AN310 AN340 AN 320 AN345 AN315 AN316 AN335 AN 350 FICURE 6-6. Non-self-locking nuts.
device. It is used for miscellaneous light-tension requirements.
The plain check nut, AN316, is employed as a locking device for plain nuts, set screws, threaded rod ends, and other devices.
The wing nut, AN350, is intended for use where the desired tightness can be obtained with the fingers and where the assembly is frequently removed.
Self-Locking Nuts As their name implies, self-locking nuts need no auxiliary means of safetying but have a safetying feature included as an integral part of their construction. Many types of self-locking nuts have been designed and their use has become quite widespread. Common applications are: (1) Attachment of antifriction bearings and control pulleys; (2) Attachment of accessories, anchor nuts around inspection holes and small tank installation openings; and (3) Attachment of rocker box covers and exhaust stacks. Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions of the manufacturer.
Self-locking nuts are used on aircraft to provide tight connections which will not shake loose under severe vibration. Do not use self-locking nuts at joints which subject either the nut or bolt to rotation. They may be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by the nut and bolt. Plates must be attached to the structure in a positive manner to eliminate rotation or misalignment when tightening the bolts or screws.
The two general types of self-locking nuts currently in use are the all-metal type and the fiberlock type. For the sake of simplicity, only three typical kinds of self-locking nuts are considered in this handbook: The Boots self-locking and the stainless steel self-locking nuts, representing the all-metal types; and the elastic stop nut, representing the fiber-insert type.
Boots Self-Locking Nut The Boots self-locking nut is of one-piece, allmetal construction, designed to hold tight in spite of severe vibration. Note in figure 6-7 that it has two sections and is essentially two nuts in one, a locking nut and a load-carrying nut. The two sections are connected with a spring which is an integral part of the nut. The spring keeps the locking and load-carrying sections such a distance 127
threaded insert and causes it to clench the bolt tightly. The cross-sectional view in figure 6-8 shows how the key of the case fits into the slotted keyway of the insert so that when the case is turned the threaded insert is turned with it. Note that Boots aircraft nut Elastic anchor nut the slot is wider than the key. This permits the slot to be narrowed and the insert to be compressed when the nut is tightened.
Flexloc nut Fiber locknut Elastic stop nut FIGURE 6-7. Self-locking nuts.
apart that the two sets of threads are out-of-phase;
that is, so spaced that a bolt which has been screwed through the load-carrying section must push the locking section outward against the force of the spring to engage the threads of the locking section properly.
Thus, the spring, through the medium of the locking section, exerts a constant locking force on the bolt in the same direction as a force that would tighten the nut. In this nut, the load-carrying section has the thread strength of a standard nut of comparable size, while the locking section presses against the threads of the bolt and locks the nut firmly in position. Only a wrench applied to the nut will loosen it. The nut can be removed and reused without impairing its efficiency.
Boots self-locking nuts are made with three different spring styles and in various shapes and sizes.
The wing type, which is the most common, ranges in size for No. 6 up to ¼ inch, the Rol-top ranges from ¼ inch to 16 inch, and the bellows type ranges in size from No. 8 up to ⅜ inch. Wing.
type nuts are made of anodized aluminum alloy, cadmium plated carbon steel, or stainless steel. The Rol-top nut is cadmium-plated steel, and the bellows type is made of aluminum alloy only.
Stainless Steel Self-Locking Nut The stainless steel self-locking nut may be spun on and off with the fingers, as its locking action takes place only when the nut is seated against a solid surface and tightened. The nut consists of two parts; a case with a beveled locking shoulder and key, and a threaded insert with a locking shoulder and slotted keyway. Until the nut is tightened it spins on the bolt easily, because the threaded insert is the proper size for the bolt. However, when the nut is seated against a solid surface and tightened, the locking shoulder of the insert is pulled downward and wedged against the locking shoulder of the case. This action compresses the Elastic Stop Nut The elastic stop nut is a standard nut with the height increased to accommodate a fiber-locking collar. This fiber collar is very tough and durable and is unaffected by immersion in hot or cold water or ordinary solvents such as ether, carbon tetrachloride, oils, and gasoline. It will not damage bolt threads or plating.
