Phase 1 Inspection Procedures PDF

Fasteners.1 Screw threads General Fasteners with screw threads allow parts to be fastened The same principle is utilized by a simple wedge. Anybody who together with all of the strength unthreaded fasteners provide....

Fasteners.1 Screw threads General Fasteners with screw threads allow parts to be fastened The same principle is utilized by a simple wedge. Anybody who together with all of the strength unthreaded fasteners provide. has been involved with wood-splitting by means of a wedge However, unlike rivets and pins, threaded fasteners may be knows about this. When the wedge is driven into a piece of disassembled and reassembled an almost infinite number of wood by exerting a force on the short edge, the forced times. multiplies outwards along the slopes of the long sides, opening the wood. You can also use the wedge to lift an object. History of threads The first real practical thread type in use is the Helix (or Principle of a thread Archimedes screw) used as a simple mechanical transportation A wedge is a simple machine which can be used to apply a device. The helix is a properly defined mathematical device. It load. If a wedge is driven under an object it will raise it, the describes the path of a point that travels along the distance being governed by the inclination of the wedge. A circumference at the same time as the circle advances along a steeper wedge will move it a greater distance, but a shallower line through its centre. The axial advance per turn depends of wedge will raise a greater weight. Of course, a wedge may also the slope of the helical path (or thread). The slope angle is be used to prevent movement as in a door stop. called pitch when we relate it to threaded fasteners. A screw may be considered as an inclined plane or wedge An object which has entered the rotating helix will be forced into wrapped around a cylinder or shaft to form a helix. The distance a circumferential movement. The shortest distance from the along the cylinder by one full turn of the helix is the ‛pitch‘ )P(. feed port to the discharge port is of course a straight line. An object travels actually much longer than necessary. The It can be seen from the diagram that a shallow wedge angle steeper the turns (or the higher the pitch) the higher advance produces a fine pitch thread while a steeper angle produces a per revolution. At first glance, this seems to be an advantage. coarse pitch thread. But, suppose you operate the helix by muscle power only, you will realize that it requires a high effort to turn the helix (in fact it may be impossible to turn it). If a low pitch helix was used instead you will realize that each revolution is now easily achieved. However, if the helix was used as a water pump you will see that the water supply has decreased! In other words; the helix is a simple and clever gear ratio device where the gear ratio depends primarily on the pitch of the helical thread. Egyptair Training Academy © Copyright 2023 1/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Male and female components of a screw thread The use of a wedge to exert a force Wedge angle and pitch Egyptair Training Academy © Copyright 2023 2/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners When a male thread is engaged with a female thread, e.g. a Thus, the force exerted in turning (or wrenching) the screw/bolt bol t in a nut the full surface area o f the female ‛wedge‘ i s in is multiplied by a mechanical advantage (or ratio) proportional contact with the male. More force (F) may be exerted between to the slope of the thread angle (pitch). This is how threaded the two surfaces of a shallow wedge than when the wedge fasteners develop large clamping forces. angle is greater. Also, as friction is dependent on applied load and surface area, a shallow wedge angle will produce more The head stops, the thread continues to advance and the body friction and greater resistance to both tightening and loosening. of the bolt/screw must stretch (or strain) in reaction to the This is a desirable property in threaded fasteners, particularly tensile stress developed in the bolt. When we stop turning the in smaller threads where the surface area is already limited by head of the bolt, the strain and residual stress remain and size. generate a clamping force proportional to the residual stress. When a thread is used to produce motion, e.g. a worm screw, If the tension force is sufficient the clamping force will prevent greater travel (P) will be produced by a coarse thread but again the bolt or screw from loosening. We say that the bolt is more force can be applied by a fine one. preloaded in tension. The correct preloading is secured by applying the correct torque while wrenching. All treaded fasteners consist of two items: The correct torque depends on the materials being clamped, The male thread (a screw or bolt), the dimension and thread type of the fastener, the temperature The female thread (a nut or a threaded hole in a range that is met during operation, etc. component). Notice that the female part is kept in compression. The higher Both the helix and the wedge principles apply to mechanical the compression load is on the female part and the higher the thread physics. The clue is that for a small tuning force tension load is on the male part, the higher is the clamping (moment or torque) large axial screw or bolt tension loadings force. The clamping force can be regarded as a spring force. can be obtained. When we talk about clamping forces, there are two distinctive The screw thread is nothing more than a wedge wrapped characteristics of the helical thread we must understand: First, around a cylinder. In effect, the force that we apply in turning when the male helix is turned through a female mating part, the the screw pushes against the short edge of the wedge. The helix must advance as it turns; and second, when the thread upper surface of the male thread, as it engages the lower advances to the point where the underside of the screw/bolt surface of the matching nut-thread, acts like the angled long head bottoms against the mating surface, further turning will side of the wedge. bring the wedging action into play. Egyptair Training Academy © Copyright 2023 3/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners The effect of coarse, medium and fine threads on applied force (F) and distance travelled (P) Stresses in a thread assembly Stresses in a thread Egyptair Training Academy © Copyright 2023 4/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Screw nomenclature Threads are either of the left or right-hand type. Threads are The major diameter is the distance measured from normally Right Handed (RH) and unless otherwise stated this crest to crest. is the norm. This means that the nut screws on with a clockwise The minor diameter is the distance measured from root rotation. Left Hand (LH) threads are of course the opposite. LH to root. threads are used extensively to secure rotating parts such as drive shafts, spur gears, etc. where the normal angular rotation An important part of any thread is the root and crest form (or would tend to tighten the nut. radii). Usually, but not always, this takes the form of a radius or sometimes a flat. The lead or thread pitch (axial advance per turn) is usually expressed in threads per inch (TPI) or as an absolute The root and crest form may also vary on the male and female dimension for one single pitch (1mm. 0.2mm.75mm etc.). threads. Production of correct radii is very difficult. A 100% Another important tread property is the thread angle. sharp "V" is undesirable as it may form the stress point for fracture and on bolts, cut fingers, etc. Apart from a number of specialist threads the included angles for the most common thread angle is 60°. This goes for these The pitch diameter is difficult to explain. On a parallel thread, thread types: it is the diameter of an imaginary cylinder which would pass through the threads at such a point that both male and female UNF, UNC, ANF, ANC, thread were at the same width. This point is usually ½ the Metric (or ISO), thread depth, but not always. These are examples of other thread types with different angles: It is only at ½-depth when the root and crest radii are the same. BA (47½°), The bolt head (or screw head) is an enlarged shape that is BSW and BSF (55°), formed at one end of the bolt