Many special fasteners produce high strength with lightweight and can be used in place of conventional AN bolts and nuts. When AN bolts are tightened with the nut, the bolt stretches, narrowing the diameter and then the bolt is no longer tight in the hole. Special fasteners eliminate this loose fit, because they are held in place by a collar that is squeezed into position. These fasteners are not under the same tensile loads as a bolt during installation. Special fasteners are also used extensively for light sport aircraft (LSA). Always follow the aircraft manufacturer’s recommendations.
Pin rivets are essentially threadless bolts. The pin is headed at one end and is grooved about the circumference at the other. A metal collar is swaged onto the grooved end effecting a firm, tight fit. [Figure 1] Pin rivets are fabricated in a variety of materials but should be used only in shear applications. They should never be used where the grip length is less than the shank diameter.
Part numbers for pin rivets can be interpreted to give the diameter and grip length of the individual rivets. A typical part number breakdown would be:
NAS177-14-17
NAS = National Aircraft Standard
177 = 100° countersunk head rivet
OR 178 = flathead rivet
14 = Nominal diameter in 32nds of an inch
17 = Maximum grip length in 16ths of an inch
The self-locking, threaded HI-LOK™ collar has an internal counterbore at the base to accommodate variations in material thickness. At the opposite end of the collar is a wrenching device that is torqued by the driving tool until it shears off during installation, leaving the lower portion of the collar seated with the proper torque without additional torque inspection. This shear-off point occurs when a predetermined preload or clamp-up is attained in the fastener during installation.
Note: For these fasteners, “Preload” is defined as the maximum tensile load experienced by a fastener in a joint during the fastener installation sequence. Consequently, the term “Residual Tension” is defined as the remaining tensile load experienced by a fastener in a joint after the fastener installation sequence is complete, and after any residual relaxation of the joint assembly.
The advantages of HI-LOK™ two-piece fastener include its lightweight, high fatigue resistance, high strength, and its inability to be over-torqued. The pins, made from alloy steel, corrosion-resistant steel, nickel, or titanium alloy, come in many standard and oversized shank diameters. The collars are made of aluminum alloy, corrosion-resistant steel, titanium, or alloy steel. The collars have wrenching flats, fracture point, threads, and a recess. The wrenching flats are used to install the collar. The fracture point has been designed to allow the wrenching flats to shear when the proper torque has been reached. The threads match the threads of the pins and have been formed into an ellipse that is distorted to provide the locking action. The recess serves as a built-in washer. This area contains a portion of the shank and the transition area of the fastener.
The hole shall typically be prepared so that the maximum interference fit does not exceed 0.002-inch. This avoids build up of excessive internal stresses in the work adjacent to the hole. The HI-LOK™ pin has a slight radius under its head to increase fatigue life. After drilling, deburr the edge of the hole to allow the head to seat fully in the hole. The HI-LOK™ is typically installed in interference fit holes for aluminum structure and a clearance fit for steel, titanium, and composite materials.
The grommet is made of aluminum or aluminum alloy material. It acts as a holding device for the stud. Grommets can be fabricated from 1100 aluminum tubing, if none are available from normal sources.
The spring is made of steel, which is cadmium plated to prevent corrosion. The spring supplies the force that locks or secures the stud in place when two assemblies are joined.
The studs are fabricated from steel and are cadmium plated. They are available in three head styles: wing, flush, and oval. Body diameter, length, and head type may be identified or determined by the markings found on the head of the stud. [Figure 5] The diameter is always measured in sixteenths of an inch. Stud length is measured in hundredths of an inch and is the distance from the head of the stud to the bottom of the spring hole.
A quarter of a turn of the stud (clockwise) locks the fastener. The fastener may be unlocked only by turning the stud counterclockwise. A Dzus key or a specially ground screwdriver locks or unlocks the fastener.
The Camloc fastener is used to secure aircraft cowlings and fairings. It consists of three parts: a stud assembly, a grommet, and a receptacle. Two types of receptacles are available: rigid and floating. [Figure 6]
The stud and grommet are installed in the removable portion; the receptacle is riveted to the structure of the aircraft. The stud and grommet are installed in either a plain, dimpled, countersunk, or counter bored hole, depending upon the location and thickness of the material involved.
A quarter turn (clockwise) of the stud locks the fastener. The fastener can be unlocked only by turning the stud counterclockwise.
The total amount of material thickness to be secured with the Airloc fastener must be known before the correct length of stud can be selected for installation. The total thickness of material that each stud satisfactorily locks together is stamped on the head of the stud in thousandths of an inch (0.040, 0.070, 0.190, and so forth). Studs are manufactured in three head styles: flush, oval, and wing.
The cross pin is manufactured from chrome-vanadium steel and heat-treated to provide maximum strength, wear, and holding power. [Figure 7] It should never be used the second time; once removed from the stud, replace it with a new pin.
