RIVETS AND RIVETED JOINTS
Riveting is among the oldest methods of joining materials, dating back as far as the use of metals in construction practice.. Rivets were the most popular fasteners during the first half of this century,
but their use has declined steadily since the introduction of the high-strength bolts. At the present time they are rarely used in either field or shop connections; either high-strength bolts or welds are used almost exclusively in new work. Nevertheless, the increasing importance of evaluation and
retrofitting of existing structures will require that the designer be knowledgeable about riveted connections.
Classes and Types of Riveted Joints
Riveted joints may be classified by application as:
1) pressure vessel;
2) structural;
3) machine member.
Basically there are two kinds of riveted joints, the lap-joint and the butt-joint.
In the ordinary lap-joint, the plates overlap each other and are held together by one or more rows of rivets.
In the butt-joint, the plates being joined are in the same plane and are joined by means of a cover plate or butt strap, which is riveted to both plates by one or more rows of rivets.
The term single riveting means one row of rivets in a lap-joint or one row on each side of a butt-joint;
double riveting means two rows of rivets in a lap-joint or two rows on each side of the joint in butt riveting
Joints are also triple and quadruple riveted. Lap-joints may also be made with inside or outside cover plates
Tubular Rivets
In tubular rivets, the end opposite the head is made with an axial hole (partway) to form a thin-walled, easily up settable end. As the material at the edge of the rivet hole is rolled over against the surface of the joint, a clinch is formed
Two-part tubular rivets have a thin-walled head with attached thinwalled rivet body and a separate thin-walled expandable plug. The head-body is inserted through a hole in the joint from one side, and the plug from the other. By holding an anvil against the plug bottom and hammering on the head, the plug is caused to expand within the head, thus locking both parts together
Blind rivets are inserted and set all from one side of a structure. This is accomplished by mechanically expanding, through the use of the rivet’s built-in mandrel, the back (blind side) of the rivet into a bulb or upset head after insertion. Blind rivets include the pull type and drive-pin type.
The pull-type rivet is available in two configurations: a self-plugging type and a pull-through type. In the self-plugging type, part of the mandrel remains permanently in the rivet body after setting, contributing additional shear strength to the fastener. In the pull-through type, the entire mandrel is pulled through, leaving the installed rivet empty. In a drive-pin rivet, the rivet body is slotted. A pin is driven forward into the rivet, causing both flaring of the rivet body and upset of the blind side
Forms and Proportion of Rivets The forms and proportions of small and large rivets have been standardized and conform to ANSI B18.1.1
General Considerations for Riveted Joints
Factors to be considered in the design or specification of a riveted joint are: type of joint; spacing of rivets; type and size of rivet; type and size of hole; and rivet material.
Spacing of Rivets: The spacing between rivet centers is called pitch and between row center lines, back pitch or transverse pitch. The distance between centers of rivets nearest each other in adjacent rows is called diagonal pitch. The distance from the edge of the plate to the center line of the nearest row of rivets is called margin.When fastening thin plate, it is particularly important to maintain accurate spacing to avoid buckling.
where t is the thickness of the plate
Size and Type of Hole: Rivet holes may be punched, punched and reamed, or drilled. Rivet holes are usually made 1⁄16 inch larger in diameter than the nominal diameter of the rivet although in some classes of work in which the rivet is driven cold, as in automatic machine riveting, the holes are reamed to provide minimum clearance so that the rivet fills the hole completely.
Rivet Material:Rivets for structural and machine-member purposes are usually made of wrought iron or soft steel, but for aircraft and other applications where light weight or resistance to corrosion is important, copper, aluminum alloy, Monel, Inconel, etc., may be used as rivet material.
Simplified Design Assumptions: In the design of riveted joints, a simplified treatment is frequently used in which the following assumptions are made:
1) The load is carried equally by the rivets.
2) No combined stresses act on a rivet to cause failure.
3) The shearing stress in a rivet is uniform across the cross-section under question.
4) The load that would cause failure in single shear would have to be doubled to cause failure in double shear.
5) The bearing stress of rivet and plate is distributed equally over the projected area of the rivet.
6) The tensile stress is uniform in the section of metal between the rivets.
Formulas for Riveted Joint Design.
A riveted joint may fail by shearing through the rivets (single or double shear), crushing the rivets, tearing the plate between the rivets, crushing the plate or by a combination of two or more of the foregoing causes. Rivets placed too close to the edge of the plate may tear or shear the plate out to the edge but this type of failure is avoided by placing the center of the rivet 1.5 times the rivet diameter away from the edge.
The efficiency of a riveted joint is equal to the strength of the joint divided by the strength of the unriveted plate, expressed as a percentage. In the following formulas, let
d=diameter of holes
t =thickness of plate
tc =thickness of cover plates
p=pitch of inner row of rivets
P=pitch of outer row of rivets
Ss= shear stress for rivets
St =tensile stress for plates
Sc =compressive or bearing stress for rivets or plates
Allowable Stresses.
The design stresses for riveted joints are usually set by codes, practices, or specifications. The American Institute of Steel Construction issues specifications for the design, fabrication, and erection of structural steel for buildings in which the allowable stress permitted in tension for structural steel and rivets is specified at 20,000 pounds per square inch, the allowable bearing stress for rivets is 40,000 psi in double shear and 32,000 psi in single shear, and the allowable shearing stress for rivets is 15,000 psi. The
American Society of Mechanical Engineers in its Boiler Code lists the following ultimate stresses: tensile, 55,000 psi; shearing, 44,000 psi; compressive or bearing, 95,000 psi. The design stresses usually are one-fifth of these, that is tensile, 11,000 psi; shearing, 8800 psi; compressive or bearing, 19,000 psi. In machine design work, values close to these or somewhat lower are commonly used.
Failure of Riveted Joints.
Rivets may fail by:
1) Shearing through one cross-section (single shear)
2) Shearing through two cross-sections (double shear)
3) Crushing Plates may fail by:
4) Shearing along two parallel lines extending from opposite sides of the rivet hole to the edge of the plate
5) Tearing along a single line from middle of rivet hole to edge of plate
6) Crushing
7) Tearing between adjacent rivets (tensile failure) in the same row or in adjacent rows
Nice Blog, Blind-Rivets
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