Par 7-101 Page 7-309 (and 7-18)
SECTION 6. PINS
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.
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.
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.
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.
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.
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.
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 sp ring 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.
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.
7-1067-121. [RESERVED.]
Par 7-101 Page 7-310 (and 7-18)
Par 7-122 Page 7-311
SECTION 7. SAFETYING
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.
Wire, either soft brass or steel is used on cylinder studs, control cable turnbuckles , and engine accessory attaching bolts.
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.
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.)
Pal or speed nuts include designs which force the nut thread against the bolt or sc rew thread when tightened.
These nuts should never be reused and should be replaced with new ones when removed.
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.
There are two methods of safety wiring; the doubletwist 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.)
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 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.
A secure seal indicates that the component has not been opened. Some emergency devices require installation of brass or soft
Par 7-122 Page 7-312
FIGURE 7-3. Securing screws, nuts, bolts, and snaprings.
FIGURE 7-3a. Wire twisting by hand.
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.
SAFETY-WIRING PROCEDURES. There are many combinations of safety wiring with certain basic rules common to all applications.
These rules are as follows.
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.
Drilled boltheads and screws need not be safety wired if installed with self-locking nuts.
To prevent failure due to rubbing or vibration, safety wire must be tight after installation.
Safety wire must be installed in a manner that will prevent the tendency of the part to loosen.
Safety wire must never be over-stressed. Safety wire will break under vibrations if twisted too tightly. Safety wire must be pulled taut when being twisted, and
Par 7-124 Page 7-313 maintain a light tension when secured. (See figure 7-3a.)
Safety-wire ends must be bent under and inward toward the part to avoid sharp or projecting ends, which might present a safety hazard.
Safety wire inside a duct or tube must not cross over or obstruct a flow passage when an alternate routing can be used.
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.
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.
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.
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.
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.
Pull the knob, and the spiral rod spins and twists the wire.
Squeeze handles together to release wire.
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 valve,
Par 7-124 Page 7-314
FIGURE 7-4. Use of a typical wire twister.
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 genera l 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.
NOTE: 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 7-4a. Securing oil caps, drain cocks, and valves.
Par 7-126 Page 7-315
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 loos ening tendency of either part is counteracted by tightening of the other part. The direction of twist fr om the second to the third unit is counterclockwise in examples 1, 3, and 4 to keep the loop in positi on 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 thir d 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 bolt.
Examples 5, 6, 7, & 8 show methods for wiri ng various standard items, NOTE: Wire may be wrapped over the unit rather than around it when wiring cast ellated nuts or on other items when there is a clearance problem.
Example 9 shows the method for wiring bolts in different planes. Note that wire should always be applied so that tension is in the tightening direction.
Hollow-head plugs shall be wired as shown with the tab bent inside the hole to avoid snags and possible injury to personnel working on the engine.
Correct application of single wire to closely spaced multiple group.
FIGURE 7-5. Safety-wiring procedures.
Par 7-127 Page 7-316
Examples 12 and 13 show methods for attaching lead seal to protect critical adjustments.
Example 14 shows bolt wired to a right-angle bracket with the wire wrapped around the bracket.
Example 15 shows correct method for wiring adjustable connecting rod.
Example 16 shows correct method for wiring the coupling nut on flexible line to the straight connector brazed on rigid tube.
Fittings incorporating wire lugs shall be wired as shown in Examples 17 and 18. Where no lock- wire lug is provided, wire should be applied as shown in examples 19 and 20 with caution being exerted to ensure that wi re is wrapped tightly around the fitting.
Small size coupling nuts shall be wired by wrapping the wire around the nut and inserting it through the holes as shown.
FIGURE 7-5a. Safety-wiring procedures.
Par 7-126 Page 7-317
Coupling nuts attached to straight connectors shall be wired as, shown, when hex is an integral part of the connector.
Coupling nuts on a tee shall be wired, as shown above, so that tension is always in the tightening direction.
Straight Connector (Bulkhead Type)
Examples 26, 27, and 28 show the proper met hod for wiring various standard fittings with checknut wired independently so that it need not be disturbed when removing the coupling nut.
