Torque, Rivets, and Installation Practices

7-48 Plain AN 960 Split-Lock Internal toothlock External toothlock Ball seat & socket AC9950 & AC955 Taper pin AN975 Plain AN 935 Star lock washers Special washers factor must be considered. Oversized or elongated holes in noncritical members can usually be drilled or reamed to the next larger size. Many bolt holes, particularly those in primary connecting elements, have close tolerances. Generally, it is permissible to use the first lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close tolerance bolts or AN Clevis bolts are used.

Light-drive fits for bolts (specified on the repair drawings as 0.0015 inch maximum clearance between bolt and hole) are required in places where bolts are used in repair, or where they are placed in the original structure. The fit of holes and bolts cannot be defined in terms of shaft and hole diameters; it is defined in terms of the friction between bolt and hole when sliding the bolt into place. A tight drive fit, for example, is one in which a sharp blow of a 12- or 14-ounce hammer is required to move the bolt. A bolt that requires a hard blow and sounds tight is considered too tight a fit. A light-drive fit is one in which a bolt moves when a hammer handle is held against its head and pressed by the weight of the body.

Installation Practices Examine the markings on the bolt head to determine that each bolt is of the correct material. It is extremely important to use like bolts in replacement. In every case, refer to the applicable Maintenance Instructions Manual and Illustrated Parts Breakdown. Be sure that washers are used under both the heads of bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum- alloy washer should be used under the head and nut of a steel bolt securing aluminum alloy or magnesium alloy members.

Any corrosion that occurs attacks the washer rather than the members. Steel washers should be used when joining steel members with steel bolts. Whenever possible, place the bolt with the head on top or in the forward position. This positioning tends to prevent the bolt from slipping out if the nut is accidentally lost. Be certain that the bolt grip length is correct. Grip length is the length of the unthreaded portion of the bolt shank. The grip length should equal the thickness of the material being bolted together. However, bolts of slightly greater grip length may be used if washers are placed under the nut or the bolt head. In the case of plate nuts, add shims under the plate.

Safetying of Bolts & Nuts It is very important that all bolts or nuts, except the self- locking type, be safetied after installation. This prevents them from loosening in flight due to vibration. Methods of safetying are discussed later in this chapter. Repair of Damaged Internal Threads Installation or replacement of bolts is simple when compared to the installation or replacement of studs. Bolt heads and nuts are cut in the open, whereas studs are installed into internal threads in a casting or built-up assembly. Damaged threads on bolts or nuts can be seen and only require replacement of the defective part. If internal threads are damaged, two alternatives are apparent: the part may be replaced or the threads repaired or replaced. Correction of the thread problem is usually cheaper and more convenient. Two methods of repairing are by replacement bushings or helicoils.

Replacement Bushings Bushings are usually special material (steel or brass spark plug bushings into aluminum cylinder heads). A material that resists wear is used where removal and replacement is frequent. The external threads on the bushing are usually coarse. The bushing is installed, a thread lock compound may or may not be used, and staked to prevent loosening. Many bushings have left-hand threads external and right- hand threads internal. With this installation, removal of the 7-49 bolt or stud (right-hand threads) tends to tighten the bushing. Bushings for common installations, such as spark plugs, may be up to 0.040 oversize (in increments of 0.005). Original installation and overhaul shop replacements are shrunk fit: a heated cylinder head and a frozen bushing.

Helicoils Helicoils are precision-formed screw thread coils of 18-8 stainless steel wire having a diamond-shaped cross section. [Figure 7-31] They form unified coarse or unified fine thread classes 2-band 3B when assembled into (helicoil) threaded holes. The assembled insert accommodates UNJ (controlled radius root) male threaded members. Each insert has a driving tang with a notch to facilitate removal of the tang after the insert is screwed into a helicoil tapped hole. They are used as screw thread bushings. In addition to being used to restore damaged threads, they are used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to protect and strengthen tapped threads in light materials, metals, and plastics, particularly in locations that require frequent assembly and disassembly and/ or where a screw locking action is desired.

