Note: This bender can be slipped over partially connected tubes as it is Raise slide bar upward applied at direct point of bend.
Hight hand
2. Place tube Left hand
3. Drop clip over tube Note: Zero mark coincides with mark on block Left hand With right hand continue to bend tube to desired angle by pressing handle as shown below.
90° mark To remove bent tube. lift slide bar handle to original position and raise clip Right hand Left hand FIGURE 5-11. Tube bending.
over a forming block shaped to the specified radius.
In a modified version of the filler method, a fusible alloy is used instead of sand. In this method, the tube is filled under hot water with a fusible alloy that melts at 160° F. The alloy-filled tubing is then removed from the water, allowed to cool, and bent slowly by hand around a forming block or with a tube bender. After the bend is made, the alloy is again melted under hot water and removed from the tubing.
When using either filler method, make certain that all particles of the filler are removed so that none will be carried into the system in which the tubing is installed. Store the fusible alloy filler where it will be free from dust or dirt. It can be 110
Grip die Flaring tool " shown in vise Yake Perfect bend Flattened bend Wrinkled bend Kinked bend FIGURE 5-12. Correct and incorrect tubing bends.
re-melted and re-used as often as desired. Never heat this filler in any other than the prescribed method, as the alloy will stick to the inside of the tubing, making them both unusable.
Tube Flaring Two kinds of flares are generally used in aircraft plumbing systems, the single flare and the double fare. Flares are frequently subiected to extremely high pressures; therefore, the flare on the tubing must be properly shaped or the connection will leak or fail.
A flare made too small produces a weak joint, which may leak or pull apart; if made too large it interferes with the proper engagement of the screw thread on the fitting and will cause leakage.
crooked flare is the result of the tubing not being cut squarely. If a flare is not made properly, flaws cannot be corrected by applying additional torque when tightening the fitting. The flare and tubing must be free from cracks, dents, nicks, scratches, or any cther defects.
The Haring tool used for aircraft tubing has male and female dies ground to produce a flare of 35° to 37°. Under no circumstances is it permissible to use an automotive type flaring tool which produces a flare of 45°.
Single Flare A hand flaring tool similar to that shown in figure 5-13 is used for flaring tubing. The tool Tubing, grip die Tubing Grip die 00000 Flaring tool Steel pilots Top view plunger Side view FIGURE 5-13. Hand faring tool (single flare).
consists of a flaring block or grip die, a yoke, and a faring pin. The flaring block is a hinged double bar with holes corresponding to various sizes of tubing. These holes are countersunk on one end to form the outside support against which the flare is formed. The yoke is used to center the flaring pin over the end of the tube to be flared.
To prepare a tube for flaring, cut the tube squarely and remove all burrs. Slip the fitting nut and sleeve on the tube and place the tube in the proper size hole in the flaring tool. Center the plunger or flaring pin over the end of the tube.
Then project the end of the tubing slightly from the top of the flaring tool, about the thickness of a dime, and tighten the clamp bar securely to prevent slippage.
Make the flare by striking the plunger several light blows with a lightweight hammer or mallet.
Turn the plunger a half turn after each blow and be sure it seats properly before removing the tube from the flaring tool. Check the flare by sliding the sleeve into position over the flare.
the outside diameter of the flare should extend approximately one-sixteenth inch beyond the end of the sleeve, but should not be larger than the major outside diameter of the sleeve.
Double Flare A double flare should be used on 5052-0 and 6061-T aluminum alloy tubing for all sizes from ⅛- to g-inch outside diameter. This is necessary to prevent cutting off the flare and failure of the tube assembly under operating pressures. Double flaring is not necessary on steel tubing. See figure 5-14 for an illustration of single- and doublefared tubing. The double flare is smoother and more concentric than the single flare and, therefore, seals better. It is also more resistant to the shearing effect of torque.
of the tube is lubricated with light oil to reduce the friction between the rollers during beading.
The sizes, marked in sixteenths of an inch on the rollers, are for the outside diameter of the tubing that can be beaded with the rollers.
Separate rollers are required for the inside of each tubing size, and care must be taken to use the correct parts when beading. The hand-beading tool works somewhat like the tube cutter in that the roller is screwed down intermittently while rotating the beading tool around the tubing. In A. Single-flared end addition, a small vise (tube holder) is furnished with the kit.
