Special Purpose Fasteners: Part One

Top viewSide viewBottom view A. Driven correctly B. Unsteady tool C. Driven excessively D. Separation of sheets E. Unsteady rivet set F. Excessive shank length Imperfection Cause Remedy Action None Cut head Excessively flat head, resultant head cracks Sheet separation Sloping head Buckled shank None Improperly held tools Excessive driving, too much pressure on bucking bar Work not held firmly together and rivet shank swelled a. Bucking bar not held firmly b. Bucking bar permitted to slide and bounce over the rivet Improper rivet length, and E above None Hold riveting tools firmly against work Improve riveting technique Fasten work firmly together to prevent slipping Hold bucking bar firmly without too much pressure E above and rivet of proper length None Replace rivet A B C D E F Damaged head Swelled shank Sloping head Buckled shankCracks holes show elongation, replace the rivets with the next larger size. Sheet failures such as tear-outs, cracks between rivets, and the like usually indicate damaged rivets. The complete repair of the joint may require replacement of the rivets with the next larger size.

The general practice of replacing a rivet with the next larger size ( 1⁄32-inch greater diameter) is necessary to obtain the proper joint strength of rivet and sheet when the original rivet hole is enlarged. If the rivet in an elongated hole is replaced by a rivet of the same size, its ability to carry its share of the shear load is impaired and joint weakness results. Removal of Rivets When a rivet has to be replaced, remove it carefully to retain the rivet hole’s original size and shape. If removed correctly, the rivet does not need to be replaced with one of the next larger size. Also, if the rivet is not removed properly, the strength of the joint may be weakened and the replacement of rivets made more difficult.

When removing a rivet, work on the manufactured head. It is more symmetrical about the shank than the shop head, and there is less chance of damaging the rivet hole or the material around it. To remove rivets, use hand tools, a power drill, or a combination of both. The procedure for universal or protruding head rivet removal is as follows: 1. File a flat area on the head of the rivet and center punch the flat surface for drilling. Note: On thin metal, back up the rivet on the upset head when center punching to avoid depressing the metal. 2. Use a drill bit one size smaller than the rivet shank to drill out the rivet head.

Note: When using a power drill, set the drill on the rivet and rotate the chuck several revolutions by hand before turning on the power. This procedure helps the drill cut a good starting spot and eliminates the chance of the drill slipping off and tracking across the metal. 3. Drill the rivet to the depth of its head, while holding the drill at a 90° angle. Do not drill too deeply, as the rivet shank will then turn with the drill and tear the surrounding metal. Note: The rivet head often breaks away and climbs 4-44 the drill, which is a signal to withdraw the drill. 4. If the rivet head does not come loose of its own accord, insert a drift punch into the hole and twist slightly to either side until the head comes off.

5. Drive the remaining rivet shank out with a drift punch slightly smaller than the shank diameter. On thin metal or unsupported structures, support the sheet with a bucking bar while driving out the shank. If the shank is unusually tight after the rivet head is removed, drill the rivet about two-thirds through the thickness of the material and then drive the rest of it out with a drift punch. Figure 4-98 shows the preferred procedure for removing universal rivets. The procedure for the removal of countersunk rivets is the same as described above except no filing is necessary. Be careful to avoid elongation of the dimpled or the countersunk holes. The rivet head should be drilled to approximately one- half the thickness of the top sheet. The dimple in 2117–T rivets usually eliminates the necessity of filing and center punching the rivet head.

To remove a countersunk or flush head rivet, you must: 1. Select a drill about 0.003-inch smaller than the rivet shank diameter. 2. Drill into the exact center of the rivet head to the approximate depth of the head. 3. Remove the head by breaking it off. Use a punch as a lever. 4. Punch out the shank. Use a suitable backup, preferably wood (or equivalent), or a dedicated backup block. If the shank does not come out easily, use a small drill and drill through the shank. Be careful not to elongate the hole. Replacing Rivets Replace rivets with those of the same size and strength whenever possible. If the rivet hole becomes enlarged, deformed, or otherwise damaged, drill or ream the hole for the next larger size rivet. Do not replace a rivet with a type having lower strength properties, unless the lower strength is adequately compensated by an increase in size or a greater number of rivets. It is acceptable to replace 2017 rivets of 3⁄16-inch diameter or less, and 2024 rivets of 5⁄32-inch diameter or less with 2117 rivets for general repairs, provided the replacement rivets are 1⁄32-inch greater in diameter than the rivets they replace.

