Par 7-64 Page 7-13 TABLE 7-2. MINIMUM PREVAILING TORQUE VALUES FOR REUSED
SELF-LOCKING NUTS
FINE THREAD SERIES
THREAD SIZE MINIMUM PREVAILING TORQUE 7/16-20 8 inch-pounds 1/2-20 10 inch-pounds 9/16-18 13 inch-pounds 5/8-18 18 inch-pounds 3/4-16 27 inch-pounds 7/8-14 40 inch-pounds 1-14 55 inch-pounds 1-1/8-12 73 inch-pounds 1-1/4-12 94 inch-pounds
COARSE THREAD SERIES
THREAD SIZE MINIMUM PREVAILING TORQUE 7/16-14 8 inch-pounds 1/2-13 10 inch-pounds 9/16-12 14 inch-pounds 5/8-11 20 inch-pounds 3/4-10 27 inch-pounds 7/8-9 40 inch-pounds 1-8 51 inch-pounds 1-1/8-8 68 inch-pounds 1-1/4-8 88 inch-pounds
g. Self-locking nut plates are produced in a variety of forms and materials for riveting or welding to aircraft structures or parts. Certain applications require the installation of self-locking nuts in channel arrangement permitting the attachment of many nuts in a row with only a few rivets.
7-65. NUT IDENTIFICATION FINISHES. Several types of finishes are used on self-locking nuts. The particular type of finish is dependent on the application and temperature requirement.
The most commonly used finishes are described briefly as follows.
a. Cadmium-Plating. This is an electrolytically deposited silver-gray plating which provides exceptionally good protection against corrosion, particularly in salty atmosphere, but is
Par 7-65 Page 7-14 not recommended in applications where the temperature exceeds 450 ºF. The following additional finishes or refinements to the basic cadmium can be applied.
(1) Chromic Clear Dip. Cadmium surfaces are passivated, and cyanide from the plating solution is neutralized. The protective film formed gives a bright, shiny appearance, and resists staining and finger marks.
(2) Olive Drab Dichromate. Cadmium-plated work is dipped in a solution of chromic acid, nitric acid, acetic acid, and a dye which produces corrosion resistance.
(3) Iridescent Dichromate. Cadmium-plated work is dipped in a solution of sodium dichromate and takes on a surface film of basic chromium chromate which resists corrosion.
Finish is yellow to brown in color.
NOTE: Cadmium-plated nuts are restricted for use in temperatures not to exceed 450 ºF. When used in temperatures in excess of 450 ºF, the cadmium will diffuse into the base material causing it to become very brittle and subject to early failure.
b. Silver plating. Silver plating is applied to locknuts for use at higher temperatures.
Important advantages are its resistance to extreme heat (1,400 ºF) and its excellent lubricating characteristics. Silver resists galling and seizing of mating parts when subjected to heat or heavy pressure.
c. Anodizing for Aluminum. An inorganic oxide coating is formed on the metal by connecting the metals and anodes in a suitable electrolyte. The coating offers excellent corrosion resistance and can be dyed in a number of colors.
d. Solid Lubricant Coating. Locknuts are also furnished with molybdenum disulfide for lubrication purposes. It provides a clean, dry, permanently-bonded coating to prevent seizing and galling of threads. Molybdenum disulfide is applied to both cadmium and silver-plated parts.
Other types of finishes are available, but the finishes described in this chapter are the most widely used.
7-66. CASTLE NUT (AN310). The castle nut is used with drilled shank hex head bolts, clevis bolts, drilled head bolts, or studs that are subjected to tension loads. The nut has slots or castellations cut to accommodate a cotter pin or safety wire as a means of safetying.
7-67. CASTELLATED SHEAR NUT (AN320). The castellated shear nut is designed for use with hardware subjected to shear stress only.
7-68. PLAIN NUT (AN315 AND AN335). The plain nut is capable of withstanding large tension loads; however, it requires an auxiliary locking device, such as a checknut or safety wire.
