100° COUNTERSUNK UNIVERSAL MS 20426 MS 20470 For countersunk applications. For protruding head applications.
100° COUNTERSUNK UNISINK NAS 1097 For thin top sheet machine countersunk applications for use in very thin top sheets. Strength equal to A combination countersunk & protruding head double-dimpling without the high cost.
156° COUNTERSUNK A large diameter, shallow countersunk head providing wide bearing area for honey-comb applications.
FIGURE 6-43. Cherrylock rivet heads.
marked on the rivet head and have a total grip range of 16 inch. Figure 6-44 demonstrates a typical grip accommodation.
To determine the proper grip rivet to use, measure the material thickness with a grip selection gage (available from blind rivet manufacturers).
The proper use of a grip selector gage is shown in figure 6-45.
READ READ Max. Min.
Grip 1/4" 3/16" Grip Rivet Grip Number to be used: -4 FICURE 6-14. Typical grip length. FIGURE 6-15. Grip gage use.
Olympic Screw and Rivet Corporation- RV 2 0 0 - 4 - 2 -Grip range in 16ths of an inch.
-Shank diameter in 32nds of an inch:
4 = ⅛ inch. 6 = 16 inch.
5 = ⅔2 inch. 8 = ¼ inch.
A - Thickness of material (grip range) -Head style:
B - 3/64 to 1/8 inch 0 = universal head.
C - Total rivet shank length 1 = 100° countersunk.
FIGURE 6-46. Determining rivet length.
Material composition of shank:
The thickness of the material being riveted determines the overall length of the shank of the rivet. 0 = 2017 aluminum alloy.
As a general rule, the shank of the rivet should 5 = 5056 aluminum alloy.
extend beyond the material thickness approxi- 7 = mild steel.
mately 64 inch to /8 inch before the stem is pulled (see figure 6-46). Rivet type:
Rivet Identification 2 = self-plugging (friction lock).
Each company that manufactures self-plugging 5 = hollow pull-thru.
(friction lock) rivets has a code number to help users obtain the correct rivet for the grip range Manufacturer:
or material thickness of a particular installation. Olympic Rivet and Screw Corporation.
In addition, MS numbers are used for identification purposes. The following examples of part Figure 6-48.
numbers for self-plugging (friction lock) rivets are Townsend Company, Cherry Rivet Divisionrepresentative of each. CR 163 - 6 - 6 Huck Manufacturing Company- 9SP-B - A 6 - 3 -Grip. range (material thickness) in 16ths of an inch.
-Shank diameter in 32nds of an -Grip range (material thickness):
knob stem in 32nds of an inch; serrated stem in 16ths of an inch.
inch: -Rivet shank diameter in 32nds of an inch:
4 = ⅛ inch. 6 = 16 inch.
5 = ⅔2 inch. 8 = ¼ inch. 4 = ⅛ inch. 6 = 16 inch.
5 = ⅔2 inch. 8 = ¼ inch.
Material composition of shank:
A = 2017 aluminum alloy. Series number:
B = 5056 aluminum alloy. Designates rivet material, type of rivet, R = mild steel. and head style (163 = 2117 aluminum Head style: alloy, self-plugging (friction lock) rivet, protruding head).
9SP-B = brazier or universal head.
9SP-100 = 100° countersunk head. Cherry Rivet.
Figure 6-47. Figure 6-49.
Military Standard Number- MS 20600 B 4 K 2 Grip range (material thickness) in ⅙ths Flat-0.32 Head Thickness of an inch.
6-45 6-75 6-100 Type of stem: 8-45 8-75 8-100 K = knot head stem. 10-45 6845 10-75 6875 10-100 68100 W = serrated stem. 8B45 8875 88100 Shank diameter in 32nds of an 10845 6K45 10875 6K73 108100 6K100 inch: 8K45 8K75 8K100 10K45 10K75 10K100 4 = ⅛ inch. 6 = ¾6 inch. 6K845 6KB75 6K#100 5 = ⅔g inch. 8 = ¼ inch. 10KB45 6K345 10KB75 BKB75 10K8100 SKB100 Material composition of sleeve: 100°-0.48 Head Thickness AD = 2117 aluminum alloy.