Helicoil installation is a 5 or 6 step operation, depending upon how the last step is classed. [Figure 7-32] Step 1: Determine what threads are damaged. Step 2: (a) New installation of helicoil—drill out damaged threads to minimum depth specified. (b) Previously installed helicoil—using proper size extracting tool, place edge of blade in 90° from the edge of the insert. Tap with hammer to seat tool. Turn to left, applying pressure, until insert backs out. Threads are not damaged if insert is properly removed. Step 3: Tap—use the tap of required nominal thread size. The tapping procedure is the same as standard thread tapping. Tap length must be equal to or exceed the requirement.

Step 4: Gauge—threads may be checked with a helicoil thread gauge. Step 5: Insert assembly—using proper tool, install insert to a depth that puts end of top coil 1⁄4 to 1⁄2 turn below the top surface of the tapped hole. Step 6: Tang breakoff—select proper breakoff tool. Tangs should be removed from all drilled through holes. In blind holes, the tangs may be removed when necessary if enough hole-depth is provided below the tang of the installed insert. These are not to be considered specific instructions on helicoil installation. The manufacturer’s instruction must be followed when making an installation.

Helicoils are available for the following threads: unified coarse, unified fine, metric, spark plug, and national taper pipe threads.

Fastener Torque

Torque As the speed of an aircraft increases, each structural member becomes more highly stressed. It is therefore extremely important that each member carry no more and no less than the load for which it was designed. To distribute the loads safely throughout a structure, it is necessary that proper torque be applied to all nuts, bolts, studs, and screws. Using the proper torque allows the structure to develop its designed strength and greatly reduces the possibility of failure due to fatigue. Torque Wrenches The three most commonly used torque wrenches are the flexible beam, rigid frame, and the ratchet types.

[Figure 7-33] When using the flexible beam and the rigid frame torque wrenches, the torque value is read visually on a dial or scale mounted on the handle of the wrench. To use the ratchet type, unlock the grip and adjust the handle to the desired setting on the micrometer-type scale, then relock the grip. Install the required socket or adapter to the square drive of the handle. Place the wrench assembly on the nut or bolt, and pull the wrench assembly on the nut or bolt in a clockwise direction with a smooth, steady motion. (A fast or jerky motion results in an improperly torqued unit.) When the applied torque reaches the torque value indicated on the handle setting, the handle automatically releases or “breaks” and moves freely for a short distance. The release and free travel is easily felt, so there is no doubt about when the torqueing process is completed.

To assure getting the correct amount of torque on the fasteners, all torque wrenches must be tested at least once a month or more often if necessary. Note: It is not advisable to use a handle length extension on a flexible beam-type torque wrench at any time. A handle extension alone has no effect on the reading of the other types. The use of a drive end extension on any type of torque wrench makes the use of the formula mandatory. When applying the formula, force must be applied to the handle of the torque wrench at the point from which the measurements were taken. If this is not done, the torque obtained is incorrect.

7-50 Drill Tap Gauge Install Torque Tables Use the standard torque table as a guide in tightening nuts, studs, bolts, and screws whenever specific torque values are not called out in maintenance procedures. The following rules apply for correct use of the torque table: [Figure 7-34] 1. To obtain values in foot-pounds, divide inch-pounds by 12. 2. Do not lubricate nuts or bolts except for corrosion- resistant steel parts or where specifically instructed to do so. 3. Always tighten by rotating the nut first if possible. When space considerations make it necessary to tighten by rotating the bolt head, approach the high side of the indicated torque range. Do not exceed the maximum allowable torque value.

4. Maximum torque ranges should be used only when materials and surfaces being joined are of sufficient thickness, area, and strength to resist breaking, warping, or other damage. 5. For corrosion-resisting steel nuts, use torque values given for shear-type nuts. 6. The use of any type of drive end extension on a torque wrench changes the dial reading required to obtain the actual values indicated in the standard torque range tables. When using a drive end extension, the torque wrench reading must be computed by use of the proper formula, which is included in the handbook accompanying the torque wrench.

Cotter Pin Hole Line Up When tightening castellated nuts on bolts, the cotter pin holes may not line up with the slots in the nuts for the range of recommended values. Except in cases of highly-stressed engine parts, the nut may not be over torque. Remove hardware and realign the holes. The torque loads specified may be used for all unlubricated cadmium-plated steel nuts of the fine or coarse thread series, which have approximately equal number of threads and equal face bearing areas. These values do not apply where special torque requirements are specified in the maintenance manual. If the head end, rather than the nut, must be turned in the tightening operation, maximum torque values may be increased by an amount equal to shank friction, provided the latter is first measured by a torque wrench.