Other methods and types of beading tools and machines are available, but the hand-beading tool is used most often. As a rule, beading machines are limited to us. with large-diameter tubing, over 11¾6 inch, unk ss special rollers are supplied.
The grip-die method of beading is confined to small tubing.
Flareless-Tube Assemblies Although the use of flareless-tube fittings eliminates all tube flaring, another operation, referred to as presetting, is necessary prior to installation B. Double-flared end of a new flareless-tube assembly. Figure 5-15 (steps 1, 2, and 3) illustrates the presetting oper- FIGURE 5-14. Cutaway view of single- and ation, which is performed as follows:
double-flared tube ends.
(a.) Cut the tube to the correct length, with the ends perfectly square. Deburr the inside and outside of the tube. Slip the nut, then To make the double flare, separate the clamp the sleeve, over the tube (step 1).
blocks of the double-flaring tool and insert and (b.) Lubricate the threads of the fitting and clamp the tubing with the burred end flush with nut with hydraulic fluid. Place the fitting in the top of the clamp. Insert the starting pin into a vise (step 2), and hold the tubing firmly and the flaring pin guide and strike the pin sharply squarely on the seat in the fitting. (Tube with a hammer until the shoulder of the pin stops must bottom firmly in the fitting.) Tighten against the clamp blocks. Remove the starting pin the nut until the cutting edge of the sleeve and insert the finishing pin; hammer it until its grips the tube. This point is determined by shoulder rests on the clamp block. slowly turning the tube back and forth while tightening the nut. When the tube no longer Beading turns, the nut is ready for final tightening.
(c.) Final tightening depends upon the tub- Tubing may be beaded with a hand-beading ing. For aluminum alloy tubing up to and intool, with machine-beading rolls, or with grip dies. cluding ½-inch outside diameter, tighten the The method to be used depends on the diameter nut from one to one and one-sixth turns.
and wall thickness of the tube and the material For steel tubing and aluminum alloy tubing from which it was made. over ½-inch outside diameter, tighten from The hand-beading tool is used with tubing one and one-sixth to one and one-half turns.
having ¼- to 1-inch outside diameter. The bead is formed by using the beader frame with the After presetting the sleeve, disconnect the tubproper rollers attached. The inside and outside ing from the fitting and check the following points 112
Sleeve pilot Tube nut for aluminum alloy tubing or 0.015 inch for Fitting.
Sleeve • Tube steel tubing.
(c.) A slight collapse of the tube at the sleeve cut is permissible. No movement of the sleeve pilot, except rotation, is permissible.
REPAIR OF METAL TUBE LINES Sleeve cutting edge Scratches or nicks no deeper than 10 percent Step 1 of the wall thickness in aluminum alloy tubing may be repaired, if they are not in the heel of a bend. Replace lines with severe die marks, seams, or splits in the tube. Any crack or deformity in a flare is also unacceptable and is cause for rejection. A dent of less than 20 percent of the tube diameter is not objectionable, unless it is in the heel of a bend. Dents can be removed by drawing a bullet of proper size through the tube by means of a length of cable.
A severely damaged line should be replaced.
However, the line can be repaired by cutting out the damaged section and inserting a tube section of the same size and material. Flare both ends of the undamaged and replacement tube sections and make the connection by using standard unions, sleeves, and tube nuts. If the damaged portion is short enough, omit the insert tube and repair by using one union and two sets of con- Step 2 necting fittings.
3/32 to When repairing a damaged line, be very careful 1/8 inch to remove all chips and burrs. Any open line that is to be left unattended for some time should be sealed, using metal, wood, rubber, or plastic plugs or caps.
When repairing a low-pressure line using a flexible fluid connection assembly, position the Step 3 hose clamps carefully in order to prevent overhang Slight deformation permissible of the clamp bands or chafing of the tightening screws on adjacent parts. If chafing can occur, .005 inch maximum - aluminum the hose clamps should be repositioned on the alloy tubing hose. Figure 5-16 illustrates the design of a flexible 015 inch maximum - corrosion fluid connection assembly and gives the maximum resistant steel tubing allowable angular and dimensional offset.
