30079_35a.pdf

CHAPTER 35

MECHANICAL FASTENERS

Murray J. Roblin

Chemical and Materials Engineering Department

California State Polytechnic University

Pomona, California

35.1 INTRODUCTION

11 36

35.8 THEORETICAL BEHAVIOR

OF THE JOINT UNDER

TENSILE LOADS

35.2 BOLTED AND

RIVETED JOINT

TYPES

11 46

35.8.1 Critical External

Load

11 37

11 48

35.8.2 Very Large

External Loads 11 49

35.3 EFFICIENCY

11 38

35.4 STRENGTH OF A

SIMPLE LAP JOINT

(BEARING-TYPE

CONNECTION)

35.9 EVALUATION OF SLIP

CHARACTERISTICS 1153

35.10 INSTALLATION OF HIGH-

STRENGTH

BOLTS

11 38

1153

35.5 SAMPLE PROBLEM

OF A COMPLEX BUTT

JOINT (BEARING-

TYPE CONNECTION) 11 39

35.5.1 Preliminary Calculations 1140

35.11 TORQUE AND TURN

TOGETHER

1155

35.12 ULTRASONIC

MEASUREMENT

OF BOLT STRENGTH

OR TENSION

35.6 FRICTION-TYPE

CONNECTIONS

11 42

1156

35.7 UPPER LIMITS ON

CLAMPING FORCE 11 44

35.7.1 Yield Strength

of the Bolt 11 44

35.7.2 Thread Stripping Strength 1144

35.7.3 Design-Allowable

Bolt Stress and

Assembly Stress

Limits

35.13 FATIGUE FAILURE

AND DESIGN FOR

CYCLICAL TENSION

LOADS 1158

35.13.1 Rolled Threads 1158

35.13.2 Fillets 1158

35.13.3 Perpindicularity 1158

35.13.4 Overlapping

Stress

Concentrations 1158

35.13.5 Thread Run-Out 1158

35.13.6 Thread Stress

Distribution 1158

35.13.7 Bending 1159

35.13.8 Corrosion 1159

35.13.9 Surface

Conditions 1159

35.13.10 Reduce Load

Excursions

1144

35.7.4 Torsional Stress

Factor

11 44

35.7.5 Shear Stress

Allowance 1145

35.7.6 Flange Rotation 11 45

35.7.7 Gasket Crush 11 45

35.7.8 Stress Cracking 11 45

35.7.9 Combined Loads 11 45

1159

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

35.15 COOLING RATES AND

THE HEAT-AFFECTED ZONE

(HAZ)

IN WELDMENTS

35.14 WELDED JOINTS

1159

35.14.1 Submerged Arc

Welding (SAW) 1160

35.14.2 Gas Metal Arc Welding 1162

35.14.3 Flux-Cored Arc

Welding: FCAW 1166

35.14.4 Shielded Metal

Arc Welding

(SMAW)

1170

1167

35.1 INTRODUCTION

Most of the information in this chapter is not original. I am merely passing along the information I

have gained from many other people and from extensive reading in this subject.

For an in-depth understanding of this inexact field of study, I would recommend two excellent

books that I used extensively in the preparation of this chapter.1'2 For a full comprehension of this

topic, it is necessary to read both volumes, as they approach the topic from distinctly different points

of view.

Two or more components may need to be joined in such a way that they may be taken apart

during the service life of the part. In these cases, the assembly must be fastened mechanically. Other

reasons for choosing mechanical fastening over welding could be:

1. Ease of part replacement, repair, or maintenance

2. Ease or lower cost to manufacture

3. Designs requiring movable joints

4. Designs requiring adjustable joints

The most common mechanical joining methods are bolts (threaded fasteners), rivets, and welding

(welding will be covered in a later section).

To join two members by bolting or riveting requires holes to be drilled in the parts to accommodate

the rivets and bolts. These holes reduce the load-carrying cross-sectional area of the members to be

joined. Because this reduction in area as a result of the holes is at least 10-15%, the load-carrying

capacity of the bolted structure, is reduced, which must be accounted for in the design. Alternatively,

when one inserts bolts into the holes, only the cross section of the bolt or rivet supports the load. In

this case, the reduction in the strength of the joint is reduced even further than 15%.