Use of this type on aircraft structures is limited.
7-69. LIGHT HEX NUTS (AN340 AND AN345). These nuts are used in nonstructural applications requiring light tension. Like the AN315 and AN335, they require a locking device to secure them.
Par 7-70 Page 7-15
7-70. CHECKNUT (AN316). The checknut is used as a locking device for plain nuts, screws, threaded rod ends, and other devices.
7-71. WINGNUTS (AN350). The wingnut is used where the desired torque is obtained by use of the fingers or hand tools. Wingnuts are normally drilled to allow safetying with safety wire.
7-72. SHEET SPRING NUTS (AN365). Sheet spring nuts are commonly called speed nuts.
They are used with standard and sheet metal self-tapping screws in nonstructural applications.
They are used to support line and conduit clamps, access doors, etc. Their use should be limited to applications where they were originally used in assembly of the aircraft.
7-73.–7-84. RESERVED.
Par 7-85 Page 7-16 SECTION 5. WASHERS
7-85. GENERAL. The type of washers used in aircraft structure are plain washers and special washers. Typical washer types are shown in Table 7-14.
7-86. PLAIN WASHERS (AN960 AND AN970). Plain washers are widely used with hex nuts to provide a smooth bearing surface, act as a shim to obtain the proper grip length, and to position castellated nuts in relation to drilled cotter pin holes in bolts. Use plain washers under lock washers to prevent damage to bearing surfaces. Cadmium-plated steel washers are recommended for use under bolt heads and nuts used on aluminum alloy or magnesium structures to prevent corrosion. The AN970 steel washer provides a larger bearing surface than the plain type and is often used in wooden structures under bolt heads and nuts to prevent local crushing of the surface.
7-87. LOCK WASHERS (AN935 AND AN936). Lock washers may be used with machine screws or bolts whenever the self-locking or castellated type nut is not applicable. Do not use lock washers where frequent removal is required, in areas subject to corrosion, or in areas exposed to airflow. Use a plain washer between the lock washer and material to prevent gouging the surface of the metal.
CAUTION: Lock washers are not to be used on primary structures, secondary structures, or accessories where failure might result in damage or danger to aircraft or personnel.
7-88. BALL SOCKET AND SEAT WASHERS (AN950 AND AN955). Ball socket and seat washers are used in special applications where the bolt is installed at an angle to the surface or when perfect alignment with the surface is required. These washers are used together as a pair.
7-89. TAPER PIN WASHERS (AN975). Taper pin washers are used with the threaded taper pin. NAS143 and MS20002 washers are used with NAS internal wrenching bolts and internal wrenching nuts. They may be plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolt head shank radius, and the plain washer is used under the nut.
7-90.–7-100. [RESERVED.]
Par 7-101 Page 7-17 SECTION 6. PINS
7-101. TAPER PINS. Plain (AN385) and threaded (AN386) taper pins are used in joints which carry shear loads and where the absence of play is essential. The plain taper pin is usually drilled and secured with wire. The threaded taper pin is used with a taper pin washer (AN975) and shear nut (safetied with a cotter pin) or self-locking nut (if undrilled). Typical pin types are shown in Table 7-15.
7-102. FLATHEAD PINS (AN392 THROUGH AN406). Commonly called a clevis pin, this pin is used in conjunction with tie-rod terminals and in secondary controls which are not subject to continuous operation. The pin is normally installed with the head up, or forward, to prevent loss should the cotter pin fail or work out.
7-103. COTTER PINS (AN380). Cotter pins are used for securing bolts, screws, nuts, and pins.
Use AN381 or MS24665 cotter pins in locations where nonmagnetic material or resistance to corrosion is desired. Cotter pins should not be reused.
