B-6b Aircraft Hardware 2023-02-15.pdf

MODULE 06 Category B Licence CASA B-06b Aircraft Hardware  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the wr...

MODULE 06 Category B Licence CASA B-06b Aircraft Hardware  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the written permission of Aviation Australia. CONTROLLED DOCUMENT 2023-02-15 B-06b Materials and Hardware Page 2 of 211 CASA Part 66 - Training Materials Only  Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2023-02-15 B-06b Materials and Hardware Page 3 of 211 CASA Part 66 - Training Materials Only  Table of Contents Aircraft Fastening Devices (6.5.1) 10 Learning Objectives 10 Aircraft Hardware 11 Screw Principles 11 Screw Terminology 11 Screws and Threads Used in Aircraft 14 Aircraft Hardware Standards 14 Bolts and Screws in Aircraft 16 Classification of Threads 18 Single and Multiple Start Threads 18 Right and Left Hand Threads 19 American Standard Threads 20 Unified Standard Threads 20 SI Metric Threads 21 Classes of Fit 22 Measuring Screw Threads 24 Thread-Pitch Gauge 24 Bolts, Studs and Screws (6.5.2) 25 Learning Objectives 25 Threaded Aircraft Fasteners 26 Threaded Fastener Aircraft Applications 26 Thread Type and Fits 26 Designation Codes 27 Standard Aircraft Bolts 28 Standard Airframe Bolts 29 Clevis Bolts - AN21 to AN36 31 Eyebolts - AN42 through AN49 32 Drilled-Head Engine Bolts - AN73 to AN81 33 Close Tolerance Bolts - AN173 to AN186 34 Internal Wrenching Bolts - MS20004 to MS20024 35 Precision Airframe Bolts 37 NAS: National Aerospace Standards (Aerospace Industries Association) 37 MS: Military Standard 38 Standard Airframe Bolts: NATO Standardisation Agency 38 2023-02-15 B-06b Materials and Hardware Page 4 of 211 CASA Part 66 - Training Materials Only  Nuts 40 Types of Aircraft Nuts 40 Self-Locking Nuts 40 Low-Temperature Self-Locking Nuts 41 Metal Self-Locking Nuts 42 Standard Nuts 44 AN310 Castle Nut 44 AN320 Shear Castle Nut 44 AN315 Plain Nut 44 AN316 Check Nut 45 AN340 and AN345 (Light Hex Nuts) 45 AN355 Slotted Engine Nut 46 AN360 Plain Engine Nut 46 AN350 Wing Nut 47 Anchor Nut 47 Tinnerman Nuts 48 Rivnuts 49 Studs 51 Studs Used in Aircraft 51 Standard Studs 51 Waisted Stud 52 Stepped Stud 52 Shouldered Stud 53 Insertion and Removal of Studs 54 Stud Replacement 54 Stud Replacement - Locknut Method 54 Stud Replacement - Stud Box 55 Stud Replacement - Stud Tool 56 Stud Removal 57 Damaged Studs 59 Sheared or Broken Studs 59 Screws 61 Aircraft Screw Applications 61 Machine Screws 61 Structural Screws 62 Self-Tapping Screws 63 2023-02-15 B-06b Materials and Hardware Page 5 of 211 CASA Part 66 - Training Materials Only  Washers 65 Types of Washers 65 Plain Washers 65 Lock Washers 66 Special Washers 67 Clevis Pins 68 Dowels 70 Types of Dowels 70 Threaded Dowels 70 Smooth Solid Dowels 70 Seamless Hollow Dowels 71 Split Hollow Dowels 72 Applications of Dowel Pins 73 Locking Devices (6.5.3) 76 Learning Objectives 76 Safety and Locking Mediums 77 Aviation Locking / Safety Mediums 77 Lock Washers 77 Tab Washers 78 Lockwire 79 Lock-Wiring Methods 80 Locking Plates 82 Pal Nuts 82 Grub Screws 83 Fasteners 85 Quick-Release Fasteners 85 Circlips 85 Taper and Cotter Pins 86 Roll Pins 87 Clevis Pins 88 Split Pins 89 Locating Devices 91 Keys 91 Woodruff Key 91 Square Key 91 Pratt and Whitney Key 92 2023-02-15 B-06b Materials and Hardware Page 6 of 211 CASA Part 66 - Training Materials Only  Gib Head Key 92 Springs (6.7) 94 Learning Objectives 94 Springs in Aircraft 95 Spring Applications 95 Spring Steel 95 Spring Varieties 96 Torsion Bar 101 Disc Springs 102 Spring Terminology 103 Bearings (6.8) 108 Learning Objectives 108 Bearings and Bearing Loads 109 Bearing Characteristics 109 Axial Loads 110 Radial Loads 111 Combination Loads 112 Bearing Supports 112 Bearing Types 114 Aircraft Bearing Applications 114 Plain Bearings 114 Bushings 118 Plain Bearing Diametrical Clearance 119 Anti-friction Bearings 119 Ball Bearings 120 Ball Bearing Uses 124 Ball Bearings 124 Angular-Contact Ball Bearings 124 Roller Bearings 124 Needle Roller Bearings 126 Tapered Roller Bearings 127 Self-Aligning Bearings 129 Thrust Bearings 133 Diametrical Clearance 135 Bearing Retention Methods 137 Bearing Fit 137 Interference Fit 137 2023-02-15 B-06b Materials and Hardware Page 7 of 211 CASA Part 66 - Training Materials Only  Circlips 138 Retaining Plates 139 Staking 142 Impression Staking 142 Swaging 145 Gears and Transmissions (6.9) 148 Learning Objectives 148 Gears 149 The Purpose of Gears 149 Spur Gears 149 Bevel Gears 152 Hypoid Gears 153 Worm Gears 154 Rack and Pinion 155 Sector Gear 155 Reduction Gear Assembly 157 Reduction Gears 157 Epicyclic (Planetary) Gears 157 Differential Gears 159 Gear Ratio/Speed Ratio Relationship 161 Gears 161 Speed Ratio 161 Gear Ratio 162 Idler Gears 163 Direction of Rotation 166 Spur Planetary Gear Systems 166 Backlash 169 Belts, Chains and Sprockets 171 Chains 171 Sprockets 174 Belts 175 Screw Jacks in Aircraft 178 Screw Jacks 178 The Worm-and-Nut Mechanism 178 The Recirculating Ball Mechanism 179 Control Rods 180 Bellcranks 182 2023-02-15 B-06b Materials and Hardware Page 8 of 211 CASA Part 66 - Training Materials Only  Torque Tubes 183 Aircraft Control Cables (6.10) 185 Learning Objectives 185 Cable Systems 186 Introduction to Cable Systems 186 Cable Construction 186 Lockclad Cable 189 Cable Termination 190 Nicopress Process 190 Swaged Terminals 191 Shackle Pins 193 Proof Load Test 194 Cable Installation 195 Introduction to Cable Installation 195 Control Cable Pulleys 196 Cable Inspection 197 Travel Adjustment and Cable Tension 198 Cable Tension 198 Control Springback 200 Turnbuckles 200 Turnbuckle Locking 200 Cable Drums 203 Cable Quadrants 203 Broken Cable Compensator 204 Bowden and Teleflex Control Cables 205 Swivelling Couplings 209 Bowdenflex Cables 210 2023-02-15 B-06b Materials and Hardware Page 9 of 211 CASA Part 66 - Training Materials Only  Aircraft Fastening Devices (6.5.1) Learning Objectives 6.5.1.1 Describe screw nomenclature (Level 2). 6.5.1.2.1 Describe standard thread forms used in aircraft (Level 2). 6.5.1.2.2 Describe dimensions for standard thread forms used in aircraft (Level 2). 6.5.1.2.3 Describe tolerances for standard thread forms used in aircraft (Level 2). 6.5.1.3 Describe the measurement of screw threads (Level 2). 2023-02-15 B-06b Materials and Hardware Page 10 of 211 CASA Part 66 - Training Materials Only  Aircraft Hardware Screw Principles A wedge is a simple machine which can be used to apply a load. If a wedge is driven under an object it will raise it, the distance being governed by the inclination of the wedge. A steeper wedge will move it a greater distance, but a shallower wedge will raise a greater weight. A screw may be considered as an inclined plane or wedge wrapped around a cylinder or shaft to form a helix. The distance along the cylinder by one full turn of the helix is the 'pitch' (P). It can be seen from the diagram that a shallow wedge angle produces a fine pitch thread while a steeper angle produces a coarse pitch thread. When a male thread is engaged in a female thread, e.g. a bolt in a nut the full surface area of the female "wedge" is in contact with the male. More force (F) may be exerted between the two surfaces of a shallow wedge than when the wedge angle is greater. Also, as friction is dependent on applied load and surface area, it can be seen that a shallow wedge angle will produce more friction and greater resistance to both tightening and loosening. This is a desirable property in threaded fasteners, particularly in smaller threads where the surface area is already limited by size. When a thread is used to produce motion, e.g. a worm screw, greater travel (P) will be produced by a coarse thread but again more force can be applied by a fine one. © TTS Courseware Screw principles 2023-02-15 B-06b Materials and Hardware Page 11 of 211 CASA Part 66 - Training Materials Only  Screw Terminology © TTS Courseware Screw nomenclature Pitch The pitch of a screw thread is the distance in inches or millimetres from any point on a thread to the corresponding point on the next thread measured parallel to the axis. The pitch is equal to 1 thread per square inch. Effective Diameter This may also be called the pitch diameter, and is the diameter of an imaginary cylinder (Pitch Cylinder) which splits the fundamental triangles exactly in half. Minor Diameter The minor diameter is the distance measured between the roots of the thread, in the case of a male thread and between the crests of the thread in the case of a female thread. Major Diameter Is the measured diameter over the crests of the thread (roots in the female). Crest The top surface joining the flanks of the thread. 