B = 5056 aluminum alloy. 6-91 8-91 8-121 6-121 8-146 6-146 Type of rivet and head style: 10-91 6891 10-121 68121 10-146 68146 20600 = self-plugging (friction lock) 8891 8B121 88146 protruding head. 10891 10B121 108146 20601 = self-plugging (friction lock) 100°-0.63 Head Thickness 100° countersunk head. 6-106 6-136 6-161 8-106 8-136 8-161 Military Standard. 10-106 6B106 10-136 68136 10-161 6B161 Figure 6-50. 88106 86136 8฿T61 108106 108136 10B161 6K106 6K136 6K161 8K106 8K136 8K161 Rivnuts 10K106 10K136 10K161 This is the trade name of a hollow, blind rivet SK 106 8KB106 8KB136 6KB136 8KB161 6KB161 made of 6053 aluminum alloy, counterbored and T0KB106 10KB136 10KB161 threaded on the inside. Rivnuts can be installed by one person using a special tool which heads the rivet on the blind side of the material. The Rivnut is threaded on the mandrel of the heading tool and inserted in the rivet hole. The heading tool is held at right angles to the material, the handle is squeezed, and the mandrel crank is turned clockwise after each stroke. Continue squeezing the handle and turning the mandrel crank of the heading tool until a solid resistance is felt, which indicates that the rivet is set.
The Rivnut is used primarily as a nut plate and in the attachment of deicer boots to the leading edges of wings. It may be used as a rivet in secondary structures or for the attachment of accessories such as brackets, instruments, or soundproofing materials.
Rivnuts are manufactured in two head types, each with two ends; the flat head with open or closed end, and the countersunk head with open or closed end. All Rivnuts, except the thin-head countersunk type, are available with or without small projections (keys) attached to the head to FIGURE 6-51. Rivnut data chart.
keep the Rivnut from turning. Keyed Rivnuts are used as a nut plate, while those without keys are used for straight blind riveting repairs where no torque loads are imposed. A keyway cutter is needed when installing Rivnuts which have keys.
The countersunk style Rivnut is made with two different head angles; the 100° with .048- and .063-inch head thickness, and the 115° with .063- inch head thickness. Each of these head styles is made in three sizes, 6-32, 8-32, and 10-32. These numbers represent the machine screw size of the threads on the inside of the Rivnut. The actual outside diameters of the shanks are 6 inch for the 6-32 size, ⅔2 inch for the 8-32 size, and ¼ inch for the 10-32 size.
Open-end Rivnuts are the most widely used and are recommended in preference to the closedend type wherever possible. However, closed-end Rivnuts must be used in pressurized compartments.
Rivnuts are manufactured in six grip ranges.
The minimum grip length is indicated by a plain head, and the next higher grip length by one radial dash mark on the head. Each succeeding grip range is indicated by an additional radial dash mark until five marks indicate the maximum range.
Notice in figure 6-51 that some part number codes consist of a "6", an "g", or a "10", a "dash", and two or three more numbers. In some, the dash is replaced by the letters "K" or "KB".
The first number indicates the machine screw size of the thread, and the last two or three numbers indicate the maximum grip length in thousandths of an inch. A dash between the figures indicates that the Rivnut has an open end and is keyless; a "B" in place of the dash means it has a closed end and is keyless; a "K" means it is an open end and has a key; and a"KB" indicates that it has a closed end and a key. If the last two or three numbers are divisible by five, the Rivnut has a flathead; if they are not divisible by five, the Rivnut has a countersunk An example of a part number code is:
10 KB 106 -Grip length.
Closed end and key.
Screw and thread size.
Dill Lok-Skrus and Dill Lok-Rivets Dill "Lok-Skru" and "Lok-Rivet" (see figure 6-52) are trade names for internally threaded rivets. They are used for blind attachment of such accessories as fairings, fillets, access door covers, door and window frames, floor panels, and the like. Lok-Skrus and Lok-Rivets are similar to the Rivnut in appearance and application;
however, they come in two parts and require more clearance on the blind side than the Rivnut to accommodate the barrel.