National Advisory Committee for Aeronautics (NACA) Method of Double Flush Riveting A rivet installation technique known as the National Advisory Committee for Aeronautics (NACA) method has primary applications in fuel tank areas. [Figure 4-99] To make a NACA rivet installation, the shank is upset into a 82° countersink. In driving, the gun may be used on either the head or shank side. The upsetting is started with light blows, then the force increased and the gun or bar moved on the shank end so as to form a head inside the countersink well. If desired, the upset head may be shaved flush after driving. If utilizing this method, it is important to reference the manufacturer’s instructions for repair or replacement.

Special Purpose Fasteners Special purpose fasteners are designed for applications in which fastener strength, ease of installation, or temperature properties of the fastener require consideration. Solid shank rivets have been the preferred construction method for metal aircraft for many years because they fill up the hole, which results in good load transfer, but they are not always ideal. For example, the attachment of many nonstructural parts (aircraft interior furnishings, flooring, deicing boots, etc.) do not need the full strength of solid shank rivets. To install solid shank rivets, the aircraft technician must have access to both sides of a riveted structure or structural part. There are many places on an aircraft where this access is impossible or where limited space does not permit the use of a bucking bar. In these instances, it is not possible to use solid shank rivets, and special fasteners have been designed that can be bucked from the front. [Figure 4-100] There are also areas of high loads, high fatigue, and bending on aircraft.

Although the shear loads of riveted joints are very good, the tension, or clamp-up, loads are less than ideal. Special purpose fasteners are sometimes lighter than solid shank rivets, yet strong enough for their intended use. These fasteners are manufactured by several corporations and have unique characteristics that require special installation tools, special installation procedures, and special removal procedures. Because these fasteners are often inserted in locations where one head, usually the shop head, cannot be seen, they are called blind rivets or blind fasteners. Typically, the locking characteristics of a blind rivet are not as good as a driven rivet. Therefore, blind rivets are usually not used when driven rivets can be installed. Blind rivets shall not be used: 1. In fluid-tight areas.

2. On aircraft in air intake areas where rivet parts may be ingested by the engine. 3. On aircraft control surfaces, hinges, hinge brackets, flight control actuating systems, wing attachment fittings, landing gear fittings, on floats or amphibian 4-45 Rivet Removal Remove rivets by drilling off the head and punching out the shank as illustrated. 1. File a flat area on the manufactured head of non-flush rivets. 2. Place a block of wood or a bucking bar under both flush and nonflush rivets when center punching the manufactured head. 3. Use a drill that is 1/32 (0.0312) inch smaller than the rivet shank to drill through the head of the rivet. Ensure the drilling operation does not damage the skin or cut the sides of the rivet hole.

4. Insert a drift punch into the hole drilled in the rivet and tilt the punch to break off the rivet head. 5. Using a drift punch and hammer, drive out the rivet shank. Support the opposite side of the structure to prevent structural damage. 1. File a flat area on manufactured head 2. Center punch flat 5. Punch out rivet with machine punch 3. Drill through head using drill one 4. Remove weakened head with size smaller than rivet shank machine punch Shop head formed in countersink Rivet factory head 4-46 hulls below the water level, or other heavily stressed locations on the aircraft. Note: For metal repairs to the airframe, the use of blind rivets must be specifically authorized by the airframe manufacturer or approved by a representative of the Federal Aviation Administration (FAA).

Blind Rivets The first blind fasteners were introduced in 1940 by the Cherry Rivet Company (now Cherry ® Aerospace), and the aviation industry quickly adopted them. The past decades have seen a proliferation of blind fastening systems based on the original concept, which consists of a tubular rivet with a fixed head and a hollow sleeve. Inserted within the rivet’s core is a stem that is enlarged or serrated on its exposed end when activated by a pulling-type rivet gun. The lower end of the stem extends beyond the inner sheet of metal. This portion contains a tapered joining portion and a blind head that has a larger diameter than the stem or the sleeve of the tubular rivet.

