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only 25 percent of initial preload in that bolt tightened first. Worst-case minimum clamping force in the vicinity of that bolt, therefore, becomes FC(min) = FP(min) FX 1 kB FP(min)PZ kJ + kB 2.265 106 (7.316 + 2.265) 106 (22.20)
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FC(min) = 6.91 103 5000 1
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6.91 103(0.05 + 0.25) = 1019 lb (4.53 kN) So our computed anticipated minimum is greater than the zero minimum acceptable clamping force determined earlier. If it had not been acceptable, we would, of course, have had to readjust our target preload and/or select more accurate tools and/or revise our design. 22.3.5 Achieving a Desired Preload In most cases we will try to achieve our target preload by using a wrench of some sort to apply torque to the nut or to the head of the bolt, accepting the resulting scatter in preload which this implies. We will discuss the torque-preload relationship at length in the next section. First, however, let us take a brief look at some of our other options for tightening bolts. Hydraulic Tensioners and Bolt Heaters. On large bolts we do not have to use a wrench. We can use a hydraulic tensioner which exerts a pure tension on the bolt, grabbing a few threads which stick out past the nut. Once the tool has stretched the bolt by the desired amount, the nut is run down to retain the tension. Then the tool lets go. At first glance this sounds like a perfect answer to some of the torquing and friction uncertainties we shall consider in Sec. 22.4, but there are other problems.The amount of preload retained by the fastener s nut is never the same as the preload introduced by the tool, because the nut must embed itself in the joint to pick up the loads originally supported by the much larger feet of the tensioner.This elastic recovery loss can equal 10 to 80 percent of the initial tension, depending on whether the fastener is relatively long (smaller loss) or short, and depending on how much torque was applied to the nut when it was run down (more torque, less loss). Hydraulic tensioners, however, are superb tools when it comes to preloading large fasteners. They can be gang-driven from a single hydraulic pump and so can tighten several fasteners simultaneously with the same initial tension in each. This can be a very important feature when you are tightening large joints, especially if they are gasketed. Tensioners also eliminate the galling problems often encountered when we attempt to torque large fasteners (3 in in diameter or more). The male and female threads are not turned relative to each other under heavy contact pressure with the tensioner. So tensioners have a place, but they do not provide perfect control of bolt preload. Another nonwrench sometimes used to preload large fasteners is a bolt heater. This is inserted in an axial hole running down the center of the bolt. The bolt gets longer as it gets hot. When it is hot, the nut is run down against the joint to retain the increase in length produced by the heat. Since this is a crude way to preload bolts,
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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.
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the process must be controlled by other means. Dial gauges or micrometers are usually used to measure the net change in length of the bolts after they have cooled. If they have been stretched too much or too little, the bolts must be reheated and the nuts run down again. The process takes skill but is widely used on large fasteners (again 3 in or so and larger). Microprocessor-Controlled Torque-Turn Tools. Hydraulic tensioners and bolt heaters make it possible to tighten fasteners without suffering the uncertainties of the torque-preload relationship, but they can be used only on large-diameter fasteners. For smaller ones, we need something else. One relatively new approach microprocessor-controlled tools measures both applied torque and the turn of the nut to monitor and/or control fastener preload. Most of the presently available systems are designed for automatic or semiautomatic assembly in mass-production operations (automotive, primarily), but there are manual versions of some of them. They can control preload to 2 to 5 percent if the joints are relatively soft (preload builds up smoothly as the bolts are tightened) and reasonable control is maintained over fastener dimensions and lubricity. Some of these systems are designed to tighten every fastener to the yield point. This provides good preload accuracy (control is based on the act of yield rather than the torque-friction-preload relationship). But not all joints can be tightened safely to the yield point of the fastener. Although there is considerable debate on this point, many designers feel that yield-point tightening can lead to fatigue failure or rupture unless future external loads can be predicted and controlled. Turn-of-the-Nut Control. There is one place where tightening to or past the yield point is the norm; structural steel joints have been tightened this way for half a century using a carefully designed process called turn-of-the-nut. The fastener is first tightened to 60 to 80 percent of yield by the application of torque (usually with an air-powered impact wrench). The location of one corner of the nut is then noted, and a wrench is used to give the nut a specified half turn or so (depending on the size of the fastener and whether or not it is being used on a flat or tapered joint member). This amount of turn always takes the bolt beyond yield. Since external loads can be predicted, however, and are generally static rather than dynamic, the process is a safe and effective way to control preload. Ultrasonic Control of Preload. Ultrasonic instruments are sometimes used instead of torque and/or turn-of-the-nut to control preload (Ref. [22.6], p. 157). This technology has been used in a few aerospace applications for nearly a decade and is just starting to emerge in the commercial marketplace. Presently available instruments send bursts of sound through the fastener and measure the time it takes for these wavefronts to travel through the fastener, echo off the far end, and return to the transducer. As the fastener is tightened, the time required for this round trip increases because the fastener gets longer, and so the path length is increased. Also, the velocity of the sound waves decreases as the stress level increases. Microprocessors in the instruments sort out the change-in-length effect from the velocity effect and display either the change in length of the fastener or the average stress level in the tensile-stress area of the threads. Either of these quantities can be used to estimate fastener preload with better accuracy than is possible with torque or torque-turn controls. One advantage of ultrasonics is that it can also be used in some cases to measure residual or working loads in the fasteners, as well as initial loads. You can use it to
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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.
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