Receptacles for Airloc fasteners are manufactured in two types: rigid and floating. Number—No. 2, No. 5, and No. 7, classifies sizes. They are also classified by the center-to-center distance between the rivet holes of the receptacle: No. 2 is 3⁄4 inch; No. 5 is 1 inch; and No. 7 is 13⁄8 inch. Receptacles are fabricated from high-carbon, heat-treated steel. An upper wing assures ejection of the stud when unlocked and enables the cross pin to be held in a locked position between the upper wing, cam, stop, and wing detent, regardless of the tension to which the receptacle is subjected.
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Pin Rivets
Pin (Hi-Shear) rivets are classified as special rivets but are not of the blind type. Access to both sides of the material is required to install this type of rivet. Pin rivets have the same shear strength as bolts of equal diameters, are about 40 percent of the weight of a bolt, and require only about one-fifth as much time for installation as a bolt, nut, and washer combination. They are approximately three times as strong as solid shank rivets.Pin rivets are essentially threadless bolts. The pin is headed at one end and is grooved about the circumference at the other. A metal collar is swaged onto the grooved end effecting a firm, tight fit. [Figure 1] Pin rivets are fabricated in a variety of materials but should be used only in shear applications. They should never be used where the grip length is less than the shank diameter.
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| Figure 1. Pin (Hi-Shear) rivet |
Part numbers for pin rivets can be interpreted to give the diameter and grip length of the individual rivets. A typical part number breakdown would be:
NAS177-14-17
NAS = National Aircraft Standard
177 = 100° countersunk head rivet
OR 178 = flathead rivet
14 = Nominal diameter in 32nds of an inch
17 = Maximum grip length in 16ths of an inch
Taper-Lok
Taper-Loks are the strongest special fasteners used in aircraft construction. The Taper-Lok exerts a force on the walls of the hole because of its tapered shape. The Taper-Lok is designed to completely fill the hole, but unlike the rivet, it fills the hole without deforming the shank. Instead, the washer head nut squeezes the metal with tremendous force against the tapered walls of the hole. This creates radial compression around the shank and vertical compression lines as the metals are squeezed together. The combination of these forces generates strength unequaled by any other fastener. [Figure 2] |
| Figure 2. Taper-Lok special fasteners |
HI-LOK™ Fastening System
The threaded end of the HI-LOK™ two-piece fastener contains a hexagonal shaped recess. The hex tip of an Allen wrench engages the recess to prevent rotation of the pin while the collar is being installed. The pin is designed in two basic head styles. For shear applications, the pin is made in countersunk style and in a compact protruding head style. For tension applications, the MS24694 countersunk and regular protruding head styles are available.The self-locking, threaded HI-LOK™ collar has an internal counterbore at the base to accommodate variations in material thickness. At the opposite end of the collar is a wrenching device that is torqued by the driving tool until it shears off during installation, leaving the lower portion of the collar seated with the proper torque without additional torque inspection. This shear-off point occurs when a predetermined preload or clamp-up is attained in the fastener during installation.
Note: For these fasteners, “Preload” is defined as the maximum tensile load experienced by a fastener in a joint during the fastener installation sequence. Consequently, the term “Residual Tension” is defined as the remaining tensile load experienced by a fastener in a joint after the fastener installation sequence is complete, and after any residual relaxation of the joint assembly.
The advantages of HI-LOK™ two-piece fastener include its lightweight, high fatigue resistance, high strength, and its inability to be over-torqued. The pins, made from alloy steel, corrosion-resistant steel, nickel, or titanium alloy, come in many standard and oversized shank diameters. The collars are made of aluminum alloy, corrosion-resistant steel, titanium, or alloy steel. The collars have wrenching flats, fracture point, threads, and a recess. The wrenching flats are used to install the collar. The fracture point has been designed to allow the wrenching flats to shear when the proper torque has been reached. The threads match the threads of the pins and have been formed into an ellipse that is distorted to provide the locking action. The recess serves as a built-in washer. This area contains a portion of the shank and the transition area of the fastener.
The hole shall typically be prepared so that the maximum interference fit does not exceed 0.002-inch. This avoids build up of excessive internal stresses in the work adjacent to the hole. The HI-LOK™ pin has a slight radius under its head to increase fatigue life. After drilling, deburr the edge of the hole to allow the head to seat fully in the hole. The HI-LOK™ is typically installed in interference fit holes for aluminum structure and a clearance fit for steel, titanium, and composite materials.