FIGURE 5-15. Presetting flareless-tube assembly. Layout of Lines (illustrated in step 3):
(a.) The tube should extend ⅔2 to ⅛ inch beyond the sleeve pilot; otherwise blowoff may occur.
(b.) The sleeve pilot should contact the tube or have a maximum clearance of 0.005 inch Remove the damaged or worn assembly, taking care not to further damage or distort it, and use it as a forming template for the new part. If the old length of tubing cannot be used as a pattern, make a wire template, bending the pattern by hand as required for the new assembly. Then bend the tubing to match the wire pattern.
Never select a path that does not require bends 113
Clamps 1/4" Min.
Ref.
Nut Socket Fitting Hose Tubing Nipple 3° Max.
Angular deviation 1/16" Max offset Minimum gap "G" shall be 1/2" or Tube OD/4, whichever is greater.
Maximum gap "G" is not limited except on suction lines using other than self-sealing hose. On such suction lines, maximum G shall be 1-1/2 inch or one tube diameter, whichever is greater.
FIGURE 5-16. Flexible fluid connection assembly.
in the tubing. A tube cannot be cut or flared accurately enough so that it can be installed without bending and still be free from mechanical strain. Bends are also necessary to permit the tubing to expand or contract under temperature changes and to absorb vibration. If the tube is small (under one-fourth inch) and can be hand formed, casual bends may be made to allow for this. If the tube must be machine formed, definite bends must be made to avoid a straight assembly.
Start all bends a reasonable distance from the fittings, because the sleeves and nuts must be slipped back during the fabrication of flares and during inspections. In all cases the new tube assembly should be so formed prior to installation that it will not be necessary to pull or deflect the assembly into alignment by means of the coupling nuts.
FABRICATION AND REPLACEMENT OF
FLEXIBLE HOSE Hose and hose assemblies should be checked for deterioration at each inspection period. Leakage, separation of the cover or braid from the inner tube, cracks, hardening, lack of flexibility, and FIGURE 5-17. Sleeve-type fittings.
excessive "cold flow" are apparent signs of deterioration and reason for replacement. The term "cold flow" describes the deep, permanent impressions in the hose produced by the pressure of hose clamps or supports.
When failure occurs in a flexible hose equipped with swaged end fittings, the entire assembly must be replaced. Obtain a new hose assembly of the correct size and length, complete with factoryinstalled end fittings.
When failure occurs in hose equipped with reusable end fittings, a replacement line can be fabricated with the use of such tooling as may be necessary to comply with the assembly instructions of the manufacturer.
Assembly of Sleeve-Type Fittings Sleeve-type end fittings for flexible hose are detachable and may be reused if determined to be serviceable. The inside diameter of the fitting is the same as the inside diameter of the hose to which it is attached. Common sleeve-type fittings are shown in figure 5-17.
To make a hose assembly, select the proper size hose and end fittings. Cut the hose to the correct length using a fine-tooth hacksaw. Place the socket in a vise. Screw the hose into the socket counterclockwise until the hose bottoms on the shoulder of the socket (figure 5-18); then back off one-quarter turn. Lubricate inside of hose and nipple threads liberally. Mark the hose position 114
assembly prior to tightening the cap or plug.
Hose tests, using a gas, are conducted underwater.
In all cases follow the hose manufacturer's instructions for proof-test pressure and fluid to be used when testing a specific hose assembly.
Place the hose assembly in a horizontal position and observe for leakage while maintaining the test pressure. Proof-test pressures should be maintained for at least 30 seconds.
<I T FIGURE 5-18. Assembly of MS fitting to flexible hose.
around the hose at the rear of the socket using a grease pencil or painted line. Insert the nipple into the nut and tighten the nipple and nut on the assembly tool. If an assembly tool is not available, a mating AN815 adapter may be used.
Using a wrench on the assembly tool, screw the nipple into the socket and hose. A ½2- to %-inch clearance between the nut and sleeve is required so that the nut will swivel freely when the assembly tool is removed. After assembly, always make sure all foreign matter is removed from inside the hose by blowing out with compressed air.
