• Threaded fasteners are tightened to clamp parts together and transmit loads. In gasketed joints, the purpose is to prevent leakage. In other joints, the clamping force is developed to prevent the parts from separating or getting loose and transmit load (e.g. gear couplings). If the fasteners are over tightened, they may break either during the tightening itself or when the working load is added to the pre-load in applications such as gasketed joints. If too loose, the fastener will shake loose in vibration. If they are not adequately tightened, they will be subjected to cyclic loading and fail due to fatigue. Thus proper amount of tightening (pre-loading) is very important. Information about various methods of tightening, sequence of tightening, tightening of stainless steel fasteners and recommended tightening torque values for preloading fasteners is given in this article.

    Tightening Methods

    In gasketed joints fasteners shall be tightened to create a seal. Thus for this application, one can calculate required pre-load based on internal pressure and number of fasteners. For other applications, required pre-load depends on application and typical values vary from 50% to 80% of the yield strength of the fastener material. For critical applications designer specifies preloading values.

    All fastener materials are slightly elastic and must be stretched a small amount to develop clamping force. Within elastic limit, stress is proportional to strain and modulus of elasticity, Young’s modulus (stress / strain = constant) is a property of material. Young’s modulus (tensile or compressive loads) for steel is 30,000,000 psi. Thus for steel fasteners, a stretch of 0.001 inch per inch length of a fastener will develops 30,000 psi clamping force. From this information, one can easily calculate fastener to be stretched for desired pre-load. This method of preloading is very accurate but it requires that the ends of the bolts be properly prepared and also that all measurements be very carefully made. In addition, direct measurements are only possible where both ends of the fastener are available for measurement after installation. Since in most cases it is not possible to measure fastener elongation easily, other indirect methods are used for pre-loading.

    Six methods are used to control tightness of a threaded fastener. In order of increasing accuracy they are:

     

    Method Accuracy
    Feel +/- 35%
    Torque wrench +/- 25%
    Turn-of-the-nut +/- 15%
    PLI washers +/- 10%
    Bolt elongation +/- 3 to 5%
    Strain gages +/- 1%

     

    The decision as to which tightening method to use depends primarily on the criticality of the joint. As cost increases with higher accuracy method, generally the method selected will almost always lie between the two extremes. Some applications will allow the high inaccuracy of the "feel" method, while the high cost, highly accurate strain gages are used almost entirely in the laboratory. Information on various methods is as under.

    Feel

    In this method, fasteners are tightened as per feel of a person based on his work experience. As there is no measurement, quality of work depends on person’s expertise.

    Torque wrench

    Torque wrench is a manual wrench which incorporates a gauge or other method to indicate the amount of torque transferred to the fastener. In this method, fasteners are tightened by calibrated torque wrench set to a desired torque value.

    When torque is applied to a fastener it is used to overcome friction to turn the fastener and stretch the fastener to develop the clamping force. The latter is considered the useful part of the torque. Generally 85% of the torque is used to overcome friction and only 15% is available to produce bolt load. Thus change in the coefficient of friction for different conditions can have a very significant effect on fastener loading. Thus though this is the least expensive and the most popular method for preloading fasteners, it is the least accurate.

    Fastener manufacturers usually recommend seating torques for each size and fastener material based on test carried out by them. In absence of information about torque values, the torque tightening equation may be used to decide torque value. Within the elastic range, before permanent stretch is induced, the relationship between torque and tension is essentially linear. Studies have found that there are many variables that have an effect on this relationship. To overcome this, a torque tightening equation (formula) has been developed based on empirical test results as under.

    T = KDP Where,
    T = torque, Nm (in-pounds).
    D = fastener nominal diameter, m (inch)
    P = preload, N (pounds)
    K = “nut factor,” “tightening factor,” or “k-value”

    The nut factor is often assumed to be 0.2 for steel nuts and bolts tightened without lubrication.
    More information on friction and nut factor are given in an article reproduced from Loctite, “Torque/Tension Relationship – The Forgotten Factor” at the end of this article.

    Turn-of-the-nut

    The process of pulling parts of a joint together is called snugging. During snugging most of the input turn is absorbed in the joint with little tension being given to the bolt. The torque required to pull plates together so that direct contact occurs is called snug torque. The snug torque is usually determined experimentally on the actual joint. The snug torque ensures that metal to metal contact occurs at all the interfaces within the joint. At this point the nut or bolt is turned to a predetermined number of degrees in turn-of-the-nut method of tightening. This method also utilizes change in bolt length. In theory, one bolt revolution (360° rotation) should increase the bolt length by the thread pitch. This method is used for slip critical (friction grip) connection of structural members using structural bolts. It eliminates the friction factor. However its accuracy is affected by the care of the workman in measuring the angle the nut or bolt is turned. For more information on this method, please refer article on Structural Bolts.