7-51 150 250 350 450 55 650 750 0 Basic formula F x L = T F = Applied force L = Lever length between centerline of drive and centerline of applied force (F must be 90° to L) T = Torque Formula for use with extensions Tw = A = Lever length of wrench B = Lever length of wrench plus extension Te = Required torque on bolt Tw = Torque reading on wrench dial A B 90° L F T Flexible beam Ratchet type Rigid frame 90° Te x A B

Aircraft Rivets

Sheets of metal must be fastened together to form the aircraft structure, and this is usually done with solid aluminum-alloy rivets. A rivet is a metal pin with a formed head on one end when the rivet is manufactured. The shank of the rivet is inserted into a drilled hole, and its shank is then upset (deformed) by a hand or pneumatic tool. The second head, formed either by hand or by pneumatic equipment, is called a “shop head.” The shop head functions in the same manner as a nut on a bolt. In addition to their use for joining aircraft skin sections, rivets are also used for joining spar sections, for holding rib sections in place, for securing fittings to various parts of the aircraft, and for fastening innumerable bracing members and other parts together. The rivet creates a bond that is at least as strong as the material being joined.

Two of the major types of rivets used in aircraft are the common solid shank type, which must be driven using a bucking bar, and the special (blind) rivets, which may be installed where it is impossible to use a bucking bar. Aircraft rivets are not hardware store rivets. Rivets purchased at a hardware store should never be used as a substitute for aircraft quality rivets. The rivets may be made from very different materials, the strength of the rivets differs greatly, and their shear strength qualities are very different. The countersunk heads on hardware store rivets are 78°, whereas countersunk aircraft rivets have 100° angle heads for more surface contact to hold it in place.

Standards and Specifications The FAA requires that the structural strength and integrity of type-certificated aircraft conform to all airworthiness requirements. These requirements apply to performance, structural strength, and integrity as well as flight characteristics. To meet these requirements, each aircraft must meet the same standards. To accomplish standardization, all materials and hardware must be manufactured to a standard of quality. Specifications and standards for aircraft hardware are usually identified by the organization that originated them. Some of the common standardizing organizations include: AMS Aeronautical Material Specifications AN Air Force-Navy AND Air Force-Navy Design AS Aeronautical Standard ASA American Standards Association ASTM American Society for Testing Materials MS Military Standard NAF Naval Aircraft Factory 7-52 NAS National Aerospace Standard SAE Society of Automotive Engineers When a MS20426-AD4-6 rivet is required, the specifications have already been written for it in the Military Standard (MS) specifications. That information is available to the aircraft manufacturers, the rivet manufacturers and the mechanic.

The specifications designate the material to be used as well as the head type, diameter, and length of the rivet. The use of standardized materials in the production of aircraft makes each aircraft exactly the same as the previous one and makes them less expensive to build. Aircraft rivets are manufactured to much higher standards and specifications than rivets manufactured for general use. When aircraft manufacturers started building all-metal aircraft in the 1930s, different manufacturers had different rivet head designs. Brazier heads, modified brazier heads, button heads, mushroom heads, flatheads, and 78° countersunk heads were used. As aircraft standardized, four rivet head designs almost completely replaced all the others. Rivets exposed to the airflow over the top of the structure are usually either universal head MS20470 or 100° countersunk head MS20426 rivets. For rivets used in internal structures, the roundhead MS20430 and the flathead MS20442 are generally used.

Solid Shank Rivets Solid shank rivets are generally used in repair work. They are identified by the kind of material of which they are made, their head type, size of shank, and their temper condition. The designation of the solid shank rivet head type, such as universal head, roundhead, flathead, countersunk head, and brazier head, depends on the cross-sectional shape of the head. [Figure 7-35] The temper designation and strength are indicated by special markings on the head of the rivet. The material used for most aircraft solid shank rivets is aluminum alloy. The strength and temper conditions of aluminum-alloy rivets are identified by digits and letters similar to those adopted for the identification of strength and temper conditions of aluminum and aluminum-alloy stock.