As shown in figure 6-9, the fiber-locking collar is not threaded and its inside diameter is smaller than the largest diameter of the threaded portion or the outside diameter of a corresponding bolt.
When the nut is screwed onto a bolt, it acts as an ordinary nut until the bolt reaches the fiber collar.
When the bolt is screwed into the fiber collar, however, friction (or drag) causes the fiber to be pushed upward. This creates a heavy downward pressure on the load-carrying part and automatically throws the load-carrying sides of the nut and bolt threads into positive contact. After the bolt has been forced all the way through the fiber collar, the downward pressure remains constant.
This pressure locks and holds the nut securely in place even under severe vibration.
Nut case Locking shoulder Untightened nut Threaded nut core Keyway Tightened nut FIGURE 6-8. Stainless steel self-locking nut.
Do not install elastic stop nuts in places where the temperature is higher than 250° F., because the effectiveness of the self-locking action is reduced beyond this point. Self-locking nuts may be used on aircraft engines and accessories when their use is specified by the engine manufacturer.
Self-locking nut bases are made in a number of forms and materials for riveting and welding to aircraft structure or parts. (See figure 6-10.)
„Nut Certain applications require the installation of selflocking nuts in channels, an arrangement which Fiber collar• permits the attachment of many nuts with only a few rivets. These channels are track-like bases with regularly spaced nuts which are either removable or nonremovable. The removable type carries a floating nut, which can be snapped in or out of the channel, thus making possible the easy re moval of damaged nuts. Nuts such as the clinch type and spline-type which depend on friction for their anchorage are not acceptable for use in aircraft structures.
FIGURE 6-9. Elastic stop nut.
Nearly all elastic stop nuts are steel or aluminum alloy. However, such nuts are available in practically any kind of metal. Aluminum alloy elastic stop nuts are supplied with an anodized finish.
Steel nuts are cadmium plated Normally, elastic stop nuts can be used many times with complete safety and without detriment to their locking efficiency. When reusing elastic stop nuts, be sure the fiber has not lost its locking friction or become brittle. If a nut can be turned with the fingers, replace it.
After the nut has been tightened, make sure the rounded or chamfered end of the bolts, studs, or screws extends at least the full round or chamfer through the nut. Flat end bolts, studs, or screws should extend at least 32 inch through the nut.
Bolts of 16-inch diameter and over with cotter pin holes may be used with self-locking nuts, but only if free from burrs around the holes. Bolts with damaged threads and rough ends are not acceptable. Do not tap the fiber-locking insert.
The self-locking action of the elastic stop nut is the result of having the bolt threads impress themselves into the untapped fiber.
Sheet Spring Nuts Sheet spring nuts, such as speed nuts, are used with standard and sheet-metal self-tapping screws in nonstructural locations. They find various uses in supporting line clamps, conduit clamps, electrical equipment, access doors, and the like, and are available in several types. Speed nuts are made from spring steel and are arched prior to tightening. This arched spring lock prevents the screw from working loose. These nuts should be used only where originally used in the fabrication of the aircraft.
Internal and Externa! Wrenching Nuts Two commercial types of high-strength internal or external wrenching nuts are available; they are the internal and external wrenching elastic-stop nut and the Unbrako internal and external wrenching nut. Both are of the self-locking type, are heat-treated, and are capable of carrying highstrength bolt-tension loads.
Identification and Coding Part numbers designate the type of nut. The common types and their respective part numbers are: Plain, AN315 and AN335; castle AN310;
Boots aircraft channel assembly Elastic stop nut channel assembly FIGURE 6-10. Self-locking nut bases.
plain check, AN316; light hex, AN340 and AN345;
and castellated shear, AN320. The patented selflocking types are assigned part numbers ranging from MS20363 through MS20367. The Boots, the Flexloc, the fiber locknut, the elastic stop nut, and the self-locking nut belong to this group.
Part number AN350 is assigned to the wing nut.
Letters and digits following the part number indicate such items as material, size, threads per inch, and whether the thread is right or left hand The letter "B" following the part number indicates the nut material to be brass; a "D" indicates 2017-T aluminum alloy; a "DD' indicates 2024-T aluminum alloy; a "C" indicates stainless steel;
and a dash in place of a letter indicates cadmiumplated carbon steel.
