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Rivets: Keeping it all together
By H.G. Frautschy (originally published in EAA Sport Aviation, June 2000)
Have you ever tried to pull apart two pieces of stuck-together duct tape? Tug as you may on the ends of the tape, the adhesive seems to stick to itself with a tenacity that would stymie a tractor pull. But stick it on a surface and pull vertically and the tape comes off with comparatively little effort. What you’ve witnessed is the difference between two stress forces being applied to the tape’s adhesive: Shear, trying to separate the two pieces by pulling on the ends, and tension, the motion of pulling the piece up from the table.
Rivets resist these same forces in ways useful to those who want to build structures that will be light enough to fly but strong enough to tolerate the substantial loads imposed by the forces of flight. A rivet acts as a clamp that holds two or more pieces of material together, but its main job is to resist shear forces. A properly installed rivet will resist tension to a lesser degree, but its primary job is to transmit loads along the piece of material, not at a major angle away from it.
Rivets have been in use since aviation’s earliest days. Until welded structures became more common, riveting a structure or component was the preferred method. When the temper and/or dimensions of an aluminum or steel sheet must be maintained, riveting is still the most common method used to join the various components.
The two rivets we see most often are the round universal head MS 20470 and the 100-degree countersunk MS20426. They are identical to the older AN426 and AN470 rivets that became the standards during World War II. Before the war, manufacturers used a bewildering array of rivets, including the round head (AN430), with a higher-diameter head than the newer universal head; the brazier head (AN455 and AN456), with a flatter and wider head; and the flat head (AN442), among others. Most of these types have gone by the wayside, but you can still find them at times, particularly when someone is restoring an older airplane and wishes to maintain the same look.
And there are "blind rivets," so named because they can be installed in places where it would be impossible to get a bucking bar to form the bucked head. (Some people call them "pop rivets," after the sound they make when the stem separates.)
A blind rivet is installed using a hand-operated puller or a pneumatic gun that pulls the rivet’s stem. After the rivet is set, the pulling force separates the stem from the rivet at a manufactured weak point in the stem. The remainder of the stem then stays in the rivet (part of the rivet’s strength comes from the stem), sealing the rivet. Blind rivets come in both round head and countersunk versions.
There are other high-strength "rivets" used in airframe building, including the Huck and Hi-Shear rivets, where a collar is installed on a manufactured head and shank using special tools. These close-tolerance rivets are sometimes seen in built-up spars and other highly stressed components. The line between rivets and high-strength, close-tolerance fasteners can seem a bit blurred. Think of it this way-if you have to destroy part of the fastener to remove it, it’s a rivet!
Making It Strong
As written back in the late 1930s edition of Brimm and Boggess’ Aircraft Maintenance for the Airplane Mechanic, "The strength of a riveted joint depends upon six factors: (1) The diameter of the rivets; (2) the number of rivets; (3) the strength, in shear and bearing, of the material from which the rivet is made; (4) the strength in bearing of the material being joined; (5) the thickness of the material being joined; (6) the number of planes along which shear is resisted. This list does not include such items as improperly headed rivets, holes too large for rivets, etc., but assumes that the workmanship is satisfactory and that the diameter of the hole is not more than 5 percent greater than the diameter of the rivet."
The bearing strength is the amount of force applied to an installed rivet that causes the rivet to elongate the rivet hole. It can"bear" so much load before the rivet holes deform and the joint fails. Well-designed joints will have a bearing strength close to the shear strength of the rivets included in the joint.
Another way to increase the shear strength of a riveted joint in thinner sheet materials is to dimple the material and use countersunk rivets. Reducing overall drag is the primary reason for using countersunk rivets, and an added benefit is a stronger joint-by dimpling the hole, you add the cross section of the material to the overall strength of the union. In thicker sheets, the hole is drilled and countersunk using a special tool. The "microstop countersink" allows the operator to accurately set the depth of the countersink, so each hole is consistently bored.
Like many building basics, riveting is an acquired skill-one you’ll gain with practice. Good workmanship includes drilling accurate holes in the material to be joined and deburring each hole so the rivet can rest directly on the material.
There are a few different methods for setting a solid rivet. Using a hammer and bucking bar is the simplest method, and it requires a surprising amount of practice to do well. Another tool often used is a handheld or bench-mounted rivet squeezer. As the name implies, the tool squeezes each rivet a set amount to upset the shank and set the rivet.
The most common method is using a pneumatic rivet gun with a rivet set and a bucking bar. The rivet set (the part that goes in the rivet gun) for a round head rivet set is concave to ensure that the driving force is applied to the rivet’s center. For a countersunk rivet the set has a slight convex curve to it, again to ensure that riveting force is applied to the rivet and not to the surrounding material. A bucking bar (used with hearing protection, please) held against the rivet’s shank upsets it and forms the other side of the rivet’s "clamp."
Riveting is a skill easily learned and one mastered with practice. To avoid ruining parts of your airplane, it’s good to practice on scrap material before you start making parts. With this skill mastered you can create a beautiful aircraft from a pile of sheet aluminum and a couple of pounds of rivets. Don’t be intimidated by the concept-you can do it!
Aircraft Inspection and Repair, AC 43.13-1B Stock # F00191
Aircraft Sheet Metal Stock # F37118
Both these books are available from EAA through its website at www.eaa.org or by calling 800/843-3612.