Proof-test After Assembly All flexible hose must be proof-tested after assembly by plugging or capping one end of the hose and applying pressure to the inside of the hose assembly. The proof-test medium may be a liquid or a gas. For example, hydraulic, fuel, and oil lines are generally tested using hydraulic oil or water, whereas air or instrument lines are tested with dry, oil-free air or nitrogen. When testing with a liquid, all trapped air is bled from the Instaliation of Flexible Hose Assemblies Flexible hose must not be twisted on installation, since this reduces the life of the hose considerably and may also loosen the fittings. Twisting of the hose can be determined from the identification stripe running along its length. This stripe should not spiral around the hose.
Flexible hose should be protected from chafing by wrapping it with tape, but only where necessary.
The minimum bend radius for flexible hose varies according to size and construction of the hose and the pressure under which the hose is to operate. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value (figure 5-19).
Flexible hose should be installed so that it will be subject to a minimum of flexing during operation. Although hose must be supported at least every 24 inches, closer supports are desirable. A flexible hose must never be stretched tightly between two fittings. From 5 percent to 8 percent of its total length must be allowed for freedom of movement under pressure. When under pressure, flexible hose contracts in length and expands in diameter.
Protect all flexible hose from excessive temperatures, either by locating the lines so they will not be affected or by installing shrouds around them.
INSTALLATION OF RIGID TUBING Before installing a line assembly in an aircraft, inspect the line carefully. Remove dents and scratches, and be sure all nuts and sleeves are snugly mated and securely fitted by proper flaring of the tubing. The line assembly should be clean and free of all foreign matter.
Connection and Torque Never apply compound to the faces of the fitting or the flare, for it will destroy the metal-to-metal contact between the fitting and fare, a contact 115
PLANNING HOSE LINE INSTALLATIONS WRONG
WRONG RIGHT
2... observe linear stripe. The hose must not be twisted. High pressures applied to a twisted hose may cause failure or loosen the nut.
RIGHT
1... provide slack or bend in the hose line to provide for changes in length that will occur when pressure is applied.
WRONG
WRONG RIGHT RIGHT
3.., relieve sharp bends, avoid strain or hose collapse nd make cleaner installations by using Aeroquir elbows or other adapter fittings. Provide as large a bend radius as possible. Never use less than the recommended minimum bend radius specified for the hose.
4... provide additional bend radius when lines are subject to flexing and remember that the metal end fittings are not flexible. Place line support clamps so as not to restrict hose flexing.
FrcURE 5-19. Flexible hose installation.
which is necessary to produce the seal. Be sure Do not deflect into place that the line assembly is properly aligned before Replace tube assembly tightening the fittings. Do not pull the installation into place with torque on the nut. Correct and Incorrect - Will damage flare or threads, or cause sleeve to incorrect methods of installing flared-tube assemcrack under vibration if tight- blies are illustrated in figure 5-20. Proper torque ened values are given in figure 5-21. It must be remembered that these torque values are for flared- Incorrect - May pull off or type fittings only. Always tighten fittings to the distort flare if tightened correct torque value when installing a tube assembly. Overtightening a fitting may badly damage Correctly fitted or completely cut off the tube flare, or it may ruin and tightened the sleeve or fitting nut. Failure to tighten suf- .025 clearance between Flare and shoulder allow the line to blow out of the assembly or to ficiently also can be serious, as this condition may before tightening leak under system pressure.
The use of torque wrenches and the prescribed torque values prevents overtightening or under- FIGURE 5-20. Correct and incorrect methods of tightening. If a tube fitting assembly is tightened properly, it can be removed and retightened many tightening flared fittings. times before re-flaring is necessary.
Hose End Fittings Minimum bend and Hose Assemblies radii (inches) Aluminum Alloy Tubing, Bolt, Steel Tubing, Bolt MS28740 or Equivalent Alum.