to provide a bearing surface and Acme (29°). a part to attach a spanner to. The bearing surface is the supporting surface of the fastener with respect to the part that Do not be tempted to use male and female threads with it is fastening. The bolt is loaded through its bearing surface. differing thread angles. All loads are transferred to the thread The shank is the length of the cylindrical part between the head crests and cause high stress levels, leading to slackening in and the point of the fastener. The shank is divided into the grip service and premature failure. part and the thread part. Egyptair Training Academy © Copyright 2023 5/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Thread dimension nomenclature Thread dimension nomenclature Thread pitch comparison Egyptair Training Academy © Copyright 2023 6/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Root Truncation The root is the bottom of the groove joining adjacent sides or The distance measured radially from the crest or root of the flanks of the thread, whether of the male screw or of the female thread to the adjacent apex of the fundamental triangle. screw. Single and multiple threads Flank or side When formed by one continuous groove a thread is said to be The surface of the thread form which connects the crest with single or single start. Most threads used for adjustment and the root. fastening are single thread. In such threads the lead, that is, the distance travelled axially by an engaged threaded part in a Thread angle complete turn, is always equal to the pitch of the thread. The included angle between the flanks measured in the axial plane. Multiple start threads consist of two or more ridges left by grooves cut side by side. In this way, the axial travel or lead of Lead the thread is increased without changing the pitch. For The distance a screw thread advances axially in one complete example, a nut engaged with a double start thread will travel turn (i.e. same as the pitch for single start thread). twice as far in one complete turn as one engaged with a single thread of similar pitch. In multiple threads, the lead is equal to Length of engagement the pitch of the thread (P) multiplied by the number of starts. The axial distance over which two mating threads are designed to make contact. Note that: Angular depth lead (L) = pitch (P) x starts The triangle formed by the intersection of the extended flanks. The vertical height of this triangle is the angular depth. Right and left-hand threads A right-hand thread is one on which the thread is cut so that Actual depth turning of the nut in a clockwise direction will tighten it on a bolt. The distance between the crest and the root of the thread A left-hand thread requires the nut to be turned anti-clockwise measured perpendicular to the axis to tighten it. Left-hand threads are only used for special purposes. Fundamental triangle The triangle formed by the intersection of the extended flanks (XYZ). Egyptair Training Academy © Copyright 2023 7/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Thread pitch comparison Right -hand and left-hand thread Thread starts, pitch (p) and lead (L) relationships Egyptair Training Academy © Copyright 2023 8/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Fine thread and coarse thread Threaded fasteners are typical available in two distinctive types regardless of country of origin: Fine thread (or fine pitch), Coarse thread (or coarse pitch). There is no difference in the fatigue resistance between the two types. However, if the female fastener (the nut, etc.) is the weakest part, a coarse pitch is preferred. The reason is that the smaller the minor diameter of the female part, the greater the thread area (flank engagement) will be. This gives the highest static strength and highest fatigue limit. Except in cases where fine adjustment is needed coarse threads are therefore always preferred. Coarse threads have higher resistance to ”stripping“ )the thread sare ripped off), and, consequently, can therefore be wrenched (torqued) to higher clamping load and higher assembly strength. Coarse threads are also easier to assemble and misalignment is rare. Egyptair Training Academy © Copyright 2023 9/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Coarse pitch and fine pitch Fine pitch and coarse pitch on a pipe fitting Egyptair Training Academy © Copyright 2023 10/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Tolerances and classes of fit A s with all mechanical ‛fits‘ screw thread s are subje ct to Class 4 and 5 are little used and give a wrench or force fit tolerances. This tolerance is designated by Thread Class. requiring the application of high torque for semi-permanent assembly. In the Unified thread system, there are five classes of fit, Aircraft bolts are almost always manufactured in Class 3, A Class 1 fit thread is a loose fit, medium fit. A Class 2 fit is a free fit, A Class 3 fit is a medium fit, In the metric thread system, there are separate tolerances for A Class 4 fit s a close fit, and the major and minor dimensions of a thread. These are A Class 5 fit is a tight fit. indicated by upper case letters for female threads and lower case for male. There are: In any specification, the thread will have the letter A added for male (external) threads and B for female (internal) threads. 5 tolerance grades (grades 4 to 8) available for the minor Both partners must be of the same class. diameter of the nut thread. 3 tolerance grades (grades 4, 6 and 8) for the major The Class 1 fit allows you to turn the nut all the way down using diameter of the bolt thread. only your fingers. Wing nuts are a good example of a Class 1 5 tolerance grades (grades 4 to 8) for the pitch diameter fit. A Class 4 and 5 fit requires a wrench to turn a nut down from tolerance of the nut thread. start to finish. Aircraft bolts are usually fine threaded with a 7 tolerance grades (grades 3 to 9) for the pitch diameter Class 3 fit, whereas screws are Class 2 or 3 fit. tolerance of the nut thread. Classes 1A and 1B are intended for special applications involving replacement parts, for quick and easy assembly even when the threads are slightly damaged or dirty. Classes 2A and 2B are general purpose threads used for most commercial applications. Classes 3A and 3B are used for closer tolerances than those available from classes 2A and 2B. This makes them suitable for high-performance precision applications. Egyptair Training Academy © Copyright 2023 11/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners.2 Bolts, studs and screws General A modern commercial aircraft is composed of many individual Fastener types often met are: component parts, units and assemblies. Amongst the most numerous of these are the fasteners, the nuts, bolts, screws Bolts, and rivets, that hold the whole thing together. In fact, a Screws, Boeing 747 contains over 2,400,000 individual fasteners of Nuts, many different types and sizes. Wing nuts Studs, The word ‛fastener‘ i sused as a general term to describe all the Dowel pins, various types of fastening device employed in the construction Clamps, of an aeroplane. It is necessary to use this term because the e…tc meaning so fthe word s‛bolt‘ and ‛screw‘ for instance, ma ybe interpreted differently by separate organisations. Bolts and screws are similar in the fact, that both have a head at one end and a screw thread at the other. However, there are One commonly used definition says the word ‛bol‘t describes a several differences between them. The threaded end of a bolt threaded fastener with a plain (unthreaded) shank portion, is always relatively blunt. A screw may be either blunt or whil st‛screw‘ refers to a full ythreaded item regardle sso fthread pointed. The threaded end of a bolt must be screwed into a nut. size, head style or material. Another definition holds that any The threaded end of the screw may fit into a nut or directly into external wrenching (e.g. hex drive) is a bolt, while and internally the material being secured. A bolt has a fairly short threaded driven (e.g. slotted or Allen socket) item is a screw. section and a comparatively long grip length (the unthreaded To avoid confusion, it is easier to use the term threaded part). A screw may have a longer threaded section and no fastener although ‛bolt‘ and ‛screw‘ are often used and are often clearly defined grip length. A bolt assembly is generally