The digit (or two digits) following the dash or the material code letter is the dash number of the nut, and it indicates the size of the shank and threads per inch of the bolt on which the nut will fit. The dash number corresponds to the first figure appearing in the part number coding of general-purpose bolts. A dash and the number 3, for example, indicates that the nut will fit an AN3 bolt (10-32); a dash and the number 4 means it will fit an AN4 bolt (¼-28); a dash and the number 5, an AN5 bolt (516-24); and so on.
The code numbers for self-locking nuts end in three- or four-digit numbers. The last two digits refer to threads per inch, and the one or two preceding digits stand for the nut size in 16ths of an inch.
Some other common nuts and their code numbers are:
Code Number AN310D5R:
AN310 = aircraft castle nut.
D = 2024-T aluminum alloy.
• 5 = 6-inch diameter.
R = right-hand thread (usually 24 threads per inch).
Code Number AN320-10:
AN320 = aircraft castellated shear nut, cadmium-plated carbon steel.
10 = g-inch diameter, 18 threads per inch (this nut is usually right-hand thread).
Code Number AN350B1032:
AN350 = aircraft wingnut.
B = brass.
10 = number 10 bolt.
32 = threads per inch.
AIRCRAFT WASHERS Aircraft washers used in airframe repair are either plain, lock, or special type washers.
Plain Washers Plain washers (figure 6-11), both the AN960 and AN970, are used under hex nuts. They provide a smooth bearing surface and act as a shim 130
Plain Ball seat & socket taper pin AN 960 AC950 & AC955 AN975 Special washers AN 935 Star lock washers FIGURE 6-11. Various types of washers.
in obtaining correct grip length for a bolt and nut assembly. They are used to adjust the position of castellated nuts in respect to drilled cotterr pin holes in bolts. Plain washers should be used under lockwashers to prevent damage to the surface material.
Aluminum and aluminum alloy washers may be used under boltheads or nuts on aluminum alloy or magnesium structures where corrosion caused by dissimilar metals is a factor. When used in this manner, any electric current flow will be between the washer and the steel bolt. However, it is common practice to use a cadmiumplated steel washer under a nut bearing directly against a structure as this washer will resist the cutting action of a nut better than an aluminum alloy washer.
The AN970 steel washer provides a greater bearing area than the AN960 washer and is used on wooden structures under both the head and the nut of a bolt to prevent crushing the surface.
Lockwashers Lockwashers, both the AN935 and AN936, are used with machine screws or bolts where the selflocking or castellated type nut is not appropriate.
The spring action of the washer (AN935) provides enough friction to prevent loosening of the nut from vibration. (These washers are shown in figure 6-11.)
Lockwashers should never be used under the following conditions:
1. With fasteners to primary or secondary structures.
2. With fasteners on any part of the aircraft where failure might result in damage or danger to the aircraft or personnel.
3. Where failure would permit the opening of a joint to the airflow.
4. Where the screw is subject to frequent removal.
5. Where the washers are exposed to the airflow.
6. Where the washers are subject to corrosive conditions.
7. Where the washer is against soft material without a plain washer underneath to prevent gouging the surface.
Shakeproof Lockwashers Shakeproof lockwashers are round washers designed with tabs or lips that are bent upward across the sides of a hex nut or bolt to lock the nut in place. There are various methods of securing the lockwasher to prevent it from turning, such as an external tab bent downward 90° into a small hole in the face of the unit, or an internal tab which fits a keyed bolt.
Shakeproof lockwashers can withstand higher heat than other methods of safetying and can be used under high-vibration conditions safely. They should be used only once because the tabs tend to break when bent a second time.
Special Washers The ball-socket and seat washers, AC950 and AC955, are special washers used where a bolt is installed at an angle to a surface, or where perfect alignment with a surface is required.
These washers are used together. They are shown in figure 6-11.
The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144 through NAS158 series. This washer is either plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used 131
to seat the bolt head shank radius, and the plain washer is used under the nut.
INSTALLATION OF NUTS AND BOLTS Bolt and Hole Sizes Slight clearances in boltholes are permissible wherever bolts are used in tension and are not subject to reversal of load. A few of the applications in which clearance of holes may be permitted are in pulley brackets, conduit boxes, lining trim, and miscellaneous supports and brackets.