The 1100, 2017-T, 2024-T, 2117-T, and 5056 rivets are the five grades usually available. The 1100 rivet, which is composed of 99.45 percent pure aluminum, is very soft. It is for riveting the softer aluminum alloys, such as 1100, 3003, and 5052, which are used for nonstructural parts (all parts where strength is not a factor). The riveting of map cases is a good example of where a rivet of 1100 aluminum alloy may be used. The 2117-T rivet, known as the field rivet, is used more than any other for riveting aluminum alloy structures. The field rivet is in wide demand, because it is ready for use as received and needs no further heat-treating or annealing. It also has a high resistance to corrosion.

The 2017-T and 2024-T rivets are used in aluminum-alloy structures where more strength is needed than is obtainable with the same size 2217-T rivet. These rivets are known as “ice box rivets,” are annealed and must be kept refrigerated until they are to be driven. The 2017-T rivet should be driven within approximately 1 hour and the 2024-T rivet within 10 to 20 minutes after removal from refrigeration. The 5056 rivet is used for riveting magnesium-alloy structures because of its corrosion-resistant qualities in combination with magnesium. Mild steel rivets are used for riveting steel parts. The corrosion- resistant steel rivets are for riveting corrosion-resistant steels in firewalls, exhaust stack brackets, and similar structures.

Monel rivets are used for riveting nickel-steel alloys. They can be substituted for those made of corrosion-resistant steel in some cases. The use of copper rivets in aircraft repair is limited. Copper rivets can be used only on copper alloys or nonmetallic materials, such as leather. Metal temper is an important factor in the riveting process, especially with aluminum alloy rivets. Aluminum-alloy rivets have the same heat-treating characteristics as aluminum-alloy stock. They can be hardened and annealed in the same manner as aluminum. The rivet must be soft, or comparatively soft, before a good head can be formed. The 2017-T and 2024-T rivets are annealed before being driven. They harden with age.

The process of heat-treating (annealing) rivets is much the same as that for stock. Either an electric air furnace, a salt bath, or a hot oil bath is needed. The heat-treating range, depending on the alloy, is 625 °F to 950 °F. For convenient handling, rivets are heated in a tray or a wire basket. They are quenched in cold water (70 °F) immediately after heat-treating. The 2017-T and 2024-T rivets, which are heat-treatable rivets, begin to age harden within a few minutes after being exposed to room temperature. Therefore, they must be used immediately after quenching or else be placed in cold storage.

The most commonly used means for holding heat-treatable rivets at low temperature (below 32 °F) is to keep them in a refrigerator. They are referred to as “icebox” rivets. Under this storage condition, they remain soft enough for driving for up to 2 weeks. Any rivets not used within that time should be removed for reheat-treating. 7-53 Bolt, Stud, or Screw Size On standard bolts, studs, and screws having a tensile strength of 125,000 to 140,000 psi On bolts, studs, and screws having a tensile strength of 140,000 to 160,000 psi On high-strength bolts, studs, and screws having a tensile strength of 160,000 psi and over Shear-type nuts (AN320, AN364, or equivalent) Tension-type nuts and threaded machine parts (AN-310, AN365, or equivalent) Any nut, except shear type Any nut, except shear type TORQUE VALUES FOR TIGHTENING NUTS 8–32 10–24 ¼–20 ½–13 ¾–10 7⁄8–9 1–8 11⁄8–8 1¼–8 8–36 10–32 ¼–28 3⁄8–24 7/16–20 ½–20 ¾–16 1–14 11⁄8–12 1¼–12 7–9 12–15 25–30 30–40 48-55 60–85 95–110 140–155 270–300 240–290 290–410 300–420 480–600 420–540 660–780 700–950 1,300–1,500 1,300–1,800 1,500–1,800 22,00–3,000 2,200–3,300 3,300–4,000 3,000–4,200 4,000–5,000 5,400–6,600 12–15 20–25 40–50 50–70 80–90 100–140 160–185 160–190 235–255 450–500 400–480 480–690 500–700 800–1,000 700–900 1,100–1,300 1,150–1,600 2,300–2,500 2,200–3,000 2,500–3,000 3,700–5,000 3,700–5,500 5,500–6,500 5,000–7,000 6,500–8,000 9,000–11,000 14–17 23–30 45–49 60–80 85–117 120–172 173–217 175–271 245–342 475–628 440–636 585–840 600–845 900–1,220 800–1,125 1,200–1,730 1,380–1,925 2,400–3,500 2,600–3,570 2,750–4,650 4,350–5,920 4,600–7,250 6,000–8,650 6,000–10,250 7,250–11,000 10,000–16,750 15–18 25–35 50–68 70–90 90–144 140–203 185–248 190–351 255–428 500–756 480–792 690–990 700–990 1,000–1,440 900–1,350 1,300–2,160 1,600–2,250 2,500–4,500 3,000–4,140 3,000–6,300 5,000–6,840 5,500–9,000 6,500–10,800 7,000–13,500 8,000–14,000 11,000–22,500 (INCH-POUNDS) 5/16–18 3/8–16 7/16–14 9/16–12 5/8–11 5/16–24 9/16–18 5/8–18 7/8–14 7-54 Icebox rivets attain about one-half their maximum strength in approximately 1 hour after driving and full strength in about 4 days. When 2017-T rivets are exposed to room temperature for 1 hour or longer, they must be subject to reheat-treatment. This also applies to 2024-T rivets exposed to room temperature for a period exceeding 10 minutes.