interchangeable. tightened by turning a nut. The scope of this sub-module is to introduce the common types The bolt head may or may not be designed to be turned. A of fasteners used and the categories, identification and screw is always designed to be turned by its head. Another common application. minor difference between a screw and a bolt is that a screw is usually made of lower strength materials. Egyptair Training Academy © Copyright 2023 12/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Aircraft hardware is identified for use by its specification number or trade name. Threaded fasteners and rivets are identified by Air Force-Navy (AN), National Aircraft Standard (NAS), and Military Standard (MS) numbers. You should know the names of bolt parts and be aware of the bolt dimensions that must be considered in selecting a bolt. The nomenclature shown below applies to all types of bolts and screws. Bolts types depends on their head style. Common bolt types are: Hex head bolts (AN, MS, NAS), 12-point head bolts (MS, NAS), Flush head bolts/screws (MS, NAS), Internal wrenching bolts (AN, MS,), Socket head cap (hexagon) bolts/screws (MS, NAS). Flush head screws require different types of screw driver bits depending on their type. Socket head cap bolts/screws are also known as Allen screws. Most head types are available as drilled (prepared for safety wire) or un-drilled. Egyptair Training Academy © Copyright 2023 13/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Bolt types General The fastener systems discussed in this section are standard threaded fasteners and their mating parts (washers, nuts, locking devices etc.). The various types of rivet, lock pin and special threaded fastener are considered elsewhere. A standard fastener comprises a threaded portion, a head and sometimes a plain shank or grip. Between the shank and the thread is a small tapered transition zone or lead-in and between the shank and the head is stress relieving fillet radius. A fastener will be defined by several parameters which are laid down in detail in its specification. These include its thread form, shank diameter, head style, grip length, material, surface finish and locking facilities. The three principal parts of a bolt are the head, thread, and grip. The head is the larger diameter of the bolt and may be one of many shapes or designs. The head keeps the bolt in place in one direction, and the nut used on the threads keeps it in place in the other direction. Egyptair Training Academy © Copyright 2023 14/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Bolt dimensions Various bolt types Various bolt types Bolt dimensions and terms Egyptair Training Academy © Copyright 2023 15/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Head style The design of a fastener will be dictated by the specification Tri-wing screws are easy to identify as they are the three-point requirements and the environment that it is intended to operate slots, however, it is important to select the correct bit size. in. Torx is a trade name for the hexalobular internal driving slot or Where a flush surface is required, the fastener hole may be star head. It is a standardised design, so different sized drivers countersunk to accommodate the head. The standard will fit fasteners from different manufacturers. countersink angle is 100° although variations do occur. The hexagonal socket or Allen head is driven by a hexagonal Flush fastener heads are provided with a recessed drive or key sized in fractions of an inch. ‛slot‘. This slo tmay be of one of the following forms: Protruding head fasteners may have any of the above recess straight; drive slots in a variety of head styles or may be external radiused straight )‛Hi-Torque‘); wrenching. cruciform (cross-shaped e.g. ‛Philip‘s(; offset cruciform (‛Torq-set‘); External wrenching fasteners have flats on the outer edges and offset three-point (‛Tri-wing‘); may be turned with a spanner or socket. They are normally splined )e.g. ‛Torx‘(; or hexagonal (6-point) or Bi-Hex (12-point). The head size for both hexagonal (Allen). types is specified as the distance Across the opposite Flats of the hexagon (AF) as a fraction of an inch or metric It is important to use the correct type and size of screwdriver or measurement. Therefore a 7⁄16" AF Bi-Hex socket will fit both a ‛bit‘ a sthe wrong tool will damage the head and ma ypreven t 7 ⁄16" AF hex bolt head and a 7⁄16" AF 12-point bolt head. successful removal of the fastener. Many fasteners have the tool size stamped on the head. Note: BA, BSW and BSF head sizes are directly related to thread size, and spanner sizes are specified by this, e.g. The straight screwdriver should fit snugly into the slot to its full a ¼ BSF spanner only fits a bolt or nut with a ¼ BSF depth most of its width. It should not be used on the Hi-Torque thread. head which has a special curved slot and a dedicated screwdriver bit. Cruciform fasteners should be carefully identified as Philips, Reed and Prince, Posidrive and Torque-set fasteners each have their own dedicated drivers, use of the wrong one will damage the fastener slot and impair or prevent removal. Egyptair Training Academy © Copyright 2023 16/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Recess drive protruding head Hex and Bi-hex head designs Recess drive slots Egyptair Training Academy © Copyright 2023 17/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Bolt drive types Some common bolt drive types are shown below: View G shows a hi-torque style driving slot. This single View A shows an eyebolt, often used in flight control slot is narrower at the centre than at the outer portions. systems. This design, and the centre dimple, provides the slot with View B shows a countersunk-head, close-tolerance bolt. a bow tie appearance. The recess is also undercut in a View C shows an internal-wrenching bolt. Both the taper from the centre to the outer ends, producing an countersunk-head bolt and the internal-wrenching bolt inverted keystone shape. These bolts must be installed have hexagonal recesses (six-sided holes) in their with a special hi-torque driver adapter. They must also heads. They are tightened and loosened by use of be driven with some type of torque-limiting or torque- appropriately sized Allen wrenches. measuring device. Each diameter of bolt requires the View D shows a clevis bolt with its characteristic round proper size of driver for that bolt. The bolts are available head. This head may be slotted, as shown, to receive a in standard and reduced 100-degree flush heads. The common screwdriver or recessed to receive a Reed- reduced head requires a driver one size smaller than the and-Prince or a Phillips screwdriver. standard head. View E shows a torque-set wrenching recess that has four driving wings, each one offset from the one opposite Fastener loads it. There is no taper in the walls of the recess. This Fasteners holding parts together may be subjected to two types permits higher torque to be applied with less of a o f‛load‘. Load scarried along the length o fa fastener are known tendency for the driver to slip or cam out of the slots. as Tension loads whilst those felt across the fastener are called View F shows an external-wrenching head that has a shear loads. A fastener in tension will require a head and nut of washer face under the head to provide an increased sufficient strength to spread the load to the surrounding bearing surface. The 12-point head gives a greater structure. A fastener taking only shear loads may have the size wrench-gripping surface. of the head reduced to save weight. Most fasteners take a combination of both loads and will be designed to carry them with an adequate safety margin. Stress Raisers No matter how well designed a certain bolt, screw or stud is or how high quality they have, they still have the risk of failing if installed wrongly or if misused. Aircraft technician have no real need to know either the detail of stress analysis or the calculation of fatigue strength. But we need to know what to look after and what to watch out for. Egyptair Training Academy © Copyright 2023 18/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Fastener loads Bolt drive types Stress raisers Egyptair Training Academy © Copyright 2023 19/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Grip length Bolt diameter If a bolt is too long or too short, its grip is usually