Boltholes are to be normal to the surface involved to provide full bearing surface for the bolthead and nut and must not be oversized or elongated. A bolt in such a hole will carry none of its shear load until parts have yielded or deformed enough to allow the bearing surface of the oversized hole to contact the bolt. In this respect, remember that bolts do not become swaged to fill up the holes as do rivets.
In cases of oversized or elongated holes in critical members, obtain advice from the aircraft or engine manufacturer before drilling or reaming the hole to take the next larger bolt. Usually, such factors as edge distance, clearance, or load factor must be considered. Oversized or elongated tea or beated e holes in noncritical members can usually be drilled or reamed to the next larger size.
Many boltholes, particularly those in primary connecting elements, have close tolerances. Generally, it is permissible to use the first lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in lightdrive fit (reamed) applications and where NAS close-tolerance bolts or AN clevis bolts are used.
Light-drive fits for bolts (specified on the repair drawings as .0015-inch maximum clearance between bolt and hole) are required in places where bolts are used in repair, or where they are placed in the original structure.
The fit of holes and bolts cannot be defined in terms of shaft and hole diameters; it is defined in terms of the friction between bolt and hole when sliding the bolt into place. A tight-drive fit, for example, is one in which a sharp blow of a 12- or 14-ounce hammer is required to move the bolt. A bolt that requires a hard blow and sounds tight is considered to fit too tightly. A light-drive fit is one in which a bolt will move when a hammer handle is held against its head and pressed by the weight of the body.
Installation Practices Examine the markings on the bolthead to determine that each bolt is of the correct material.
It is of extreme importance to use like bolts in replacement. In every case, refer to the applicable Maintenance Instructions Manual and Illustrated Parts Breakdown.
Be sure that washers are used under both the heads of bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum alloy washer should be used under the head and nut of a steel bolt securing aluminum alloy or magnesium alloy members. Any corrosion that occurs then attacks the washer rather than the members. Steel washers should be used when joining steel members with steel bolts.
Whenever possible, the bolt should be placed with the head on top or in the forward position.
This positioning tends to prevent the bolt from slipping out if the nut is accidently lost.
Be certain that the bolt grip length is correct.
Grip length is the length of the unthreaded portion of the bolt shank. Generally speaking, the grip length should equal the thickness of the material being bolted together. However, bolts of slighily greater grip length may be used if washers are placed under the nut or the bolthead. In the case of plate nuts, add shims under the plate.
Safetying of Bolts and Nuts It is very important that all bolts or nuts, except the self-locking type, be safetied after installation.
This prevents them from loosening in flight due to vibration. Methods of safetying are discussed later in this chapter.
TORQUE AND TORQUE WRENCHES As the speed of an aircraft increases, each structural member becomes more highly stressed. It is therefore extremely important that each member carry no more and no less than the load for which it was designed. In order to distribute the loads safely throughout a structure, it is necessary that proper torque be applied to all nuts, bolts, studs and screws. Using the proper torque allows the structure to develop its designed strength and greatly reduces the possibility of failure due to fatigue.
Torque Wrenches The three most commonly used torque wrenches are the flexible beam, rigid frame, and the ratchet types (figure 6-12). When using the flexible beam and the rigid frame torque wrenches, the torque value is read visually on a dial or scale mounted on the handle of the wrench.
To use the ratchet type, unlock the grip and adjust the handle to the desired setting on the micrometer type scale, then relock the grip.
Install the required socket or adapter to the square drive of the handle. Place the wrench assembly on the nut or bolt and pull the wrench assembly on the nut or bolt and pull in a clockwise direction with a smooth, steady motion. (A fast or jerky motion will result in an improperly torqued unit.)
When the applied torque reaches the torque value which indicated on the handle setting, the handle will automatically release or "break" and move Basic formula F X L = T F = Applied force L = Lever length betwen centerline of drive and centerline of applied force (F must be 90 degrees to L) T = Torque 90 DEGREES 90 _DEGREES
RATCHET
RIGID FRAME Formula for use with extensions Tw = Te X A A Lever length of wrench B Lever length of wrench plus extension Te Required torque on bolt Tw Torque reading on wrench dial
FLEXIBLE BEAM FIGURE 6-12. Common torque wrenches.