7-104. SPRING PINS. The spring pin is designed for use in double-shear applications. The pins are manufactured with the diameter greater than the holes in which they are to be used. Spring pins are stronger than mild carbon steel straight pins, taper pins, or grooved pins of the equivalent size. The spring pin is compressed as it is driven into the hole and exerts continuous spring pressure against the sides of the hole to prevent loosening by vibration. Spring pins require no other means of securing and can be used inside one another to increase shear strength.
a. Be careful when using these pins, since spring-pin performance depends entirely on the fit and the durability of the fit under vibration or repeated load conditions (especially in soft materials, such as aluminum alloys and magnesium). They should not be used in an aircraft component or system where the loss or failure of the pin might endanger safe flight.
b. The joints where spring pins are used for fastening shall be designed like riveted and bolted joints. Spring pins should not be mixed with other structural fasteners in the same joint.
These pins, for primary structural applications, should be used only where there will be no rotation or relative movement of the joint. Spring pins may be reused if a careful inspection reveals no deformation of the pin or hole.
c. Be careful to observe that the hole has not enlarged or deformed preventing proper functioning of the spring pin. Where hole misalignment results in the pin gap closing or necessitates excess inserting force, the spring pin will not be used. The spring pin should not be used as a substitute for a cotter pin. When a spring pin is used in a clevis joint, it is recommended that the pin be held by the outer members of the unit for maximum efficiency and reduced maintenance.
7-105. QUICK-RELEASE PINS. These pins are used in some applications where rapid removal and replacement of equipment is necessary. When equipment is secured with these pins, no binding of the spindle should be present. Spindle binding could cause the locking balls to remain in the open position which could result in the pin falling out under vibration.
7-106.–7-121. [RESERVED.]
Par 7-122 Page 7-18 SECTION 7. SAFETYING
7-122. GENERAL. The word safetying is a term universally used in the aircraft industry.
Briefly, safetying is defined as, “Securing by various means any nut, bolt, turnbuckle etc., on the aircraft so that vibration will not cause it to loosen during operation.” These practices are not a means of obtaining or maintaining torque, rather a safety device to prevent the disengagement of screws, nuts, bolts, snap rings, oil caps, drain cocks, valves, and parts. Three basic methods are used in safetying: safety wire, cotter pins, and self-locking nuts. Retainer washers and pal nuts are also sometimes used.
a. Wire, either soft brass or steel, is used on cylinder studs, control cable turnbuckles, and engine accessory attaching bolts.
b. Cotter pins are used on aircraft and engine controls, landing gear, and tailwheel assemblies, or any other point where a turning or actuating movement takes place.
c. Self-locking nuts are used in applications where they will not be removed often.
Repeated removal and installation will cause the self-locking nut to lose its locking feature. They should be replaced when they are no longer capable of maintaining the minimum prevailing torque. (See Table 7-2.)
d. Pal or speed nuts include designs which force the nut thread against the bolt or screw thread when tightened. These nuts should never be reused and should be replaced with new ones when removed.
7-123. SAFETY WIRE. Do not use stainless steel, Monel, carbon steel, or aluminum alloy safety wire to secure emergency mechanisms such as switch handles, guards covering handles used on exits, fire extinguishers, emergency gear releases, or other emergency equipment. Some existing structural equipment or safety-of-flight emergency devices require copper or brass safety wire (.020-inch diameter only). Where successful emergency operation of this equipment is dependent on shearing or breaking of the safety wire, particular care should be used to ensure that safetying does not prevent emergency operation.
a. There are two methods of safety wiring: the double-twist method that is most commonly used, and the single-wire method used on screws, bolts, and/or nuts in a closely-spaced or closed-geometrical pattern such as a triangle, square, rectangle, or circle. The single-wire method may also be used on parts in electrical systems and in places that are difficult to reach. (See Figures 7-3 and 7-3a.)