the wrong A second bolt dimension that must be considered is diameter. length. As shown below, grip length should be approximately The diameter of the bolt is the thickness of its shaft. If this the same as the thickness of the material to be fastened. If the thickness is ¼ inch or more, the bolt diameter is usually given grip is too short, the threads of the bolt will extend into the bolt in fractions of an inch (for example, ¼, 5/16, 7/16, and ½). hole and may act like a reamer when the material is vibrating. However, if the bolt is less than ¼ inch thick, the diameter is To prevent reaming, no more than two threads should extend usually expressed as a whole number. For instance, a bolt that into the bolt hole. Also, users should be certain that any threads is 0.190 inch in diameter is called a No. 10 bolt, while a bolt that that enter the bolt hole extend only into the thicker member that is 0.164 inch in diameter is called a No. 8. is being fastened. If the grip is too long, the nut will run out of threads before it can be tightened. In this event, a bolt with a The results of using a bolt of the wrong diameter should be shorter grip should be used, or if the bolt grip extends only a obvious. If the bolt is too big, it cannot enter the bolt hole. If the short distance through the hole, a washer may be used. diameter is too small, the bolt has too much play in the bolt hole, and it is likely not as strong as the correct bolt. The nominal length depends on the style of the fastener in question. If the item has a plain shank and protruding head the nominal length or ‛grip‘ i smeasured from the underside o fthe head to the end of the plain shank. If the item has a protruding head and is fully threaded the nominal length is a measure of the threaded portion. If the item has a flush head (countersunk) the depth of the head is included in the nominal length or grip. The increments in which the length is measured varies between the standards systems and must be determined before fastener identification can take place. Egyptair Training Academy © Copyright 2023 20/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Bolt nomenclature Gr iplength measurement usin g a’dipper‘ Grip length and nominal length definitions Egyptair Training Academy © Copyright 2023 21/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Nuts General Fasteners have to mate with a female thread form. This may be a threaded hole in a structure or component or, more frequently, a nut. Like fasteners, nuts come in all shapes and sizes, made from many different materials. It is, therefore, important to select replacement nuts by part number from the illustrated parts catalogue. Standard nuts are hexagonal in shape and are usually the same size across flats as a bolt of the same thread size. Often, they are symmetrical, but some have an enlarged bearing surface on one side and so must be used the right way up, these are sometimes referred to as Engine nuts. Nuts are normally six to ten threads deep but thinner lightweight versions are available for shear applications. Many are provided with locking facilities which include wire locking holes and split pin slots. Some nuts have a plain raised portion to accommodate the slots without compromising the screw thread, these are referred to as castellated nuts. Bi-hex or twelve-point nut are often used and these are of a smaller diameter than their hexagonal counterparts and are thus lighter. Wingnuts are used in non-structural applications where a quick release is required. The wing may be drilled for wire locking. Pal nuts are lightweight pressed-steel nuts of various designs which contain only a partial thread and are usually used for non-structural purposes. Egyptair Training Academy © Copyright 2023 22/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners 12-point nut Various types of nut Various types of castle nut Various types of nut Egyptair Training Academy © Copyright 2023 23/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Stiffnuts Stiffnuts are commonly used in the aerospace industry and Aerotight stiffnut have an element or feature which increases the friction This nut is made with a circular crown which is slotted across between the nut and its mating part to prevent loosening the diameter and also on each side through an arc of 150°. The caused b y vibration. The mo st common method s are ‛thread resultant wings are de-pitched and forced inwards, thus disruption‘ where par tof the thread is distorted radially or axially causing a sideways as well as downwards tension on the bolt and the ‛insert‘ type where the fastener mu stcu ta thread in a threads. plastic or fibre insert. Note: These types are seldom encountered on modern To check the effectiveness of the friction element of a small aircraft. diameter stiffnut, it is general practice to turn the nut onto its mating thread by hand. If it is possible to pass the thread through the friction element by hand, then the locking is unsatisfactory. However, certain manufacturers specify acceptable limi ts o f ‛in-built‘ or frictional torque for various thread types and sizes and in these instances, each stiffnut should be checked with a torque wrench before re-use. Oddie stiffnut The top of this nut is counterbored, slotted vertically and depressed inwards to form a circle of six tongues with the diameter slightly smaller than the bolt thread core diameter. As the nut installed the threads of the bolt displace the tongues upwards, and a load is applied to the contacting thread faces. Philidas stiffnut This nut is made with a circular crown in which two slots are cut, one above the other with an arc of about 270°. The wings are displaced downward thus providing locking tension. Egyptair Training Academy © Copyright 2023 24/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Philidas nut Aerotight nut Egyptair Training Academy © Copyright 2023 25/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Nyloc stiffnut Kayloc This nut is counterbored at the top and spun over to retain a This nut is pressed from sheet steel and is both light and strong. nylon insert, the insert is not threaded and has an internal The rear portion o fthe threaded ‛tube‘ i sdeformed to produce diameter slightly less than the diameter of the bolt to which it is an ellipse. The mating thread will overcome the distortion but fitted. On assembly, the bolt displaces the nylon in forming a the resilience of the nut will maintain a grip on the bolt. These thread, and a high friction value is set up between the load nuts have the additional advantage that an internal Kaynar carrying sides of the thread in contact with the nylon. They may socket can be used in areas of restricted access. be used only once and are not suitable for hot locations. Lightweight stiffnut Nyloc cap nut Like the Kayloc, the top of this nut is distorted to an oval shape. Like the Nyloc nut but incorporates a nylon insert in cap form to When the nut is screwed onto the bolt, the top of the nut is seal the end of the bolt or screw to which the nut is fitted. A cap forced to assume the round shape of the bolt and thus provides nut is used for special applications, such as in pressurised a locking device. When the nut is removed from the bolt it cabins, fuel and oil tanks etc., when leakage or seepage along assumes its original shape. the bolt thread is undesirable. Care must be taken not to reject these stiffnuts as They may also be used in parts of the aircraft where there is a unserviceable due to the distortion which is an integral feature likelihood of an exposed bolt thread catching and tearing a of their design. passenger‘ s clothing, for example, on the passenger entry structure. The Nyloc cap nut makes for a smoother bolt thread The term lightweight is used because this type of stiffnut has termination and therefore is less likely to snag clothing and or the equivalent strength of larger nuts with the same thread passenger baggage. diameter. They are available in both hex and bi-hex form. Egyptair Training Academy © Copyright 2023 26/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Types of stiffnuts Capnuts Lightweight stiffnuts Nyloc nuts Kayloc stiffnuts Egyptair Training Academy © Copyright 2023 27/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Anchor nuts Anchor nuts are stiffnuts which are retained (either fixed or floating) on a plate which is riveted to