Aircraft Fasteners
  • Thread Types and Fits - NC and NF have more threads than the UNF and UNC fasteners
    • American National Coarse - NC -
    • American National Fine - NF - 14 threads per inch
    • American Standard Unified Coarse - UNC
    • American Standard Unified Fine - UNF - 12 threads per inch
    • Classes of Fit
      • Class 1 - is a loose fit and allows you to turn the nut all the way down using only your fingers
      • Class 2 - Is a free fit, is the standard for most screws
      • Class 3 - Is a medium fit, and is the standard for most aircraft fasteners
      • Class 4 - is a close fit, requires a wrench to turn a nut down from start to finish
      • Class 5 - Is a tight fit, requires a wrench to turn a nut down from start to finish
    • Designation Codes - AN4-7
      • The first designation is the numbering system used
      • The second is the diameter in 1/16 inch
        • Threaded fasteners 1/4 inch in diameter and smaller are dimensioned in screw sizes rather than 1/8 increments and range from 0-12
      • The third is the length in 1/8 of an inch
        • For bolts longer than 7/8 the code changes, essentially there are no -8 or -9 bolts. It goes to 10 when the length hits 1 inch
      • The letter "A" at the end of the designation indicates whether or not the bolt has a hole drilled through it. The A meaning the absence of a hole.
      • The letter "H" following the diameter is use to identify bolts that have been drilled through the head for safety wire
  • Standard Aircraft Bolts - The FAA forbids the use of aluminum alloy bolts and alloy steel bolts smaller than AN3 on structural components and are not used in areas where the must be removed and installed frequently.
    • Grip Length Is the length of the unthreaded portion
    • Are available in Cadmium-plated nickel steel, corrosion resistant steel, and in 2024 aluminum alloy
      • Unless other wise specified a bolt is made of cadmium-plated nickel steel
        • Aluminum alloy nuts can be used with cadmium-plated steel bolts loaded in shear, but only on land aircraft.
      • A corrosion resistant bolt is identified by the letter "C" inserted between the diameter and length designators
      • Aluminum alloy bolts are identified by the letters "DD"
    • Drilled-Head Engine Bolts - AN73 - AN81 bolts are hex headed nickle-steel bolts that are similar in appearance to the AN# through AN20 series.
      • The advantage of these bolts is that they are made in either fine or coarse threads. A fine threaded bolt is ID'd by the absence of an 'A' preceding the dash number while coarse threaded bolt is ID'd by the presence of the letter 'A'
    • Close Tolerance Bolts - Are designated AN173 - AN186 and are ground to a tolerance of +0.000 to -0.0005 inches
      • Must be used in areas that are subject to pounding loads or in a structure that is required to be both riveted and bolted and the application of cadmium plating is typically not possible.
      • A thin layer of grease is usually applied to the shank of a close tolerance bolt before installation to prevent corrosion.
    • Clevis Bolts - Are loaded in Shear only
      • Designated AN21-AN36 and has a domed head that is usually slotted for a screw driver.
      • Only a short portion of the shank is threaded and there is a small notch between the threads and the shank which creates greater shear strength and allows the bolt to rotate in its hole.
      • When installed on fork end cable terminals, the bolt is secured with a shear nut tightened to a snug fit, but with no strain imposed on the fork.
    • Internal Wrenching Bolts - MS20004 - MS20024 they are high strength bolts used primarily in areas that are subjected to high tensile loads.
      • They accept an allen wrench of the proper size.
      • These bolts have a radius between the head and the shank and when installed in steel parts the steel must be counter bored to accept the radius
      • When installed on an aluminum alloy structure a MS20002C washer must be used under the head to provide the needed bearing area.
    • Bolt Selection and Instillation - when joining two pieces of material, their combined thickness determines the correct length of bolt to use. The grip length should be identical to the total thickness of the materials being fastened.
      • When joining Aluminum alloy or magnesium, the washers should be made of aluminum to minimize the possibility of dissimilar metal corrosion.
      • Unless otherwise specified in an assembly drawing, bolts should be installed with their head on top or forward.
        • An acronym used to help remember this is IDA, which stands for inboard, down or aft
      • Never assume a control cable is clear of a bolt until the bell-crank is inspected for full travel
  • Nuts - All nuts used in aircraft construction must have some sort of locking device to prevent them from loosening and falling off.
    • The size of a nut is indicated in the part code by a dash number which denotes the size of bolt it fits.