FIGURE 7-3. SECURING SCREWS, NUTS, BOLTS, AND SNAPRINGS
SMALL SCREWS IN CLOSELY SPACED CLOSED GEOMETRICAL OUBLE-TWIST METHO INGLE- FASTENER APPLICATIO
PATTERN
SINGLE - WIRE METHOD
CREWHEADS
OUBLE-TWIST METHO
BOLT HEADS CASTLE NUTS NOTE
HE SAFETYWIRE IS SHOWN INSTALLED FOR RIGHT-HAND THREADS THE SAFETYWIRI
S ROUTED IN THE OPPOSITE DIRECTION FOR LEFT-HAND THREADS FIGURE 7-3A. WIRE TWISTING BY HAND 6 TO 8
TWIST PER INCH 1 | INCH
b. When using double-twist method of safety wiring, .032-inch minimum diameter wire should be used on parts that have a hole diameter larger than .045 inch. Safety wire of .020-inch Par 7-123 Page 7-19
Par 7-123 Page 7-20 diameter (double strand) may be used on parts having a nominal hole diameter between .045 and .062 inch with a spacing between parts of less than 2 inches. When using the single-wire method, the largest size wire that the hole will accommodate should be used. Copper wire (.020-inch diameter), aluminum wire (.031-inch diameter), or other similar wire called for in specific technical orders should be used as seals on equipment such as first-aid kits, portable fire extinguishers, emergency valves, or oxygen regulators.
CAUTION: Care should be taken not to confuse steel with aluminum wire.
c. A secure seal indicates that the component has not been opened. Some emergency devices require installation of brass or soft copper shear safety wire. Particular care should be exercised to ensure that the use of safety wire will not prevent emergency operation of the devices.
7-124. SAFETY-WIRING PROCEDURES. There are many combinations of safety wiring with certain basic rules common to all applications. These rules are as follows:
a. When bolts, screws, or other parts are closely grouped, it is more convenient to safety wire them in series. The number of bolts, nuts, screws, etc. that may be wired together depends on the application.
b. Drilled bolt heads and screws need not be safety wired if installed with self-locking nuts.
c. To prevent failure due to rubbing or vibration, safety wire must be tight after installation.
d. Safety wire must be installed in a manner that will prevent the tendency of the part to loosen.
e. Safety wire must never be overstressed. Safety wire will break under vibrations if twisted too tightly. Safety wire must be pulled taut when being twisted and maintain a light tension when secured. (See Figure 7-3a.)
f. Safety wire ends must be bent under and inward toward the part to avoid sharp or projecting ends, which might present a safety hazard.
g. Safety wire inside a duct or tube must not cross over or obstruct a flow passage when an alternate routing can be used.
(1) Check the units to be safety wired to make sure that they have been correctly torqued, and that the wiring holes are properly aligned to each other. When there are two or more units, it is desirable that the holes in the units be aligned to each other. Never overtorque or loosen to obtain proper alignment of the holes. It should be possible to align the wiring holes when the bolts are torqued within the specified limits. Washers may be used (see paragraph 7-37) to establish proper alignment. However, if it is impossible to obtain a proper alignment of the holes without undertorquing or overtorquing, try another bolt which will permit proper alignment within the specified torque limits.
Par 7-124 Page 7-21
(2) To prevent mutilation of the twisted section of wire, when using pliers, grasp the wires at the ends. Safety wire must not be nicked, kinked, or mutilated. Never twist the wire ends off with pliers, and, when cutting off ends, leave at least four to six complete turns (1/2- to 5/8-inch long) after the loop. When removing safety wire, never twist the wire off with pliers.
Cut the safety wire close to the hole, exercising caution.
h. Install safety wire where practicable with the wire positioned around the head of the bolt, screw, or nut, and twisted in such a manner that the loop of the wire fits closely to the contour of the unit being safety wired.
7-125. TWISTING WITH SPECIAL TOOLS. Twist the wire with a wire twister as follows.
(See Figure 7-4.)
CAUTION: When using wire twisters, and the wire extends 3 inches beyond the jaws of the twisters, loosely wrap the wire around the pliers to prevent whipping and possible personal injury. Excessive twisting of the wire will weaken the wire.
a. Grip the wire in the jaws of the wire twister and slide the outer sleeve down with your thumb to lock the handles or lock the spring-loaded pin.
b. Pull the knob, and the spiral rod spins and twists the wire.
c. Squeeze handles together to release wire.