2023-02-15 B-06b Materials and Hardware Page 12 of 211 CASA Part 66 - Training Materials Only  Root The root is the bottom of the groove joining adjacent sides or flanks of the thread, whether of the male screw or of the female screw. © TTS Courseware Screw thread components Flank or Side The surface of the thread form which connects the crest with the root. Thread Angle The included angle between the flanks measured in the axial plane. Lead The distance a screw thread advances axially in one complete turn (i.e. same as pitch for single start thread). Length of Engagement The axial distance over which two mating threads are designed to make contact. 2023-02-15 B-06b Materials and Hardware Page 13 of 211 CASA Part 66 - Training Materials Only  Angular Depth The triangle formed by the intersection of the extended flanks. The vertical height of this triangle is the angular depth. Actual Depth The distance between the crest and the root of the thread measured perpendicular to the axis Fundamental Triangle The triangle formed by the intersection of the extended flanks (XYZ). Truncation The distance measured radially from the crest or root of the thread to the adjacent apex of the fundamental triangle. Screws and Threads Used in Aircraft Aircraft hardware is the term used to describe various types of fasteners and miscellaneous small items used in the manufacture and repair of aircraft. The safe and efficient operation of any aircraft is greatly dependent on the correct selection and use of aircraft hardware. Vibration is always present during aircraft operation. Consequently, there must be provision for safe tying or locking of fasteners to prevent them from vibrating loose in flight, and anyone involved in aircraft maintenance must be familiar with the methods used. Aircraft hardware is generally identified by the organisation originating it, engineering data, materials, and processors used to design and produce it. This process eliminates the need for elaborate descriptions of individual items of hardware. Thread measuring - thread callout 2023-02-15 B-06b Materials and Hardware Page 14 of 211 CASA Part 66 - Training Materials Only  Aircraft Hardware Standards Various standards are used for hardware specifications in the aircraft industry. The most common standards you will encounter are: American Standards AN – Airforce Navy MS – Military Standards NAS – National Aerospace Standards European Standards NSO – NATO Standardisation Office 2023-02-15 B-06b Materials and Hardware Page 15 of 211 CASA Part 66 - Training Materials Only  Aircraft Manufacturers' Standards BAC – Boeing Aircraft Corporation FON – Fokker AMS – Aeronautical Materials Specifications. Standard hardware, which is available from aviation suppliers, is identified by a specification number. Special fasteners must be replaced with those having the same part number and not with similar- looking standard hardware. Often the difference between a standard and a special part is the material used to manufacture it, the closer tolerance in its manufacture or a more critical inspection of the part. AN fasteners can be replaced by NAS and MS equivalent fasteners. NAS and MS standard hardware must not be replaced by AN standard hardware. Aircraft hardware - screws 2023-02-15 B-06b Materials and Hardware Page 16 of 211 CASA Part 66 - Training Materials Only  Bolts and Screws in Aircraft Various types of fastening devices allow quick dismantling of aircraft parts that must be taken apart and put back together at frequent intervals. Riveting or welding these parts each time they are serviced would soon weaken or ruin the joint. Furthermore, some joints require greater tensile strength and stiffness than rivets can provide. Bolts and screws are two types of fastening devices which give the required security of attachment and rigidity. Generally, bolts are used when great strength is required, and screws are used where strength is not the deciding factor. Bolts and screws are similar in many ways. They are both used for fastening or holding, and each has a head on one end and a screw thread on the other. Regardless of these similarities, there are several distinct differences between the two types of fasteners. The threaded end of a bolt is always blunt, while that of a screw may be either blunt or pointed. The threaded end of a bolt usually has a nut secured to it to complete the assembly. The threaded end of a screw may fit into a female receptacle, or it may fit directly into the material being secured. A bolt has a fairly short thread section and a comparatively long grip length or unthreaded portion, whereas a screw has a longer threaded section and may have a clearly defined grip length. A bolt assembly is generally tightened by turning the nut on the bolt; the head of the bolt may or may not be designed for turning. A screw is always tightened by turning its head. When it becomes necessary to replace aircraft fasteners, a duplicate of the original fastener should always be used. Aircraft panel with screws and rivets 2023-02-15 B-06b Materials and Hardware Page 17 of 211 CASA Part 66 - Training Materials Only  Classification of Threads Single and Multiple Start Threads When formed by one continuous groove a thread is said to be ‘single’ or ‘single-start’. The majority of threads used for adjustment and fastening are single thread. For any single-start thread, the ‘lead’ is always equal to the pitch of the thread. Lead is the distance travelled axially by an engaged threaded part in a complete turn. Multiple start threads consist of two or more grooves cut side by side. In this way the axial travel or lead of the thread is increased without changing the pitch. For example, a nut engaged with a double- start thread will travel twice as far in one complete turn as one engaged with a single thread of similar pitch. In multiple threads the lead is equal to the Pitch of the thread (P) multiplied by the number of starts. Single and multi start threads Note that Lead (L) = Pitch (P) x Number of starts. 2023-02-15 B-06b Materials and Hardware Page 18 of 211 CASA Part 66 - Training Materials Only  Single and multiple start threads - colour coded Multi start threads are used in some screw-jack actuators and in hydraulic quick-disconnect couplings. Multi-start threads in a hydraulic quick-disconnect coupling 2023-02-15 B-06b Materials and Hardware Page 19 of 211 CASA Part 66 - Training Materials Only  Right and Left Hand Threads A right hand thread is one on which the thread is cut so that turning of the nut in a clockwise direction will tighten it on a bolt. A left hand thread requires the nut to be turned anti-clockwise to tighten it. Left hand threads are only used for special purposes. Aviation Australia A turnbuckle uses both right and left hand threads American Standard Threads ANC - American National Coarse. ANF - American National Fine. ANEF - American National Extra Fine. ANP - American National Pipe. American Standard thread 2023-02-15 B-06b Materials and Hardware Page 20 of 211 CASA Part 66 - Training Materials Only  Unified Standard Threads Note: The inclusion of a radius on the thread root for all unified standard threads. This feature increases shear strength in comparison to the American Standard thread types. UNC – Unified National Coarse. UNF – Unified National Fine. UNEF – Unified National Extra Fine. Unified Standard thread 2023-02-15 B-06b Materials and Hardware Page 21 of 211 CASA Part 66 - Training Materials Only  SI Metric Threads The SI Metric system of threads is generally used on equipment manufactured in Europe. All metric threads have a standardised thread form. SI Metric standard thread 2023-02-15 B-06b Materials and Hardware Page 22 of 211 CASA Part 66 - Training Materials Only  Classes of Fit Threads are also designated by class of fit, which indicates the tolerance allowed in manufacturing: Class 1 is a loose fit Class 2 is a free fit Class 3 is a medium fit Class 4 is a close fit Class 5 is a tight fit. Note: Aircraft bolts are almost always manufactured in a Class 3, medium fit. A Class 4 fit requires a wrench to turn a nut onto a bolt, whereas a Class 1 fit can easily be turned with the fingers. Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly. https://en.wikipedia.org/wiki/Screw_thread#Classes_of_fit Class of fit - manufacturing tolerance 2023-02-15 B-06b Materials and Hardware Page 23 of 211 CASA Part 66 - Training Materials Only  Measuring Screw Threads Thread-Pitch Gauge A thread-pitch gauge is used to identify the threads on various bolts and screws. The gauge thread is held against the bolt thread with daylight behind it to check whether the thread pitch matches the bolt exactly. The corresponding number on the gauge indicates the number of threads per inch (TPI). Thread-pitch gauge 2023-02-15 B-06b Materials and Hardware Page 24 of 211 CASA Part 66 - Training Materials Only  Bolts, Studs and Screws (6.5.2) Learning Objectives 6.5.2.1.1 Describe the types of aircraft bolts including their specifications, markings and identifying features (Level 2). 6.5.2.1.2 Describe the international standards of aircraft bolts (Level 2). 6.5.2.2.1 Describe the types, specifications and international standards of aircraft self- locking, anchor and standard nut types (Level 2). 6.5.2.2.2 Identify aircraft self-locking, anchor and standard nut types and their markings (Level 2). 6.5.2.3 Describe the aircraft specifications for machine screws (Level 2). 6.5.2.4.1 Describe the types and uses of studs (Level 2). 6.5.2.4.2 Describe stud insertion and removal (Level 2). 6.5.2.5.1 Describe self tapping screw (Level 2). 6.5.2.5.2 Describe dowels (Level 2). 6.5.2.6 Describe the use and purpose of washers used in aircraft applications (S). 2023-02-15 B-06b Materials and Hardware Page 25 of 211 CASA Part 66 - Training Materials Only  Threaded Aircraft Fasteners Threaded Fastener Aircraft Applications Threaded fasteners allow parts to be fastened together with all of the strength unthreaded fasteners provide. However, unlike rivets, threaded fasteners may be disassembled and reassembled an almost infinite number of times. Threaded fasteners are used in aircraft to allow disassembly and reassembly for maintenance 2023-02-15 B-06b Materials and Hardware Page 26 of 211 CASA Part 66 - Training Materials Only  Thread Type and Fits Most bolts used in aircraft structures are either general-purpose, internal-wrenching or close- tolerance AN, NAS or MS bolts. Aircraft bolts, screws and nuts are threaded in the American National Coarse (NC), the American National Fine (NF), the American Standard Unified Coarse (UNC) or the American Standard Unified Fine (UNF) series. In addition to being identified as either coarse or fine, threads are also designated by class of fit from 1 to 5: A Class 1 thread is a loose fit A Class 2 is a free fit A Class 3 is a medium fit A Class 4 is a close fit A Class 5 fit is a tight fit. A Class 1 fit allows you to turn the bolt all the way down using only your fingers. Wing nuts are a good example of a Class 1 fit but they can also exist as a Class 2 fit. Class 4 and 5 fits require a wrench to tighten a bolt from start to finish. Aircraft bolts are usually fine-threaded with a Class 3 fit, whereas screws are typically a Class 2 or 3 fit. Class of fit 2023-02-15 B-06b Materials and Hardware Page 27 of 211 CASA Part 66 - Training Materials Only  Designation Codes An aircraft bolt is given a part code indicating its diameter in 1/16-in. increments and its length in 1/8-in. increments. For example, an AN4-7 identifies a bolt that measures 4/16 or 1/4 in. in diameter and 7/8 in. in length. For bolts that are longer than 7/8 in., the code changes. For example, a 1-in. bolt is identified by a ‑10 representing 1 in. and no fraction. In other words, there are no -8 or -9 lengths. Dash numbers go from -7 to -10, from -17 to -20 and from -27 to -30. Therefore, a bolt that is 1 1/2 in. long is identified by a -14. A bolt with the code AN5-22 identifies an Air Force-Navy bolt that is 5/16 in. in diameter and 2 1/4 in. long. Threaded aircraft bolts 1/4 in. in diameter and smaller are dimensioned in screw sizes rather than 1/8-in. increments. The AN3 bolt is the exception to this rule. These machine screw sizes range from 0 to 12. A number 10 fastener has a diameter of approximately 3/16 in. and a number 5 fastener has a 1/8-in. diameter. © Jeppesen NF thread series 2023-02-15 B-06b Materials and Hardware Page 28 of 211 CASA Part 66 - Training Materials Only  Standard Aircraft Bolts A bolt is designed to hold two or more items together. Bolts that are typically used for airframe structural applications have hex heads and range in size from AN3 (3/16 in.) to AN2O (2 in.). Bolts are identified by their diameter and length. A diameter represents the shank diameter, while the length represents the distance from the bottom of the head to the end of the bolt. A bolt’s grip length is the length of the unthreaded portion as shown. If the grip length is slightly longer than this thickness, washers must be added to ensure that the nut can provide the proper amount of pressure when it is tightened. If the grip length is substantially less than the thickness of the materials, the bolt’s threads will extend into the material, resulting in a weaker joint. When a bolt joins two pieces of material, their combined thickness determines the correct length of bolt to use. Coding of standard aircraft bolts The diameter of a bolt is indicated by the number immediately following the prefix such as AN. The dash number of standard bolts indicates length in 1/8-in. increments. AN3-6A: AN = Air Force/Navy 3 = diameter in 1/16 in. (3/16 in.) -6 = length in 1/8 in. (6/8 = 3/4 in.) A = not drilled for split pin (No letter = drilled for split pin). 2023-02-15 B-06b Materials and Hardware Page 29 of 211 CASA Part 66 - Training Materials Only  Standard Airframe Bolts Aircraft bolts are available in cadmium-plated nickel steel, corrosion-resistant steel and 2024 aluminium alloy. Unless specified, a bolt is made of cadmium-plated nickel steel. A corrosion-resistant bolt, on the other hand, is identified by the letter C inserted between the diameter and length designations. Aluminium alloy bolts are identified by the letters DD. For example, a bolt that is 1/4 in. in diameter, 3/4 in. long and made of cadmium-plated nickel steel is identified by the code AN4-6 (4/16 = 1/4 and 6/8 = 3/4). However, if the same bolt is made of corrosion-resistant steel, it carries the code AN4C6, whereas an aluminium alloy bolt would be AN4DD6. Standard airframe bolts In addition to the designation code, most aircraft bolts have a marking on their head identifying what the bolt is made of and, in many cases, the manufacturer. For example, AN standard steel bolts are marked with either a raised dash or an asterisk in the centre of the manufactured head, corrosion- resistant steel is marked by a single dash and AN aluminium-alloy bolts are marked with two raised dashes. When hardware was first standardised, almost all nuts were locked onto a bolt with a split pin, and therefore all bolts had holes drilled near the end of their shank to accommodate a split pin. However, when self-locking nuts became popular, many standard AN bolts were made without a drilled shank. To help you identify whether or not a bolt has a hole drilled through it, the letter A is used in the part code. For example, if an A appears immediately after the dash number, the bolt does not have a hole. However, the absence of an A indicates a hole exists in the shank. As an example, an AN6C-12A bolt is 3/8 in. in diameter, is made of corrosion-resistant steel, is 1 1/4 in. long and has an undrilled shank. Some AN bolts, such as those used to fasten a propeller into a flanged shaft, must be safe tied by passing safety wire through holes drilled through the bolt’s head. A bolt drilled for this type of safetying has the letter H following the number indicating its diameter. For example, the part number AN6H-34A identifies a bolt that is 3/8 in. in diameter, made of nickel- steel, has a drilled head, is 3 1/2 in. long and has an undrilled shank. 