Once an icebox rivet has been taken from the refrigerator, it should not be mixed with the rivets still in cold storage. If more rivets are removed from the refrigerator than can be used in 15 minutes, they should be placed in a separate container and stored for reheat-treatment. Heat-treatment of rivets may be repeated a number of times if done properly. Proper heating times and temperatures are shown in Figure 7-36. Most metals, and therefore aircraft rivet stock, are subject to corrosion. Corrosion may be the result of local climatic conditions or the fabrication process used. It is reduced to a minimum by using metals that are highly resistant to corrosion and possess the correct strength-to-weight ratio.

Ferrous metals placed in contact with moist salt air rust if not properly protected. Nonferrous metals, those without an iron base, do not rust, but a similar process known as corrosion takes place. The salt in moist air (found in the coastal areas) attacks the aluminum alloys. It is a common experience to inspect the rivets of an aircraft, which has been operated near salt water, and find them badly corroded. If a copper rivet is inserted into an aluminum-alloy structure, two dissimilar metals are brought in contact with each other. Remember, all metals possess a small electrical potential. Dissimilar metals in contact with each other in the presence of moisture cause an electrical current to flow between them and chemical byproducts to be formed. Principally, this results in the deterioration of one of the metals.

Certain aluminum alloys react to each other and, therefore, must be thought of as dissimilar metals. The commonly used aluminum alloys may be divided into the two groups shown in Figure 7-37. Members within either group A or group B can be considered as similar to each other and will not react to others within the same group. A corroding action will take place, however, if any metal of group A comes in contact with a metal in group B in the presence of moisture. Avoid the use of dissimilar metals whenever possible. Their incompatibility is a factor that was considered when the AN Standards were adopted. To comply with AN Standards, the manufacturers must put a protective surface coating on the rivets. This may be zinc chromate, metal spray, or an anodized finish.

The protective coating on a rivet is identified by its color. A rivet coated with zinc chromate is yellow, an anodized surface is pearl gray, and the metal sprayed rivet is identified by a silvery gray color. If a situation arises in which a protective coating must be applied on the job, paint the rivet with zinc chromate before it is used and again after it is driven. Identification Markings on the heads of rivets are used to classify their characteristics. These markings may be either a raised teat, two raised teats, a dimple, a pair of raised dashes, a raised cross, a single triangle, or a raised dash; some other heads have no markings.

The different markings indicate the composition of the rivet stock. As explained previously, the rivets have different colors to identify the manufacturers’ protective surface coating. Roundhead rivets are used in the interior of the aircraft, except where clearance is required for adjacent members. The roundhead rivet has a deep, rounded top surface. The head is large enough to strengthen the sheet around the hole and, at the same time, resists tension. The flathead rivet, like the roundhead rivet, is used on interior structures. It is used where maximum strength is needed and where there is not sufficient clearance to use a roundhead rivet. It is seldom, if ever, used on external surfaces. The brazier head rivet has a head of large diameter, which makes it particularly adaptable for riveting thin sheet stock (skin). The brazier head rivet offers only slight resistance to the airflow, and because of this factor, it is frequently used for riveting skin on exterior surfaces, especially on aft sections of the fuselage and empennage. It is used for riveting thin sheets exposed to the slipstream. A modified brazier head rivet is also manufactured; it is simply a brazier head of reduced diameter.