the structure to provide a blind attachment. Where more than one nut is attached to a plate they are referred to as strip nuts. Clip nuts are self-locking threaded elements retained by a spring steel clip housing. When slid over the edge of a thin flange, a small spring section centres it in the fastener hole. Clinch nuts have an unthreaded spigot which is swaged into the attachment hole in sheet metal to retain them. Egyptair Training Academy © Copyright 2023 28/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Anchor nut Clip Nuts Clinch nut Egyptair Training Academy © Copyright 2023 29/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Sheet spring nuts Sometime scalled ’Tinnerman nuts‘. These nuts are used with standard and sheet metal self-tapping screws to support line clamps, conduit clamps, electrical equipment, and access doors. The most common types are the float, the two-lug anchor, and the one-lug anchor. The nuts have an arched spring leak that prevents the screw from working loose. They should be used only where originally used in the fabrication of the aircraft. Instrument nuts Instrument mounting nuts permit mounting of aircraft instruments from the front of the panel. It is easily compressed with finger pressure to allow the legs into the clearance hole. Once it is fully inserted and pressure is released, legs spring apart. Most instrument mounting nuts take a No.6-32 machine screw. They are non-magnetic (speed nut is brass; cage is phosphor bronze). Egyptair Training Academy © Copyright 2023 30/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Instrument nut Spring nut Spring nut Egyptair Training Academy © Copyright 2023 31/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Point-wrenching nuts These nuts are generally used where a nut with a high tensile length is required. These nuts are installed with a small socket wrench. They are usually self-locking. Shear nuts These nuts are designed for use with devices such as drilled clevis bolts and threaded taper pins that are normally subjected to shearing stress only. They are self-locking, castellated, or drilled for wirelocking. Klincher locknuts Klincher locknuts are used to ensure a permanent and vibration proof, bolted connection that holds solidly and resists thread wear. It will withstand extremely high or low temperatures and exposure to lubricants, weather, and compounds without impairing the effectiveness of the locking element. The nut is installed with the end that looks like a double washer toward the metal being fastened. Notice in the figure below that the end that looks like a double hexagon is away from the metal being fastened. Barrel nuts In many high load joints, where higher load distribution is required, barrel nuts (or floating nuts) are often used (for instance engine mount attachment to the wing pylon). Barrel nut are inserted into a round access hole, located 90° in respect to the bolt or screw hole. Egyptair Training Academy © Copyright 2023 32/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Shear nuts Point- wrenching Barrel nuts nuts Clincher lock nuts Egyptair Training Academy © Copyright 2023 33/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Thread inserts General With the assembly of units and components, fasteners are Inserts should be installed strictly in accordance with the often installed in threaded holes. With the periodic disassembly manufacturer‘ s instructions, since there may be sligh t and reassembly of these parts during their maintenance cycle, variations between inserts conforming to the same threads in softer metals will become worn and damaged. specifications. Thread inserts are usually fitted in light alloy materials such as Wire thread inserts aluminium, magnesium, bronze and brass, to provide a more A wire thread insert is a precision formed wire of diamond resilient screw thread. They also produce a large diameter section (usually of spring steel or stainless steel) wound into a thread and thus a stronger attachment point in the softer helical coil, the cross-section of the wire forming a thread both materials. inside and outside the coil. In addition, thread inserts are often specified for repair work in When correctly installed, the coil provides a thread which steel components, where the original thread has been conforms to a British Standard or other specification with a damaged and fitment of an insert enables the original size bolts good surface finish and the inherent flexibility to compensate to to be used without affecting interchangeability. some degree for any errors of form in the engaging bolt or screw. The radial pressure attained in fitting the insert produces There are basically two types of inserts available. One is known good self-locking characteristics. as a wire thread insert (often referred to by the trade name ‛Helicoil‘( and is made from specially formed wire wound into a The possibility of thread failure from vibration, fatigue, corrosion helical coil. The other is known as a thin wall insert and is made or seizure is also reduced. from a tube with threads formed on both the inside and the outside surfaces. Both types are manufactured in a variety of Wire thread inserts have a tang at the inner end to facilitate materials and finishes and may have either plain or self-locking fitting with a special tool; this tang may be removed after threads. installation if required, by breaking it off at the notch, using long- nose pliers. Thread inserts should only be used when specified in the relevant manual, drawing or repair scheme and care should be Some special wire thread insert additionally features one or taken to ensure that the correct insert is used. more polygon coils that put pressure on the sides of the mounted bolt. It is especially well suited to applications that are subject to alternating loads or vibrations. Egyptair Training Academy © Copyright 2023 34/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Tang Notch Polygon coil Thread insert with tang, notch and polygon coil Egyptair Training Academy © Copyright 2023 35/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Installation Since the internal and external threads on a threaded insert Normal workshop practices should be used for tapping, with have the same number of threads per inch and the internal special emphasis on cutting the thread coaxially with the hole. thread is designed to be of standard size, then a special size Lubricant should be used according to the type of metal being tap is required to cut the threads into which the insert is fitted. cut, e.g. a light mineral oil is generally recommended for tapping light alloys. These special taps and checking gauges are provided by the insert manufacturers. Installation procedures, which comprise drilling and tapping the hole, thread gauging, insertion of the insert and removal of the tang, are outlined. Drilling The hole for the insert should be drilled to the diameter and depth specified in tables supplied by the insert manufacturer, the depth being calculated from the fitted length of the insert, plus the thread runout, plus a half pitch gap at each end of the insert. Care should be taken to ensure that the hole is drilled in the correct location and square to the surface and that all swarf is removed before tapping. In some cases, particularly when the hole is near to the edge of the component, it may be necessary to check for cracks by a specified non-destructive testing method. Thread tapping The thread should be tapped with a special tap provided by the insert manufacturer, a straight-fluted tap being used for hand tapping and a spiral-fluted tap for machine tapping where this is possible. Egyptair Training Academy © Copyright 2023 36/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Egyptair Training Academy © Copyright 2023 37/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Thread gauging After the insert thread has been cut, it should be cleaned of all When a prewind tool is used, the insert should be placed in the swarf and foreign matter. The thread should then be checked chamber with the tang towards the nozzle and the mandrel with a special GO / NO GO plug gauge provided by the insert pushed forward through the insert to engage the tang in the manufacturer to ensure that the thread is satisfactory. Any slot. The mandrel should be rotated clockwise and pushed thread imperfections indicated