When the pulling force of the rivet gun forces the blind head upward into the sleeve, its stem upsets or expands the lower end of the sleeve into a tail. This presses the inner sheet upward and closes any space that might have existed between it and the outer sheet. Since the exposed head of the rivet is held tightly against the outer sheet by the rivet gun, the sheets of metal are clamped, or clinched, together. Note: Fastener manufacturers use different terminology to describe the parts of the blind rivet. The terms “mandrel,” “spindle,” and “stem” are often used interchangeably. For clarity, the word “stem” is used in this handbook and refers to the piece that is inserted into the hollow sleeve.

Friction-Locked Blind Rivets Standard self-plugging blind rivets consist of a hollow sleeve and a stem with increased diameter in the plug section. The blind head is formed as the stem is pulled into the sleeve. Friction-locked blind rivets have a multiple-piece construction and rely on friction to lock the stem to the sleeve. As the stem is drawn up into the rivet shank, the stem portion upsets the shank on the blind side, forming a plug in the hollow center of the rivet. The excess portion of the stem breaks off at a groove due to the continued pulling action of the rivet gun. Metals used for these rivets are 2117- T4 and 5056-F aluminum alloy. Monel® is used for special applications.

Many friction-locked blind rivet center stems fall out due to vibration, which greatly reduces its shear strength. To combat that problem, most friction-lock blind rivets are replaced by the mechanical-lock, or stem-lock, type of blind fasteners. However, some types, such as the Cherry SPR ® 3⁄32-inch Self-Plugging Rivet, are ideal for securing nutplates located in inaccessible and hard-to-reach areas where bucking or squeezing of solid rivets is unacceptable. [Figure 4-101] Friction-lock blind rivets are less expensive than mechanical- lock blind rivets and are sometimes used for nonstructural applications. Inspection of friction-lock blind rivets is visual.

A more detailed discussion on how to inspect riveted joints can be found later in this chapter. Removal of friction-lock blind rivets consists of punching out the friction-lock stem and then treating it like any other rivet. Mechanical-Lock Blind Rivets The self-plugging, mechanical-lock blind rivet was developed to prevent the problem of losing the center stem due to vibration. This rivet has a device on the puller or rivet head that locks the center stem into place when installed. Bulbed, self-plugging, mechanically-locked blind rivets form a large, blind head that provides higher strength in thin sheets when installed. They may be used in applications where the blind head is formed against a dimpled sheet.

Manufacturers such as Cherry ® Aerospace (CherryMAX®, CherryLOCK®, Cherry SST®) and Alcoa Fastening Systems (Huck-Clinch ®, HuckMax ®, Unimatic ®) make many variations of this of blind rivet. While similar in design, the tooling for these rivets is often not interchangeable. The CherryMAX® Bulbed blind rivet is one of the earlier types of mechanical-lock blind rivets developed. Their main 4-47 advantage is the ability to replace a solid shank rivet size for size. The CherryMAX ® Bulbed blind rivet consists of four parts: 1. A fully serrated stem with break notch, shear ring, and integral grip adjustment cone.

2. A driving anvil to ensure a visible mechanical lock with each fastener installation. 3. A separate, visible, and inspectable locking collar that mechanically locks the stem to the rivet sleeve. 4. A rivet sleeve with recess in the head to receive the locking collar. It is called a bulbed fastener due to its large blind side bearing surface, developed during the installation process. These rivets are used in thin sheet applications and for use in materials that may be damaged by other types of blind rivets. This rivet features a safe-lock locking collar for more reliable joint integrity. The rough end of the retained stem in the center on the manufactured head must never be filed smooth because it weakens the strength of the lockring, and the center stem could fall out.

CherryMAX® bulbed rivets are available in three head styles: universal, 100° countersunk, and 100° reduced shear head styles. Their lengths are measured in increments of 1⁄16 inch. It is important to select a rivet with a length related to the grip length of the metal being joined. This blind rivet can be installed using either the Cherry ® G750A or the newly released Cherry® G800 hand riveters, or either the pneumatic- hydraulic G704B or G747 CherryMAX ® power tools. For installation, please refer to Figure 4-102. The CherryMAX ® mechanical-lock blind rivet is popular with general aviation repair shops because it features the one tool concept to install three standard rivet diameters and their oversize counterparts. [Figure 4-103] CherryMAX® rivets are available in four nominal diameters: 1⁄8, 5⁄32, 3⁄16, and 1⁄4-inch and three oversized diameters and four head styles: universal, 100° flush head, 120° flush head, and NAS1097 flush head. This rivet consists of a blind header, hollow rivet shell, locking (foil) collar, driving anvil, and pulling stem complete with wrapped locking collar. The rivet sleeve and the driving washer blind bulbed header takes up the extended shank and forms the bucktail.

