Par 7-14 Page 7-292 Monel rivets are used in special cases for rive ting high-nickel steel alloys and nickel alloys. They may be used interchangeably with stainless steel rivets as they are more easily driven. However, it is preferable to use stainless steel rivets in stainless steel parts.
Copper rivets are used for riveting copper alloys, leather, and other nonmetallic materi als. This rivet has only limited usage in aircraft.
Hi-Shear rivets are sometimes used in connections where the shearing loads are the pr imary design consideration. Its use is restricted to such connections. It should be noted that Hi-Shear rivets are not to be used for the installation of control surface hinges and hinge brackets. Do not paint the rivets before a ssembly, even where dissimilar metals
Par 7-14 Page 7-293 are being joined. However, it is advisa ble to touch up each end of the driven rivet with primer to allow the later application of the general airplane finish.
Blind rivets in the MS20600 through MS20603 series rivets and the mechanically-locked stem NAS 1398, 1399, 1738, and 1739 rivets sometimes may be substituted for solid rivets. They should not be used wh ere the looseness or failure of a few rivets will impair the airworthiness of the aircraft. Design allowables for blind rivets are specified in MIL-HDBK-5. Specific structural applicati ons are outlined in MS33522. Nonstruc tural applications for such blind rivets as MS20604 and MS20605 are contained in MS33557.
7-3.7-13. [RESERVED.]
Par 7-14 Page 7-294 SECTION 2. SCREWS
GENERAL. In general, screws differ from bolts by the following characteristics.
Screws usually have lower material strength, a looser thread fit, head shapes form ed to engage a screwdriver, and the shank may be threaded along its entire length without a clearly defined grip. Screws may be divided into three basic groups: structural screws, machine screws, and self-tapping screws . Screws are marked as required by the applicable Army Navy (AN), National Aerospace Standard (NAS), or Military Standard (MS) drawing. Normally a manufacturer places his trademark on the head of the screw. Several types of structural screws are available that differ from the standard structural bolts only in the type of head.
It would be impossible to cover all screws that are available to the aviation market; therefore, only the most frequently used screws will be discussed in this text. Design specifications are available in MIL-HDBK-5, or U.S.A.F./Navy T.O.1-1A-8/NAVAIR 01-1A-8, Structural Hardware.
Typical screw types are shown in table 7-11.
STRUCTURAL SCREWS. NAS502, NAS503, AN509, NAS220 through NAS227, and NAS583 through NAS590, may be used for structural applications, similar to structural bolts or rivets. These screws are fabricated from a material with a high-tensile strength and differ from structural bolts only in the type of head.
MACHINE SCREWS. These screws are available in four basic head styles: flathead (countersunk), roundhead, fillister, and socket head.
Flathead machine screws (AN505, AN510, AN507, NAS200, NAS514, NAS517, and NAS662) are used in countersunk holes where a flush surface is desired.
Roundhead machine screws (AN515 and AN520) are general-purpose screws for use in nonstructural applications.
Fillister head machine screws (AN500 through AN503, AN116901 through AN116912, AN116913 through AN116924, AN116962 through AN116990, AN117002 through AN117030, and AN117042 through AN117070) are general-purpose screws that may be used as capscrews in light mechanical applications and are usually drilled for safety wire.
Socket head machine screws (NAS608 and NAS609) are designed to be driven into tapped holes by means of internal wrenches. They ma y be used in applications requiring high strength, compactness of assembled parts, or sinking of heads below surfaces into fitted holes.
PANHEAD SCREWS (NAS600 THROUGH NAS606, NAS610 THROUGH NAS616, NAS623, AND NAS1402 THROUGH NAS1406). Flathead screws (MS35188 through MS35203), panhead machine screws (MS35024 through MS35219), and truss-head screws (AN526) are generalpurpose screws used where head height is not important.
