Machinery's Handbook, 31st Edition
Riveted Joints 1863 would be subject to a combination of tensile and shearing stresses and it would fail because of combined stresses, not a single stress. Furthermore, the shearing stress is usually consid ered to be distributed evenly over the cross section, which is also not the case. Rivets that are usually driven hot contract on cooling. This contraction in the length of the rivet draws the plates together and sets up a stress in the rivet estimated to be equal in magnitude to the yield point of the rivet steel. The contraction in the diameter of the rivet results in a little clearance between the rivet and the hole in the plate. The tightness in the plates caused by the contraction in length of the rivet gives rise to a condition in which quite a sizeable frictional force would have to be overcome before the plates would slip over one another and subject the rivets to a shearing force. It is European practice to design joints for resistance to this slipping. It has been found, however, that the strength-basis designs obtained in American and English practice are not very different from European designs. Allowable Stresses.— The design stresses for riveted joints are usually set by codes, prac tices, or specifications. The American Institute of Steel Construction issues specifications for the design, fabrication, and erection of structural steel for buildings in which the allow able 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 some what lower are commonly used. Analysis of Joint Strength.— The following examples and strength analyses of riveted joints are based on the six previously outlined Simplified Design Assumptions . Example 1: Consider a 12-inch section of single-riveted lap-joint made up with plates of 1 ∕ 4 -inch thickness and six rivets, 5 ∕ 8 inch in diameter. Assume that rivet holes are 1 ∕ 16 inch larger in diameter than the rivets. In this joint, the entire load is transmitted from one plate to the other by means of the rivets. Each plate and the six rivets carry the entire load. The safe tensile load L and the efficiency η may be determined in the following way: Design stresses of 8500 psi for shear, 20,000 psi for bearing, and 10,000 psi for tension are arbitrarily assigned and it is assumed that the rivets will not tear or shear through the plate to the edge of the joint. a) The safe tensile load L based on single shear of the rivets is equal to the number of riv ets n times the cross-sectional area of one rivet A r times the allowable shearing stress S s or . L n A S 6 4 0 625 8500 15,647 pounds r s 2 # # # # π = = = ^ h b) The safe tensile load L based on bearing stress is equal to the number of rivets n times the projected bearing area of the rivet A b (diameter times thickness of plate) times the allowable bearing stress S c or L = n × A b × S c = 6 × (0.625 × 0.25) × 20,000 = 18,750 pounds c) The safe load L based on the tensile stress is equal to the net cross-sectional area of the plate between rivet holes A p times the allowable tensile stress S t or L = A p × S t = 0.25[12 − 6(0.625 + 0.0625)] × 10,000 = 19,688 pounds The safe tensile load for the joint would be the least of the three loads just computed or 15,647 pounds, and the efficiency η would be equal to this load divided by the tensile strength of the section of plate under consideration if it were unperforated, or . . 12 0 25 10,000 15,647 100 522 # # # η = = percent Example 2: Under consideration is a 12-inch section of double-riveted butt-joint with main plates 1 ∕ 2 inch thick and two cover plates each 5 ∕ 16 inch thick. There are 3 rivets in the inner row and 2 on the outer and their diameters are 7 ∕ 8 inch. Assume that the diameter of
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online