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BOLTED AND RIVETED JOINTS 22.27
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BOLTED AND RIVETED JOINTS
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FIGURE 22.14 Joint diagram for a joint loaded in compression.
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Note that any external compression or tension load will alter both the tension in the fasteners and the compression in the joint. This is contrary to the widely held belief that there will be no such change in either member until and unless the external tensile load exceeds the preload in magnitude. If the external tensile load exceeds the initial preload, then all clamping force will have been removed from the joint and the bolts will see, in its entirety, any additional tension load placed on that joint. One nice feature of the joint diagram is that it allows us to derive expressions which define joint behavior, such as the following: kB = FP LB kJ = FP tj (22.17)
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FB = Fx
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(22.18)
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Let us continue the example we started in Fig. 22.12 by assuming an external tensile load of 10 kip (44.5 kN) per bolt has been placed on the system and computing the estimated effect of the external load on the bolt tension:
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
BOLTED AND RIVETED JOINTS 22.28
FASTENING, JOINING, AND CONNECTING
FB = 5000
2.265 106 = 1182 lb (5.26 kN) (7.316 + 2.265) 106
Estimated Maximum Tension in the Bolts. We can now combine all the effects which we have studied to determine the maximum anticipated tension in the bolts under the worst-case situation: FB(max) = FP(max) + FB FB(max) = 13.3 103 + 1.182 103 = 14.5 103 lb (64.5 kN) So our anticipated maximum is less than the acceptable maximum preload of 16.9 103 lb (75.2 kN). We can therefore continue with our analysis. Note that if the anticipated maximum had exceeded the acceptable maximum, we would have had to lower our target preload somewhat and try again.
22.3.4 Estimating Actual Lower Limit on the Clamping Force To determine the lower limit on clamping force, we follow a procedure similar to that used for determining the maximum tension to be expected in the fasteners, this time subtracting, not adding, from target preload the tool and operator scatter and that portion of the external load which reduces the clamping force on the joint members ( FC in Fig. 22.13). When considering the lower limit, we must also consider one other effect: the short-term relaxation of the joint following or during initial tightening. Relaxation Effects. Embedment Relaxation. When joint and fastener are first assembled, especially if we are dealing with new parts, they contact each other only on the microscopically rough high spots of their contact surfaces. These high spots will be loaded past the yield point and will creep and flow when first placed under load until enough additional surface area has been brought into play to stabilize the process. Other plastic flow can occur in thread roots, in the head-to-body fillet, and even in some whole threads, causing further relaxation. All these effects are lumped together under the term embedment relaxation. Typically they will lead to a loss of 2 to 10 percent of initial preload in the first few seconds or minutes after initial tightening. Let us assume a 5 percent loss of our ongoing example. Elastic Interactions in the Joint. Achieving perfect initial preload is not our only problem when we tighten a multibolt joint. When we tighten the first fasteners, we stretch them and partially compress the joint. When we subsequently tighten other fasteners in the same joint, we further compress the joint. Because this act allows previously tightened fasteners to contract a little, it reduces the tension in those fasteners even if we achieved perfect initial preload in each when we first tightened it. The effect a fastener has on a previously tightened fastener is illustrated in Fig. 22.14. The new fastener applies a compressive load to the joint which had been previously preloaded by the first bolt. Estimating Minimum Clamping Force. Elastic interactions can be a special problem in large-diameter and/or gasketed joints. Since our example is neither, and involves only two bolts, we will assume that these interactions cost us, worst case,
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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