STARTING THE ASSEMBLY PROCESS

We’re going to assemble a hypothetical joint, using as our example a round, gasketed, pipe flange joint held together by 16, ¹⁼⁸–8, ASTM A193 B7 bolts (see Figure 6.1). The large diameter and the presence of a gasket make this assembly a little more difficult than most, but will therefore allow us to look at a more complete range of assembly problems than would a simpler example. Most of the discussion would apply to joints in general.

We’re also going to measure the torque we apply to the nuts to control the buildup of initial preload in these bolts. This is the most common, and one of the simplest, types of control. It will be the subject of Chapter 7, so we won’t go into a lot of detail here about it’s pros and cons. We’ll just use it for now.

6.2.1 ASSEMBLING THEPARTS

We start by roughly aligning the flanges so that we can insert the bolts by hand. When we finish pushing and pulling on the flanges, their mating surfaces are not exactly parallel and the holes aren’t aligned perfectly; so we have to tap a few of the bolts with a hammer to get them through their holes, and some of them stick out a little farther, on the nut end, than do others.

Now we’re going to apply a preliminary ‘‘snugging torque’’ to run the nuts down and pull the flanges together.

6.2.2 TIGHTENING THEFIRSTBOLT

To load the joint (and gasket) evenly, we’ll apply the snugging torque in a cross or star pattern, as shown in Figure 6.2. We’d use a similar pattern on a square or rectangular joint if the bolts were all around the edge. In a rectangular, structural joint pattern, with several rows of bolts, we’d start snugging at the center of the bolt pattern and work our way out to the free edges.

We’ll use 225 lb-ft of torque for this first, snugging pass. This is about a third of the final torque we’re planning to use, and we’ll follow it with a second pass at two-thirds of final torque, and then with a third and final pass at full torque. In a structural steel joint, we would follow the snugging pass with a second (last) pass at the final torque. Note that in each case

FIGURE 6.1 This is a sketch of the large-diameter, pressure vessel joint used as an example in this chapter. We see what happens when we install and tighten the bolts.

we’re following basically a two-step procedure: pull the joint together and then tighten it.

Because this is a learning experiment we’ll use ultrasonic equipment (Chapter 9) to measure the preload in each bolt as we tighten it. We’ll also measure the angle through which the nut turns after it contacts the surface of the joint, and we’ll measure the amount by which the bolt stretches and the amount by which the joint is compressed.

We now apply the snugging torque to the first bolt and use the resulting preload, torque, and turn data to plot the curves shown in Figure 6.3. We’re doing work on this fastener as we tighten it. The amount of work is equal to the area under the torque turn curve (measured in lb-ft or N-m times radians). Ideally, all of this work would be converted to potential energy in the bolt and in those portions of the joint members which surround it. If that were the case, all of the work we do on this fastener would end up contributing to the clamping force.

Unfortunately and unavoidably, most of our input work is lost.

Typically, about 90% of the work we do on a nut is converted to heat, thanks to the frictional resistance between the face of the nut and the surface of the joint, and between male and female threads. About 50% is lost under the nut, and about 40% within the threads, as shown in Figure 6.4. Only 10% of the input work typically ends up as potential energy in the bolt; so only 10% ends up as bolt preload or as clamping force between joint members.

1 9

5 13

3

11

7 2 15

10 6

14 4 12

8 16

FIGURE 6.2 We’ll tighten the bolts of our example joint in the ‘‘star pattern’’ sequence shown here.

We’ll use three passes, at one-third, two-thirds, and final torque, following the same sequence on each pass.

Input work

Turn Torque

Torque

Initial preload

FIGURE 6.3 As we tighten the first bolt in our example joint we plot the buildup of initial preload versus applied torque (left-hand diagram) and applied torque versus the angle through which the nut turns (diagram on the right). The area under the torque–turn curve is equal to the amount of work we’re doing on the nut and to the energy delivered to the fastener joint system.

We’d like to apply a given torque to each bolt and create a given amount of initial preload (the same amount) in each bolt. But the fact that most of the work we do on the nuts is converted to heat makes this virtually impossible, because these frictional losses are extremely difficult to predict or control. Let’s assume, for example, that this first nut we’re tightening is a little drier than average. As a result, let’s assume that 52% of the input work is converted to heat at the nut joint interface, rather than the typical 50%. A 4% increase in friction—from 50% to 52% of the input work—is easy to come by.