The Lok-Rivet and the Lok-Skru are alike in construction, except the Lok-Skru is tapped internally for fastening an accessory by using an attaching screw, whereas the Lok-Rivet is not tapped and can be used only as a rivet. Since both Lok-Skrus and Lok-Rivets are installed in the same manner, the following discussion for the FIGURE 6-52. Internally threaded rivet The main parts of a Lok-Skru are the barrel, the head, and an attachment screw. The barrel is made of aluminum alloy and comes in cither closed or open ends. The head is either aluminum alloy or steel, and the attachment screw is made of steel. All of the steel parts are cadmium plated, and all of the aluminum parts are anodized to resist corrosion. When installed, the barrel screws up over the head and grips the metal on the blind side. The attaching screw is then inserted if needed. There are two head types, the flathead and the countersunk head. The Lok-Skru is tapped for 7-32, 8-32, 10-32, or 10-24 screws, and the diameters vary from .230 inch for 6-32 screws, to .292 inch for 10-32 screws. Grip ranges vary from .010 inch to.225 inch.
Deutsch Rivets This rivet is a high-strength blind rivet used on late model aireraft. It has a minimum shear strength of 75,000 p.s.i., and can be installed by The Deutsch rivet consists of two parts, the stainless steel sleeve and the hardened steel drive pin (see figure 6-53). The pin and sleeve are coated with a lubricant and a corrosion inhibitor.
FIGURE 6-53. Deutsch rivet.
The Deutsch rivet is available in diameters of 16 4, or ⅛ inch. Grip lengths for this rivet range from ¾6 to 1 inch. Some variation is allowed in grip length when installing the rivet; for example, a rivet with a grip length of ¾6 inch can be used where the total thickness of materials is between 0.198 and 0.228 inch.
When driving a Deutsch rivet, an ordinary hammer or a pneumatic rivet gun and a flathead set are used. The rivet is seated in the previously drilled hole and then the pin is driven into the sleeve. The driving action causes the pin to exert pressure against the sleeve and forces the sides of the sleeve out. This stretching forms a shop head on the end of the rivet and provides positive fastening. The ridge on the top of the rivet head locks the pin into the rivet as the last few blows are struck.
Pin Rivets Pin (Hi-shear) rivets are classified as special rivets but are not of the blind type. Access to both sides of the material is required to install this type of rivet. Pin rivets have the same shear strength as bolts of equal diameters, are about 40 percent of the weight of a bolt, and require only about one-fifth as much time for installation as a bolt, nut, and washer combination. They are approximately three times as strong as solidshank rivets.
Pin rivets are essentially threadless bolts. The pin is headed at one end and is grooved about the circumference at the other. A metal collar is swaged onto the grooved end effecting a firm, Stud Collar FIGURE 6-54. Pin (Hi-shear) rivet.
Pin rivets are fabricated in a variety of materials but should be used only in shear applications. They should never be used where the grip length is less than the shank diameter.
Part numbers for pin rivets can be interpreted to give the diameter and grip length of the individual rivets. A typical part number breakdown would be:
NAS 17-14 - -Maximum grip length in 16ths of an inch.
Nominal diameter in 32nds of an inch.
|177 = 100° countersunk head rivet.
178 = flathead rivet.
National Aircraft Standard.
PLASTICS Plastics are used in many applications throughout modern aircraft. These applications range from structural components of thermosetting plastics reinforced with fiber glass to decorative trim of thermoplastic materials.
Transparent Plastics Transparent plastic materials used in aircraft canopies, windshields, and other similar transparent enclosures may be divided into two major classes or groups. These plastics are classified according to their reaction to heat. The two classes are Thermoplastic and Thermosetting.
Thermoplastic materials will soften when heated and harden when cooled. These materials can be heated until soft, and then formed into the desired shape. When cooled, they will retain this shape.
The same piece of plastic can be reheated and reshaped any number of times without changing the chemical composition of the material.
Thermosetting plastics harden upon heating, and reheating has no softening effect. These plastics cannot be reshaped after once being fully cured by the application of heat.