    For using this method with gasket, value of nut or bolt to be rotated after snugging must be developed by tests for each joint because of the "rubberiness" of the joint. The snugging also produces a large variation in preload due to rubberiness of gasket material.

    PLI washers

    Preload indicating (PLI) washers utilize compression of plastically deformable material of washer under load. The use of load indicating washers is widespread in structural engineering. One type of such washers have small raised pips on their surface which plastically deform as the bolt is tensioned. The correct preload is achieved when a predetermined gap is present between the washer and the under head of the bolt. This is measured using feeler gauges. The smaller the gap, greater is the tension in the bolt. Generally they are not used in mechanical engineering. They are also known as direct tension indicators. For more information on direct tension indicators, please refer – other useful information section in article about Gaskets.

    Bolt elongation

    Since stress/strain is a constant relationship for any given material, the relationship is used to preload bolt by staining (stretching) it for desired stress (loading). Various methods are used to stretch bolt or measure stretch as the bolt is being loaded. Two such methods used for large size bolts are heat tightening and use of hydraulic tensioner.

    Heat Tightening

    Heat tightening utilizes the thermal expansion characteristics of the bolt. The bolt is heated to expand. After it has expanded, the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil or any other suitable method. This is not a widely used method and is generally used only on very large size bolts.

    Hydraulic Tensioner

    In this method a hydraulic tool is used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched, the nut is run down the thread to snug up with the joint. The hydraulically applied load is then removed resulting in tension being induced into the fastener.

    Some times special bolts are used which have provision to measure stretch as they are tightened. One such fastener is the Rotabolt which measures bolt stretch (extension) by the use of a central gauge pin which passes down a centrally drilled hole in the bolt.

    Bolt elongation can be measured by ultrasonic measurement also.

    Strain gages

    By using strain gages, the strain produced can be directly detected. Strain gages may be applied directly to the outside surface of the bolt or by having a hole drilled in the center of the bolt and the strain gage installed internally. The output from these gages need instrumentation to convert the gage electrical measurement method. It is an expensive method and not always practical.

    Sequence of tightening

    The sequence in which bolts or studs are tightened has a substantial effect on the distribution of preload in a joint. Since in most cases, all bolts of a joint are not tightened simultaneously, tightening of a bolt effects preload in other previously tightened bolts in the group. Such effects are called elastic interaction or bolt cross talk. To minimize this, bolts are tightened in a cross bolt tightening pattern. If the joint is critical it is recommended considering a multiple pass tightening sequence. In such a sequence, each bolt is tightened more than once so as to reduce the preload reduction caused by the tightening of the other bolts in the joint. Tightening of bolts as per tightening sequence uniformly preloads all the bolts of a joint. Always run the nuts or bolts down by hand. This gives an indication that the threads are satisfactory (if the nuts will not run down by hand, then there is probably some thread defect – check again and, if necessary, replace defective parts). Cross bolt tightening sequence for various patterns of joints is shown below.

    Sequence of Tightening

    Tightening of stainless steel fasteners

    Stainless steel fasteners can unpredictably sustain galling (cold welding) during preloading. Stainless steel self-generates an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the thread surfaces, protective oxides are broken, possibly wiped off, and interface metal’s high points and shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme case this leads to seizing – the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out. The problem can be eliminated by lubricating the threads. In very unfavorable assembling conditions MoS2 or similar lubricants are recommended. It is also recommended to use a very low revolution screw-driving machine or they should be tightened by hand to minimize heat generation during tightening. High temperature increases tendency for galling. Be careful however, if you are using the stainless steel fasteners in food related applications as some lubricants may be unacceptable.

    Recommended tightening torque values for preloading fasteners

    In case, torque tightening values are not specified, steel fasteners may be preloaded by tightening them to following torque values.

    Tightening Torques in Kilograms Meters / Pound Feet

     

    Grade Nominal Diameter – Regular Pitch
    M4 M5 M6 M7 M8 M10 M12 M14 M16 M18 M20 M22 M24 M27 M30
    8.8 kgm 0.29 0.57 1 1.6 2.5 5 8 13 20 26 36 51 65 98 134
    Pound Feet 2 4 7 11 18 32 58 94 144 190 260 368 470 707 967
    10.9 kgm 0.4 0.8 1.4 2.3 3.5 6 12 18 27 37 51 72 92 138 188
    Pound Feet 2.9 6 10 16 25 47 83 133 196 269 366 520 664 996 1357
    12.9 kgm 0.5 1 1.6 2.7 4 8 14 22 33 45 61 87 110 167 226
    Pound Feet 3.6 7 11 20 29 58 100 159 235 323 440 628 794 1205 1630

     

    For ready reference tables showing tightening torque and forces for stainless steel fasteners are reproduced below from website of Bufab Stainless AB, Sweden – http://www.bufab-stainless.se