Torque Values in Inch-Pounds for Tightening Nuts On standard bolts, studs, and On bolts, studs, On high-strength screws having a tensile strength and serews having bolts, studs, of 125,000 to 140,000 p.s.i. a tensile strength and screws having of 140,000 to a tensile strength Bolt, Stud 160,000 p.s.i. 160,000 p.s.i.
or Screw Size and over Shear type nuts Tension type nuts (AN320, AN364 and threaded Any nut, Any nut, or equivalent) machine parts except, except (AN-310,A.V365 shear type shear type or equivalent) 8-32 8-36 7-9 12-15 14-17 15-18 10-24 10-32 12-15 20-25 23-30 25-35 1/4-20 25-30 40-50 45-49 50-68 1/4-28 30-40 50-70 60-80 70-90 5/16-18 48-55 80-90 85-117 90-144 5/16-24 60-85 100-140 120-172 140-203 3/8-16 95-110 160-185 173-217 185-248 3/8-24 95-110 160-190 175-271 190-351 7/16-14 140-155 235-255 245-342 255-428 7/16-20 270-300 450-500 475-628 500-756 1/2-13 240-290 400-480 440-636 480-792 1/2-20 290-410 480-690 585-840 690-990 9/16-12 300-420 500-700 600-845 700-990 9/16-18 480-600 800-1,000 900-1.220 1,000-1,440 5/8-11 420-540 700-900 800-1,125 900-1.350 5/8-18 660-780 1,100-1,300 1,200-1,730 1,300-2,160 3/4-10 700-950 1,150-1,600 1,380-1,925 1,600-2,250 3/4-16 1,300-1,500 2,300-2,500 2,400-3,500 2,500-4,500 7/8-9 1,300-1,800 2,200-3,000 2,600-3,570 3,000-4,140 7/8-14 1,500-1,800 2,500-3,000 2,750-4,650 3,000-6,300 1-8 2,200-3,000 3,700-5,000 4,350-5,920 5,000-6,840 I"-14 2,200-3,300 3,700-5,500 4,600-7,250 5,500-9,000 11/8-8 3,300-1,000 5,500-6,500 6,000-8,650) 6,500-10,800 1 1/8-12 3,000-4,200 5,000-7,000 6,000-10,250 7,000-13,500 1 1/4-8 4,000-5,000 6,500-8,000 7,250-11,000 8,000-14,000 1 1/4-12 5,400-6,600 9,000-11,000 10,000-16,750 11,000-22,500 FIGURE 6-13. Standard torque table (inch-pounds).
next slot with the cotter pin hole. The torque loads specified may be used for all unlubricated cadmium-plated steel nuts of the fine- or coarsethread series which have approximately equal number of threads and equal face bearing areas.
These values do not apply where special torque requirements are specified in the maintenance manual.
If the head end, rather than the nut, must be turned in the tightening operation, maximum torque values may be increased by an amount equal to shank friction, provided the latter is first measured by a torque wrench.
AIRCRAFT SCREWS Screws are the most commonly used threaded Torque Tables fastening devices on aircraft. They differ from bolts inasmuch as they are generally made of lower strength materials. They can be 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 or un- 6-13) : threaded portion while others are threaded along their entire length.
Several types of structural screws differ from the standard structural bolts only in head style.
The material in them is the same, and a definite grip length is provided. The AN525 washer-head screw and the NAS220 through NAS227 series are such screws.
Commonly used screws are classified in three groups: (1) Structural screws which have the same strength as equal size bolts; (2) machine screws, which include the majority of types used for general repair; and (3) self-tapping screws, which are used for attaching lighter parts. A tourth group, drive screws, are not actually screws other damage. but nails. They are driven into metal parts with a mallet or hammer and their heads are not slotted or recessed.
Structural Screws Structural screws are made of alloy steel, are properly heat treated, and can be used as structural bolts. These screws are found in the NAS204 through NAS235 and AN509 and AN525 series.
They have a definite grip and the same shear strength as a bolt of the same size. Shank tolerances are similar to AN hex-head bolts, and the threads are National Fine. Structural screws are available with round, brazier, or countersunk heads. The recessed head screws are driven by either a Phillips or a Reed and Prince screwdriver.