Fitting or Nut Fitting or Nut End Fitting alloy Tubing Fitting Bolt Torque Torque 1100-H14 O.D. or Nut Size inch-Ibs. inch--Ibs. Minimum Maximum 5052-0 Steel 1/8 -2 3/8 20 - 30 3/16 -3 30 - 40 90 - 100 70 120 7/16 21/32 1/4 40 - 65 135 - 150 100 250 9/16 7/8 5/16 60 85 180 - 200 210 420 3/4 1 1/8 3/8 75 - 125 270 - 300 300 480 15/16 1 5/16 1/2 150 - 250 450 - 500 500 850 1 1/4 1 3/4 5/8 --10 200 - 350 650 - 700 700 1150 1 1/2 2 3/16 3/4 - 12 300 - 500 900 - 1000 1 3/4 2 5/8 7/8 -14 500 - 600 1000 - 1100
- 16 500 - 700 1200 - 1400 3 3 1/2 1-1/4 -20 600 - 900 1200 - 1400 3 3/4 4 3/8 1-1/2 -24 600-- 900 1500 - 1800 5 5 1/4 I -3/4 -28 850 - 1050 7 6 1/8 -32 950 - 1150 7 FIGURE 5-21. Flared fitting data.
Flareless Tube Installation Tighten the nut by hand until an incrcase in resistance to turning is encountered. Should it be impossible to run the nut down with the fingers, use a wrench, but be alert for the first signs of bottoming. It is important that the final tightening commence at the point where the nut just begins to bottom.
With a wrench, turn the nut % turn (one flat on a hex nut). Use a wrench on the connector to prevent it from turning while tightening the nut.
After the tube assembly is installed, the system should be pressure tested. Should a connection leak, it is permissible to tighten the nut an additional % turn (making a total of ⅓ turn). If, after tightening the nut a total of ½ turn, leakage still exists, the assembly should be removed and the components of the assembly inspected for scores, cracks, presence of foreign material, or damage from overtightening.
NOTE: Overtightening a flareless-tube nut drives the cutting edge of the sleeve deeply into the tube, causing the tube to be weakened to the point where normal in-flight vibration could cause the tube to shear. After inspection (if no discrepancies are found), reassemble the connections and repeat the pressure test procedures.
CAUTION: Do not in any case tighten the nut beyond ⅓ turn (two flats on the hex nut) ; this is the maximum the fitting may be tightened without the possibility of permanently damaging the sleeve and nut.
Common faults are:
1. Flare distorted into nut threads.
2. Sleeve cracked.
3. Flare cracked or split.
4. Flare out of round.
5. Inside of flare rough or scratched.
6. Fitting cone rough or scratched.
7. Threads of nut or union dirty, damaged or broken.
Some manufacturers service instructions will specify wrench torque values for flareless tubing installations (e.g., see figure 5-22).
PLUMBING ASSEMBLY PRECAUTIONS Make certain that the material in the fittings used is similar to that of the tubing; for example, use steel fittings with steel tubing and aluminum alloy fittings with aluminum alloy tubing. Brass fittings plated with cadmium may be used with aluminum alloy tubing.
For corrosion prevention, aluminum alloy lines and fittings are usually anodized. Steel lines and fittings, if not stainless steel, are plated to prevent rusting or corroding. Brass and steel fittings are usually cadmium plated, although some may come plated with nickel, chromium, or tin.
To ensure proper sealing of hose connections and to prevent breaking hose clamps or damaging the hose, follow the hose clamp tightening instructions carefully. When available, use the hose clamp torque-limiting wrench. These wrenches are available in calibrations of 15 and 25 inch-pounds. In the absence of torque-limiting wrenches, the fingertight-plus-turns method should be followed. Be- 117
WRENCH TORQUE FOR 304 1/8 H STEEL TUBES Tube Wrench Torque Outside Diameter Wall Thickness Inch-Pounds 3/16 0.016 90 - 110 3/16 0.020 90 - 110 1/4 0.016 110 - 140 1/½ 0.020 110 - 140 5/16 0.020 100 - 120 3/8 0.020 170 - 230 3/8 0.028 200-250 1/2 0.020 300 - 400 1/2 0.028 400 - 500 1/2 0.035 500 - 600 5/8 0.020 300 - 400
0.035 600 - 700 5/8 0.042 700 - 850 3/4 0.028 650 - 800 3/4 0.049 800 - 960
0.020 800 - 950
0.065 1600 - 1750 WRENCH TORQUE FOR 304-1A or 3471A STEEL T IBES 3/8 0.042 145 - 175 1/2 0.028 300 - 400 1/2 0.049 500 - 600
0.035 750 - 900 WRENCH TORQUE FOR 6061-T6 OR T4 TUBES 1/4 0.035 110 - 140 3/8 0.035 145 - 175 1/2 0.035 270 - 330 1/2 0.049 320 - 380 5/8 0.035 360 - 440 5/8 0.049 425 -- 525 3/4 0.035 380 - 470
0.035 750 - 900 1 1/4 0.035 900 - 1100 FIGURE 5-22. Torque values for flareless fittings.
