nuts & b o l t s

building basics

Driven to Rivet

Solid rivets add strength, but must be installed with care Jack Dueck, EAA 337912 ll aircraft homebuilders will work with aluminum sheet metal and fasten components with rivets. Even composite aircraft have sheet metal parts. Exercising a little quality control in riveted joints will contribute to your airplane's structural strength as well as the aesthetic outcome of the project. Rivets can be classified as blind (or pulled) and solid (or driven). Each category has its own parameters, but the solid (driven) rivet requires greater care during installation. The process of driving a rivet correctly is as much an art as it is a science, so for now we'll focus on driven rivets. The design of all riveted joints is analyzed for both shear and bearing strengths. Shear strength refers to the shearing action on the rivet that results when a riveted connection is pulled in the same direction as the components. Figure 1 shows this action on a rivet in a single shear joint, whereas in Figure 2 you can see this action in a double shear joint, since the shearing action occurs at two points on the rivet. It should also be noted that the double shear connection would give

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Figure 1 P

twice the shear strength to the joint as the single shear connection. Bearing strength refers to the same force applied to the plate by the rivet, whereas shear strength represents force applied to the rivet by the plate. This value will obviously vary with the thickness of the plate. To calculate the bearing strength of a joint with differing thicknesses, the designer will base calculations on the thinner plate. The most efficient joint will occur if it has approximately the same strength in shear and in bearing. The ideal joint would have these two values identical, but this is rarely possible. Joint strengths will be based on the lower of the two values. If the

2D Edge Distance (4D Max.)

2D Edge Distance (4D Max.) 3D Between Rows (Win.) 3D to 12D Pitch

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shear value is lower than the bearing value, the joint is called critical in shear. If the bearing value is lower, the joint is considered critical in bearing. Another design consideration is the rivet diameter/thickness ratio. The chosen rivet diameter should not be larger than 5.5 times the thickness of the thinner plate. If the rivet diameter is too large, there will be a tendency for the sheet to buckle and distort locally around the rivet because of the expansion forces caused by the swelling of the rivet when it is driven. For example, a 3/32 inch (0.09375 inch) diameter rivet will tend to distort a sheet size thinner than 0.017 inch.