Aerospace Structural Fasteners

Aerospace Structural Fasteners

• Aerospace structural fasteners securely join sheet metal structures and are available in many specialized shapes and sizes for certain aircraft.
• This section focuses on the more frequently used fasteners common to all aircraft.
• Fasteners are divided into solid shank rivets and special purpose fasteners, including blind rivets.

Solid Shank Rivets

• The solid shank rivet is the most common rivet used in aircraft construction.
• Solid shank rivets join aircraft structures, and is one of the oldest and most reliable types of fastener.
• Solid shank rivets are low-cost and permanently installed, making them widely used in the aircraft manufacturing industry.
• Solid shank rivets install faster than bolts and nuts and adapt well to automatic, high-speed installation tools.
• Rivets are difficult to lock in thick materials where the total grip length is longer.
• Rivets should not be used in tensile applications because the tensile strengths are quite low relative to their shear strength.
• Riveted joints are not airtight or watertight unless special seals or coatings are applied.
• Permanently installed rivets removed by drilling is a laborious task

Solid Shank Rivet Description

• A rivet consists of a smooth cylindrical shaft with a factory head at one end.
• Two or more pieces of sheet metal is secured together when the rivet is placed into a hole cut slightly larger in diameter than the rivet.
• The bucktail or shop head is upset (squished or deformed) using methods ranging from hand-held hammers to pneumatic squeezing tools.
• The rivet expands to 1 1/2 times the original shaft diameter, forming a second head that firmly holds the material in place.

Rivet Head Shape

• Solid rivets are available in universal and 100° countersunk head shapes, the most commonly used in aircraft structures.
• Universal head rivets were developed for the aircraft industry to replace older round, flat, and brazier head rivets.
• All protruding head rivets are replaced by the universal rivet and are used where protruding head is not aerodynamically significant.
• Universal head rivets have a flat area on the head and a head diameter twice the shank diameter and are not flush with the skin of a Boeing 737.
• The countersunk head angle can vary from 60° to 120°, but 100° has been adopted as standard.
• The 100° standard provide the best possible compromise between tension/shear strength and flushness requirements.
• The countersunk rivet is used where flushness is required because the rivet is flat-topped and undercut to allow the head to fit into a countersunk or dimpled hole.
• Countersunk rivets are used when aerodynamic smoothness is critical, such as on a high-speed aircraft's external surface.

Rivet Identification

• Rivets come in a wide variety of alloys, head shapes, sizes, and have a wide variety of uses in aircraft structure and can be unsatisfactory for other parts.
• Knowing the strength and driving properties of rivets and how to identify them is important.
• Solid rivets are classified with an identification code by head shape, material, and size.
• Identifier codes used are derived from MS (Military Standard) and NAS (National Aerospace Standard) systems and AN (Army/Navy).
• The identification code starts with MS, NAS, and AN, followed by a number identifies the rivets Head Shape (AN470).
• A Letter or Letters (AD) following the head-shaped code identifies the material or alloy
• Rivets are fabricated from aluminum alloys, like 2017-T4, 2024-T4, and 2117-T4, 5056, 7050 but can use Titanium, nickel-based alloys, such as Monel, mild steel, iron, and copper rivets.
• Heads of solid rivets are marked to identify the specific material of the rivet.
  • 1100 solid rivets are heads are plain in appearance signaling that they are very soft in composition
  • Heads of 2117 solid rivets have a dimple and are received as is (in received condition)
  • Heads of 5056 solid rivets have a raised cross and are made of Magnesium
  • Heads of 2017 solid rivets have a raised dot and are refrigerated
  • Heads of 2024 solid rivets have two raised dashes and are refrigerated
  • 7050 solid rivets have a raised ring and are received as is (in received condition)
• The alloy code is followed by two numbers separated by a dash
• The first number is the numerator of a fraction specifying the shank diameter in thirty-seconds of an inch
• The second number is the numerator of a fraction identifying the length of the rivet in sixteenths of an inch.
• Rivets manufactured in accordance with AN/MS standards are identified by a four-part code:
  • The AN or MS number specifies the Head Type.
  • One or two letters indicate the Material.
  • Shank Diameter is shown in 1/32nd inch increments.
  • A dash followed by a number indicates Rivet length in 1/16th inch increments.
• AN470AD4-5 or MS20470AD4-5 is a standard universal head solid rivet made of 2117-T4 aluminum, has a 1/8 inch diameter, and measures 5/16 inch in length.
• AN470 or MS20470 specifies universal head types.
• A=1100, B=5056, C=copper, D=2017, DD=2024, F=stainless and M=Monel