    • Self-Locking Nuts - Locks onto a bolt on its own, and the two general types are nylon and metal.
      • Low-Temperature Self-Locking Nuts
        • AN365 - are used on bolts and machine screws that are held in position by a nylon insert above the threads and allows a Class 3 fit.
          • They may be reused as long as there is enough friction between the nut and bolt so that the nut cannot be turned down by hand.
          • Must be screwed onto a bolt until all the chamfer on the bolt's end protrudes through the insert. (1-3 threads)
          • Bolts that have been drilled for a cotter pin can use self locking nuts as long as the bolt is 5/16 inch or larger in diameter and the edges of the cotter pin hole are chamfered so there are no burrs around the hole.
          • You may not use self-locking nuts in areas which subject either the bolt or nut to rotation.
          • May be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by a nut and bolt.
          • Nylon self locking nuts should not be used any where the temperature exceeds 250 degrees F
        • Metal Self-Locking Nuts - In applications where the temperature exceeds 250 degrees F, all metal lock nuts such as the AN363, are used. These nuts have a protion of their end slotted and the slots swaged together.
          • Available in either NF or NC threads, the nuts dash number identifies both diameter and threads per inch.
          • AN364 Shear Self-Locking Nuts - have a much lower profile and therefore are only allowable in shear applications, typically made for use on clevis bolts that do not have a drilled shank.
    • Non Self-Locking Nuts - Relies on a cotter pin, check nut, or lock washer to hold it in place.
      • AN310 Castle Nut - Designed to fit on a standard airframe bolt with a Class 3 fit, and are used when the bolt is subjected to either shear or tensile loads.
        • Available in cadmium nickel plated steel, corrosion resistant steel, and 2024 aluminum alloy.
            • Unless specified, a castle nut is made of cadmium plated nickel steel
            • A corrosion resistant nut has is identified by the letter 'C' inserted before the dash number in the part code.
            • Aluminum alloy nuts are identified by the letter 'D' in the part code
        • AN320 Shear Castle Nut - Made of the same material and has the same type of thread as the AN310 nut.
          • Are only used for shear loads on clevis bolts.
        • AN315 Plain Nut
          • Has no castellations and can't be held in place using a cotter pin.
          • A spring type lock washer must be used in combination with the nut
          • They are used in either tensile or shear loads and come in either nickel steel, corrosion resistant steel and aluminum alloy
          • They are made with both left and right hand threads
        • AN316 Check Nut - is simply a second nut tightened against the first nut so it cannot turn off
          • Its made of cadmium plated steel and is available in both right and left hand threads
            • The designations after the length indicator tell whether it is a left hand thread (L) or right hand thread (R)
        • AN340 Machine Screw Nut - Made in sizes from number 2 up through 1/4 in and have coarse threads.
          • Available in carbon steel, corrosion resistant steel (C), brass (B), and 2024 aluminum alloy (DD)
        • AN345 Machine Screw Nut - Similar to AN340 nuts except they have national fine series threads
          • Available in carbon steel, corrosion resistant steel (C), brass (B), and 2024 aluminum alloy (DD)
        • AN355 Slotted Engine Nut - Designed for use on aircraft engines and is not approved for use on airframes.
          • Made of a heat treated steel and has national fine threads that produce a Class 3 fit.
          • Available in sizes ranging from AN355-3 (3/16 inch ) to AN355-12 (3/4 inch) and has slots for a cotter pin.
        • AN360 Plain Engine Nut - Similar to the AN355 in that it is approved for use on engines only
          • Does not have cotter pin slots and has a black rustproof finish.
        • AN350 Wing Nut - Used when it is necessary to remove a part frequently without the use of tools
          • Made of either cadmium plated steel, or brass and are available in sizes to fit a number six machine screw up to a 1/2 bolt.
          • Have national fine threads that produce a Class 2 fit.
      • Anchor Nuts - are permanently mounted nut plates that enable inspection plates and access doors to be easily removed and installed.
        • Ganged Anchor Nuts - are strips of anchor nuts that are installed in a channel that is riveted to the structure.
        • Tinnerman Nuts - Cost effective nuts that are stamped out of sheet metal.
          • Commonly used on light aircraft to mount instruments to the instrument panel.
          • To reduce magnetic interference the nuts are made out of brass and the cage that holds the nut is constructed of phosphor bronze.
          • They are approved for no structural inspection plates.
