Forma de afectación del bien jurídico por el delito de encubrimiento

Rivets and bolts are part of a general group called fasteners. Under the same heading there is a large range of proprietary fasteners which offer particular advantages for special purposes, such as shear strength above the normal to be expected from a rivet, but with a lower installed weight than a bolt. Another special fastener acts in a similar way to a lightly loaded bolt, but is designed to be released by only a 90° twist with a screw-driver. This last example belongs to a subgroup called quarter-turn fasteners and, although they do not have any relevance to major struc-ture, they are used to retain light fairings, engine cowlings and access panels.

Some indication of the range of fasteners is shown in Fig. 9.14 and notes on the methods of reference by number are given in Chapter 7.

In Chapter 7 reference was made to the grip length of bolts. When spec-ifying bolt length it is important that the smooth shank of the bolt extends right through the hole so that no thread is against a bearing surface, so

Fig. 9.12(a–c) Structural components machined from forgings or heavy plate. (Courtesy of British Aerospace.)

Fig. 9.12(b)

the grip length is the plain length and is approximately equal to the thick-ness of the materials being fastened together plus the thickthick-ness of any washer under the bolt head. For countersunk or flat-head bolts the length is measured from the top of the head, and for other bolts from under the head (see Fig. 7.8). In almost all cases, specifying the correct length of bolt shank will result in having a small amount of plain shank exposed past the hole so that a washer or washers will be needed under the nut to allow tightening.

When specifying the length of a rivet, allowance must be made for forming a head. This allowance is about one and a half times the nominal rivet diameter but the design offices of the aircraft manufacturers have their own standards for this important dimension.

In a normal structural use of bolts the fastening is completed with a nut.

All so-called ‘self-locking’ nuts (see Fig. 7.8) are better referred to as stiff nuts and (for structures) must not be used singly; that is, the minimum number of bolts and stiff nuts in a group must not be less than three. For some applications nuts are wire locked as shown in Fig. 7.8. If bolts are used through fittings which are unlikely to be removed for checking during the service life of the aircraft then serious consideration should be given to the use of lighter fasteners of the type produced by the HiShear Corporation (see Fig. 9.14).

In addition to the solid rivets, there are various types of rivets which can be placed without access to both sides of the structure. These are referred to under the general heading of blind rivets. An indication of the general principles is shown in Fig. 9.14. The method for using almost all the blind rivets involves having a hole right down the length of the shank Fig. 9.12(c)

Fig. 9.13 Notches.

(a) A steel mandrel which has an opposite taper on the head is drawn through from the tail end of the rivet, expanding the rivet tail around the rear side of the hole forming a shoulder.

(b and c) The mandrel continues to be pulled through the rivet, symmetrically expanding the rivet shank to fill the hole.

(d) This ensures the rivet has good bearing in the hole and a parallel bore is left in the rivet. The cycle is completed and the next rivet is ready to be placed into the prepared hole.

How the Avdel MBC Rivet system works

1. The MBC Rivet is loaded into the nose of the placing tool, and the tool applied to a prepared hole in the workpiece.

2. When the tool is actuated, jaws in the nose of the tool grip the rivet stem and exert an axial pull, drawing the stem through the rivet shell to give a high-clench joint and complete hole-fill.

3. The placing tool automatically shears the stem flush with the rivet head, and is mechanically locked into the rivet shell.

Fig. 9.14 Some proprietary fasteners. (Courtesy of Avdel Ltd (a and b), HiShear Corpora-tion (c), Dzus Fastener, Europe Ltd (d), Cherry Rivet (e), Huck Fasteners (f).)

Fig. 9.14 Cont.

is installed. In other types the hole is plugged automatically as part of the rivet-setting process. All the various patterns have their own advantages and there is a great deal of commercial competition in this market. For discussion purposes, and since we have referred above to Chobert rivets, the companion self-plugging rivet from the same manufacturer is the Avdel MBC. Chobert are for light duties, comparatively cheap to buy, very cheap (in labour terms) to place and are produced in several different materials for different purposes. Avdel MBC are much more expensive to buy and to place than Chobert but are very strong (at least as strong as solid rivets) and apart from the facility of being able to place them from one side of a structure, the fact of mechanical installation means that vul-nerable surfaces such as wing skins are not damaged by the hammering which is required to set up the strong types of solid rivet.

Wherever possible, aircraft structures are designed so that bolts and rivets are used in shear. Some companies allow rivets to be used where they may be subjected to a small amount of tension but, in general, fasteners in tension must be bolts. The exception to this rule is the use of rivets for attaching the skin. On the top surface of wings, where there is negative pressure, and on the skin of pressurised fuselages, where there is positive internal pressure, the skin rivets are in tension. The justification for this bending of the rules is that there are a lot of rivets required to hold the skin to the structure members and carry the shears mentioned before so the tension in each rivet is small.