Rivet Usage Considerations

• When deciding which rivet to use, decide on the rivet head, rivet size and rivet strength
• Countersunk heads are used where a smooth aerodynamic surface is required, while universal head rivets may be used in most other areas.
• Excessive force to drive an excessively large rivet into thin material causes bulging around the rivet head.
• If an excessively small rivet diameter is selected for thick material, the shear strength of the rivet is not great enough to carry the load in the joint.
• Rivets chosen for aircraft assembly and repair range from 3/32-inch to 3/8-inch in diameter.
• You can determine the proper sized rivets to use for repairs by referring to the rivets in the next parallel row inboard on the wing or forward on the fuselage
• To determine rivet size, multiply the skin's thickness by 3 where rivet diameter should be 2 1/2 to 3 times the thicker sheet's thickness.
  • For example, if the skin is 0.040 inch thick, multiply 0.040 inch by 3 to get 0.120 inch; then, use the next larger size of rivet, 1/8-inch (0.125 inch).
• Rivet Strength is determined by factors like forces at the point riveted, the kind and thickness of material to be riveted, and the location of the part on the aircraft.
• The AD rivet is the most frequently used repair rivet because it can be installed in the received condition.
• DD rivets (alloy 2024-T4) are too hard to drive in the received condition and must be annealed before installation.
• Annealed rivets are stored in a freezer to retard hardening, giving them the nickname "ice box rivets" and are removed just before use.
• Most DD rivets have been replaced by E-type rivets which can be installed in the received condition.

Drilling

• Rivet holes are drilled using with a pneumatic hand drill.
• The standard shank twist drill is most commonly used.
• The drill bit size for common rivet diameters can be identified
• Rivet holes should be the correct size and free from burrs.
• The protective coating is scratched from the rivet and when the hole is too small.
• If the hole is too large, the rivet does not fill the hole completely, so the joint's structural failure occurs because it does not develop its full strength.
• Drill bit sizes for rivet holes should be the smallest size and allows for easy insertion of the rivet, approximately 0.003-inch greater than the largest tolerance of the shank diameter.
• The best approach to make a rivet hole the correct size is to first drill a slightly undersized hole and then drill the pilot hole with a twist drill of the correct size.
• To drill, follow these steps:
  • Ensure the drill bit is the correct size.
  • Place the drill in the center of cross, then rotate the bit a few times before starting motor.
  • Hold the drill at a 90° angle to the work, or the curvature of the material, while drilling.
  • Avoid excessive pressure and never push the drill bit through metal.
  • Remove all burrs with a deburrer.
  • Clean up all drill chips (swarf).
• Small burrs form around sheet metal rivet holes with a drill punch, and then should be removed with a deburr tool or larger size drill bit before riveting.
• Transferring holes from a drilled metal part to another metal part is done by placing the first part over the second and using the drilled holes as a guide, using a drill bit the same size as the hole