2023-02-15 B-06b Materials and Hardware Page 30 of 211 CASA Part 66 - Training Materials Only  Different bolt heads 2023-02-15 B-06b Materials and Hardware Page 31 of 211 CASA Part 66 - Training Materials Only  Clevis Bolts - AN21 to AN36 Clevis bolts ranging from AN21 to AN36 are to be loaded in shear only. For example, a control cable must be attached to a control horn with a bolt that is loose enough to allow the cable to pivot freely as the control surface moves, but not so loose that excess play exists. For these applications, a clevis bolt is used. The AN21 through AN36 clevis bolt has a domed head that is typically slotted or recessed to accept a screwdriver. A unique feature of a clevis bolt is that only a short portion of the shank is threaded, and there is a small notch between the threads and the shank. This results in a long grip length, which increases the bolt’s shear strength and allows it to rotate more freely in its hole. The diameter of a clevis bolt is given in 1/16-in. increments. The length of a clevis bolt is more critical than that of the other types of bolts and therefore is also measured in 1/16-in. increments with a dash number indicating the length. For example, AN29-20 identifies a 9/16-in. diameter clevis bolt that is 20/16 (1 1/4) in. long. Clevis bolts 2023-02-15 B-06b Materials and Hardware Page 32 of 211 CASA Part 66 - Training Materials Only  Eyebolts - AN42 through AN49 Eyebolts ranging from AN42 to AN49 are used in applications where external tension loads are to be applied. The head of this type of bolt is specially designed for attaching a turnbuckle, a clevis or a cable shackle. The threaded shank may or may not be drilled for safetying. © Aviation Australia Eyebolts - AN42 through AN49 2023-02-15 B-06b Materials and Hardware Page 33 of 211 CASA Part 66 - Training Materials Only  Drilled-Head Engine Bolts - AN73 to AN81 AN73 through AN81 bolts are hex-headed nickel-steel bolts that are similar in appearance to the AN3 through AN2O series. However, unlike standard bolts, drilled-head engine bolts have a thicker head that is drilled with a small hole in each of the flats and in the centre of the head. As with most bolts, the diameters of drilled-head engine bolts are in 1/16-in. increments, while bolt lengths are in 1/8-in. increments. The diameter is indicated by the second number following the AN designation, while the bolt length is indicated by a dash number. For example, a drilled-head engine bolt designated as AN746 has a diameter of 1/4 in. and a length of 3/4 in. Drilled head bolt 2023-02-15 B-06b Materials and Hardware Page 34 of 211 CASA Part 66 - Training Materials Only  Close Tolerance Bolts - AN173 to AN186 Close tolerance bolts are designated AN173 to AN186 and are ground to a tolerance of +0.000– 0.0005 in. This is much tighter than standard AN3 through AN14 bolts, which are manufactured with a tolerance of +0.000–0.0025 in., or AN16 through AN2O bolts, which are manufactured with a tolerance of +0.000–0.0055 in. Close tolerance bolts must be used in areas that are subject to pounding loads or in a structure that is required to be both riveted and bolted. Close tolerance bolts carry a triangle mark on their heads and are ground to a much tighter tolerance than standard bolts. Close tolerance bolt 2023-02-15 B-06b Materials and Hardware Page 35 of 211 CASA Part 66 - Training Materials Only  Internal Wrenching Bolts - MS20004 to MS20024 MS20004 through MS20024 internal wrenching bolts are high-strength steel bolts used primarily in areas subjected to high tensile loads. A six-sided hole is machined into the centre of their heads to accept an Allen wrench of the proper size. These bolts have a radius between the head and shank and, when installed in steel parts, the hole must be counter-bored to accommodate this radius. When an internal wrenching bolt is installed in an aluminium alloy structure, a MS20002C washer must be used under the head to provide the needed bearing area. The strength of interim wrenching bolts is much higher than that of a standard steel AN bolt, and for this reason an AN bolt must never be substituted for an internal wrenching type. Military Specification - example bolt table 2023-02-15 B-06b Materials and Hardware Page 36 of 211 CASA Part 66 - Training Materials Only  Precision Airframe Bolts NAS: National Aerospace Standards (Aerospace Industries Association) Developed and updated by the National Aerospace Standards Committee, Aerospace Standards include many standards for precision fasteners as well as for other aerospace hardware. The length is always displayed as a dash number and is generally in increments of 1/16 inch for standard fasteners. Grip length is measured from under the head to the start of the threaded section. © Aviation Australia NAS bolt The basic NAS number identifies the part. The suffix letters and dash numbers identify different sizes, plating material, drilling specifications, etc. Refer to the National Aerospace Standard to identify a specific NAS bolt by part number. 2023-02-15 B-06b Materials and Hardware Page 37 of 211 CASA Part 66 - Training Materials Only  © Aviation Australia NAS part number breakdown MS: Military Standard All aircraft bolts, except AN bolts, are measured by their grip length, not by their overall length. Standard-issue bolts have even dash numbers. In most applications, these bolts will do, but where the grip length is critical and a (standard) grip is either too long or too short, an ODD dash number bolt is available. Aircraft bolts must be fitted correctly; just any length bolt will not do. The load must be on the shank and not on the thread. Tensile strength ranges from 160 000 to 180 000 psi and shear strength from 95 000 to 108 000 psi, depending on the type of bolt. MS bolt 2023-02-15 B-06b Materials and Hardware Page 38 of 211 CASA Part 66 - Training Materials Only  Standard Airframe Bolts: NATO Standardisation Agency The European NSA bolts should not be confused with American NAS bolts. Always refer to the aircraft illustrated parts catalogue when purchasing fasteners. Often bolts may look the same but the maintenance engineer is responsible for ensuring that all hardware that is fitted to the aircraft meets the manufacturers specifications. NSA Specification is a four-digit number identifying the type of bolt, e.g. NSA5022 The first dash number is the diameter in 1/16 in., e.g. -4. The second dash number is the grip length in 1/16 in., e.g. -22. Example: NSA5022-4-22 is a 1/4 × 1 3/8 in. hex-head bolt. NOTE: To prevent dangerous cross-use between metric and imperial sizes, all Western aerospace bolts are manufactured to imperial specifications. NAS bolt 2023-02-15 B-06b Materials and Hardware Page 39 of 211 CASA Part 66 - Training Materials Only  Nuts Types of Aircraft Nuts All nuts used in aircraft construction must have some sort of locking device to prevent them from loosening and falling off. There are two basic types of nuts: self-locking and non-self-locking. As the name implies, a self- locking nut locks onto a bolt on its own, while a non-self-locking nut relies on a split pin, check nut or lock washer to hold it in place. Variety of nuts 2023-02-15 B-06b Materials and Hardware Page 40 of 211 CASA Part 66 - Training Materials Only  Self-Locking Nuts Self-locking nuts, or lock nuts, employ a locking device in their design to keep them from coming loose. The two general types of self-locking nuts used in aviation are the fibre, or nylon type (low temperature) and the all-metal type. A self-locking nut must be screwed onto a bolt until all of the chamfer on the bolt’s end protrudes through the insert. If the bolt is not chamfered, at least one thread but not more than three threads should protrude through the nut. If more than three