In addition to the above classes, transparent plastics are manufactured in two forms, monolithic (solid) and laminated. Laminated transparent plastics are made from transparent plastic face sheets bonded by an inner-layer material, usually polyvinyl butyral. Because of its shatterresistant qualities, laminated plastic is superior 163
to solid plastics and is used in many pressurized aircraft. Most of the transparent sheet used in aviation is manufactured in accordance with various military specifications.
A new development in transparent plastics is stretched acrylic. Stretched acrylic is a type of plastic which, before being shaped, is pulled in both directions to rearrange its molecular structure. Stretched acrylic panels have a greater resistance to impact and are less subject to shatter;
its chemical resistance is greater, edging is simpler, and crazing and scratches are less detrimental.
Individual sheets of plastic are covered with a heavy masking paper to which a pressure-sensitive adhesive has been added. This paper helps to prevent accidental scratching during storage and handling. Care should be taken to avoid scratches and gouges which may be caused by sliding sheets against one another or across rough or dirty tables.
Sheets should be stored in bins which are tilted at approximately 10° from vertical, if possible. If they must be stored horizontally, piles should not be over 18 inches high, and small sheets should be stacked on the larger ones to avoid unsupported overhang. Storage should be in a cool, dry place away from solvent fumes, heating coils, radiators, and steampipes. The temperature in the storage room should not exceed 120° F.
While direct sunlight does not harm acrylic plastic, it will cause drying and hardening of the masking adhesive, making removal of the paper difficult. If the paper will not roll off easily, place the sheet in an oven at 250° F. for 1 minute, maximum. The heat will soften the masking adhesive for easy removal of the paper.
If an oven is not available, hardened masking paper may be removed by softening the adhesive with aliphatic naphtha. Rub the masking paper with a cloth saturated with naphtha. This will soften the adhesive and free the paper from the plastic. Sheets so treated must be washed immediately with clean water, taking care not to scratch the surfaces.
NOTE: Aliphatic naphtha is not to be confused with aromatic naphtha and other dry-cleaning solvents which are definitely harmful in their effects on plastic. However, aliphatic naphtha is flammable and all precautions regarding the use of flammable liquids must be observed.
Reinforced Plastic Reinforced plastic is a thermosetting material used in the manufacture of radomes, antenna covers, and wingtips, and as insulation for various pieces of electrical equipment and fuel cells. It has excellent dielectric characteristics which make it ideal for radomes; however, its high strengthweight ratio, resistance to mildew, rust, and rot, and ease of fabrication make it equally suited for other parts of the aircraft.
Reinforced plastic components of aircraft are formed of either solid laminates or sandwich-type laminates. Resins used to impregnate glass cloths are of the contact-pressure type (requiring little or no pressure during cure). These resins are supplied as a liquid which can vary in viscosity from a waterlike consistency to a thick sirup.
Cure or polymerization is effected by the use of a catalyst, usually benzoyl peroxide.
Solid laminates are constructed of three or more layers of resin-impregnated cloths "wet laminated" together to form a solid sheet facing or molded shape.
Sandwich-type laminates are constructed of two or more solid sheet facings or a molded shape enclosing a fiberglass honeycomb- or foam-type core. Honeycomb cores are made of glass cloths impregnated with a polyester or a combination of nylon and phenolic resins. The specific density and cell size of honeycomb cores varies over a considerable latitude. Honeycomb cores are normally fabricated in blocks that are later cut to the desired thickness on a bandsaw.
Foam-type cores are formulated from combinations of alkyd resins and metatoluene di-isocyanate. Sandwich-type fiberglass components filled with foam-type cores are manufactured to exceedingly close tolerances on overall thickness of the molded facing and core material. To achieve this accuracy, the resin is poured into a closetolerance, molded shape. The resin formulation immediately foams up to fill the void in the molded shape and forms a bond between the facing and the core.
RUBBER Rubber is used to prevent the entrance of dirt, water, or air, and to prevent the loss of fluids, 164
gases, or air. It is also used to absorb vibration, reduce noise, and cushion impact loads.
The term "rubber" is as all inclusive as the term "metal." It is used to include not only natural rubber, but all synthetic and silicone rubbers.
Natural Rubber Natural rubber has better processing and physical properties than synthetic or silicone rubber.