SELF-TAPPING SCREWS. The self-tapping screw taps their own mating thread when driven into untapped or punched holes slightly smaller than the diameter of the screw. Selftapping machine screws (AN504 and AN530), may be used to attach minor
Par 7-34 Page 7-295 nonstructural parts. Self-tapping sheet metal screws (AN504, AN530, AN531 and NAS548) may be used in blind applications for the temporary attachment of sheet meta l for riveting and the permanent assembly of nonstructural assemblies. The MS21318 is a roundhead drive screw used in the attachment of nameplates or in sealing drain holes, and is not intended to be removed after installation. They are normally installed by driving the screw into a drilled hole with a hammer.
Par 7-34 Page 7-296 CAUTION: Self-tapping screws should never be used as a replacement for standard screws, nuts, bolts, or rivets in any aircraft structure.
WOOD SCREWS AN545 and AN550, MS35492 and MS35493 are screws used in wood structures of aircraft.
7-20.7-33. [RESERVED.]
Par 7-34 Page 7-297 SECTION 3. BOLTS
GENERAL. “Hardware” is the term used to describe the various types of fasteners and small items used to assemble and repair aircraft structures and components. Only hardware with traceability to an approved manufacturing process or source should be used. This traceability will ensure that the hardware is at least equal to the original or properly-altered condition. Hardware that is not traceable or is improperly altered, may be substandard or counterfeit, since their physical properties cannot be substantiated. Selection and use of fasteners are as varied as the types of aircraft; therefore, care should be taken to ensure fasteners are approved by the Federal Aviation Administration (FAA) for the intended installation, repair, or replacement. Threaded fasteners (bolts/screws) and rivets are the most commonly used fasteners because they are designed to carry shear and/or tensile loads.
BOLTS. Most bolts used in aircraft structures are either general-purpose, internal-wrenching, or close-tolerance AN, NAS, or MS bolts. In certain cases, fastener manufacturers produce bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it is of extreme importance to use like bolts in replacement. Design specifications are available in MIL-HDBK-5 or USAF/Navy T.O. 1-1A-8/NAVAIR 01-1A-8. References should be made to military specifications and industry design standards such as NAS, the Society of Automotive Engineers (SAE), and Aerospace Material Standards (AMS). Typical bolt types are shown in table 7-12.
IDENTIFICATION. Aircraft bolts may be identified by code markings on the bolt heads.
These markings generally denote the material of which the bolt is made, whether the bolt is a standard AN-type or a special-purpose bolt, and sometimes include the manufacturer.
AN standard steel bolts are marked with either a raised dash or asterisk, corrosion-resi stant steel is marked by a single dash, and AN aluminum-alloy bolts are marked with two raised dashes.
Special-purpose bolts include high-strength, lowstrength, and close-tolerance types. These bolts are normally inspected by magnetic particle inspection methods. Typical markings include “SPEC” (usually heat-treated for strength and durability), and an aircraft manufacturer’s part number stamped on the head. Bolts with no markings are low strength. Close-tolerance NAS bolts are marked with e ither a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except th ey may be either raised or recessed. Bolts requiring non-destructive inspection (NDI) by magnetic particle inspection are identified by means of colored lacquer, or head markings of a distinctive type. (See figure 7-1.)
GRIP LENGTH. In general, bolt grip lengths of a fastener is the thickness of the material the fastener is designed to hold when two or more parts are being assembled. Bolts of slightly greater grip length may be used, provided washers are placed under the nut or bolthead.
The maximum combined height of washers that should be used is 1/8 inch. This limits the use of washers necessary to compensate for grip, up to the next standard grip size. Over the years, some fasteners specifications have been changed. For this reason, it is recommended when making repairs to an aircraft, whose original hardware is being replaced, that you must first measure the bolt before ordering, rather than relying on the parts manual for
Par 7-40 Page 7-298
FIGURE 7-1. Typical aircraft bolt markings.
identification. In the case of plate nuts, if proper bolt grip length is not available, add shims under the plate.