The AN509 (100°) flathead screw is used in 135
countersunk holes where a flush surface is neces- The AN525 washer-head structural screw is used where raised heads are not objectionable.
The washer-head screw provides a large contact area.
Machine Screws Machine screws are usually of the flathead (countersunk), roundhead, or washer-head types.
These screws are general-purpose screws and are available in low-carbon steel, brass, corrosionresistant stees, and aluminum alloy.
Roundhead screws, AN515 and AN520, have either slotted or recessed heads. The AN515 screw has coarse threads and the AN520 has fine Countersunk machine screws are AN505 and AN510 for 82°, and AN507 for 100°.
The AN505 and AN510 correspond to the AN515 and AN520 roundhead in material and usage.
The fillister-head screw, AN500 through AN503, is a general-purpose screw and is used as a capscrew in light mechanisms. This could include attachments of cast aluminum parts such as gearbox cover plates.
The AN500 and AN501 screws are available in low-carbon steel, corrosion-resistant steel, and brass. The AN500 has coarse threads while the AN501 has fine threads. They have no clearly defined grip length. Screws larger than No. 6 have a hole drilled through the head for safetying The AN502 and AN503 fillister-head screws are made of heat-treated alloy steel, have a small grip, and are available in fine and coarse threads.
These screws are used as capscrews where great strength is required. The coarse-threaded screws are commonly used as capscrews in tapped aluminum alloy and magnesium castings because of the softness of the metal.
Self-tapping Screws Machine self-tapping screws are listed as AN504 and AN506. The AN504 screw has a roundhead, and the AN506 is 82° countersunk. These screws are used for attaching removable parts, such as nameplates, to castings and parts in which the screw cuts its own threads.
AN530 and AN531 self-tapping sheet-metal screws, such as the Parker-Kalon Z-type sheetmetal screw, are blunt on the end. They are used in the temporary attachment of sheet metal for riveting, and in the permanent assembly of nonstructural assemblies. Self-tapping screws should not be used to replace standard screws, nuts, bolts, or rivets.
Drive Screws Drive screws, AN535, correspond to the Parker- Kalon U-type. They are plain-head self-tapping screws used as capscrews for attaching nameplates in castings and for sealing drain holes in corrosion proofing tubular structures. They are not intended to be removed after installation.
Identification and Coding The coding system used to identify screws is similar to that used for bolts. There are AN and NAS screws. NAS screws are structural screws.
Part numbers 510, 515, 550, and so on, catalog screws into classes such as roundhead, flathead, washerhead, and so forth. Letters and digits indicate their material composition, length, and thickness. Examples of AN and NAS code numbers follow.
AN501B-416-7 AN = Air Force-Navy standard.
501 = fillister-head, fine thread.
B = brass.
416 - 6-inch diameter.
7 = 16-inch length.
The letter "D" in place of the "B" would indicate that the material is 2017-T aluminum alloy. The letter "C" would designate corrosionresistant steel. An "A" placed before the materia.
code letter would indicate that the head is drilled for safetying.
NAS144DH-22 NAS = National Aircraft Standard.
144 = head style; diameter and thread- 4-28 bolt, internal wrenching.
DH = drilled head.
22 = screw length in 16ths of an inch- 1g inches long.
The basic NAS number identifies the part. The suffix letters and dash numbers separate different sizes, plating material, drilling specifications, etc.
The dash numbers and suffix letters do not have standard meanings. It is necessary to refer to a specific NAS page in the Standards book for the 136
REPAIR OF DAMAGED INTERNAL THREADS Installation or replacement of bolts is simple when compared to the installation or replacement of studs. Bolt heads and nuts are cut in the open, whereas studs are installed into internal threads in a casting or built-up assembly. Damaged threads on bolts or nuts can be seen and only require replacement of the defective part. If internal threads are damaged, two alternatives are apparent: the part may be replaced or the threads repaired or replaced. Correction of the thread problem is usually cheaper and more convenient.
ods of repairing are by replacement bushings or heli-coils.
Replacement Bushings Bushings are usually special material (steel or brass spark plug bushings into aluminum cylinder heads). A material that will resist wear is used where removal and replacement is frequent. The external threads on the bushing are usually coarse.