threads are exposed, you risk the danger of ‘bottoming out’ the nut and under-torquing the assembly, thus creating a stress point that could fail. If more than three threads are exposed, either replace the bolt with one of the correct length or install a washer. A self-locking nut’s dash number specifies both diameter and number of threads per inch. For example, a -524 represents a self-locking nut that fits a 5/16-in. fine-thread bolt with 24 threads per inch. © Aviation Australia Metal self-locking nut 2023-02-15 B-06b Materials and Hardware Page 41 of 211 CASA Part 66 - Training Materials Only  Low-Temperature Self-Locking Nuts Nylon self-locking nuts should not be used in any location where the temperature could exceed approximately 120 °C. However, you may use them on engines in locations specified by the engine manufacturer. AN365 self-locking nuts are used on bolts and machine screws and are held in position by a nylon insert above the threads. This insert has a hole slightly smaller than the thread diameter on which it fits. The nut’s Class 3 fit allows it to run down on a bolt’s threads easily until the bolt enters the insert. Aviation Australia Nylon lock nut AN364 nuts resemble the AN365 self-locking nut, but they are thin and are approved only for shear loads, not to be used in tension. AN364 nuts are typically made to be used on clevis bolts that do not have drilled shanks. Nylon lock nuts 2023-02-15 B-06b Materials and Hardware Page 42 of 211 CASA Part 66 - Training Materials Only  Metal Self-Locking Nuts In applications where temperatures exceed approximately 120 °C, all-metal lock nuts such as the AN363 are used. Some of these nuts have a portion of their end slotted and the slots swaged together. This gives the end of the nut a slightly smaller diameter than its body, allowing the threads to grip the bolt. Others have the end of the nut squeezed into a slightly oval shape, and as the bolt screws up through the threads, it must make the hole round, creating a gripping action. Metal lock nuts 2023-02-15 B-06b Materials and Hardware Page 43 of 211 CASA Part 66 - Training Materials Only  Standard Nuts AN310 Castle Nut These fine-thread nuts are 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. The size of a nut is indicated in the part code by a dash number which denotes the size of the bolt it fits. For example, an AN310-6 nut fits an AN6 bolt which has a diameter of 3/8 in. Castle nuts are available in cadmium-plated nickel steel, corrosion-resistant steel and 2024 aluminium alloy. Unless specified, a castle nut is made of cadmium-plated nickel steel. A corrosion- resistant nut, on the other hand, is identified by the letter C inserted before the dash number in the part code. Aluminium alloy nuts are identified by the letter D. For example, the part code AN31D-6 identifies an aluminium alloy nut that has an inside diameter of 6/16 in. (or 3/8 in.). Aviation Australia AN310 Castle Nut AN320 Shear Castle Nut The AN320 shear castle nut is made of the same material and has the same type of thread as an AN310 nut. However, shear castle nuts are much thinner than standard castle nuts and therefore are used only for shear loads on clevis bolts. An AN320-6 nut is a shear castle nut that is used on an AN26 clevis bolt. An aluminium alloy (2024) nut is identified as an AN320D6. Aviation Australia AN320 shear castle nut 2023-02-15 B-06b Materials and Hardware Page 44 of 211 CASA Part 66 - Training Materials Only  AN315 Plain Nut The AN315 plain nut has no castellations and therefore cannot be held in place using a split pin. Since these fine-thread nuts have no locking provisions, a spring-type lock washer must be used in combination with the nut. The lock washer applies a spring force to prevent the nut from shaking loose. AN315 nuts are used with either tensile or shear loads and are made of either nickel steel, corrosion-resistant steel or aluminium alloy. The type of material used is indicated in the designation code in the same way it is for bolts. In other words, the absence of an additional letter identifies nickel steel, whereas the letter C preceding the dash number identifies corrosion-resistant steel, and a U identifies 2024 aluminium alloy. Furthermore, plain nuts are made with both right- and left-hand threads. For example, an AN315-7R is a nickel-steel nut with right-hand threads that fits an AN7 bolt. An AN315C-4L, on the other hand, is a 1/4-in. diameter corrosion-resistant steel plain nut with left- hand threads. Aviation Australia AN315 plain nut AN316 Check Nut In some instances, a plain nut is locked in place using a check nut. A check nut is simply a second nut that is tightened against the primary nut so it cannot turn off. An AN316 check nut is made of cadmium-plated steel and is available in both right- and left-hand threads. An AN316-4R is a right- hand check nut that fits a 1/4-in. thread, while an AN316-4L has a left-hand thread. Aviation Australia AN316 check nut 2023-02-15 B-06b Materials and Hardware Page 45 of 211 CASA Part 66 - Training Materials Only  AN340 and AN345 (Light Hex Nuts) AN340 and AN345 light hex nuts are used in non-structural applications requiring light tension. Like the AN315 and AN335, they require a locking device to secure them. Aviation Australia AN340 machine screw nut - coarse thread Aviation Australia AN345 machine screw nut - fine thread AN355 Slotted Engine Nut The AN355 nut is designed for use on an aircraft engine and is not approved for airframe use. It is made of heat-treated steel and has National Fine threads that produce a Class 3 fit. It is available in sizes from AN355-3 (3/16 in.) to AN355-12 (3/4 in.) and has slots cut in it for a split pin. Aviation Australia AN355 slotted engine nut 2023-02-15 B-06b Materials and Hardware Page 46 of 211 CASA Part 66 - Training Materials Only  AN360 Plain Engine Nut The AN360 engine nut is similar to the AN355 in that it is approved for use on engines only. However, an AN360 differs from an AN355 in that it does not have split pin slots and has a black rustproof finish. An AN360-7 is a plain engine nut that fits a 7/16-in. bolt. Aviation Australia AN360 plain engine nut AN350 Wing Nut Wing nuts are used when it is necessary to remove a part frequently without the use of tools. Aircraft wing nuts are made of either cadmium-plated steel or brass and are available in sizes to fit number (gauge) 6 machine screws up to 1/2-in. bolts. All of these nuts have National Fine threads that produce a Class 2 fit. Nuts for machine screw sizes are designated by the series number. However, nuts used on bolts have a bolt size given in 1/16-in. increments followed by the number 16. For example, for an AN350-616 wing nut, the -6 indicates that the nut will fit a 3/8 (6/16)-in. bolt. Aviation Australia AN350 wing nut 2023-02-15 B-06b Materials and Hardware Page 47 of 211 CASA Part 66 - Training Materials Only  Anchor Nut Anchor nuts are permanently mounted nut plates that enable inspection plates and access doors to be easily removed and installed. To make the installation of an access door easier where there are a great number of screws, a floating anchor nut is often used. With a floating anchor nut, the nut fits loosely into a small bracket which is riveted to the skin. Since the nut is free to move within the bracket, it aligns itself with a screw. To speed the production of aircraft, ganged anchor nuts are installed around inspection plate openings. These are floating- type anchor nuts installed in a channel that is riveted to the structure. Each nut floats in the channel with enough play so that a screw can move the nut enough to align it. © Jeppesen Anchor nut variations 2023-02-15 B-06b Materials and Hardware Page 48 of 211 CASA Part 66 - Training Materials Only  Tinnerman Nuts Tinnerman nuts are economical nuts that are stamped out of sheet metal. Because of their semi-rigid construction, Tinnerman nuts can be adapted for use in many situations. For example, Tinnerman nuts are commonly used on light aircraft to mount instruments to the instrument panel as well as to attach inspection panels and cowlings. Tinnerman nuts used to mount instruments