All bolt installations which involve self-locking or plain nuts should have at least one thread of the bolt protruding through the nut.
LOCKING OR SAFETYING OF BOLTS. Lock or safety all bolts and/or nuts, except self-locking nuts. Do not reuse cotter pins or safety wire.
BOLT FIT. Bolt holes, particularly those of primary connecting elements, have close tolerances. Generally, it is permissible to use the first-lettered drill size larger than the nominal bolt diameter, except when the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS closetolerance bolts or AN clevis bolts are used. A light-drive fit can be defined as an interference of 0.0006 inch for a 5/8 inch bolt. Bolt holes should be flush to the surface, and free of debris to provide full bearing surface for the bolt head and nut. In the event of over-sized or elongated holes in structural members, reaming or drilling the hole to accept the next larger bolt size may be permissible. Care should be taken to ensure items, such as edge distance, clearance, and structural integrity are maintained. Consult the manufacturer’s structural repair manual, the manufacturer’s engineering department, or the FAA before drilling or reaming any bolt hole in a critical structural member.
TORQUES. The importance of correct torque application cannot be overemphasized.
Undertorque can result in unnecessary wear of nuts and bolts, as well as the parts they secure. Overtorque can cause failure of a bolt or nut from overstressing the threaded areas.
Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple, but important procedures, that should be followed to ensure that correct torque is applied.
NOTE: Be sure that the torque applied is for the size of the bolt shank not the wrench size.
Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.
Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.
Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.
Par 7-40 Page 7-299 Add the friction drag torque to the desired torque.
This is referred to as “final torque, ” which should register on the indicator or setting for a snap-over type torque wrench.
Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs during final torque, back off the nut and retorque.
NOTE: Many applications of bolts in aircraft/engines require stretch checks prior to reuse. This requirement is due primarily to bolt stretching caused by overtorquing.
When installing a castle nut , start alignment with the cotter pin hole at the minimum recommended torque plus friction drag torque.
NOTE: Do not exceed the maximum torque plus the friction drag. If the hole and nut castellation do not align, change washer or nut and try again. Exceeding the maximum recommended torque is not recommended.
When torque is applied to bolt heads or capscrews, apply the recommended torque plus friction drag torque.
If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly. Determine the torque wrench indication or setting with adapter installed as shown in figure 7-2.
Table 7-1 shows the recommended torque to be used when specific torque is not supplied by the manufacturer. The table includes standard nut and bolt combinations, currently used in aviation maintenance.
For further identification of hardware, see chapter 7, section 11.
STANDARD AIRCRAFT HEX HEAD BOLTS (AN3 THROUGH AN20). These are all-purpose structural bolts used for general applications that require tension or shear loads. Steel bolts smaller than No. 10-32, and aluminum alloy bolts smaller than 1/4 inch diameter, should not be used in primary structures. Do not use aluminum bolts or nuts in applications requiring frequent removal for inspection or maintenance.
DRILLED HEAD BOLTS (AN73 THROUGH AN81). The AN drilled head bolt is similar to the standard hex bolt, but has a deeper head which is drilled to receive safety wire. The physical differences preventing direct interchangeability are the slightly greater head height, and longer thread length of the AN73 through AN81 series. The AN73 through AN81 drilled head bolts have been superseded by MS20073, for fine thread bolts and MS20074 for coarse thread bolts. AN73, AN74, MS20073, and MS20074 bolts of like thread and grip lengths are universally, functionally, and dimensionally interchangeable.
ENGINE BOLTS. These are hex head bolts (AN101001 through AN101900), drilled shank hex head bolts (AN101901 through AN102800), drilled hex head (one hole) bolts (AN102801 through AN103700), and drilled hex head (six holes) bolts (AN103701 through AN104600).