Driving the Rivet

• Riveting equipment used to drive solid shank rivets in airframe repair work can be stationary or portable.
• Before driving rivets into sheet metal parts, make sure holes line up perfectly, shavings and burrs are removed, and parts are securely fastened with temporary fasteners (cleco's).
• Riveting may require 1 or 2 people.
• In solo riveting, the riveter holds a bucking bar with one hand and operates a riveting gun with the other.
• If the job requires two aircraft technicians, a shooter (gunner) and a bucker work together as a team to install rivets.
• An efficient signaling system is important in team riveting communicates the status of the riveting process ususally by tapping the bucking bar against the work (tap code). Radio sets can also allow for communication
• Once a rivet is installed, there should be no looseness of rivets or the riveted parts.
• Rivet installation is satisfactory when the rivet head is seated snugly against the item and the stem is tight.
• Any looseness indicates a problem usually caused by an oversized hole and requires replacing the rivet with an oversize shank diameter rivet.

Solid Rivet Installation

• 2 Aluminum sheets that are .032" and 2024-T6 are joined with rivets
• Installation requires:
  • A rivet gun with a spring, and a snap
  • A bucking bar
  • A drill
    • Drill bits
    • Chuck Key
  • Clecos
  • Cleco pliers
• Installation Steps:
  • Drill holes with a #40, then a #30 drill bit for 1/8" rivets
  • Deburr the holes by rotating (by hand) a larger drill bit in the holes to remove any metal shavings caused by drilling
  • Secure pieces together with clecos after drilling a few holes
  • Place the rivet in the hole, using the correct rivet type, material, and length
  • Align the rivet gun on the rivet head while holding the bucking bar on the opposite end of the rivet
  • Pull the trigger on the rivet gun and buck (squish) the rivet to produce the proper size shop head.
• Solid rivets can also be installed using a rivet squeezer instead of a pneumatic rivet gun and can be hand or air operated and uses various inserts depending on the type and size of rivet being installed

Countersunk Rivets

• When using countersunk rivets, it is necessary to create a conical recess in the skin for the head.
• Two methods of creating a countersunk conical recess in the skin are drill countersinking and press countersinking (dimpling)
• The proper countersinking method for any application depends on the thickness of the sheets, depth of rivet head, height and angle of the countersunk head, tools, and accessibility.
• Use the proper degree and diameter countersink, only cutting deep enough for the rivet head and metal, to yield a flush surface.
• Countersinking is an important factor in the design of fastener patterns.
• Removing material necessitates using more fasteners for load-transfer strength.
• The edge distance needed when using countersunk fasteners is greater than when using universal head fasteners.
• A knife edge/enlarging of the hole results if countersinking is done on metal below specifications; and an improper countersink reduces the strength of a flush-riveted joint.
• Countersunk holes can cause fatigue cracks in pressurized skins and also a high stress concentration causing skin cracks and fastener failures.
• In primary structure and pressurized skin repairs, no more the 2/3 or .020-inch shank depth is advised for the countersink depth- larger value is used.
• For skins that are too thin, dimple
• If you dimple, dimpling pressures must be controlled to prevent undue work hardening
• Keep the rivet head a little high before driving to ensure the force of riveting is applied to the rivet to prevent work hardening of skin during driving.

Countersinking Tools

• To form countersunk holes, use tools creating angles from 82° to 120°; the most commonly used has an angle of 100°.
• A six-fluted countersink works best in aluminum.
• Four and three-fluted countersinks are harder to control from a chatter standpoint.
• A single-flute type works best for corrosion-resistant steel.
• The microstop countersink is preferable since its' adjustable-sleeve cage functions as a stop and holds the revolving countersink bit in a vertical position, and have threaded and replaceable cutters including an integral pilot that keeps the cutter centered.
• On scrap material, test micro-countersink before countersinking repair/replacement parts to test for fitment.
• Freehand countersinking is used when a microstop countersink cannot fit where the drill motor is steady and holding it perpendicular is very important.