Hose clamp tightening, finger-tight-plus turns method cause of the variations in hose clamp design and Initial installation Worm screw type Clamps-radiat and hose structure, the values given in figure 5-23 are only per inch lamp 10 threads other type-28 threads per inch approximate. Therefore, use good judgment when Self sealing hose approximately 15 Finger-tight-plus 2 complete turns 2-1/2 complete turns Finger-tight-plus connections are subject to "cold flow" or a setting tightening hose clamps by this method. Since hose inch-pounds process, a followup tightening check should be All other aircraft Finger-tight Finger-tight made for several days after installation.
hose approximately 25 Plus 1¼ complete Plus 2 complete turns inch-pounds turns Retightening of Hose Clamps If Clamps do not seal at specified tightening, examine hose connections and replace parts as necessary The above is for initial installation and should not be used for loose clamps For re-tightening loose hose clamps in service proceed as follows:
Non-self-scaling hose - If the clamp screw cannot be tightened with the fingers do not disturb unless leakage is evident. If leakage is present tighten 1/4 turn.
2. Self-sealing hose - If looser than finger-tight, tighten to finger tight and add 1/4 turn.
FIGURE 5-23. Hose clamp tightening.
SUPPORT CLAMPS Support clamps are used to secure the various lines to the airframe or powerplant assemblies.
Several types of support clamps are used for this purpose. The rubber-cushioned and plain are the most commonly used clamps. The rubber-cushioned clamp is used to secure lines subject to vibration;
the cushioning prevents chafing of the tubing. The plain clamp is used to secure lines in areas not subject to vibration.
A Teflon-cushioned clamp is used in areas where the deteriorating effect of Skydrol ® 500, hydraulic fluid (MIL-0-5606), or fuel is expected. However, because it is less resilient, it does not provide as 118
good a vibration-damping effect as other cushion materials. Tube OD (in.) Distance between supports (in.)
Use bonded clamps to secure metal hydraulic, Aluminum Alloy Steel fuel, and oil lines in place. Unbonded clamps 9½ 11½ should be used only for securing wiring. Remove 12 13½ 14 16 any paint or anodizing from the portion of the tube 15 18 at the bonding clamp location. Make certain that 16½ 19 23 20 clamps are of the correct size. Clamps or support- 22 25% ing clips smaller than the outside diameter of the hose may restrict the flow of fluid through the hose. 1 26% 24 30 27½ All plumbing lines must be secured at specified intervals. The maximum distance between sup- FIGURE 5-24. Maximum distance between ports for rigid fluid tubing is shown in figure 5-24. supports for fluid tubing.
119 ......
CHAPTER 6
AIRCRAFT HARDWARE, MATERIALS, AND PROCESSES
AIRCRAFT HARDWARE Aircraft hardware is the term used to describe the various types of fasteners and miscellaneous small items used in the manufacture and repair of aircraft.
The importance of aircraft hardware is often overlooked because of its small size; however, the safe and efficient operation of any aircraft is greatly dependent upon the correct selection and use of aircraft hardware.
Identification Most items of aircraft hardware are identified by their specification number or trade name.
Threaded fasteners and rivets are usually identified by AN (Air Force-Navy), NAS (National Aircraft Standard), or MS (Military Standard) numbers. Quick-release fasteners are usually identified by factory trade names and size designations.
THREADED FASTENERS Various types of fastening devices allow quick dismantling or replacement of aircraft parts that must be taken apart and put back together at frequent intervals. Riveting or welding these parts each time they are serviced would soon weaken or ruin the joint. Furthermore, some joints require greater tensile strength and stiffness than rivets can provide. Bolts and screws are two types of fastening devices which give the required security of attachment and rigidity. Generally, bolts are used where great strength is required, and screws are used where strength is not the deciding factor.