The stem and rivet sleeve work as an assembly to provide radial expansion and a large bearing footprint on the blind side of the fastened surface. The lock collar ensures that the stem and sleeve remain assembled during joint loading and unloading. Rivet sleeves are made from 5056 aluminum, Monel® and INCO 600. The stems are made from alloy steel, CRES, and INCO® X-750. CherryMAX® rivets have an ultimate shear strength ranging from 50 KSI to 75 KSI. Removal of Mechanically-Locked Blind Rivets Mechanically-locked blind rivets are a challenge to remove because they are made from strong, hard metals. Lack of 1 2 3 4 The CherryMAX® rivet is inserted into the prepared hole. The pulling head (installation tool) is slipped over the rivet’s stem.

Applying a firm, steady pressure, which seats the rivet head, the installation tool is then actuated. The pulling head holds the rivet sleeve in place as it begins to pull the rivet stem into the rivet sleeve. This pulling action causes the stem shear ring to upset the rivet sleeve and form the bulbed blind head. The continued pulling action of the installation tool causes the stem shear ring to shear from the main body of the stem as the stem continues to move through the rivet sleeve. This action allows the fastener to accommodate a minimum of 1/16" variation in structure thickness. The locking collar then contacts the driving anvil.

As the stem continues to be pulled by the action of the installation tool, the Safe-Lock locking collar deforms into the rivet sleeve head recess. The safe-lock locking collar fills the rivet sleeve head recess, locking the stem and rivet sleeve securely together. Continued pulling by the installation tool causes the stem to fracture at the break notch, providing a flush, burr-free, inspectable installation. 4-48 Bulbed blind head Pulling stem Driving anvil Rivet sleeve Safe-lock locking collar access poses yet another problem for the aviation technician. Designed for and used in difficult to reach locations means there is often no access to the blind side of the rivet or any way to provide support for the sheet metal surrounding the rivet’s location when the aviation technician attempts removal.

The stem is mechanically locked by a small lock ring that needs to be removed first. Use a small center drill to provide a guide for a larger drill on top of the rivet stem and drill away the upper portion of the stem to destroy the lock. Try to remove the lock ring or use a prick punch or center punch to drive the stem down a little and remove the lock ring. After the lock ring is removed, the stem can be driven out with a drive punch. After the stem is removed, the rivet can be drilled out in the same way as a solid rivet. If possible, support the back side of the rivet with a backup block to prevent damage to the aircraft skin.

Pin Fastening Systems (High-Shear Fasteners) A pin fastening system, or high-shear pin rivet, is a two-piece fastener that consists of a threaded pin and a collar. The metal collar is swaged onto the grooved end, effecting a firm tight fit. They are essentially threadless bolts. High-shear rivets are installed with standard bucking bars and pneumatic riveting hammers. They require the use of a special gun set that incorporates collar swaging and trimming and a discharge port through which excess collar material is discharged. A separate size set is required for each shank diameter. Installation of High-Shear Fasteners Prepare holes for pin rivets with the same care as for other close tolerance rivets or bolts. At times, it may be necessary to spot-face the area under the head of the pin to ensure the head of the rivet fits tightly against the material. The spot-faced area should be 1⁄16-inch larger in diameter than the head diameter. Pin rivets may be driven from either end.

Procedures for driving a pin rivet from the collar end are: 1. Insert the rivet in the hole. 2. Place a bucking bar against the rivet head. 3. Slip the collar over the protruding rivet end. 4. Place previously selected rivet set and gun over the collar. Align the gun until it is perpendicular to the material. 5. Depress the trigger on the gun, applying pressure to the rivet collar. This action causes the rivet collar to swage into the groove on the rivet end. 6. Continue the driving action until the collar is properly formed and excess collar material is trimmed off. Procedures for driving a pin rivet from the head end are: 1. Insert the rivet in the hole.

2. Slip the collar over the protruding end of rivet. 3. Insert the correct size gun rivet set in a bucking bar and place the set against the collar of the rivet. 4. Apply pressure against the rivet head with a flush rivet set and pneumatic riveting hammer. 4-49