FIGURE 7-4. USE OF A TYPICAL WIRE TWISTER 7-
126. SECURING OIL CAPS, DRAIN COCKS, AND VALVES. (See Figure 7-4a.) When securing oil caps and drain cocks, the safety wire should be anchored to an adjacent fillister head screw. This method of safety wiring is applied to wingnuts, filler plugs, single-drilled head bolts, fillister head screws, etc., which are safety wired individually. When securing valve handles in the vertical position, the wire is looped around the threads of the pipe leading into one side of the
Par 7-126 Page 7-22 valve, double-twisted around the valve handle, and anchored around the threads of the pipe leading into the opposite side of the valve. When castellated nuts are to be secured with safety wire, tighten the nut to the low side of the selected torque range, unless otherwise specified, and, if necessary, continue tightening until a slot lines with the hole. In blind tapped hole applications of bolts or castellated nuts on studs, the safety wiring should be in accordance with the general instructions of this chapter. Hollow-head bolts are safetied in the manner prescribed for regular bolts.
NOTE: Do not loosen or tighten properly tightened nuts to align safety wire holes.
FIGURE 7-4A. SECURING OIL CAPS, DRAIN COCKS, AND VALVES NO TE: Although there are numerous safety wiring techniques used to secure aircraft hardware, practically all are derived from the basic examples shown in Figures 7-5 through 7-5B.
FIGURE 7-5. SAFETY-WIRING PROCEDURES EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 Examples 1, 2, 3, and 4 apply to all types of bolts, fillister-head screws, square-head plugs, and other similar parts which are wired so that the loosening tendency of either part is counteracted by tightening of the other part. The direction of twist from the second to the third unit is counterclockwise in examples 1, 3, and 4 to keep the loop in position against the head of the bolt. The direction of twist from the second to the third unit in example 2 is clockwise to keep the wire in position around the second unit.
The wire entering the hole in the third unit will be the lower wire, except example 2, and by making a counterclockwise twist after it leaves the hole, the loop will be secured in place around the head of that EXAMPLE 5 EXAMPLE 6 EXAMPLE 7 EXAMPLE 8 Examples 5, 6, 7, & 8 show methods for wiring various standard items, NOTE: Wire may be wrapped over the unit rather than around it when wiring castellated nuts or on other items when there is a clearance problem.
EXAMPLE 9 EXAMPLE 10 EXAMPLE 11 Example 9 shows the Hollow-head plugs shall Correct application of single method for wiring bolts in be wired as shown with wire to closely spaced multidifferent planes. Note that the tab bent inside the ple group.
wire should always be ap- hole to avoid snags and plied so that tension is in _ possible injury to personthe tightening direction. nel working on the engine.
Par 7-126 Page 7-23
FIGURE 7-5A. SAFETY-WIRING PROCEDURES EXAMPLE 12 EXAMPLE 13 Examples 12 and 13 show methods for attaching lead seal to protect critical adjustments.
EXAMPLE 14 EXAMPLE 15 EXAMPLE 16 Example 14 shows bolt Example 15 shows correct Example 16 shows correct wired to a right-angle method for wiring adjustable method for wiring the coubracket with the wire connecting rod pling nut on flexible line to wrapped around the the straight connector bracket. brazed on rigid tube.
EXAMPLE 17 EXAMPLE 18 EXAMPLE 19 EXAMPLE 20 EXAMPLE 21 Fittings incorporating wire lugs shall be wired as shown in Exam- Small size coupling nuts ples 17 and 18. Where no lock-wire lug is provided, wire should shall be wired by wrapping be applied as shown in examples 19 and 20 with caution being the wire around the nut exerted to ensure that wire is wrapped tightly around the fitting. and inserting it through the holes as shown.
Par 7-126 Page 7-24