HI-TIGUE™ Fastening System
The HI-TIGUE™ fastener offers all the benefits of the HI-LOK™ fastening system along with a unique radius contour on the thread lead-in, or a raised bead design that enhances the fatigue performance of the structure making it ideal for situations that require a controlled interference fit. The HI-TIGUE™ fastener assembly consists of a pin and collar. These pin rivets have a radius at the transition area. During installation in an interference fit hole, the radius area will “cold-work” the hole. These fastening systems can be easily confused, and visual reference should not be used for identification. Use part numbers to identify these fasteners. [Figure 3] |
| Figure 3. HI-TIGUETM special fasteners |
HI-LITE™ Fastening System
The HI-LITE™ fastener is similar in design and principle to the HI-LOK™ fastener, has the controlled radius from full diameter section to the threaded area of the HI-TIGUE™ fastener, and has a shorter transition area between the shank and the first load-bearing thread. HI-LITE™ fasteners have approximately one less thread. These differences reduce the weight of the HI-LITE™ fastener without lessening the shear strength. HI-LITE™ fasteners are available in the same materials and head configurations as the HI-LOK™ system, and can also be installed in high interference like the HI-TIGUE™ fastener. HI-LITE™ collars are also different and thus are not interchangeable with HI-LOK™ collars or HI-TIGUE™ collars.Captive Fasteners
Captive fasteners are used for quick removal of engine nacelles, inspection panels, and areas where fast and easy access is important. A captive fastener can turn in the body in which it is mounted, but will not drop out when it is unscrewed from the part it is holding. Some of the most commonly used are the Dzus, Camloc, and Airloc.Turn Lock Fasteners
Turn lock fasteners are used to secure inspection plates, doors, and other removable panels on aircraft. Turn lock fasteners are also referred to by such terms as quick opening, quick action, and stressed panel fasteners. The most desirable feature of these fasteners is that they permit quick and easy removal of access panels for inspection and servicing purposes. Turn lock fasteners are manufactured and supplied by several manufacturers under various trade names.Dzus Fasteners
The Dzus turn lock fastener consists of a stud, grommet, and receptacle. Figure 4 illustrates an installed Dzus fastener and the various parts. |
| Figure 4. Dzus fastener |
The grommet is made of aluminum or aluminum alloy material. It acts as a holding device for the stud. Grommets can be fabricated from 1100 aluminum tubing, if none are available from normal sources.
The spring is made of steel, which is cadmium plated to prevent corrosion. The spring supplies the force that locks or secures the stud in place when two assemblies are joined.
The studs are fabricated from steel and are cadmium plated. They are available in three head styles: wing, flush, and oval. Body diameter, length, and head type may be identified or determined by the markings found on the head of the stud. [Figure 5] The diameter is always measured in sixteenths of an inch. Stud length is measured in hundredths of an inch and is the distance from the head of the stud to the bottom of the spring hole.
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| Figure 5. Dzus identification |
A quarter of a turn of the stud (clockwise) locks the fastener. The fastener may be unlocked only by turning the stud counterclockwise. A Dzus key or a specially ground screwdriver locks or unlocks the fastener.
Camloc Fasteners
Camloc fasteners are made in a variety of styles and designs. Included among the most commonly used are the 2600, 2700, 40S51, and 4002 series in the regular line, and the stressed panel fastener in the heavy-duty line. The latter is used in stressed panels, which carry structural loads.The Camloc fastener is used to secure aircraft cowlings and fairings. It consists of three parts: a stud assembly, a grommet, and a receptacle. Two types of receptacles are available: rigid and floating. [Figure 6]
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| Figure 6. Camloc fastener |
The stud and grommet are installed in the removable portion; the receptacle is riveted to the structure of the aircraft. The stud and grommet are installed in either a plain, dimpled, countersunk, or counter bored hole, depending upon the location and thickness of the material involved.
A quarter turn (clockwise) of the stud locks the fastener. The fastener can be unlocked only by turning the stud counterclockwise.
Airloc Fasteners
The Airloc fastener consists of three parts: a stud, a cross pin, and a stud receptacle. [Figure 7] The studs are manufactured from steel and case hardened to prevent excessive wear. The stud hole is reamed for a press fit of the cross pin. |
| Figure 7. Airloc fastener |
The total amount of material thickness to be secured with the Airloc fastener must be known before the correct length of stud can be selected for installation. The total thickness of material that each stud satisfactorily locks together is stamped on the head of the stud in thousandths of an inch (0.040, 0.070, 0.190, and so forth). Studs are manufactured in three head styles: flush, oval, and wing.
The cross pin is manufactured from chrome-vanadium steel and heat-treated to provide maximum strength, wear, and holding power. [Figure 7] It should never be used the second time; once removed from the stud, replace it with a new pin.
Receptacles for Airloc fasteners are manufactured in two types: rigid and floating. Number—No. 2, No. 5, and No. 7, classifies sizes. They are also classified by the center-to-center distance between the rivet holes of the receptacle: No. 2 is 3⁄4 inch; No. 5 is 1 inch; and No. 7 is 13⁄8 inch. Receptacles are fabricated from high-carbon, heat-treated steel. An upper wing assures ejection of the stud when unlocked and enables the cross pin to be held in a locked position between the upper wing, cam, stop, and wing detent, regardless of the tension to which the receptacle is subjected.
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