by tightness of the GO gauge gently forward to engage the insert coil in the nozzle threads, should be removed by further use of the original tap or, if this is the rotation being continued until the insert is about to emerge ineffective, by use of a new tap. from the outer end of the nozzle. The tool should then be placed squarely over the tapped hole and the handle rotated to transfer Fitting the insert the insert from the tool into the tapped hole; no forward An insert should be screwed into the tapped hole using either pressure should be used. an inserting key or an inserting tool of the prewind type, depending upon which is recommended for the insert. A Unless otherwise stated, inserts should be installed so that the different sized key or tool is provided for each size of the insert. outer coil is at least half a pitch below the component surface. The inserting key should be used by sliding the insert onto it so Absolute cleanliness of the tapped hole and freedom from burrs that the tang is engaged in the driving slot at its forward end; is essential to prevent distortion of an insert. When a jointing the assembly should then be applied to the tapped hole, compound or anti-corrosive compounds are specified, they compressing the insert downwards with the thumb and should be applied strictly according to the relevant instructions forefinger of one hand while turning the key with the other hand; and surplus compound should be removed as specified after no downward pressure should be applied on the key. The insert installing the insert. will wind into the thread and should be installed so that the outer end of the insert is at least half a pitch below the surface of the Removal of the tang component. It is not normally necessary to remove the tang of a wire thread insert from a blind hole, but removal is usually specified in through holes for screw clearance or product appearance. A tang in a through hole is removed by use of the inserting key used as a punch, with the tang outside the engaging slot, or by use of a special punch. A sharp blow with a hammer on the key or punch will fracture the wire at the notch where the tang joins the coil. To remove the tang from an insert fitted in a blind hole, long round-nosed pliers are required; the tang should be bent backwards and forwards through the insert bore until it fractures at the notch and can be removed. Egyptair Training Academy © Copyright 2023 38/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Wire insert fitting tool Egyptair Training Academy © Copyright 2023 39/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Removal of inserts If an insert has to be removed because of bad fitting, damage or wear, this can be done by carefully picking out the top coil and bending it inwards to form a rough tang and unscrewing it with the insertion tool or a pair of pliers. Some manufacturers recommend the use of a tapered left- hand tap of appropriate size, which grips the top coils internally and unwinds the insert when rotated. Others provide a range of extractor tools which are fitted with hardened and tempered blades, the blade will bite into the inner surface of the insert, which can then be unscrewed. After removal of an insert, the threads in the hole should be carefully examined for damage before fitting a new insert. If thread damage is excessive, a repair insert, or ‛twinsert‘, can be fitted into the re-drilled and tapped hole and a standard insert installed into this. Egyptair Training Academy © Copyright 2023 40/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Wire insert removal tool Egyptair Training Academy © Copyright 2023 41/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Thin wall inserts Key-locked inserts A thin wall insert comprises a tube with threads formed on its internal and external surfaces. These inserts do not exert any Drilling and tapping outward radial pressure on the threaded holes into which they Tables provided by the manufacturer give details of the drill fit, and are locked in position by various methods. Inserts are diameters, hole depths and taps to be used to form the supplied in a variety of types, materials and finishes, and the threaded holes for each size of insert. When preparing the internal thread may be non-locking, or self-locking by means of threaded holes, the general precautions outlined should be a deformed thread or nylon insert; inserts are identified and carefully followed. ordered by manufacturer‘s part number.s Installation The inserts may be screwed in by hand or using an installation tool until the keys butt against the component surface, this being the correct installed depth. The keys are then driven into place by hammer blows or a press, using the installation tool as a punch. When the keys are flush with the top of the insert, installation is complete. Removal Should it become necessary to remove a key-locked insert, this may be done as follows: Drill out the insert to a diameter equal to the distance between two opposing key slots and to the depth of the key heads. Deflect the keys inward with a punch and break them off. Remove the insert with a standard extractor. After removal of the insert, the threads on the part should be inspected for damage. If the threads are undamaged a replacement insert of the same size may be fitted, but care should be taken to ensure that the keys are in different places than the original ones. Egyptair Training Academy © Copyright 2023 42/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Key locked inserts Key locked insert with stud fitted Thin walled insert Egyptair Training Academy © Copyright 2023 43/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Swaged inserts Drilling and tapping Removal The holes for these inserts must be drilled with a special drill Where necessary, swaged inserts may be removed in the and counterboring tool supplied by the insert manufacturer for following way: each size of insert. Holes should be drilled so that the counterbore depth is as specified in the relevant tables for the Using a drill of the relevant diameter, drill the insert to insert concerned, and the precautions outlined should be the depth specified in the manufacturer‘ s tables to observed. The hole should be tapped using a tap of the relevant separate the swaged portion of the insert. size, to the drawing requirements. All swarf should then be Carefully remove the swaged portion with a scriber or removed and the thread inspected. similar tool. Using the installation tool, unscrew and remove the Installation threaded part of the insert. A special insertion tool is used for installing these inserts (see The thread and counterbore should be checked for size graphic). The insert internal thread is deformed in such a way and damage. If satisfactory, a replacement insert of the as to permit the insertion of the hexagonal driver (unified same size may be fitted. threads), or has three axial grooves (metric threads) so that the insert can be rotated. The insert should be screwed into the threaded hole until it is the specified distance below the component surface. The insert is swaged by hammer blows on the end of the tool and installation is complete when the stop washer face contacts the component surface. Note: Insertion tools for the larger sizes of inserts are power operated. Egyptair Training Academy © Copyright 2023 44/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Swaged inserts Swaged insert fitting and fitting tool Egyptair Training Academy © Copyright 2023 45/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Ring-locked inserts Drilling and tapping Removal The holes for these inserts should be drilled and counterbored When necessary, ring-locked inserts may be removed in the in a similar way to those for swaged inserts. Similar following way: drilling/counterboring tools should be used and the dimensions of the holes should conform to those listed in the tables Drill out the insert to the depth of the counterbore, using a drill provided by the manufacturer; the precautions outlined should of the diameter specified in the tables provided by the also be observed. The holes should be tapped using a tap of manufacturer. the specified size, to drawing requirements. All swarf should then be removed and the thread should be inspected. Remove the insert by use of a standard stud extractor or a left- hand threaded tap of suitable