can be installed either in an instrument panel or in the instrument case itself. To reduce the chance of magnetic interference, the nuts are made of brass and the cage that holds the nut is constructed of phosphor bronze. If the instrument is rear-mounted, the legs of the nut are long enough to pass through the instrument case. If the instrument is front- mounted, the nut fastens into the screw hole in the instrument panel. Anchor-type Tinnerman nuts are riveted to a structure to hold screws used to secure inspection plates. The cowlings on some light aircraft are held on with self-tapping sheet metal screws. To prevent the sheet metal screws from enlarging the holes in the cowling by repeated insertion and extraction, a U- type Tinnerman nut is slipped over the edge of the inside cowling so that it straddles the screw hole. When a screw is tightened into the nut, the spring action of the nut holds the screw tight. Tinnerman nuts - Anchor type Tinnerman nut (left) and U type Tinnerman nut (right) 2023-02-15 B-06b Materials and Hardware Page 49 of 211 CASA Part 66 - Training Materials Only  Rivnuts Goodrich Rivnuts were developed by the BF Goodrich Company to attach rubber de-icer boots to aircraft wing and tail surfaces. To install a Rivnut, a hole is drilled in the skin to accommodate the Rivnut, and a special cutter is used to cut a small notch in the circumference of the hole. This notch locks the Rivnut into the skin to prevent it from turning when it is used as a nut. A Rivnut of the proper grip length is then screwed onto the puller and inserted into the hole with its key aligned with the keyway cut in the hole. When the handle of the puller is squeezed, the hollow shank of the Rivnut upsets and grips the skin. The tool is then unscrewed from the Rivnut, leaving a threaded hole that accepts machine screws for attaching a de-icer boot. Rivnuts are now used in many areas on aircraft and in the automotive industry. Rivnuts 2023-02-15 B-06b Materials and Hardware Page 50 of 211 CASA Part 66 - Training Materials Only  Studs Studs Used in Aircraft Studs are metal rods which are threaded at each end. They are used where it is not desirable or possible to drill through both components for the fitting of bolts. One end of the stud is screwed, to the full extent of the thread, into a tapped hole in one component – the 'fast' end. Studs can have the same thread cut on either end but may have fine thread at one end and course thread at the other. A second component is placed onto the exposed plain portion of the stud and clamped by a nut. They also provide a means of alignment control, particularly when they are irregularly spaced. © Aviation Australia Stud 2023-02-15 B-06b Materials and Hardware Page 51 of 211 CASA Part 66 - Training Materials Only  Standard Studs Standard studs are supplied in the following sizes: 3/16, 1/4, 5/16 and 3/8 Unified National Fine. The plain portion is the same diameter as the major diameter of the thread and the length is indicated by the part number. The length of the threaded portion is dictated by the specification. Standard stud Waisted Stud The diameter of the plain portion of the waisted stud is reduced to the minor diameter of the threaded ends, making the stud lighter in weight without impairing its ultimate strength. Waisted stud 2023-02-15 B-06b Materials and Hardware Page 52 of 211 CASA Part 66 - Training Materials Only  Stepped Stud The stepped-type stud is made with one threaded end of larger diameter than the other. The large end screws into the unit, which is usually made of soft metal, providing greater holding power. Stepped studs are also used as replacements for damaged studs where the stud hole in the job, which may also have been damaged, has to be drilled and tapped to a larger diameter. Stepped stud Shouldered Stud The integral shoulder, machined on the plain portion of the stud, sits firmly on the surface of the job into which the stud is screwed, providing a more rigid assembly than could be obtained with the use of an ordinary stud. Shouldered stud Where greater depth of thread engagement is required, i.e. when using soft material, a coarse series thread may be employed at the fast (secured) end and a Fine Series thread at the clamping nut end of the stud. 2023-02-15 B-06b Materials and Hardware Page 53 of 211 CASA Part 66 - Training Materials Only  Insertion and Removal of Studs Stud Replacement A stud must be a good fit and should remain in position when the nut is removed. Studs that are damaged or loose are to be removed and new ones fitted. There are a number of accepted methods of stud replacement; some of the more common ones are detailed in the following. Note: If an anti-seize or locking compound is specified, this must be applied prior to replacement and in accordance with the manufacturer’s instructions. Aviation Australia Damaged studs 2023-02-15 B-06b Materials and Hardware Page 54 of 211 CASA Part 66 - Training Materials Only  Stud Replacement - Locknut Method Two plain nuts are screwed onto the top thread and locked against each other, the lower nut being held by a spanner while the upper nut is tightened down onto it. The complete assembly is screwed in using the top nut. When the stud is finally screwed down into position, both locknuts are removed and discarded. For removal, the two nuts are locked in the same way and the lower one turned to loosen the stud. Installation and removal of lock nuts 2023-02-15 B-06b Materials and Hardware Page 55 of 211 CASA Part 66 - Training Materials Only  Stud Replacement - Stud Box The stud box consists of a hexagonal body with two differently sized threads at each end and is suitable for the insertion of two sizes of studs. The stud box is screwed onto the stud and locked by a bolt. A soft metal disc between them is used to prevent damage to the stud and the locking bolt. The stud is then fitted by turning the box body with a suitable spanner. Stud box removal is effected by slackening the locking bolt and unscrewing the box body from the stud. Stud replacement - stud box 2023-02-15 B-06b Materials and Hardware Page 56 of 211 CASA Part 66 - Training Materials Only  Stud Replacement - Stud Tool The stud tool consists of a hollow body with a handle attached. The upper end is threaded to accommodate a locating screw; the other end contains internally machined cam faces. Located in this end is a cage containing three hardened steel rollers which are free to move radially within the limit of their axis holes in the cage. The cage assembly is retained within the body by an end plate. The stud to be inserted or extracted is passed through the hole in the end plate until the plain portion of the stud is positioned within the cage. The locating screw is adjusted to prevent further entry of the stud into the tool and prevent damage to the threads. When the tool body is rotated, the light frictional grip of the rollers on the stud shank causes them to rotate within the housing and ride round the cam faces. The rotating cam faces force the rollers inwards, thus providing a tight grip on the stud shank. The stud then turns with the tool in the direction of rotation. Partial rotation in the opposite direction will allow the rollers to disengage from the stud shank, thus permitting the tool to be removed. This tool is not suitable for waisted studs. Stud tools 2023-02-15 B-06b Materials and Hardware Page 57 of 211 CASA Part 66 - Training Materials Only  Stud Removal Loose or undamaged studs may be removed using locknuts (use a spanner on the lower nut), the stud tool or universal stud extractors. Universal stud extractors consist of a body machined to accommodate the square drive socket bush (for use with a ratchet handle or knuckle bar) and an eccentrically mounted knurled wheel. The body is bored to allow the insertion of the largest diameter stud of the tools range. The stud is placed in the extractor until the plain portion aligns with the knurled wheel, and a suitable handle is inserted in the square drive socket. Initial movement of the handle rotates the socket bush, forcing the knurled wheel to contact and grip the plain portion of the stud. Further