    Tigthening torque and forces for A2, A4 and Bumax fasteners

     

    Thread size and class Class M3 M4 M5 M6 M8 M10 M12 M14 M16 M18 M20
    Tigthening Torque
    Mv in Nm 1), 3)
    Bumax 109
    Bumax 88
    Bumax Lock
    80
    70
    50
    1.7
    1.3

    1.2
    0.9
    0.4
    4.1
    2.9

    2.7
    2
    1
    8.1
    5.7
    6.6
    5.4
    4
    2
    14
    10
    12
    9
    7
    3
    34
    25
    29
    22
    17
    8
    66
    47
    54
    44
    33
    15
    115
    82
    94
    76
    57
    27
    162
    129

    121
    91
    43
    248
    198
    228
    187
    140
    65
    344
    275

    261
    195
    91
    481
    385
    442
    364
    273
    127
    Preload applied
    KN ± 23% 2)
    Bumax 109
    Bumax 88
    Bumax Lock
    80
    70
    50
    2.9
    2.1

    2.0
    1.5
    0.8
    5.2
    3.6

    3.4
    2.6
    1.4
    8.6
    5.9

    5.5
    4.2
    1.9
    12
    8.4
    8.4
    7.8
    5.9
    2.7
    21
    15
    15
    14
    11
    5
    34
    24
    24
    23
    17
    8
    49
    35
    35
    33
    25
    12
    60
    48

    45
    34
    16
    81
    65
    65
    61
    47
    21
    100
    80

    76
    57
    27
    128
    102
    102
    96
    72
    33
    Failure load KN Bumax 109
    Bumax 88
    Bumax Lock
    80
    70
    50
    5
    4

    4
    3.5
    2.5
    8.8
    7

    7
    6.1
    4.4
    14
    11
    11
    11
    9.9
    7.1
    20
    16
    16
    16
    14
    10
    37
    29
    29
    29
    26
    18
    58
    46
    46
    46
    41
    29
    84
    67
    67
    67
    59
    42
    115
    92

    92
    81
    58
    157
    126
    126
    126
    110
    79
    192
    154

    154
    134
    96
    245
    196
    196
    196
    172
    123
    Yield Load KN Bumax 109
    Bumax 88
    Bumax Lock
    80
    70
    50
    4.5
    3.2

    3
    2.2
    1.3
    8
    6

    5
    4
    2
    13
    9
    9
    8
    6
    3
    18
    13
    13
    12
    9
    4
    33
    23
    23
    22
    16
    8
    52
    37
    37
    35
    26
    12
    76
    54
    54
    51
    38
    18
    92
    74

    69
    52
    24
    125
    100
    100
    94
    71
    33
    154
    123

    115
    86
    40
    196
    157
    157
    147
    110
    51
    Nominal stress area mm²   5.03 8.78 14.2 20.1 36.6 58.0 84.3 115 157 192 245
    Pitch of thread   0.5 0.7 0.8 1.0 1.25 1.5 1.75 2.0 2.0 2.5 2.5

     

    Thread size and class Class M24 M27 M30 M36
    Tigthening Torque
    Mv in Nm 1), 3)
    Bumax 88
    Bumax Lock
    80
    70
    50
    665
    765
    629
    472
    220
    961

    909
    682
    318
    1310

    1240
    930
    434
    2280

    2160
    1620
    755
    Preload applied
    KN ± 23% 2)
    Bumax 88
    Bumax Lock
    80
    70
    50
    181
    181
    138
    103
    48
    235

    179
    134
    63
    287

    219
    164
    77
    418

    319
    239
    112
    Failure load KN Bumax 88
    Bumax Lock
    80
    70
    50
    282
    282
    282
    247
    177
    367

    367
    321
    230
    449

    449
    393
    281
    654

    654
    572
    409
    Yield Load KN Bumax 88
    Bumax Lock
    80
    70
    50
    226
    226
    212
    159
    74
    294

    275
    207
    96
    359

    337
    253
    118
    523

    490
    368
    172
    Nominal stress area mm²   353 459 561 817
    Pitch of thread   3.0 3.0 3.5 4.0

     