size. Installation A special tool is used to install these inserts the bore having If necessary, use a punch to separate and remove the serration‘s which fit the serrations of one particular size of remaining portion of the locking ring. insert. The insert should be screwed into the prepared hole until Provided the hole thread is not damaged, a replacement insert its upper surface is 0.25 to 0.5 mm (0.010 to 0.020") below the and locking ring of the same size may be fitted. component surface. The locking ring should then be placed over the insert so that the inner serrations engage those of the insert. Installation is completed by fitting the drive tool into the locking ring (ensuring that it is square to the component surface), and hammering the end of the tool so that the outer serrations on the locking ring bite into the material surrounding the counterbore. The installed locking ring should be flush with the surface of the component. Egyptair Training Academy © Copyright 2023 46/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Ring locked insert and fitting tool Egyptair Training Academy © Copyright 2023 47/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Screws General The most common machine screw used in aviation is the Screws are probably the most commonly used threaded fillister-head screw, which can be wire-locked using the drilled fastener in aircraft construction. They differ from bolts in that hole in the head. The flat-head (countersunk-head) screw is they are generally made from lower-strength materials. They available with single or cross-point slotted heads. The round- can be fastened by a variety of tools, including screwdrivers, head screw and the truss-head (mushroom-head) screw, spanners and Allen keys. Most screws are threaded along their provide good holding properties on thin metal sheets. complete length, whilst some have a plain portion for part of their length. The commonly used machine screws are the flush-head, round-head, fillister-head, socket-head, pan-head and truss- There are several different types of screw, which, can be used head types. for a wide range of tasks. It is common sense that great care must be taken to replace screws with the correct items, by using Flush-head – Flush-head machine screws are used in the markings on the screw, the IPC and any other systems in countersunk holes where a flush finish is desired. These current use within the supply department, to protect against screws are available in 82 and 100 degrees of head incorrect screws being installed. angle and have various types of recesses and slots for driving. Another point, requiring care, is the difference in terminology Round-head – Round-head machine screws are between the British and American names for screw heads. frequently used in assembling highly stressed aircraft What the British refer to a sa ‛countersunk – headed‘ screw, the components. American s cal l a ‛flat-head‘ or ‛flush‘ screw. Similarly, Fillister-head – Fillister-head machine screws are used ‛mushroom-headed‘ screws are known as ‛truss-heads‘ in the as general-purpose screws. They may also be used as USA. cap screws in light applications such as the attachment of cast aluminium gearbox cover plates. Machine screws Socket-head – Socket-head machine screws are Machine screws are used extensively for attaching fairings, designed to be screwed into tapped holes by internal inspection plates, fluid line clamps and other light structural wrenching. They are used in applications that require parts. The main difference between aircraft bolts and machine high-strength precision products, compactness of the screws is that the threads of a machine screw usually run the assembled parts, or sinking of the head into holes. length of the shank, whereas bolts usually have an unthreaded grip length. Egyptair Training Academy © Copyright 2023 48/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Pan – and Truss-head – Pan-head and truss – head screws are general-purpose screws used where head height is unimportant. These screws are available with cross-recessed heads only. Egyptair Training Academy © Copyright 2023 49/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Structural screws Structural screws are used for assembling structural parts. They are made of alloy steel and are heat-treated. Structural screws have a definite grip length and the same shear and tensile strengths as the equivalent size bolt. They differ from structural bolts only in the type of head. These screws are available in round-head, countersunk-head, and brazier-head types, either slotted or recessed for the various types of screwdrivers. Egyptair Training Academy © Copyright 2023 50/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Screw types Egyptair Training Academy © Copyright 2023 51/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Self-tapping screws Self-tapping screws have coarse threads and are used to hold thin sheets of metal, plastic and plywood together. The type A screw has a gimlet (sharp) point, and type B has a blunt point with threads that are slightly finer than the type A. There are four types of head in normal use: round head countersunk oval-head truss or mushroom-head flat countersunk head. Egyptair Training Academy © Copyright 2023 52/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Self-tapping screw types Egyptair Training Academy © Copyright 2023 53/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Washers Plain washers are used to provide a smooth bearing surface between the nut and the structure or component to protect it when tightening the nut. They may be used to spread the load on a tightened bolt across a larger area. They may also be used to adjust the grip length of a fastener. Washers are produced in a variety of materials, sizes and thickne ss’s and mu st be identified by part number and used accordingly. Those intended for use under a bolt head often have a chamfer or countersink to accommodate the under-head radius and must be used in the correct orientation. Various anti-vibration and locking devices may be incorporated in washer design. Among these are spring washers and shakeproof washers which are axially deformed to increase friction between the nut or bolt and the structure it is installed on. These should be used once and discarded. When used on light alloy structures a plain washer is also installed to protect the surface. Tab washers have a lug, which is located on the edge of a surface or in a hole provided to prevent rotation, and two tabs which are bent up against the flats of the nut, once tightened, to prevent loosening. The ‛Tinnerman‘ type washer i s a large diameter aluminium washer with a countersunk recess. It is used to attach fibreglass and composite panels, the large flange spreading the load of the countersunk fastener to the surrounding material. Egyptair Training Academy © Copyright 2023 54/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Use of tinnerman washers Egyptair Training Academy © Copyright 2023 55/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners.3 Locking devices The purpose of this section is to provide guidance and advice A 2nd Class locking device is one in which friction is the primary on the methods of locking screw-threaded components and the locking method. These include spring washers and stiff nuts. retention or location of circular parts in various assemblies. Regulations require that an approved means of locking must be Generally, a 1st Class locking method is preferred on a thread provided on all connecting elements in the primary structure, which is used on a moving component such as a control fluid systems, controls and other mechanical systems essential surface linkage. Some manufacturers maintain for reasons of to the safe operation of an aircraft. safe,ty tha ti fa ’friction‘ locking device i sto be used on a moving component such as a control surface linkage, no less than Purpose three threaded components are to be used to hold the components together. The purpose of a locking device is to prevent loosening or disengagement of mating components under varying conditions of stress, vibration and temperature as its effectiveness may be of the utmost importance to the safety of an aircraft. Locking devices should be fitted in such a way as to prevent the possibility of fretting, distortion, displacement or uneven stressing of the locked