movement of the handle will turn the extractor body and stud. Slight rotation in the opposite direction causes the knurled wheel to disengage from the stud shank, allowing the extractor to be removed. The stud-removal wrench is a one-piece tool that works on the same principle. Note: These tools damage the plain portion of the stud, which must be discarded after removal. Universal stud extractor 2023-02-15 B-06b Materials and Hardware Page 58 of 211 CASA Part 66 - Training Materials Only  Stud extractor tool Damaged Studs Studs damaged or broken above the surface of the component may be removed by one of the following methods: Unscrew stud with a suitable pipe wrench or stud removal tool. File flats on projecting part of stud and use an open-ended spanner or tap wrench to unscrew. Cut and file screwdriver slot in projecting part and unscrew with a screwdriver. Stud removal 2023-02-15 B-06b Materials and Hardware Page 59 of 211 CASA Part 66 - Training Materials Only  Sheared or Broken Studs For studs broken flush with or below the surface of the component, one of the following methods should be used: Drill out stud, tap over-size and fit stepped stud. Drill out stud, tap and fit threaded insert (or Twinsert). Twinsert Drill (minor diameter of stud), pick out old loose thread, re-tap to standard size – use only when accurate drilling and marking out facilities are available. Drill a hole approximately half the stud diameter, drive in a square or splined tapered drift, unscrew with a spanner – care must be taken not to expand the stud. Drill and tap the stud with a thread opposite that of the stud, insert a bolt and unscrew. 2023-02-15 B-06b Materials and Hardware Page 60 of 211 CASA Part 66 - Training Materials Only  Screws Aircraft Screw Applications Screws are probably the most commonly used threaded fastener in aircraft. They differ from bolts in that they are generally made of lower strength materials, but not always. Screws are typically installed with a loose-fitting thread, and the head shapes are made to engage a screwdriver or wrench. Some screws have a clearly defined grip length, while others are threaded along their entire length. Screw fasteners in a panel There are three basic classifications of screws used in aircraft construction: Machine screws, which are the most widely used Structural screws, which have the same strength as bolts Self-tapping screws, which are typically used to join lightweight materials. 2023-02-15 B-06b Materials and Hardware Page 61 of 211 CASA Part 66 - Training Materials Only  Machine Screws Machine screws are used extensively for attaching fairings, inspection plates, fluid line clamps and other light structural parts. The main difference between aircraft bolts and machine screws is that the threads of a machine screw usually run the full length of the shank, whereas bolts have an unthreaded grip length. Machine screws Screws normally have a Class 2, or free, fit and are available in both National Coarse and National Fine threads. The most common machine screws used in aviation are the fillister head screw, the flat- head screw, the round-head screw and the truss-head screw. 2023-02-15 B-06b Materials and Hardware Page 62 of 211 CASA Part 66 - Training Materials Only  Structural Screws Structural screws Structural screws are made of alloy steel, are heat treated and can be used as structural bolts. They have a definite grip and the same shear strength as a bolt of the same size. Shank tolerances are similar those of to AN hex-head bolts, and the threads are National Fine. Structural screws are available with fillister, flat or washer heads. These head types are not interchangeable. The correct screwdriver must be used to avoid damage to the screw head, especially to titanium screws. Never use a Philips screwdriver on a Torq-Set screw or a slotted screwdriver on a Hi-Torque screw. 2023-02-15 B-06b Materials and Hardware Page 63 of 211 CASA Part 66 - Training Materials Only  Self-Tapping Screws Self-tapping screws have coarse threads and are used to hold thin sheets of metal, plastic or plywood together. The Type A screw has a gimlet (sharp) point, and the Type B has a blunt point with threads that are slightly finer than those of a Type A screw. Self-tapping screws There are four types of heads available on self-tapping screws: Round head Truss head Countersunk head, which is flat on top Countersunk oval screw. The truss-head is rounded, similar to the round-head screw, but is considerably thinner. 2023-02-15 B-06b Materials and Hardware Page 64 of 211 CASA Part 66 - Training Materials Only  Washers Types of Washers 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. They are also used to adjust the position of castellated nuts with respect to drilled split pin holes in bolts as well as to apply tension between a nut and a material surface to prevent the nut from vibrating loose. The three most common types of washers used in airframe repair are the plain washer, lock washer and special washer. Washers 2023-02-15 B-06b Materials and Hardware Page 65 of 211 CASA Part 66 - Training Materials Only  Plain Washers All AN washers are in the 900 series. The AN960 plain washer provides a smooth surface between a nut and the material being clamped. These washers are made of cadmium-plated steel, commercial brass (B), corrosion-resistant steel (C) and 2024 aluminium alloy (D). They are available in sizes that range from those that fit a number 2 machine screw to those that fit a 1-in. bolt. If a thin washer is needed, a light series washer that is half the thickness of a regular washer is available. An example of when a light series washer should be used is if the castellations of an AN310 nut do not line up with a split pin hole. Plain washers When the nut is properly torqued, a light series washer can be substituted for the regular washer to align the holes. A light series washer is identified by the letter L added to the code. For example, the code ANO6OL identifies a light series washer. When working with wood or composite structures, washers with a large surface area are used to spread the fastener load over a wider area. These large-area washers carry the code AN970 and are all made of cadmium-plated steel with inside diameters from 3/16 to 1/2 in. 2023-02-15 B-06b Materials and Hardware Page 66 of 211 CASA Part 66 - Training Materials Only  Lock Washers In some instances, it is not convenient to use self-locking nuts or split pins on bolts. For these applications, a lock washer is often used between the nut and joint surface if the joint is not structurally critical. Lock washers are made of steel and are twisted so that when a nut is tightened against them, the spring action of the washer creates a strong friction force between the bolt threads and those in the nut. Two types of lock washers are used in aircraft construction. The most common is the AN935 split lock washer. These washers are available in sizes that fit from a number 4 machine screw to a 1/2-in. bolt. The second type of lock washer is the thinner AN936 shakeproof lock washer, which is available with both internal and external teeth. Split washer Shakeproof washer 2023-02-15 B-06b Materials and Hardware Page 67 of 211 CASA Part 66 - Training Materials Only  Special Washers Some high-strength internal wrenching bolts have a radius between their shaft and the underside of the bolt head. To provide a tight mating surface, MS20002C countersunk washers are used under the heads of internal wrenching bolts. These washers have a countersunk edge to accommodate the radius on the bolt head. Countersunk washers are made of heat-treated steel and are cadmium plated. Radiused washer Finishing washers are often used in aircraft interiors to secure upholstery and trim. These washers have a countersunk face to accommodate flush screws. Finishing washers bear against a large area to avoid damaging fragile interior components. In many instances, keyed washers can be used as a safety device. Keyed washers have small keys or protrusions to engage slots cut into bolts or panels. 