Bolts and screws are similar in many ways.
They are both used for fastening or holding, and each has a head on one end and screw threads on the other. Regardless of these similarities, there are several distinct differences between the two types of fasteners. The threaded end of a bolt is always blunt while that of a screw may be either blunt or pointed.
The threaded end of a bolt usually has a nut screwed onto it to complete the assembly. The threaded end of a screw may fit into a female receptacle, or it may fit directly into the material being secured. A bolt has a fairly short threaded section and a comparatively long grip length or unthreaded portion, whereas a screw has a longer threaded section and may have no clearly deñned grip length. A bolt assembly is generally tightened by turning the nut on the bolt; the head of the bolt may or may not be designed for turning.
A screw is always tightened by turning its head.
When it becomes necessary to replace aircraft fasteners, a duplicate of the original fastener should be used if at all possible. If duplicate fasteners are not available, extreme care and caution must be used in selecting substitutes.
Classification of Threads Aircraft bolts, screws, and nuts are threaded in either the NC (American National Coarse) thread series, the NF (American National Fine) thread series, the UNC (American Standard Unified Coarse) thread series, or the UNF (American Standard Unified Fine) thread series. There is one difference between the American National series and the American Standard Unified series that should be pointed out. In the 1-inch-diameter size, the NF thread specified 14 threads per inch (1-14NF), while the UNF thread specifies 12 threads per inch (1-12UNF). Both type threads are designated by the number of times the incline (threads) rotates around a 1-inch length of a given diameter bolt or screw. For example, a 4-28 thread indicates that a 4-inch-diameter bolt has 28 threads in 1 inch of its threaded length.
Threads are also designated by Class of fit.
The Class of a thread indicates the tolerance allowed in manufacturing. Class 1 is a loose fit, Class 2 is a free fit, Class 3 is a medium fit, and Class 4 is a close fit. Aircraft bolts are almost always manufactured in the Class 3, medium fit.
* Standard head bolt Drilled hey head bolt ank head bolt Internal hex head bolt Evebolt Clevis bolt 4037
SOCIA M COOPER
AN STANDARD AN STANDARD AN STANDARD AN STANDARD AN STANDARD STEEL BOLT STEEL BOLT STEEL BOLT STEFI. BOLT STEEL BOLT (CORROSION RESISTANT) SPEC
AN STANDARD AN STANDARI AN STANDARD AN STANDARD SPECIAL BOLT
STEEL BOLT STEEL BOUT STEEL BOLT STEEL BOLT (63-59131
SPECIAL BOLT DRILLED HEAD SPECIAL BOLT NAS CLOSE ALUMINUM
BOLT TOLERANCE BOLT ALLOY (2021) BOLT
MAGNETICALLY ORANCE-DYED CLEVIS BOLT REWORKED BOLT LOW STRENCTH INSPECTED MAGNETIC INS. MATERIAL BOLT FIGURE 6-1. Aircraft bolt identification.
A Class 4 fit requires a wrench to turn the nut onto a bolt, whereas a Class 1 fit can easily be turned with the fingers. Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly.
Bolts and nuts are also produced with righthand and left-hand threads. A right-hand thread tightens when turned clockwise; a left-hand thread tightens when turned counterclockwise.
AIRCRAFT BOLTS Aircraft bolts are fabricated from cadmium- or zinc-plated corrosion-resistant steel, unplated corrosion-resistant steel, and anodized aluminum alloys. Most bolts used in aircraft structures are either general-purpose, AN bolts, or NAS internalwrenching or close-tolerance bolts, or MS bolts.
In certain cases, aircraft manufacturers make 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.
Special bolts are usually identified by the letter "S" stamped on the head.
AN bolts come in three head styles-hex-head, clevis, and eyebolt (see figure 6-1). NAS bolts are available in hex-head, internal-wrenching, and countersunk head styles. MS bolts come in hexhead and internal-wrenching styles.
General-Purpose Bolts The hex-head aircraft bolt (AN-3 through AN-20) is an all-purpose structural bolt used for general applications involving tension or shear loads where a light-drive fit is permissible (.006- inch clearance for a ⅛-inch hole, and other sizes in proportion).
Alloy steel bolts smaller than No. 10-32 and 122