These properties include: Flexibility, elasticity, tensile strength, tear strength, and low heat buildup due to flexing (hysteresis). Natural rubber is a general-purpose product; however, its suitability for aircraft use is somewhat limited because of its inferior resistance to most influences that cause deterioration. Although it provides an excellent seal for many applications, it swells and often softens in all aircraft fuels and in many solvents (naphthas, etc.). Natural rubber deteriorates more rapidly than synthetic rubber. It is used as a sealing material for water/methanol systems.
Synthetic Rubber Synthetic rubber is available in several types, each of which is compounded of different materials to give the desired properties. The most widely used are the butyls, Bunas, and neoprene.
Butyl is a hydrocarbon rubber with superior resistance to gas permeation. It is also resistant to deterioration; however, its comparative physical properties are significantly less than those of natural rubber. Butyl will resist oxygen, vegetable oils, animal fats, alkalies, ozone, and weathering.
Like natural rubber, butyl will swell in pe.
troleum or coal tar solvents. It has a low waterabsorption rate and good resistance to heat and low temperature. Depending on the grade, it is suitable for use in temperatures ranging from
- 65° F. to 300° F. Butyl is used with phosphate ester hydraulic fluids (Skydrol), silicone fluids, gases, ketones, and acetones.
Buna-S rubber resembles natural rubber both in processing and performance characteristics.
Buna-S is as water-resistant as natural rubber, but has somewhat better aging characteristics.
It has good resistance to heat, but only in the absence of severe flexing. Generally, Buna-S has poor resistance to gasoline, oil, concentrated acids, and solvents. Buna-S is normally used for tires and tubes as a substitute for natural rubber.
Buna-N is outstanding in its resistance to hydrocarbons and other solvents; however, it has poor resilience in solvents at low temperature.
Buna-N compounds have good resistance to temperatures up to 300° F., and may be procured for low temperature applications down to - 75° F.
Buna-N has fair tear, sunlight, and ozone resistance. It has good abrasion resistance and good breakaway properties when used in contact with metal. When used as a seal on a hydraulic piston, it will not stick to the cylinder wall. Buna-N is used for oil and gasoline hose, tank linings, gaskets, and seals.
Neoprene can take more punishment than natural rubber and has better low-temperature characteristics. It possesses exceptional resistance to ozone, sunlight, heat, and aging. Neoprene looks and feels like rubber. Neoprene, however, is less like rubber in some of its characteristics than butyl or Buna. The physical characteristics of neoprene, such as tensile strength and elongation, are not equal to natural rubber but do have a definite similarity. Its tear resistance as well as its abrasion resistance is slightly less than that of natural rubber. Although its distortion recovery is complete, it is not as rapid as natural rubber.
Neoprene has superior resistance to oil. Although it is good material for use in nonaromatic gasoline systems, it has poor resistance to aromatic gasolines. Neoprene is used primarily for weather seals, window channels, bumper pads, oil-resistant hose, and carburetor diaphragms. It is also recommended for use with Freons and silicate ester lubricants.
Thiokol, known also as polysulfide rubber, has the highest resistance to deterioration but ranks the lowest in physical properties. Thiokol, in general, is not seriously affected by petroleum, hydrocarbons, esters, alcohols, gasoline, or water.
Thiokols are ranked low in such physical properties as compression set, tensile strength, elasticity, and tear abrasion resistance. Thiokol is used for oil hose, tank linings for aromatic aviation gasolines, gaskets, and seals.
Silicone rubbers are a group of plastic rubber Year marking materials made from silicon, oxygen, hydrogen, Year Threads Color and carbon. The silicons have excellent heat stability and very low temperature flexibility. They 1968_ 2 blue are suitable for gaskets, seals, or other applications 1969- 2 yellow where elevated temperatures up to 600° F. are 1970- 2 black prevalent. Silicone rubbers are also resistant to 1971- 2 green temperatures down to - 150° F. Throughout this 1972- 2 red temperature range, silicone rubber remains extremely flexible and useful with no hardness or Quarter marking gumminess. Although this material has good re- Quarter Threads sistance to oils, it reacts unfavorably to both aromatic and nonaromatic gasolines. Jan., Feb., Mar.
Apr., May, June 1 Silastic, one of the best known silicones, is used July, Aug., Sept...