The bushing is installed, a thread lock compound may or may not be used, and staked to prevent loosening. Many bushings have left-hand threads external and right-hand threads internal.
this installation, removal of the bolt or stud (righthand threads) tends to tighten the bushing.
Bushings for common installations such as spark plugs may be up to .040 oversize (in increments of .005). Original installation and overhaul shop replacements are shrunk fit: a heated cylinder head and a frozen bushing.
Heli-coils are precision formed screw thread coils of 18-8 stainless steel wire having a diamond shaped cross-section (figure 6-14). They form unified coarse or unified fine thread classes 2-band 3B when assembled into (heli-coil) threaded holes.
The assembled insert accommodates UN] (controlled radius root) male threaded members. Each insert has a driving tang with a notch to facilitate removal of the tang after the insert is screwed into a heli-coil tapped hole.
They are used as screw thread bushings. In addition to being used to restore damaged threads, they are used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to protect and strengthen tapped threads in light materials, metals, and plastics, particularly in locations which require frequent assembly and disassembly, and/or where a screw locking action is desired.
FIGURE 6-14. Heli-coil insert.
Heli-Coil Installation Heli-coil installation (figure 6-15) is a 5 to 6 step operation, depending upon how the last step is classed. The following steps are for instructional purposes only. The manufacturer's instructions should be followed during installation.
Step 1: Determine what threads are damaged.
Step 2: (a) New installation of heli-coil, drill out damaged threads to minimum depth specified.
(b) Previously installed heli-coil.
Using proper size extracting tool, place edge of blade in 90° from the edge of the insert. Tap with hammer to seat tool. Turn to left, applying pressure, until insert backs Threads are not damaged if insert is properly removed.
Step 3: Tap. Use the tap of required nominal thread size. The tapping procedure is the same as standard thread tapping.
Tap length must be equal to or exceed the requirement.
Step 4: Gage. Threads may be checked with a heli-coil thread gage.
Step 5: Insert Assembly. Using proper tool, install insert to a depth that puts end of top coil ¼ to ½ turn below the top surface of the tapped hole.
Step 6: Tang break-off.
Select proper break-off tool.
should be removed from all drilled through holes. In blind holes the tangs may be removed when necessary if enough hole depth is provided below the tang of the installed insert.
These are not to be considered specific instructions on heli-coil installation. The manufacturer's instruction must be followed when making an in- Heli-coils are available for the following threads:
unified coarse, unified fine, metric, spark plug and national taper pipe threads.
REPAIR OF DAMAGED HOLES WITH ACRES
FASTENER SLEEVES Acres fastener sleeves are thin wall, tubular, elements with a flared end. The sleeves are installed in holes to accept standard bolts and rivet type fasteners. The existing fastener holes are drilled %4-inch oversize for installation of the sleeves. The sleeves are manufactured in one inch increments. Along their length grooves provide a place to break or cut off excess length to match fastener grip range. The grooves also provide a place to hold adhesive or sealing agents when bonding sleeve into the hole.
Advantages and Limitations The sleeves are used in holes which must be drilled 64-inch oversize to clean up corrosion or other damage. The oversize hole with the sleeve installed allows the use of the original diameter fastener in the repaired hole. The sleeves can be used in areas of high galvanic corrosion where the corrosion must be confined to a readily replaceable part. Oversizing of holes reduces the net cross sectional area of a part and should not be done unless absolutely required.
The manufacturer of the aircraft, aircraft engine or aircraft component should be consulted prior to repair of damaged holes with acres sleeves.
Identification The sleeve is identified by a standard code number (figure 6-16A) which represents the type and style of sleeve, a material code, the fastener shank diameter, surface finish code letter and grip tang for the sleeve. The type and material of the sleeve is represented by the basic code number.
The first dash number represents the diameter of the sleeve for the fastener installed and the second dash represents the grip length of the sleeve. The required length of the sleeve is determined on installation and the excess is broken off of the sleeve. A JK5512A-05N-10 is a 100° low profile head sleeve of aluminum alloy. The diameter is for a 32 inch fastener with no surface finish and is g inch in length.
DRILL. TAP GAGE INSTALL FIGURE 6-15. Heli-coil installation.