FIGURE 7-5b. Safety-wiring procedures.
Par 7-140 Page 7-318 SECURING WITH COTTER PINS.
Cotter pins are used to secure such items as bolts, screws, pins, and shafts. Their use is favored because they can be removed and installed quickly. The diameter of the cotter pins selected for any application should be the largest size that will fit consistent with th e diameter of the cotter pin hole and/or the sl ots in the nut. Cotter pins should not be reused on aircraft.
To prevent injury during and after pin installation, the end of the cotter pin can be rolled and tucked.
NOTE: In using the method of cotter pin safetying, as shown in figures 7-6 and 7-7, ensure the prong, bent over the bolt, is seated firmly against the bolt sh ank, and does not exceed bo lt diameter. Also, when the prong is bent over the nut, ensure the bent prong is down and firmly flat against the nut and does not contact the surface of the washer.
Par 7-140 Page 7-319
FIGURE 7-6. Securing with cotter pins.
FIGURE 7-7. Alternate method for securing with cotter pins.
7-128.7-139. [RESERVED.]
Par 7-140 Page 7-320 SECTION 8. INSPECTION AND REPAIR OF CONTROL CABLES
AND TURNBUCKLES
GENERAL. Aircraft control cables are generally fabricated from carbon steel or corrosionresistant steel wire of either flexible or nonflexible-type construction.
CABLE DEFINITIONS. The following cable components are defined in accordance with Military Specifications MIL-W-83420, MIL-C-18375, and MIL-W-87161.
Wire Center. The center of all strands shall be an individual wire and shall be designated as a wire center.
Strand Center or Core. A strand center is a single, straight strand made of preformed wires, similar to the other strands comprising the cable, in arrangement and number of wires.
Independent Wire Rope Center (IWRC) 7 by 7. A 7 by 7 independent wire rope center as specified herein shall consist of a cable or wire rope of six strands of seven wires each, twisted or laid around a strand center or core consisting of seven wires.
FLEXIBLE CABLES. Flexible, preformed, carbon steel, Type I, composition A cables, MIL-W-83420, are manufactured from steel made by the acid-open-hearth, basic-open hearth, or electric-furnace process. The wire used is coated with pure tin or zinc. Flexible, preformed, corrosion-resistant, Type I, composition B cables, MIL-W-87161, MIL-W-83420, and MIL-C-18375 are manufactured from steel made by the electricfurnace process. (See table 7-3 and figure 7-8.)
These cables are of the 3 by 7, 7 by 7, 7 by 19, or 6 by 19 IWRC construction, according to the diameter as specified in table 7-3. The 3 by 7 cable consists of three strands of seven wires each. There is no core in this construction.
The 3 by 7 cable has a length of lay of not more than eight times or less than five times the nominal cable diameter. The 7 by 7 cable consists of six strands, of seven wires each, laid around a center strand of seven wires. The wires are laid so as to develop a cable which has the greatest bending and wearing properties. The 7 by 7 cable has a length of lay of not more than eight times or less than six times the cable diameter. The 7 by 19 cable consists of six strands laid around a center strand in a clockwise direction. The wires composing the seven individual strands are laid around a center wire in two layers. The center core strand consists of a lay of six wires laid around the central wire in a clockwise direction and a layer of 12 wires laid around this in a clockwise direction. The six outer strands of the cable consist of a layer of six wires laid around the center wire in a counterclockwise direction and a layer of 12 wires laid around this in a counterclockwise direction. The 6 by 19 cable consists of six strands of 19 wires each, laid around a 7 by 7.
MIL-C-18375 cable, although not as strong as MIL-W-83420, is equal in corrosion resistance and superior in non-magnetic and coefficient of thermal expansion properties.
NYLON-COATED CABLES.
Nylon-coated cable is made by extruding a flexible nylon coating over corrosion-resistant steel (CRES) cable. The bare CRES cable must conform and be qualified to MIL-W-83420. After coating, the jacketed cable must still conform to MIL-W-83420.
The service life of nylon-coated cable is much greater than the service life of the sa me cable when used bare.
Most cable wear occurs at pulleys where the cable bends. Wear