Oct., Nov., Dec._-- 1 to insulate electrical and electronic equipment.
Because of its dielectric properties over a wide range of temperatures, it remains flexible and free FICURE 6-55. Shock absorber cord color coding.
from crazing and cracking. Silastic is also used for gaskets and seals in certain oil systems.
SHOCK ABSORBER CORD Shock absorber cord is made from natural rubber strands encased in a braided cover of woven cotton cords treated to resist oxidation and wear. Great tension and elongation are obtained by weaving the jacket upon the bundle of rubber strands while they are stretched about three times their original length.
There are two types of elastic shock-absorbing cord. Type I is a straight cord, and type II is a continuous ring, known as a "bungee." The advantages of the type II cord are that they are easily and quickly replaced and do not have to be secured by stretching and whipping. Shock cord is available in standard diameters from ¼ inch to 16 inch.
SEALS Seals are used to prevent fluid from passing a certain point, as well as to keep air and dirt out of the system in which they are used. The increased use of hydraulics and pneumatics in aircraft systems has created a need for packings and gaskets of varying characteristics and design to meet the many variations of operating speeds and temperatures to which they are subjected. No one style or type of seal is satisfactory for all installations. Some of the reasons for this are:
(1) Pressure at which the system operates, (2) the type fluid used in the system, (3) the metal finish and the clearance between adjacent parts, and
(4) the type motion (rotary or reciprocating), if any. Seals are divided into three main classes:
(1) Packings, (2) gaskets, and (3) wipers.
Three colored threads are braided into the outer cover for the entire length of the cord.
Two of these threads are of the same color and represent the year of manufacture; the third thread, a different color, represents the quarter of the year in which the cord was made. The code covers a 5-year period and then repeats itself. This makes it easy to figure forward or backward from the years shown in figure 6-55.
Packings Packings are made of synthetic or natural rubber. They are generally used as "running seals," that is, in units that contain moving parts, such as actuating cylinders, pumps, selector valves, etc. Packings are made in the form of O-rings, V-rings, and U-rings, each designed for a specific purpose. (See figure 6-56.)
Advances in aircraft design have necessitated new O-ring compositions to meet changed operating conditions. Hydraulic O-rings were originally established under AN specification numbers (6227, 6230, and 6290) for use in MIL-H-5606 fluid at U-ring O-ring temperatures ranging from -65° F. to +160° F.
When new designs raised operating temperatures to a possible 275° F., more compounds were developed and perfected.
Recently a compound was developed that offered improved low-temperature performance V-ring U-cup without sacrificing high-temperature performance, rendering the other series obsolete. This superior material was adopted in the MS28775 series. This series is now the standard for MIL-H-5606 systems where the temperature may vary from
- 65° F. to + 275° F.
Male V-ring adapter Female FIGURE 6-56. Packing rings.
O-Ring Packings O-ring packings are used to prevent both internal and external leakage. This type of packing ring seals effectively in both directions and is the type most commonly used. In installations subject to pressures above 1,500 p.s.i., backup rings are used with O-rings to prevent extrusion.
When an O-ring packing is to be subjected to pressure from both sides, as in actuating cylinders, two backup rings must be used (one on either side of the O-ring). When an O-ring is subject to pressure on only one side, a single backup ring is generally used. In this case the backup ring is always placed on the side of the O-ring away from the pressurc.
The materials from which O-rings are manufactured have been compounded for various operating conditions, temperatures, and fluids. An O-ring designed specifically as a static (stationary) seal will probably not do the job when installed on a moving part such as a hydraulic piston. Most O-rings are similar in appearance and texture, but their characteristics may differ widely. An O-ring will be useless if it is not compatible with the system fluid and operating temperature.
Manufacturers provide color coding on some O-rings, but this is not a reliable or complete means of identification. The color coding system does not identify sizes, but only system fluid or vapor compatibility and in some cases the manufacturer. Color codes on O-rings that are compatible with MIL-H-5606 fluid will always contain blue, but may also contain red or other colors. Packings and gaskets suitable for use with Skydrol fluid will always be coded with a green stripe, but may also have a blue, grey, red, green;
or yellow dot as a part of the color code. Color codes on O-rings that are compatible with hydrocarbon fluid will always contain red, and will never contain blue. A colored stripe around the circumference indicates that the O-ring is a boss gasket seal. The color of the stripe indicates fluid compatibility: red for fuel, blue for hydraulic fluid.
