped surfaces of the joint members — so of course there may be quite a number of bolts within the joint.
The diagram at left shows the forces acting within a pre-loaded joint. When there is no external tension forces the compressive force [Fc] in the joint members equals the pre-load force [Fp] in the bolt. In flight, the joint will be loaded with external tension forces [Ft] and shear forces [Fs]. The external tension forces decrease the pre-load joint compression. However, such joints are designed so that the members are quite stiff and the bolts resilient. So, a quite high external load will cause a decrease in joint load, but not to the point of separation, and only a slight increase in the tensile load on the bolt(s). Designers will generally opt for a larger number of smaller diameter bolts in a joint, rather than a smaller number of larger diameter bolts; for example, the centre joint of the left and right main wing spars for a twin-engine Piper aircraft utilises fourteen 3/8 inch bolts to join the top spar caps — with a similar arrangement for the bottom spar caps — and sixteen 3/16 inch bolts for joining the webs: 44 bolts in one joint.
External forces acting on a structural joint are generally not pure tension or pure shear; the force vector will have a tension component and a shear component. As long as the external load is somewhat less than the pre-load, a joint clamping load exists, but this ceases if those tension forces exceed the pre-load force. Then the tensile stress on the bolts will increase, the bolts elongate (still elastically) and the mating parts begin to slip, thereby reducing joint functionality and imposing all the shear forces in the joint onto the bolt shanks. The tensile stress may take the bolts past their yield point, and the combination of shear and tension will cause the bolts to bend so that even if the external load is released the joint will no longer be functional.
Pre-load and metal fatigue. Pre-loading has the effect of reducing the dimension of the fatigue cycles to which the fastener is exposed. The forces applied to the bolt from in-flight loads are generally much less than the pre-load, so the increases in bolt tension are comparatively slight thus reducing the level of cyclic stress and keeping it inside the fatigue limit.
Embedding. After some exposure to flight loads, joint surfaces tend to embed into each other (the rougher the surfaces, the greater the embedding) which has the effect of relaxing the bolt pre-load.
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AN3–20 bolts are identified by a multi-part code:
The AN3–20 bolts use only the UNF threads; the AN3 bolt has 32 tpi, the AN4 is 28 tpi, AN5 and AN6 are 24 tpi, and AN7 and AN8 are 20 tpi.
Thread length and grip. The threaded length of AN bolts is about 3/8" for AN3, 7/16" for AN4, 1/2" for AN5 and 9/16" for AN6–AN8. The grip is the shank length minus the threaded length, which for the AN6-H7A bolt would be 7/8" shank length minus 9/16" thread length = 5/16" grip. Thus an AN3-4 bolt would have a grip of only 1/8" and might, at first glance, present the appearance of a fully threaded shank.
The threaded length should not be subject to shear loads. The specification allows shank lengths to be from 1/32 to 3/32 inches longer than the nominal length.
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Some types of nylon insert self-lockers are claimed to seal the bolt thread against entry of fluids.
Self-lockers can't be used where the joint is subject to any rotational movement; bolts in such circumstances are locked using non-friction locking — castellated nuts (AN310) with cotter pins or safetying wire.
When a nut is fully torqued, about 33% of the total tensile load is placed on the first (most inward) thread and the mating bolt thread. The second thread takes a further 23% of the load. The third thread takes about 14% so that the stress on the first three engaged threads of both the nut and bolt is about 70% of the total, and the first six threads take about 99% of the tensile load. This indicates it is just added weight (and possible space constriction) for a nut to be longer than 6 to 8 threads; and the same for the bolt — plus an allowance for thread start and thread run-out. It also follows that when a bolt is primarily loaded in shear, a light nut with possibly only three threads is ample for the task of just keeping the bolt in position for the unthreaded shank to carry the shear load.
When using some types of self-locking nuts it should be borne in mind that the first three outward bolt threads have been slightly tapered to facilitate running on the nut and reducing the chance of cross-threading — more below.
In some circumstances where a number of bolts are aligned to form a joint it is expedient to use an additional rigid bearing plate to the joint — rather than a number of washers — which acts as a doubler and distributes the compressive forces more evenly.
There is quite a lot of information on swaged fittings in section 8 of Chapter 7 'Aircraft hardware, control cables, and turnbuckles' of AC 43.13-1B on this website for download in PDF format (2.8 MB). Section 8 commences at page 27.
4.70. GENERAL. Mechanical joints in wood are usually limited to types employing aircraft bolts. Since bolts in wood can carry a much higher load parallel to the grain of the wood than across the grain, it is generally advantageous to design a fitting and its mating wood parts so that the loads on the bolts are parallel to the grain. The use of a pair of bolts on the same grain line, carrying loads perpendicular to the grain and oppositely directed, is likely to increase the tendency to split. When a long row of bolts is used to join two parts of a structure, consideration should be given to the relative deformation of the parts, as explained in section 4.82.
4.71. USE OF BUSHINGS. Bushings are often used in wood to provide additional bearing area and to prevent crushing of the wood when bolts are tightened. See figure 4-36. When bolts of large length/diameter ratio are used, or when bolts are used through a member having high density plates on the faces, plug bushings may be used to advantage.
4.72. USE OF HIGH DENSITY MATERIAL. Wherever highly concentrated loads are introduced, greater bearing strength can be obtained by scarfing-in high-density material (section 4.63). Some high density materials are quite sensitive to stress concentrations and the possibility of the serious effects of such stress concentrations should be considered when large loads must be carried through the high-density material.
Wherever metal fittings are attached to wood members, it is generally advisable to reinforce the wood against crushing by the use of high-density bearing plates (figure 4-37) and to use a coat of bitumastic or similar material between the wood and metal to guard against corrosion. Cross banding of these plates will help to prevent splitting of the solid wood member.
4.73. MECHANICAL ATTACHMENT OF RIBS. When ribs carry heavy or concentrated loads it is sometimes desirable to insure their attachment by use of mechanical fastenings (see figure 4-39).
4.74. ATTACHMENT OF VARIOUS TYPES OF FITTINGS. Fittings should always have wide base plates to prevent crushing at edges. Wood washers have a tendency to cone under tightening loads. Where possible, it is desirable to use washer plates for bolt groups, as illustrated in figure 4-40, but if washers are used, a special type for wood, AN-970 or equivalent, are necessary to provide sufficient bearing area.
Clamps around wood members should be constructed so that they can be tightened symmetrically (figure 4-41).
4.75. USE OF WOOD SCREWS, RIVETS, NAILS, AND SELF-LOCKING NUTS. Wood screws and rivets are sometimes used for the attachment of secondary structure but should not be used in connecting primary members. Wood screws have been successfully used to prevent cleavage of plywood skin from stringers in some skin-stringer applications. Nails should never be used in aircraft to carry structural loads.
Self-locking nuts of approved types designed for use with wood and plywood structures are preferable to plate or anchor nuts. When the latter type is used, however, attachment may be made to the structure with wood screws or rivets provided that care is taken not to reduce the strength of load-carrying members. Rivetting through wood is always questionable because of the danger of crushing the rivet heads and the possibility of bending the shank while bucking the rivet. Also, there is no way of tightening the joint when dimensional changes from shrinkage occur.
[Extract from ANC-18 ends]
The next module in this metals and hardware group is fastener safetying.
Builders guide to aircraft materials – metals and hardware modules
| Guide contents | Properties of metals | Metal corrosion | Hardware fittings in aircraft structures |
| [AN, MS hardware — rivets, bolts and locking devices] | Safetying |
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