parts. During an inspection of the assembly, it is necessary to ascertain that all locking or retaining devices are of the type and material specified in the relevant drawings or the appropriate publication and that the locking or fitting operation has been correctly performed with the appropriate tools. Class of locking method Locking devices are either 1st Class, or 2nd Class. A 1st Class locking device is one in which no amount of vibration will cause loosening o f the thread. These ’positive‘ locking methods include wire locking, tab locking and split pinning. Egyptair Training Academy © Copyright 2023 56/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners “THIS PAGE INTENTIONALLY LEFT BLANK“ Egyptair Training Academy © Copyright 2023 57/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Locking washers There are several types of locking washers in general use Shakeproof washers consisting of spring washers, cup washers, shakeproof Flat washers of this type, made of steel or phosphor bronze, washers, crinkle washers and tab washers. They are described are sometimes used instead of spring washers and in certain by various specification systems. circumstances, conical shakeproof washers are used for locking countersunk screws. Either the internal diameter or Spring washers external diameter is serrated, the serrations being set to bite These washers are available in two forms, i.e. as a single coil into the component and nut to prevent rotation. Shakeproof or as a double coil. washers should be used only once. In some instances, particularly with light alloy assemblies, Note: These washers will not normally be specified in spring washers are assembled with plain facing washers assemblies where anti-corrosion treatment of components has between the spring washer and the component, to prevent been carried out. damage to the surface of the component or the protective treatment when the spring washer is compressed. Often, however, particularly in steel assemblies, plain washers are not specified. It is good practice to renew spring washers during overhaul or repair. This is essential in engines and engine components, and units with reciprocating parts, such as compressors or pumps. Crinkle washers These washers, made of copper alloy or corrosion resisting steel, are often used in lightly loaded applications in instrument and electrical installations. Cup washers These washers are manufactured in spring steel and are dished to form a spring of high rating; assembly should be in accordance with the manufacturer‘ sinstruction.s Egyptair Training Academy © Copyright 2023 58/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Spring, crinkle and cup washers Shakeproof washers Spring washers Crinkle washers Shakeproof washers Spring washers Egyptair Training Academy © Copyright 2023 59/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Tab washers Locking plates Tab washers are manufactured from thin metallic sheet materials to standard or proprietary specifications, and have Locking plates are usually manufactured from steel. They are one or more tabs projecting from the external diameter; they placed over hexagonal or bi-hex nuts or bolt heads after these may also be ganged for locking two or more nuts. When the items have been tightened down, and secured, usually by a washer is fitted, one tab (usually pre-formed) is anchored screw, to an adjacent part of the structure. A typical application against the component or fitted into a hole provided for that is shown below. purpose, whilst another tab (or tabs) is bent against a flat or flats of the nut after the nut has been correctly torqued. The Locking plates may be used repeatedly provided they remain a component tab should not be bent against a curved surface or good fit around the nut or bolt head. across the junction of two faces since this would permit movement of the nut or bolt. In certain instances, particularly where vibration is likely, locking plate screws are fitted with spring or shakeproof Before bending the second tab, an examination should be washers. Some plates may be located by countersunk screws, made of the tab already fixed to ensure that it is not disturbed, which may be locked by peening (right). Plates may also be sheared or distorted because of the washer turning with the nut. provided with a retaining screw slot which permits a limited When the second tab has been bent, this too should be amount of angular adjustment to suit the position of the nut. examined for cracks. In some assemblies, washers having a tab projecting from the inside diameter are used. The tab fits into a key slot machined in the male thread, whilst an external tab is bent up against the nut flat to lock it. Tabs must not be bent more than once. Multiple tab washers may be reused after removing the used tab, dressing sharp edges, and carefully inspecting the remaining tabs for cracks or scoring. Egyptair Training Academy © Copyright 2023 60/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Ganged tab washers Multiple use tab washer Plain and key slot tab washers Locking plate Egyptair Training Academy © Copyright 2023 61/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Split pins Split pins (referred to as ’cotter pins‘ in the USA) are Note: It will be seen that British and American practice differs manufactured from corrosion resisting steel and are used in regarding split pin diameters for different thread sizes conjunction with drilled bolts and slotted or castellated nuts. and care must be taken to ensure that the correct pin is The pins should be a reasonably close fit in the nut and selected for any drilled bolt. The size of the split pin hole bolt/stud assembly. The table indicates the diameters and in the bolt should be checked before fitting the nut. length of standard pins normally used in conjunction with bolts/studs up to 1" diameter. The legs of split pins should be turned as indicated on the design drawings, but when the method is not specified it is recommended that one of the methods illustrated below should be used. If necessary, pins should be cut to a suitable length to prevent pick-up in clothing, cleaning cloths, etc., and the surplus ends accounted for to prevent there becoming a loose article hazard. After turning and closing the legs to the nut faces, an inspection should be made to ensure that cracking or cutting has not occurred at the bends. The most common methods of split pinning are illustrated below by (A) and (B). The method shown in (C) is used where clearances are critical. For bolted joints, one pair of slots must be in alignment with the hole in the bolt when the specified degree of tightness has been obtained. Undrilled bolts should be prepared for drilling by tightening the nut to the specified torque loading and marking the hole position. The nut should be removed and the split pin hole drilled with the aid of a drilling jig. Burrs should then be removed, the nut fitted and tightened to the required torque loading and the correct size of split pin fitted. Egyptair Training Academy © Copyright 2023 62/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Split pinning methods Egyptair Training Academy © Copyright 2023 63/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Palnuts A Palnut is a locking device for nuts made of stamped sheet spring metal, and is a registered trademark. The palnut is screwed on the bolt on top of an ordinary nut, and has a series of protruding barbs that locks the nut in place when the nut is tightened. The Palnut itself is reusable. A Palnut should not be used as a substitute for a positive locking device such as a castellated nut or nuts with safety wire. Egyptair Training Academy © Copyright 2023 64/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Palnut Use of Palnuts Egyptair Training Academy © Copyright 2023 65/179 Nov 2023 :Rev.Date FOR TRAINING PURPOSE ONLY  Fasteners Wire locking General Wire locking (also known as ’lock wiring‘ and ’safety wiring‘) is Sometimes controls or switches are wire locked into one of the most commonly used methods of preventing their normal operating position using thin copper wire. threaded elements from loosening. Selection of an emergency position necessitates physically breaking the wire. Corrosion resisting steel and heat resisting nickel alloy are the The wire must be adequately tensioned; over-tensioning materials normally