Even more critical are the method and care taken in drilling the holes. When one drills a hole in

metal, not only is the cross-sectional area reduced, but the hole itself introduces "stress risers" and/

or flaws in/on the surface of the holes that may substantially endanger the structure. First, the hole

places the newly created surface in tension, and if any defects are created as a result of drilling, they

must be accounted for in a quantitative way. Unfortunately, it is very difficult to obtain definitive

information on the inside of a hole that would allow characterization of the introduced defect.

The only current solution is to make certain that the hole is properly prepared which means not

only drilling or subpunching to the proper size, but also reaming the surface of the hole. To be

absolutely certain that the hole is not a problem, one needs to put the surface of the hole in residual

compression by expanding it slightly with an expansion tool or by pressing the bolt, which is just

slightly larger than the hole. This method causes the hole to expand during insertion, creating a hole

whose surface is in residual compression. While there are fasteners designed to do this, it is not clear

that all of the small surface cracks of the hole have been removed to prevent flaws/stress risers from

existing in the finished product.

Using bolts and rivets in an assembly can also provide an ideal location for water to exist in the

crevices between the two parts joined. This trapped water, under conditions where chlorides and

sodium exist, can cause "crevice corrosion," which is a serious problem if encountered.

Obviously, in making the holes as perfect as possible, you increase the cost of a bolted and/or

riveted joint significantly, which makes welding or adhesive joining a more attractive option. Of

course, as will be shown below, welding and joining have their own set of problems that can degrade

the joint strength.

The analysis of the strength of a bolted, riveted, or welded joint involves many indeterminate

factors resulting in inexact solutions. However, by making certain simplifying assumptions, we can

obtain solutions that are acceptable and practical. We discuss two types of solutions: bearing-type

connections, which use ordinary or unfinished bolts or rivets, and friction-type connections, which

Fig. 35.1 Lap joints. Connectors are shown as rivets only for convenience.

use high-strength bolts. Today, economy and efficiency are obtained by using high-strength bolts for

field connections together with welding in the shop. With the advent of lighter-weight welding power

supplies, the use of field welding combined with shop welding is finding increasing favor.

While riveted joints do show residual clamping forces (even in cold-driven rivets), the clamping

forces in the rivet is difficult to control, is not as great as that developed by high-strength bolts, and

cannot be relied upon. Installation of hot-driven rivets involves many variables, such as the initial or

driving temperature, driving time, finishing temperature, and driving method. Studies have shown

that the holes are almost completely filled for short rivets. As the grip length is increased, the

clearances between rivet and plate material tend to increase.

35.2 BOLTED AND RIVETED JOINT TYPES

There are two types of riveted and bolted joints: lap joints and butt joints. See Figs. 35.1 and 35.2

for lap and butt joints, respectively. Note that there can be one or more rows of connectors, as shown

in Fig. 35.2a and b.

Fig. 35.2 Butt joints: (a) single-row; (b) double-row; (c) triple-row (pressure-type); (of) quadruple

row (pressure-type).

In a butt joint, plates are butted together and joined by two cover plates connected to each of the

main plates. (Rarely, only one cover plate is used to reduce the cost of the joint.) The number of

rows of connectors that fasten the cover plate to each main plate identifies the joint—single row,

double row, and so on. See Fig. 35.2.

Frequently the outer cover plate is narrower than the inner cover plate, as in Fig. 35.2c and d,

the outer plate being wide enough to include only the row in which the connectors are most closely

spaced. This is called a pressure joint because caulking along the edge of the outer cover plate to

prevent leakage is more effective for this type of joint.

The spacing between the connectors in a given row is called the pitch. When the spacing varies

in different rows, as in Fig. 35.2d, the smallest spacing is called the short pitch, the next smallest

the intermediate pitch, and the greatest the long pitch. The spacing between consecutive rows of

connectors is called the back pitch. When the connectors (rivets or bolts) in consecutive rows are

staggered, the distance between their centers is the diagonal pitch.