Dimpling

• Dimpling is for sheets thinner than the minimum specified thickness for countersinking.
• It creates an indentation (dimple) around a rivet hole to install a countersunk rivet flush.
• Dimpling is done with a male and female die (punch and die).
• The male die has a guide the size of the rivet hole and is beveled to correspond to the degree of countersink of the rivet head.
• The female die has a hole the size of the rivet hole and is beveled to a degree matching that of the countersink of the male die.
• Dimpling dies for light work are in pneumatic or hand squeezers.
• If the dies are used with a squeezer, they are adjusted accurately. Heavier parts may be dimpled without cracking by specialized hot dimpling equipment.
• Material temper, rivet size, and equipment are considered when dimpling.
• A combination of Dimpling and Countersinking of different thicknesses is used.
• A countersink made well is called a subcountersink.
• Subcountersinking is seen when connecting a thin web to a heavy structure, on thin gap seals, wear strips, and repairs for worn countersinks.

Formed Head

• The formed head/bucktail/shop head size signals proper rivet installation with minimum, preferred and maximum sizes.

Evaluating Rivets

• In manufacturing and repair of aircraft, inspection of all rivets must be done before the part is put in service.
• Inspections cover the shop and manufactured heads, the surrounding skin, and structural parts for deformations.
• A rivet gauge can check the upset rivet head for proper requirements where deformities in manufactured head can be identified by eye.
• Common causes of unsatisfactory riveting are rivet set slipping off, incorrect rivet set angle, wrong size holes/rivets, countersunk rivets not flush, improperly fastened work and burrs, excessive or deficient driving.

Removing Rivets

• To retain the rivet hole's original size/shape, carefully remove rivets marked for replacement.
• If a rivet was removed correctly, the replacement doesnt have to be a larger size.
• An improperly removed rivet will weaken the joint and make the replacement of rivets more difficult.
• When removing a rivet, work on the manufactured head ,since it is more symmetrical about the shank than the shop head, causing less damage.
• Removal steps:
  • Using a drill bit the same size as the rivet shank, drill out the rivet head.
  • By hand, rotate the chuck several times before a power drill is used to cut a starting spot and prevent slipping/tracking.
  • Drill the rivet to the depth of its head at a 90° angle; do not drill too deeply or the rivet shank will turn with the drill tear surrounding metal.
  • When the rivet head breaks away and climbs the drill, withdraw the drill.
  • If the rivet head does not come loose, insert a drift punch and twist slightly until it comes off.
  • Punch out the shank, while supporting if working on thin or unsupported structures
  • If the shank is unusually tight, drill the rivet about two-thirds through the material and drive what remains out with a drift punch while being careful not to elongate the hole

Replacing Rivets

• Replace rivets with those of similar size and strength.
• Enlarge a rivet hole for a larger size rivet, if the hole becomes enlarged, deformed, or otherwise damaged.
• Unless the lower strength is adequately compensated by an increase in the rivet size, do not replace a rivet with a lower strength product.

(NACA) Method of Double Flush Riveting

• The National Advisory Committee for Aeronautics (NACA) method has primary applications in fuel tank areas to make a rivet installations by upsetting the shank into a 82° countersink.
• A rivet installation technique known as the National Advisory Committee for Aeronautics (NACA) method has primary applications in fuel tank areas.
• 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.
• Upsettings start with light blows which are then increased with the gun/bar moved to the shank, and creates a head inside the countersunk hole.
• If required, the upset head can be shaved flush, and optimal strength results from cutting the countersink hole to published specification
• Thickness minimums must be observed.

(NACA) Method of Double Flush Riveting Dimensions

• Rivet Size 3/32: .032 Minimum Thickness, dimensions ±.005 for .141 Countersink Diameter
• Rivet Size 1/8: .040 Minimum Thickness, dimensions ±.005 for .189 Countersink Diameter
• Rivet Size 5/32: .050 Minimum Thickness, dimensions ±.005 for .236 Countersink Diameter
• Rivet Size 3/16: .063 Minimum Thickness, dimensions ±.005 for .288 Countersink Diameter
• Rivet Size 1/4: .090 Minimum Thickness, dimensions ±.005 for .400 Countersink Diameter