Figure 9.15 and other illustrations in this book show the way fasteners are set out in aircraft structures.

Some particular points in Fig. 9.15 should be noted. The distance A in Fig. 9.15(b) must be sufficient to allow a riveting tool to have access above and below unless a blind rivet is being installed from below. As a guide, for 3.2 mm (¹/₈in.) diameter solid rivets in 1.0 mm (0.04 in.) thick sheet, dimension A should be at least 7.6 mm (0.3 in.) and the edge distance should be 6.4 mm (0.25 in.). For blind rivets installed from below, A could be reduced to 6.4 mm (0.25 in.). The above dimensions should all be increased by 1.3 mm (0.05 in.) for 4.0 mm and 4.8 mm (⁵/₃₂and³/₁₆in.) diam-eter rivets but these dimensions are sufficiently important that draughts-men must check the standard practice in their particular company before quoting figures. Unsuitable edge distance allowances by the designer can cause quality control problems, e.g. a rivet hole positional tolerance of +/-0.5 mm (0.02 in.) could result in a 6.5% strength loss in a 7.6 mm (0.3 in.) edge and a 10% loss in a 5.0 mm (0.2 in.) edge, but a 7.6 mm edge is, by definition, 50% heavier than a 5 mm edge.

Some of these arguments apply to Fig. 9.15(c). In this case dimension A must be determined by the clearance needed for wrench access.

However, because of the load conditions indicated by the arrows, the edge distance must not be less than dimension A. Even if the two dimensions are equal, the tension load in the bolt will be twice the load P in the angle.

Fig. 9.15 Dimensions of riveted and bolted joints. ((a) Courtesy of British Aerospace.)

the angle, but it also helps keep A to a minimum.

The basic loading of bolts (or rivets) in a shear joint is shown in Fig.

9.16(a). Ignoring any design criticism of the whole joint (such as the pos-sibility of distortion putting the bolt in tension as shown in (b)) we can see that the shear load (see Section 5.5.2) in the bolt is P. Similarly, in Fig.

9.16(c) we have doubled the load but shared it between two bolts so the shear in each bolt is again P. Now consider (d). In this case we have increased the load three times and increased the number of bolts to three, so that we are tempted to say that the load in each bolt is still P. Unfor-tunately, this is not necessarily true for the following reason:

If we consider (c), the material between the two bolts is carrying a load and is stressed. Because it is stressed it is strained (i.e. it is extended) and because the stress is constant over the whole length between the bolts the extension is even and uniform over the length.

Now, if we imagine (c) being kept in a loaded condition while a hole is drilled through both parts to receive the third bolt shown in (d) we can see (because the extension is even) that the holes in both parts will still line up and still accept the bolt after the load is removed.

The situation is that the middle bolt can be placed in its hole when the joint is either loaded or unloaded, so it is quite clear that when the load is applied the middle bolt carries no load; i.e. the two outer bolts in (d) each carry 1.5 P.

In practice, because of minor distortions, the centre bolt does receive a load (but not a third of the total). Also we can taper the joint plates as shown in Fig. 9.16(e), which has the effect of making the strain between the outer bolts uneven over the whole distance, and so by careful design we can make the loads on the bolts very nearly equal.

Although the example above is somewhat over-simplified, the underly-ing principle is that in a line of bolts, such as shown in Fig. 9.16(f), the loads on the bolts are not equal. Also, in a joint where the plates are bonded together with an adhesive (such as epoxy resin), the shear stress in the adhesive is not constant along the joint but is concentrated at the ends.

Students who have become practised in consideration of stress and associated extension will be able to satisfy themselves that the shear distribution diagram shown in Fig. 9.16(g) is representative. (It should be noticed that the diagram must be symmetrical because the joint is symmetrical.)

9.5 Joggling

Joggles are features of the type shown in Fig. 9.4. In sheet-metal structures they are virtually unavoidable. (Figure 9.4(b) shows a method of

avoid-ance but it is heavy and expensive and only justified where strength or tooling considerations demand drastic solutions.)

As far as the strength of joggles is concerned, their problem is illus-trated in Fig. 9.4(c), but the difficulty is made more severe because, in spite of knowing that this failing exists, the designer can still be persuaded oth-erwise by the robust appearance of joggled components. If an attempt is Fig. 9.16 Joints.

some tension (Fig. 9.4(d)), and if the angle of the joggle is 45° then the load R is, by triangle of forces (see Fig. 4.3), the same as P. Load R can be reduced by increasing the length of the joggle and hence reducing the angle in the triangle of forces, and this partial solution, carefully used, is satisfactory.

The illusion of strength in joggles is most marked when angles are joggled and it disguises the fact that the joint in Fig. 9.4(e) is as poor as that in (f). A better solution is shown in (g).