In determining the strength of a joint, computations are usually made for the length of a joint

corresponding to a repeating pattern of connectors. The length of the repeating pattern, called the

repeating section, is equal to the long pitch.

To clarify how many connectors belong in a repeating section, see Fig. 35.2c, which shows that

there are five connectors effective in each half of the triple row—that is, two half connectors in row

1, two whole connectors in row 2, and one whole and two half connectors in row 3. Similarly, there

are 11 connectors effective in each half of the repeating section in Fig. 35.2d

When rivets are used in joints, the holes are usually drilled or, punched, and reamed out to a

diameter of Vie in. (1.5 mm) larger than the nominal rivet size. The rivet is assumed to be driven so

tightly that it fills the hole completely. Therefore, in calculations the diameter of the hole is used

because the rivet fills the hole. This is not true for a bolt unless it is very highly torqued. In this

case, a different approach needs to be taken, as delineated later in this chapter.

35.3 EFFICIENCY

Efficiency compares the strength of a joint to that of the solid plate as follows:

„_ . strength of the joint

Efficiency = strength of solid plate

35.4 STRENGTH OF A SIMPLE LAP JOINT (BEARING-TYPE CONNECTION)

For bearing-type connections using rivets or ordinary bolts, we use the equation

PS = A<T

For shear, this is rewritten as

nd2T

Ps=Asr= —

where

Ps = the load

A = shear area of one connector

d = diameter of connector and/or hole

For the above example, friction is neglected. Figure 35.3 shows the shearing of a single connector.

Another possible type of failure is caused by tearing the main plate. Figure 35.4 demonstrates

this phenomenon.

Fig. 35.3 Shear failure.

Fig. 35.4 Tear of plate at section through connector hole. Pt = Atat = (p - d)tat.

The above failure occurs on a section through the connector hole because this region has the

minimum tearing resistance. If p is the width of the plate or the length of a repeating section, the

resisting area is the product of the net width of the plate (p — d) times the thickness t. The failure

load in tension therefore is

^tension = &Pt = (P ~ 4W^)

A third type of failure, called a bearing failure, is shown in Fig. 35.5. For this case, there is

relative motion between the main plates or enlargement of the connector hole caused by an excessive

tensile load. Actually, the stress that the connector bears against the edges of the hole varies from

zero at the edges of the hole to the maximum value at the center of the bolt or rivet. However,

common practice assumes the stress as uniformly distributed over the projected area of the hole. See

Fig. 35.5.

The failure load in the bearing area can be expressed by

Pb = Abab = (td)orb

Other types of failure are possible but will not occur in a properly designed joint. These are

tearing of the edge of the plate back of the connector hole (Fig. 35.6a) or a shear failure behind the

connector hole (Fig. 35.6b) or a combination of both. Failures of this type occur when the distance

from the edge of the plate is ~2 or less multiplied by the diameter of the connector or hole.

35.5 SAMPLE PROBLEM OF A COMPLEX BUTT JOINT (BEARING-TYPE CONNECTION)

The strength of a bearing-type connection is limited by the capacity of the rivets or ordinary bolts

to transmit load between the plates or by the tearing resistance of the plates themselves, depending

on which is smaller. The calculations are divided as follows:

1. Preliminary calculations to determine the load that can be transmitted by one rivet or bolt in

shear or bearing neglecting friction between the plates

2. Calculations to determine which mode of failure is most likely

A repeating section 180 mm long of a riveted triple row butt joint of the pressure type is illustrated

in Fig. 35.7. The rivet hole diameter d = 20.5 mm, the thickness of the main plate t = 14 mm, and

the thickness of each cover plate t = 10 mm. The ultimate stresses in shear, bearing, and tension are

respectively r = 300 MPa, crb = 650 MPa, and crt = 400 MPa. Using a factor of safety of 5, determine

Fig. 35.5 Exaggerated bearing deformation of upper plate. Pb = Abcrb = (td)ab.

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