The universal head rivet is a combination of the roundhead, flathead, and brazier head. It is used in aircraft construction and repair in both interior and exterior locations. When replacement is necessary for protruding head rivets— roundhead, flathead, or brazier head—they can be replaced by universal head rivets. The countersunk head rivet is flat topped and beveled toward the shank so that it fits into a countersunk or dimpled hole and is flush with the material’s surface. The angle at which the head slopes may vary from 78° to 120°. The 100° rivet is the most commonly used type. These rivets are used to fasten sheets over which other sheets must fit. They are also 7-55 * New specifications are for design purposes.

Head MarkingMaterial AN Material Code AN425 78° Counter- sunk Head AN426 100° Counter- sunk Head MS20426* AN427 100° Counter- sunk Head MS20427* AN430 Round Head MS20470* AN435 Round Head MS20613* MS20615* AN441 Flat Head AN442 Flat Head MS20470* AN455 Brazier Head MS20470* AN456 Brazier Head MS20470* AN470 Universal Head MS20470*

Heat

Treat Before Use Shear Strength psi Bearing Strength psi X 25,000 100,000 113,000 126,000 136,000 90,000 X A AD D DD B F C M C X MS20613* X MS20613* X MS20615* X MS20615* X MS20426 No Yes No Yes No 10,000 30,000 34,000 38,000 41,000 27,000 35,000 65,000 23,000 49,000 95,000 Plain Recessed Dot Raised Dot Raised Dot Raised Double Dash Raised Cross Three Raised Dashes Recessed Triangle Recessed Dash Plain Recessed Large and Small Dot Recessed Double Dots 1100 2117T 2017T 2024T 5056T 2017T-HD 7075-T73 Carbon Steel Copper Brass Titanium Monel (Nickel- Copper Alloy)

Corrosion

Resistant Steel Monel 7-56 Heating Time—A ir Furnace Rivet Alloy Time at Temperature Heat-Treating Temperature 910 °F– 930 °F 925 °F– 950 °F 2024 2017 1 hour Heating Time—S alt Bath 2024 2017 30 minutes Rivet Alloy Time at Temperature Heat-Treating Temperature 910 °F– 930 °F 925 °F– 950 °F Group A 1100 3003 5052 6053 Group B 2117 2017 2124 7075 used on exterior surfaces of the aircraft, because they offer only slight resistance to the slipstream and help to minimize turbulent airflow. The markings on the heads of rivets indicate the material of which they are made and, therefore, their strength.

materials indicated by them. Although there are three materials indicated by a plain head, it is possible to distinguish their difference by color. The 1100 is an aluminum color; the mild steel is a typical steel color; and the copper rivet is a copper color. Any head marking can appear on any head style of the same material. A part number identifies each type of rivet so that the user can select the correct rivet for the job. The type of rivet head is identified by AN or MS standard numbers. The numbers selected are in series and each series represents a particular type of head. The most common numbers and the types of heads they represent are: AN426 or MS20426—countersunk head rivets (100°) AN430 or MS20430—roundhead rivets AN441—flathead rivets AN456—brazier head rivets AN470 or MS20470—universal head rivets There are also letters and numbers added to a part number.

The letters designate alloy content; the numbers designate rivet diameter and length. The letters in common uses for alloy designation are: A—Aluminum alloy, 1100 or 3003 composition AD—Aluminum alloy, 2117-T composition D—Aluminum alloy, 2017-T composition DD—Aluminum alloy, 2024-T composition B—Aluminum alloy, 5056 composition C—Copper M—Monel The absence of a letter following the AN standard number indicates a rivet manufactured from mild steel. The first number following the material composition letters expresses the diameter of the rivet shank in 32nds of an inch. For example, 3 indicates 3⁄32, 5 indicates 5⁄32, and so forth.

[Figure 7-38] The last number(s), separated by a dash from the preceding number, expresses the length of the rivet shank in 16ths of an inch. For example, 3 indicates 3⁄16, 7 indicates 7⁄16, 11 indicates 11⁄16, and so forth. [Figure 7-38] An example of identification marking of a rivet is: AN470AD3-5—complete part number AN—Air Force-Navy standard number 470—universal head rivet AD—2117-T aluminum alloy 3—3⁄32 in diameter 5—5⁄16 in length Blind Rivets There are many places on an aircraft where access to both sides of a riveted structure or structural part is impossible, or where limited space does not permit the use of a bucking bar.