The coding on some rings is not permanent.
On others it may be omitted due to manufacturing difficulties or interference with operation. Furthermore, the color coding system provides no means to establish the age of the O-ring or its temperature limitations.
Because of the difficulties with color coding, O-rings are available in individual hermetically sealed envelopes, labeled with all pertinent data.
When selecting an O-ring for installation, the basic part number on the sealed envelope provides the most reliable compound identification.
Male V-ring Adjustment nuts adapter Female V-ring adapter V-ring packing FICURE 6-57. V-ring installation.
Although an O-ring may appear perfect at first glance, slight surface flaws may exist. These flaws are often capable of preventing satisfactory O-ring performance under the variable operating pressures of aircraft systems; therefore, O-rings should be rejected for flaws that will affect their performance. Such flaws are difficult to detect, and one aircraft manufacturer recommends using a 4-power magnifying glass with adequate lighting to inspect each ring before it is installed.
By rolling the ring on an inspection cone or dowel, the inner diameter surface can also be checked for small cracks, particles of foreign material, or other irregularities that will cause leakage or shorten the life of the O-ring. The slight stretching of the ring when it is rolled inside out will help to reveal some defects not otherwise visible.
Backup Rings Backup rings (MS28782) made of Teflon do not deteriorate with age, are unaffected by any system fluid or vapor, and can tolerate temperature extremes in excess of those encountered in highpressure hydraulic systems. Their dash numbers indicate not only their size but also relate directly to the dash number of the O-ring for which they are dimensionally suited. They are procurable under a number of basic part numbers, but they are interchangeable; that is, any Teflon backup ring may be used to replace any other Teflon backup ring if it is of proper overall dimension to support the applicable O-ring. Backup rings are not color coded or otherwise marked and must be identified from package labels.
The inspection of backup rings should include a check to ensure that surfaces are free from irregularities, that the edges are clean-cut and sharp, and that scarf cuts are parallel. When checking Teflon spiral backup rings, make sure that the coils do not separate more than ¾ inch when unrestrained.
V-Ring Packings V-ring packings (AN6225) are one-way seals and are always installed with the open end of the "V" facing the pressure. V-ring packings must have a male and female adapter to hold them in the proper position after installation. It is also necessary to torque the seal retainer to the value specified by the manufacturer of the component being serviced, or the seal may not give satisfactory service. An installation using V-rings is shown in figure 6-57.
U-Ring Packings U-ring packings (AN6226) and U-cup packings are used in brake assemblies and brake master cylinders. The U-ring and U-cup will seal pressure in only one direction; therefore, the lip of the packings must face toward the pressure. U-ring packings are primarily low-pressure packings to be used with pressures of less than 1,000 p.s.i.
GASKETS Gaskets are used as static (stationary) seals between two flat surfaces. Some of the more 168
common gasket materials are asbestos, copper, cork, and rubber. Asbestos sheeting is used wherever a heat-resistant gasket is needed. It is used extensively for exhaust system gaskets. Most asbestos exhaust gaskets have a thin sheet of copper edging to prolong their life.
A solid copper washer is used for spark plug gaskets where it is essential to have a noncompressible yet semisoft gasket.
Cork gaskets can be used as an oil seal belween the engine crankcase and accessories, and where a gasket is required that is capable of occupying an uneven or varying space caused by a rough surface or expansion and contraction.
Rubber sheeting can be used where there is a need for a compressible gasket. It should not be used in any place where it may come in contact with gasoline or oil because the rubber will deteriorate very rapidly when exposed to these Gaskets are used in fluid systems around the end caps of actuating cylinders, valves, and other units. The gasket generally used for this purpose is in the shape of an O-ring, similar to O-ring WIPERS Wipers are used to clean and lubricate the exposed portions of piston shafts. They prevent dirt from entering the system and help protect the piston shaft against scoring.
Wipers may be either the metallic or the felt type. They are sometimes used together, with the felt wiper installed behind the metallic wiper.