2023-02-15 B-06b Materials and Hardware Page 68 of 211 CASA Part 66 - Training Materials Only  Clevis Pins A clevis pin is used in conjunction with tie-rod terminals and secondary controls which are not subject to continuous operation. Clevis pins are secured with a split pin or an AN416 safety pin. A washer must be placed under the split or safety pin. The pin is normally installed with the head up or forward. This prevents loss should the split pin fail or work out. Clevis pin slots Clevis pin locking mechanisms - safety pin and split pin 2023-02-15 B-06b Materials and Hardware Page 69 of 211 CASA Part 66 - Training Materials Only  Dowels Types of Dowels A dowel is a solid cylindrical rod, usually made of wood, plastic or metal. A dowel pin is cut from ‘dowel rod’. Dowels are used where the precision alignment and correct orientation of two mating surfaces is required. These are the types of dowels used in aircraft: Smooth solid dowels Seamless hollow dowels Threaded dowels Split hollow dowels. Threaded Dowels The threaded portion is permanently attached to one of the two mating components. Aviation Australia Threaded dowels 2023-02-15 B-06b Materials and Hardware Page 70 of 211 CASA Part 66 - Training Materials Only  Smooth Solid Dowels Smooth solid dowels are usually steel dowel pins made with high-quality metallic products, ensuring smooth surface finishing and high performance. Smooth solid dowels 2023-02-15 B-06b Materials and Hardware Page 71 of 211 CASA Part 66 - Training Materials Only  Seamless Hollow Dowels Hollow dowels are designed to be a direct replacement for solid ground dowels in alignment applications. These lighter subcomponents provide the same positional accuracy as solid ground dowels, and require the same hole tolerances for proper installation and retention. Some varieties of hollow dowels are 50% lighter than a solid dowel. Hollow dowels may also be used in applications where a bolt or stud needs to pass through the material that the dowel is installed. Aviation Australia Seamless hollow dowels 2023-02-15 B-06b Materials and Hardware Page 72 of 211 CASA Part 66 - Training Materials Only  Split Hollow Dowels A split hollow dowel may also be termed, a roll pin or spring pin. Roll pins are often used to provide locking for a joint where the pin is not likely to be removed or to lock something onto a shaft such as a handle or lever. A roll pin is made of flat spring steel that is rolled into a cylinder but the two ends are not joined. This allows the pin to compress when it is pressed into a hole and create a spring action that holds the pin tight against the edge of the hole. To remove a roll pin, it must be driven from a hole with a proper size pin punch. Use of a split or seamless hollow dowel enables the joint to be secured by a bolt though the hollow centre. Aviation Australia Split hollow dowels 2023-02-15 B-06b Materials and Hardware Page 73 of 211 CASA Part 66 - Training Materials Only  Applications of Dowel Pins Dowels are used to ensure accurate location of mating surfaces, for example, engine and gearbox casing flanges, or the correct alignment and orientation of a propeller to a propeller shaft flange. Dowel pins for locating a propeller on the propeller shaft flange In some instances, a hollow dowel is used not only to maintain alignment and orientation, but also act as a bushing for a rotating component – see the top-right of the image below. In this application - a shouldered bolt would be required to prevent the split dowel from being clamped rigid - enabling the wheel to spin. In the example shown in the top-left of the image below, the hollow dowel is limiting the compression applied to the clamped components. This prevents the softer or more brittle materials from being crushed. 2023-02-15 B-06b Materials and Hardware Page 74 of 211 CASA Part 66 - Training Materials Only  Applications for dowel pins 2023-02-15 B-06b Materials and Hardware Page 75 of 211 CASA Part 66 - Training Materials Only  Locking Devices (6.5.3) Learning Objectives 6.5.3.1 Describe tab washers (Level 2). 6.5.3.2 Describe spring washers (Level 2). 6.5.3.3 Describe locking plates (Level 2). 6.5.3.4 Describe split pins (Level 2). 6.5.3.5 Describe pal-nuts (Level 2). 6.5.3.6 Describe wire locking (Level 2). 6.5.3.7 Describe quick release fasteners (Level 2). 6.5.3.8 Describe keys (Level 2). 6.5.3.9 Describe circlips (Level 2). 6.5.3.10 Describe cotter pins (Level 2). 2023-02-15 B-06b Materials and Hardware Page 76 of 211 CASA Part 66 - Training Materials Only  Safety and Locking Mediums Aviation Locking / Safety Mediums Vibration is always present during aircraft operation. Consequently, there must be provision for safetying or locking all fasteners to prevent them vibrating loose in flight. There are various methods of safetying and locking. The most widely used methods are safety wire, split pins, lock washers, circlips and special nuts such as self-locking nuts, pal nuts and jam nuts. Lockwire used on bolt heads 2023-02-15 B-06b Materials and Hardware Page 77 of 211 CASA Part 66 - Training Materials Only  Lock Washers In some instances, it is not convenient to use self-locking nuts or split pins on bolts. For these applications, a lock washer is often used between the nut and joint surface if the joint is not structurally critical. Lock washers are made of steel and are twisted so that when a nut is tightened against it, the spring action of the washer creates a strong friction force between the bolt threads and those in the nut. Lock washers The AN935 lock washer may be used between the nut and the surface if the joint is not structurally critical. The AN936 shake-proof washer is thinner than the AN935 lock washer and is available with both internal and external teeth. 2023-02-15 B-06b Materials and Hardware Page 78 of 211 CASA Part 66 - Training Materials Only  Tab Washers Tab washers are often used for locking hex-head fasteners. A tab must not be bent more than once. You can re-use multiple tab washers after removing the used tab, dressing sharp edges and carefully inspecting the remaining tabs for cracks or scoring. Tab washers 2023-02-15 B-06b Materials and Hardware Page 79 of 211 CASA Part 66 - Training Materials Only  Lockwire Because aircraft vibrate, there must be some provision for safetying or locking all fasteners to keep them from vibrating loose. Self-locking nuts are used for the vast majority of applications in modern aircraft construction, but there are still places where lockwire or split pins are needed. For example, drilled-head bolts are often used in vibration-prone areas and are safety-wired together. Lock-wiring is a means of securing hardware and components and is a safety method employed in aircraft maintenance procedures. The type of lockwire most commonly used is made of stainless steel. When installing lockwire, the wire should pull the bolt head in the direction of tightening and should be twisted evenly to the next bolt. After the end of the wire is passed through the head of the second bolt, it is again twisted, this time for about three or four turns. Once this is done, the excess is cut off and the ends of the wire are bent back, where they cannot cut anyone who passes their hand over the bolts. Lock-wired bolt heads Lock-wiring is often used in critical areas, where inspection intervals may be infrequent. 2023-02-15 B-06b Materials and Hardware Page 80 of 211 CASA Part 66 - Training Materials Only  Lock-Wiring Methods Hand Lock-Wiring Method The double twist is the most commonly used method of lock-wiring. © Aviation Australia Lockwiring by hand When performing the task by hand, pull firmly. Start the twist in close to the fastener, hold the ends about 90° apart and twist in a clockwise direction (for RH threads). Apply 6 to 8 twists per inch. Avoid exceeding 8 twists per inch because this causes the wire to stretch, work-harden, and become brittle. In areas where a number of bolts must be safetied, such as a propeller, you may safety-wire the bolts in groups of three. If more than three bolts are safetied together, it is difficult to get the safety wire tight enough to be effective. The single-wire method is used on screws, bolts and nuts in a closely spaced or closed-geometrica