Also, in the attachment of many non-structural parts, such as aircraft interior furnishings, flooring, deicing boots, and the like, the full strength of solid shank rivets is not necessary. For use in such places, special rivets have been designed that can be bucked from the front. Special rivets are sometimes lighter than solid shank rivets, yet amply strong for intended use. These rivets are produced by several manufacturers and 7-57 have unique characteristics that require special installation tools, special installation procedures, and special removal procedures. That is why they are called special rivets.

Because these rivets are often inserted in locations where one head (usually the shop head) cannot be seen, they are also called blind rivets. Mechanically-Expanded Rivets Two classes of mechanically-expanded rivets are discussed here: • Non-structural—self-plugging (friction lock) rivets, pull-thru rivets • Mechanical lock—flush fracturing, self-plugging rivets Self-Plugging Rivets (Friction Lock) The self-plugging (friction lock) blind rivets are manufactured by several companies. The same general basic information about their fabrication, composition, uses, selection, installation, inspection, and removal procedures apply to all of them.

Self-plugging (friction lock) rivets are fabricated in two parts: a rivet head with a hollow shank or sleeve, and a stem that extends through the hollow shank. Figure 7-39 illustrates a protruding head and a countersunk head self-plugging rivet produced by one manufacturer. Several events, in their proper sequence, occur when a pulling force is applied to the stem of the rivet: 1. The stem is pulled into the rivet shank. 2. The mandrel portion of the stem forces the rivet shank to expand. 3. When friction (or pulling action pressure) becomes great enough, it causes the stem to snap at a breakoff groove on the stem.

The plug portion (bottom end of the stem) is retained in the shank of the rivet giving the rivet much greater shear strength than could be obtained from a hollow rivet. Self-plugging (friction lock) rivets are fabricated in two common head styles: a protruding head like the MS20470 or universal head, and a 100° countersunk head. Other head styles are available from some manufacturers. The stem of the self-plugging (friction lock) rivet may have a knot or knob on the upper portion, or it may have a serrated portion. [Figure 7-39] Self-plugging (friction lock) rivets are fabricated from several materials. Rivets are available in the following material combinations: stem 2017 aluminum alloy and sleeve 2117 aluminum alloy; stem 2017 aluminum alloy and sleeve 5056 aluminum alloy; and stem steel and sleeve steel.

Self-plugging (friction lock) rivets are designed so that installation requires only one person; it is not necessary to have the work accessible from both sides. The pulling strength of the rivet stem is such that a uniform job can always be assured. Because it is not necessary to have access to the opposite side of the work, self- plugging (friction lock) rivets can be used to attach assemblies to hollow tubes, corrugated sheet, hollow boxes, and so forth. Because a hammering force is not necessary to install the rivet, it can be used to attach assemblies to plywood or plastics. Factors to consider in the selection of the correct rivet for installation are: installation location, composition of the material being riveted, thickness of the material being riveted, and strength desired.

If the rivet is to be installed on an aerodynamically smooth surface, or if clearance for an assembly is needed, countersunk head rivets should be selected. In other areas where clearance or smoothness is not a factor, the protruding head type rivet may be utilized. Material composition of the rivet shank depends upon the type of material being riveted. Aluminum alloy 2117 shank rivets can be used on most aluminum alloys. Aluminum alloy 5056 shank rivets should be used when the material being riveted is magnesium. Steel rivets should always be selected for riveting assemblies fabricated from steel.

The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 7-40] Pull-Thru Rivets Several companies manufacture the pull-thru blind rivets. The same general basic information about their fabrication, composition, uses, selection, installation, inspection, and removal procedures apply to all of them. Pull-thru rivets are fabricated in two parts: a rivet head with a hollow shank or sleeve and a stem that extends through the hollow shank. Figure 7-41 illustrates a protruding head and a countersunk head pull-thru rivet.

Several events, in their proper sequence, occur when a pulling force is applied to the stem of the rivet: 1. The stem is pulled through the rivet shank. 7-58 Length of rivet Diameter of shankDiameter of shank Countersunk angle Protruding head Countersunk head 2. The mandrel portion of the stem forces the shank to expand forming the blind head and filling the hole. Pull-thru rivets are fabricated in two common head styles: protruding head like the MS20470 or universal head and a 100° countersunk head. Other head styles are available from some manufacturers. Pull-thru rivets are fabricated from several materials. The most commonly used are 2117-T4 aluminum alloy, 5056 aluminum alloy, Monel. Pull-thru rivets are designed so that installation requires only one person; it is not necessary to have the work accessible from both sides.