      • Torque Recommendations - The strength of a joint held together by a threaded fastener depends upon proper preloading of the fasteners threads.
        • To much torque can damage threads, to little torque can allow excessive loads to be applied on a bolt resulting in failure.
        • When torquing a self locking nut, measure the amount of torque that is needed to the nut onto a bolt before it contacts the surface. Then add this torque value to the amount of torque that is recommended for the joint.
  • Screws
    • Three Classifications of Screws
      • Machine Screw - Most widely used
        • Used extensively for attaching fairings, inspection plates, fluid line clamps and other light structural parts.
        • The threads usually run the full length of the shank
        • Types
          • Fillister-Head Machine Screw - Slotted and have a hole drilled through their head, and come in both Coarse and fine threads with a class 3 fit. Sizes 4-40 - 1/4-20
          • Flat-Head Machine Screw - Countersunk screws made of cadmium plated carbon steel, have a recessed head for cross-point screwdrivers. Fine or coarse threads and available in sizes 4-48 - 3/8-16.
          • Round-Head Machine Screw - Cadmium plated carbon steel and has either slotted or recessed heads. Also comes in brass, fine or coarse threads.
          • Truss-Head Machine Screws - Large head that provides good holding ability on thin pieces of metal. Cadmium plated carbon steel in fine or coarse threads.
      • Structural Screws - Are made of alloy steel, heat treated and can be used as structural bolts. Same shear strength as a bold of the same size.
        • Types
          • Fillister-Head Screw - cross on its head indicating that it is made of high-strength steel. Fine or Coarse threads
          • Flat-Head Screw - Made of heat treated Carbon steel that is cadmium plated and are distinguished from 100degree flat-heads by the letter 'X' on the head.
          • Washer-Head Screw - Have a washer formed onto their head to increase the screw's holding ability.
      • Self-Tapping Screws - Have coarse-threads and are used to hold thin sheets of metal, plastic, or plywood together
        • Type-A - has a gimlet (sharp) point
        • Type-B - Has a blunt point and its threads are slightly finer than those of the Type-A
        • Come in round head, truss head, countersunk head, and countersunk oval.
  • Pins - Used in shear applications and for safetying
    • Roll Pin - Used to provide a pivot point for a joint where the pin is not likely to be removed. It's made of flat spring steel that is rolled into a cylinder but the two ends are not joined.
      • To remove the pin it must be driven from the hole with a proper size pin punch.
    • Clevis Pin - Used for hinge pins in some aircraft control systems. Made of Cadmium plated steel and have grip lengths in 1/16 increments
    • Cotter Pin - Made of either cadmium plated carbon steel or corrosion-resistant steel
      • Two methods of securing cotter pins
        • Preferred Method - One leg of the cotter pin is bent up over the end of the bolt, and the other legg is bent down over one of the flats of the nut.
        • Second Method - Cotter pin is rotated 90 degrees and the legs wrapped around the castellations
    • Taper Pin - There are both plain and threaded taper pins which are used in aircraft structures to make a joint that is designed to carry shear load and they are held in place by friction.
  • Washers - Provide a bearing surface area for nuts, and act as spacers or shims to obtain the proper grip length for a bolt and nut assembly.
    • Plain Washers - provide smooth surface between a nut and the material being 'clamped'. Made of cadmium plated steel, commercial brass, corrosion-resistant steel and 2024 aluminum alloy.
      • Light series washers are identified by the letter 'L' and are used to line up the hole with the casselations of the nut.
      • Large area washers are identified by the code AN970 and are all made of cadmium plated steel with the inside diameters from 3/16 to 1/2 in
    • Lock Washers - Used between the nut and joint surface if the joint is not structurally critical. Made of steel and are twisted so that when the nut is tightened against it the spring action of the washer creates a strong friction force.
      • AN935 Split lock washer is available in number 4 screw to 1/2 in
      • AN936 Shakeproof lock washers are available which have both internal and external teeth.
    • Special Washers
      • Countersunk Washers - are used under the heads of internal wrenching bolts which have a radius that needs to accommodate via the washer
        • When the bolts are used in aluminum alloy structures a plain washer mush be used under the counter sunk one.
        • Made of heat treated steel or cadmium plated steel
      • Finishing Washers - are often used in aircraft interiors to secure upholstery and trim and have a countersunk face to accommodate flush screws.
      • Keyed Washers - can be used as safety devices. They have small tabs that can be bent up against a not or bolt head to keep it from rotating
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