The aircraft's body structure is assembled from tens of thousands of components, and the fasteners are often connected between the components. The structure of the fastener connection has high bearing capacity and good fatigue performance, and is most widely used on aircraft. There are many types of aviation fasteners, which can be roughly divided into two categories: detachable and permanent. Removable fasteners include bolts, nuts, screws, pins, and the like. Permanent fasteners include rivets, high lock bolts, ring groove nails, and the like. At present, rivets can be installed using an automatic drill riveting machine, and bolt-type fasteners still need to be done manually.
In the production process, the aircraft often appears to be inconsistent with the design state. At this time, the rejection of the structural parts will occur, which is also called the tolerance. According to statistics, the tolerances involving fasteners account for more than 80% of the total tolerance, and these tolerances are almost always caused by human factors. In order to meet the maintainability, sufficient maintenance space should be reserved when designing the structural dimensions. After structural repair, it should have the same load carrying capacity, structural rigidity and service life before repair. The repair plan should be as simple as possible and easy to operate. Pay attention to the fatigue and loss tolerance requirements while considering the static strength. For the problem of tolerance, there is no universal treatment, but each tolerance should be treated as a single design, and dealing with the tolerance is much more complicated than starting a new design. The following are some of the most common out-of-tolerance scenarios, giving analytical methods and treatment options.
1 common fasteners are out of tolerance
1.1 Insufficient margins
In general, the fastener margin is taken as 2D+1 (D is the fastener diameter), which is the value of the design margin to deal with the hole position, the aperture is out of tolerance. In general, the margin 2D is allowed, and sometimes 1.5D (the margin is perpendicular to the load direction) also meets the strength requirements. These need to be analyzed for strength to determine if they can be used as they are.
If it can't be used as it is, it can usually be handled in two ways: (1) Add a new fastener of the same specification when space permits, and meet the margin and spacing requirements. Retained; (2) Add a patch of the same material, the same shape, and a thickness of one level to the original structure. The patch size is to cover the oversized fasteners and extend the two fasteners.
1.2 Aperture tolerance
When the fastener aperture tolerance is small, a method of installing an enlarged fastener is usually employed. The standard enlarged fasteners have a diameter of 1/32 in. and 1/16 in.
When the fastener aperture is out of tolerance, it cannot be solved by installing and fixing the fastener. A bushing method can be used at this time. Cylindrical bushings and countersunk bushings are commonly used. The countersunk bushing ensures that the bushing does not slip in one direction. The bushing-excess hole uses an interference fit, and the amount of interference can be 0.0015-0.0035 in. After inserting the bushing into the excess hole, install the original fastener in the original position. When choosing a bushing, choose a bushing with a smaller diameter to ensure that the bushing has a minimum wall thickness of 0.03 in. The outer diameter of the mounting stud bushing must not exceed the diameter of the fastener head to prevent the bushing from falling out of the hole.
1.3 fastener head and fillet area interference
When the fastener head enters the structural fillet, the fastener can be reversed. If it is still not possible to avoid interference, a pad can be added between the fastener and the structural element. The minimum thickness of the pad is equal to the fillet radius plus 0.02 in. It is necessary to ensure that the fastener meets the margin requirements on the pad.
1.4 extra holes
In general, voids (excess and hole) are not allowed on the fuselage structure. The stress concentration caused by the opening will greatly reduce the fatigue strength of the structure. For structurally open holes, such as system through holes, the design will be thickened in the open area to reduce stress levels. Studies have shown that interference fit plugging can increase fatigue life to four times that of unblocked holes. Qualified plugging, the plugging member can transmit the compressive load through the hole, preventing the hole from being deformed under tensile and compressive loads, and generating a certain pre-stress by fitting the edge of the hole. Therefore, the excess holes in the structure must be blocked, and the above blocking requirements should be met.
When there are excess holes in the single-layer structure, the fasteners can be used to block the holes. The engineering commonly used rivet "AD" (2117-T3) or "DD" (2024-T31) nail plugging holes, flat or countersunk nails can be used as needed.
When the excess hole passes through the multi-layer structural member, if the margin and spacing requirements of the fastener are met, a fastener may be added at the excess hole; if the requirement is not met, the layered plugging method shall be adopted. This is because the stud passes through two or more layers of the part, and the stud is subjected to and transmits the load between the parts, which affects the load distribution on the fasteners in the area. If the diameter of the plugging hole is too large, the plug will transmit a large load corresponding to the diameter, which may cause the structural material around the plug to be cut.
If the opening is too close to the edge of the part, if the hole is equal to or less than 1.6 mm from the edge of the structure, the hole should be repaired, smoothed and then hand anodized and painted.
2 Summary
This paper introduces the fasteners that are often encountered in the production and manufacturing process, the handling principles and measures. In the actual use process, the disposal plan mentioned in the article cannot be directly applied, and the corresponding strength analysis should be carried out. This paper gives the handling ideas of fasteners that are often encountered in engineering, and can provide reference for civil aircraft design engineers and structural repair engineers.
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