Factors to consider in the selection of the correct rivet for installation are: installation location, composition of the material being riveted, thickness of the material being riveted, and strength desired. The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 7-42] Each company that manufactures pull-thru rivets has a code number to help users obtain correct rivet for the grip range of a particular installation. In addition, MS numbers are used for identification purposes. Numbers are similar to those shown on the preceding pages.

Self-Plugging Rivets (Mechanical Lock) Self-plugging (mechanical lock) rivets are like self-plugging (friction lock) rivets, except for the way the stem is retained in the rivet sleeve. This type of rivet has a positive mechanical locking collar to resist vibrations that cause the friction lock rivets to loosen and possibly fall out. [Figure 7-43] Also, the mechanical locking-type rivet stem breaks off flush with the head and usually does not require further stem trimming when properly installed. Self-plugging (mechanical lock) rivets display all the strength characteristics of solid shank rivets and, in most cases, can be substituted rivet for rivet.

Bulbed CherryLOCK® Rivets The large blind head of this fastener introduced the word “bulb” to blind rivet terminology. In conjunction with the unique residual preload developed by the high stem break load, its proven fatigue strength makes it the only blind rivet interchangeable structurally with solid rivets. [Figure 7-44] Wiredraw CherryLOCK® Rivets There is a wide range of sizes, materials, and strength levels from which to select. This fastener is especially suited for sealing applications and joints requiring an excessive amount of sheet take-up. [Figure 7-45] Huck® Mechanical Locked Rivets Self-plugging (mechanical lock) rivets are fabricated in two sections: a head and shank (including a conical recess and locking collar in the head) and a serrated stem that extends through the shank. Unlike the friction lock rivet, the Huck® mechanical lock rivet has a locking collar that forms a positive lock for retention of the stem in the shank of the rivet. This collar is seated in position during the installation of the rivet.

Material Self-plugging (mechanical lock) rivets are fabricated with 7-59 C A B A = Thickness of material (grip range) B = 3⁄64 –1⁄8" C = Total rivet shank length Protruding headCountersunk head sleeves (rivet shanks) of 2017 and 5056 aluminum alloys, Monel, or stainless steel. The mechanical lock type of self-plugging rivet can be used in the same applications as the friction lock type of rivet. In addition, because of its greater stem retention characteristic, installation in areas subject to considerable vibration is recommended. The same general requirements must be met in the selection of the mechanical lock rivet as for the friction lock rivet. Composition of the material being joined together determines the composition of the rivet sleeve.

For example, 2017 aluminum alloy rivets for most aluminum alloys and 5056 aluminum rivets for magnesium. locked blind rivet. The form and function may vary slightly between blind rivet styles and specifics should be obtained from manufacturers. Head Styles Self-plugging mechanical locked blind rivets are available in several head styles depending on the installation requirements. [Figure 7-47] Diameters Shank diameters are measured in 1⁄32-inch increments and are generally identified by the first dash number: -3 indicates 3⁄32 inch diameter, -4 indicates 1⁄8 diameter, and so forth. Both nominal and 1⁄64-inch oversize diameters are available.

Grip Length Grip length refers to the maximum total sheet thickness to be riveted and is measured in 1⁄16 of an inch. This is generally identified by the second dash number. Unless otherwise noted, most blind rivets have their grip lengths (maximum grip) marked on the rivet head and have a total grip range of 1⁄16 inch. For example, –04 grip rivet has a grip range of 3⁄16" to 1⁄4". [Figure 7-48] To determine the proper grip rivet to use, measure the material thickness with a grip selection gauge (available from blind rivet manufacturers). The proper use of a grip selector gauge is shown in Figure 7-49.

The thickness of the material being riveted determines the overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately 3⁄64 inch to 1⁄8 inch before the stem is pulled. [Figure 7-50] Rivet Identification Each company that manufactures self-plugging (friction lock) rivets has a code number to help users obtain the correct rivet for the grip range or material thickness of a particular installation. In addition, MS numbers are used for identification purposes. Figures 7-51 through 7-54 contain examples of part numbers for self-plugging (friction lock) rivets that are representative of each.