They are similar to each other except for the holes in the head and shank. Hex head bolts (AN104601 through AN105500), drilled shank hex head bolts (AN105501 through AN106400), drilled hex head (one hole) bolts (AN106401 through AN107300), and drilled hex head (six holes) bolts (AN107301 through AN108200) are similar to the bolts described in paragraph 7-42, except that this series is manufactured from corrosion-resistant steel.
NOTE
WHEN USING A TORQUE WRENCH ADAPTER WHICH
CHANGES THE DISTANCE FROM THE TORQUE
WRENCH DRIVE TO THE ADAPTER DRIVE, APPLY
THE FOLLOWING FORMULAS TO OBTAIN THE CORRECTED TORQUE READING:
Short Open End Adapter TORQUE
ADAPTER WRENCH
DRIVE HANDGRIP
CENTERLINE CENTERLINE (PREDETERMINED) WRENCH DRIVE
CENTERLINE Set Screw Adapter FORMULA 1E = Y EXAMPLE: (WITH "E" AS PLUS DIMENSION) T = 135 LB. IN. V = 10+1.5 = 11.5 = 117.39 135 × 10 Y = UNKNOWN Hose Clamp Adapter L = 10.0 IN.
E = 1.5 IN. Y = 117 LB. IN.
- LEGEND.
T = ACTUAL (DESIRED) TORQUE Y = APPARENT (INDICATED) TORQUE L = EFFECTIVE LENGTH LEVER E = EFFECTIVE LENGTH OF EXTENSION Open-End Wrench TORQUE WRENCH Adapter
WRENCH HANDGRIP
DRIVE CENTERLINE
CENTERLINE ADAPTER (PREDETERMINED) DRIVE _ CENTERLINE FORMULA IxL L-E = Y EXAMPLE: (WITH "E" AS MINUS DIMENSION) T = 135 LB. IN. Y = 10 - 1.5 = 1350 = 158.82 135 × 10 Y = UNKNOWN Spanner Wrench L = 10.0 IN. Adapter E = 1.5 IN. Y = 159 LB. IN.
FIGURE 7-2. Torque wrench with various adapters.
Par 7-40 Page 7-300
Par 7-43 Page 7-301 TABLE 7-1. Recommended torque values (inch-pounds).
CAUTION THE FOLLOWING TORQUE VALUES ARE DERIVED FROM OIL FREE CADMIUM PLATED THREADS.
TORQUE LIMITS RECOMMENDED FOR
INSTALLATION (BOLTS LOADED PRIMARILY IN SHEAR)
MAXIMUM ALLOWABLE TIGHTENING
TORQUE LIMITS Thread Size
Tension type nuts MS20365 and AN310 (40,000 psi in bolts) Shear type nuts MS20364 and AN320 (24,000 psi in bolts) Nuts MS20365 and AN310 (90,000 psi in bolts) Nuts MS20364 and AN320 (54,000 psi in bolts)
FINE THREAD SERIES 8-36 10-32 1/4-28 5/16-24 3/8-24 7/16-20 1/2-20 9/16-18 5/8-18 3/4-16 7/8-14 1-14 1-1/8-12 1-1/4-12 12-15 20-25 50-70 100-140 160-190 450-500 480-690 800-1000 1100-1300 2300-2500 2500-3000 3700-5500 5000-7000 9000-11,000 7-9 12-15 30-40 60-85 95-110 270-300 290-410 480-600 600-780 1300-1500 1500-1800 2200-3300* 3000-4200* 5400-6600* 20 40 100 225 390 840 1100 1600 2400 5000 7000 10,000 15,000 25,000 12 25 60 140 240 500 660 960 1400 3000 4200 6000 9000 15,000
COARSE THREAD SERIES 8-32 10-24 1/4-20 5/16-18 3/8-16 7/16-14 1/2-13 9/16-12 5/8-11 3/4-10 7/8-9 12-15 20-25 40-50 80-90 160-185 235-255 400-480 500-700 700-900 1150-1600 2200-3000 7-9 12-15 25-30 48-55 95-100 140-155 240-290 300-420 420-540 700-950 1300-1800 20 35 75 160 275 475 880 1100 1500 2500 4600 12 21 45 100 170 280 520 650 900 1500 2700 The above torque values may be used for all cadmium-plat ed steel nuts of the fine or coarse thread series which have approximately equal number of threads and equal face bearing areas.