SEALING COMPOUNDS Certain areas of all aircraft are sealed to withstand pressurization by air, to prevent leakage of fuel, to prevent passage of fumes, or to prevent corrosion by sealing against the weather. Most sealants consist of two or more ingredients properly proportioned and compounded to obtain the best results. Some materials are ready for use as packaged, but others will require mixing before application.
One-Part Sealants One-part sealants are prepared by the manufacturer and are ready for application as packaged.
However, the consistency of some of these compounds may be altered to satisfy a particular method of application. If thinning is desired, the thinner recommended by the sealant manufacturer should be used.
Two-Part Sealants Two-part sealants are compounds requiring separate packaging to prevent cure prior to application and are identified as the base sealing compound and the accelerator. Any alteration of the prescribed ratios will reduce the quality of the material. Generally two-part sealants are mixed by combining equal portions (by weight) of base compound and accelerator.
All sealant material should be carefully weighed in accordance with the sealant manufacturer's recommendations. Sealant material is usually weighed with a balance scale equipped with weights specially prepared for various quantities of sealant and accelerator.
Before weighing the sealant materials, both the base sealant compound and the accelerator should be thoroughly stirred. Accelerator which is dried out, lumpy, or flaky should not be used. Preweighed sealant kits do not require weighing of the sealant and accelerator before mixing when the entire quantity is to be mixed.
After the proper amount of base sealant compound and accelerator has been determined, add the accelerator to the base sealant compound.
Immediately after adding the accelerator, thoroughly mix the two parts by stirring or folding, depending on the consistency of the material. The material should be mixed carefully to prevent entrapment of air in the mixture. Too rapid or prolonged stirring must be avoided as it will build up heat in the mixture and will shorten the normal application time (working life) of the mixed sealant.
To ensure a well-mixed compound, it may be tested by smearing a small portion on a clean, flat metal or glass surface. If flecks or lumps are found, continue mixing. If the flecks or lumps cannot be eliminated, the batch should be rejected.
The working life of mixed sealant is from onehalf hour to 4 hours (depending upon the class of sealant); therefore, mixed sealant should be applied as soon as possible or placed in refrigerated storage. Figure 6-58 presents general information concerning various sealants.
The curing rate of mixed sealants varies with changes in temperature and humidity. Curing of sealants will be extremely, slow if the temperature 169
Storage (shelf) Accelerator Mixing ratio Application life after Storage (shelf) Temperature Application and Sealant Base (catalyst) by weight life (work) mixing life unmixed range limitations EC-801 (black) EC-807. 12 parts of EC- 5 days at -65°F. to Faying surfaces, MIL-S- -20°F. after fillet seals, and 7502A Class parts of EC- flash freeze packing gaps.
B-2. at -65° F.
EC-800 (red) None. Use as is. 8-12 hours. Not applicable. 6-9 months. -65° F. to Coating rivets.
200°F.
EC-612 P None. Use as is. Indefinite non- Not applicable. 6-9 months. - 40°F. to Packing voids (pink) MIL-P- drying. 200° F. up to 1/4 inch.
20628.
PR-1302HT PR-1302HT-A. 10 parts of PR- 2-4 hours. 5 days at 6 months. - 65°F. to Sealing access.
(red) MII.-S- 1302HT-A to -20°F. after 200° F. door gaskets.
8784. 100 parts of flash freeze PR-1302HT. at -65°F.
PR-727 potting PR-727A. 12 parts of PR- 1½ hours mini- 5 days at 6 months. -65°F. to Potting electrical compound 727A to 100 mum. -20° F. after 200° F. connections MIL-S- parts of PR- flash freeze and bulkhead 8516B. 727. at -65°F. seals.
HT-3 (greygreen) None. Use as is. Solvent release, Not applicable. 6 to 9 months. - 60°F. to Sealing hot air sets up in 2 850° F. ducts passing to 4 hours. through bulkheads.
EC-776 (clear None. Use as is. 8-12 hours. Not applicable. Indefinite in -65°F. to Top coating.
amber) MIL- airtight con- 250°F.
S-4383B. tainers.
FIGURE 6-58. General sealant information.