This 4% increase in friction loss, that extra 2% of the input work going into heat, means that 2% less of the input work will be converted to the thing we’re interested in, the tension in the bolt. We started with the assumption that an average of only 10% of the input would be going into preload; now we’ve lost a fifth of that. This bolt will, therefore, end up with only 80% as much preload as we expected it to. A 4% swing in friction has caused a 20% change in assembly preload, a very bad leverage situation. And, as we’ll learn in the next chapter, there are a lot of factors which can cause this kind of variation in friction.

Although in our learning experiment we’re measuring both torque and bolt tension, we won’t attempt to compensate for the frictional differences between bolts; we’ll apply the snugging torque of 225 lb-ft to the first bolt and let the initial preload end up where it may.

We also plot the deflections in the bolt and in the joint material surrounding the bolt versus the preload we create in the bolts and the presumably equal and opposite clamping force on the joint (see Figure 6.5). We then combine these force–deflection curves, plotting the preload on a common axis, as also shown in Figure 6.5. This creates what the bolting world calls a ‘‘joint diagram.’’ The pure of heart among you may complain that the preload in the bolt and the clamping force on the joint are equal and opposite, action and reaction forces, and that both should not be shown as positive values, but this joint diagram is a great convenience so we’ll draw it as shown.

Since the diagram records the forces developed in bolt and joint and the deflection of each part, it also gives us a visual indication of the stiffness of the bolt and the stiffness of the joint.

Torque

Turn

Stored energy

Thread friction loss

Nut friction loss

FIGURE 6.4 This diagram shows the approximate way in which the energy delivered to the fastener joint system is absorbed by it. About 50% of the input is lost as friction-generated heat between the face of the nut and the surface of the joint. Another 40% is lost as heat between male and female thread surfaces. Only about 10%, on the average, ends up as potential energy stored in the bolt and joint springs; and only that 10%, therefore, ends up as preload in the bolt and clamping force on the joint.

These are proportional to the slopes of the two straight lines, and, as we saw in Chapter 5, can be computed as follows:

KB¼ FP=DL (6:1)

KJ¼ FP=DT (6:2)

where

FP ¼ preload in the bolt and joint (lb, N) DL¼ deflection (stretch) of the bolt (in., mm) DT¼ deflection (compression) of the joint (in., mm) KB¼ stiffness of the bolt (lb=in., N=mm)

KJ ¼ stiffness of the joint material being loaded by this bolt (lb=in., N=mm)

Note that the areas under the bolt and joint curves also equal the amount of energy stored in these parts, as shown in Figures 5.6 and 5.17 in the last chapter. So this simple diagram contains a lot of useful information. We’ll extend this diagram in Chapter 10 to add the effects of external loads on the joint. We’ll also use joint diagrams when we design joints. For now, however, we’re merely interested in using the joint diagram to illustrate the preloading of the bolts.

Our boss, who has already read a previous edition of this book, uses Equation 7.4 in the next chapter to compute the average preload he expected us to get in this first bolt when we applied 225 lb-ft of torque to it. He tells us that we should have created 12,000 lbs of tension in the bolt. Because of the slightly higher than average friction loss described earlier, however, this bolt has ended up with only 80% of that preload, or 9,600 lbs.

FP

FP

OB

OB

FB

KB=

OJ

OJ

FCL

∆L

∆L

∆L

FCL

KJ =

∆T

∆T

∆T (A)

(B)

FIGURE 6.5 As we tighten the first bolt we also plot the buildup of preload (FP) in the bolt versus the increase in length (DL) of the bolt, and the buildup of clamping force on the joint (FCL) versus the compression or change in thickness (DT ) of the joint. At this point we assume that the preload will be equal to the clamping force. These two plots are shown at the top of this illustration. We then combine those two plots, as shown at the bottom of this illustration, to start constructing what we’ll call a joint diagram.

Has this really created 9,600 lbs of clamping force between joint members? Our joint diagram assumes it has, but the correct answer is ‘‘probably not’’—at least as far as this first bolt is concerned. Remember that we had to tap some of those bolts into their holes? This implies that there was contact between the sides of those bolts and their holes—bolt–hole interference. Furthermore, the flange surfaces were not pulled into full contact when we first assembled the parts. They were slightly misaligned, as we could tell by the fact that some of those bolts stuck out farther on the threaded end than did others. Before we go on to snug tighten the remaining 15 bolts in our example joint, let’s take a look at how hole interference and nonparallel flanges might affect the buildup of clamping force during the assembly process.