* Estimated corresponding values.
CLOSE-TOLERANCE BOLTS.
Close-tolerance, hex head, machine bolts (AN173 through AN186), 100-degree countersunk head, closetolerance, high-strength bolts (NAS333 through NAS340), hex head, close-tolerance, short thread, titanium alloy bolts (NAS653 through NAS658), 100-degree countersunk flathead, close-tolerance titanium alloy bolts (NAS663 through NAS668), and drilled hex head close-tolerance titanium alloy bolts (NAS673 through NAS678), are used in applications where two parts bolted together are subject to severe load reversals and vibration. Because of the interference fit, this type of bolt may require light tapping with a mallet to set the bolt shank into the bolt hole.
NOTE: Elimination of friction in interference fit applications may sometimes be attained by placing the bolt in a freezer prior to installation. When this procedure is used, the bolt should be allowed to warm up to ambient temperature before torquing.
CAUTION: Caution must be exercised in the use of close-tolerance bolts for all critical applications, such as
Par 7-63 Page 7-302 landing gear, control systems, and he licopter rotary controls. Do no t substitute for close-tolerance fasteners without specific instructions from the aircraft manufacturer or the FAA.
INTERNAL WRENCHING BOLTS (NAS144 THROUGH NAS158 AND NAS172 THROUGH NAS176). These are high-strength bolts used primarily in tension applications. Use a special heattreated washer (NAS143C) under the head to prevent the large radius of the shank from contacting only the sharp edge of the hole. Use a special heat-treated washer (NAS143) under the nut.
INTERNAL WRENCHING BOLTS (MS20004 THROUGH MS20024) AND SIX HOLE, DRILLED SOCKET HEAD BOLTS (AN148551 THROUGH AN149350). These are very similar to the bolts in paragraph 7-45, except these bolts are made from different alloys. The NAS144 through NAS158 and NAS172 through NAS176 are interchangeable with MS20004 through MS20024 in the same thread configuration and grip lengths. The AN148551 through AN149350 have been superseded by MS9088 through MS9094 with the exception of AN149251 through 149350, which has no superseding MS standard.
TWELVE POINT, EXTERNAL WRENCHING BOLTS, (NAS624 THROUGH NAS644). These bolts are used primarily in high-tensile, high-fatigue strength applications. The twelve point head, heatresistant machine bolts (MS9033 through
Par 7-63 Page 7-303 MS9039), and drilled twelve point head machine bolts (MS9088 through MS9094), are similar to the (NAS624 through NAS644); but are made from different steel alloys, and their shanks have larger tolerances.
CLOSE-TOLERANCE SHEAR BOLTS (NAS464). These bolts are designed for use where stresses normally are in shear only. These bolts have a shorter thread than bolts designed for torquing.
NAS6200 SERIES BOLTS. These are close tolerance bolts and are available in two oversized diameters to fit slightly elongated holes. These bolts can be ordered with an “X” or “Y” after the length, to designate the oversized grip portion of the bolt (i.e., NAS6204-6X for a 1/4 inch bolt with a 1/64 inch larger diameter). The elongated hole may have to be reamed to insure a good fit.
CLEVIS BOLTS (AN21 THROUGH AN36). These bolts are only used in applications subject to shear stress, and are often used as mechanical pins in control systems.
EYEBOLTS (AN42 THROUGH AN49). These bolts are used in applications where external tension loads are to be applied. The head of this bolt is specially designed for the attachment of a turnbuckle, a clevis, or a cable shackle. The threaded shank may or may not be drilled for safetying.
7-52.7-62. [RESERVED.]