    Thread size and class Class 1/4-20
    UNC
    5/16-18
    UNC
    3/8-16
    UNC
    1/2-13
    UNC
    5/8-11
    UNC
    3/4-10
    UNC
    7/8-9
    UNC
    1”-8
    UNC
    Tigthening Torque
    Mv in Nm 1), 3)
    Bumax 88
    80
    70
    50
    11.0
    10.0
    7.7
    3.6
    22.0
    21.0
    16.0
    7.3
    39.0
    37.0
    28.0
    13.0
    95.0
    89.0
    66.0
    31.0
    188.0
    175.0
    131.0
    61.0
    329.0
    308.0
    231.0
    108.0
    527.0
    493.0
    369.0
    172.0
    789.0
    737.0
    553.0
    258.0
    Preload applied
    KN ± 23% 2)
    Bumax 88
    80
    70
    50
    8.7
    8.0
    6.0
    2.8
    14.3
    13.2
    9.9
    4.6
    21.1
    19.5
    14.6
    6.8
    38.7
    35.7
    26.8
    12.5
    61.7
    56.9
    42.7
    19.9
    91.2
    84.2
    63.2
    29.5
    125.9
    116.2
    87.2
    40.7
    165.2
    152.5
    114.4
    53.4
    Failure load KN Bumax 88
    80
    70
    50
    17.0
    17.0
    14.3
    10.2
    28.0
    28.0
    23.6
    16.9
    41.5
    41.5
    35.0
    25.0
    75.9
    75.9
    64.1
    45.7
    121.0
    121.0
    102.2
    73.0
    179.0
    179.0
    151.2
    108.0
    247.0
    247.0
    208.6
    149.0
    325.0
    325.0
    273.7
    195.5
    Yield Load KN Bumax 88
    80
    70
    50
    13.1
    12.3
    9.2
    4.3
    22.4
    20.3
    15.2
    7.1
    33.2
    30.0
    22.5
    10.5
    60.8
    54.9
    41.2
    19.2
    96.9
    87.6
    65.7
    30.7
    143.4
    129.6
    97.2
    45.4
    197.9
    178.8
    134.1
    62.6
    259.6
    234.6
    175.9
    82.1
    Nominal stress area mm²   20.5 33.8 50.0 91.5 146.0 216.0 298.0 391.0

     

    1) The Mv recommendations refer to burr-free surfaces lubricated with a good quality lubricant.
    2) The preload applied is calculated as 65% of Rp 0.2 but in practice the value can be expected to vary between 50 – 80 %
    3) The Mv-recommendations are calculated assuming a coefficient of friction of 0.16 which requires a good-quality lubricant.

    Hankel Loctite had invented anaerobic technology for thread treatment and gasketing in 1950. They are world leader for anaerobic adhesives sealants and cyanoacrylate adhesives. An article on Torque / Tension relationship from their web site is reproduced below.

    Torque/Tension Relationship – The Forgotten Factor

    If you have used a torque wrench to ensure correct bolt tension, then you have just become a victim of friction – the forgotten factor. Only by controlling this factor can reliability be restored to the threaded fastener. The correct function of a nut and bolt is to clamp individual parts together with sufficient force, so as to prevent relative movement between the parts. When clamped parts move due to the influence of shock, stress, vibration or thermal forces, structural failures can occur rapidly. The key to reliability then is to prevent relative movement by ensuring that sufficient clamp load is generated in the fasteners. This is best achieved by understanding and controlling the friction forces which absorb so much of the applied torque. Modern design trends using smaller, fewer, high strength/low yield bolts torqued into the yield region, place more and more emphasis on good design practices and understanding of the forces involved. Improper lubrication practices can also lead to many types of failure in bolted assemblies. We tighten a screw or bolt by applying a torque to the head or nut until a balance is obtained between the torque applied and the sum of the bolt tension and friction forces. The distribution of these forces is shown in Table 1.

     

    TABLE 1 – TORQUE ABSORPTION IN A TIGHTENING BOLT
      PERCENTAGE OF TIGHTENING TORQUE

    COURSE THREAD

    FINE THREAD

    BOLT TENSION

    15%

    10%

    THREAD FRICTION

    39%

    42%

    UNDER HEAD FRICTION

    46%

    48%

    TOTAL TIGTHENING TORQUE

    100%

    100%

     

    We see, therefore, that in normal course thread fasteners only 15% of the applied torque actually produces clamp load, the rest is absorbed by friction on the tread flanks and under-head bearing surface of the nut and bolt head.

    Modern anaerobic thread-lockers such as Loctite 222, 243, 262 play a major role in improving the reliability of bolted assemblies, not simply by preventing premature loss of clamp load but more importantly, by controlling the friction characteristics of the metal surfaces of the fasteners. Even apparently identical fasteners from the same batch of steel, and having undergone the same heat treatment, can exhibit considerable difference in clamp loads, even when torqued to exactly the same levels. The explanation lies in variations in the “K” factor for the fastener. A simplified model for the relationship between the torque applied, the fastener diameter, the force achieved or required, and the “K” factor is:

    T = KDF, where
    T = Torque – Nm (inch-pounds)
    D = Nominal diameter of fastener – m (inch)
    F = Clamp Load – N (pounds)
    K = “K” factor. An empirical constant which takes into account friction and the variable diameter under the head and threads where friction is acting (it is not the coefficient of friction, although it is related to it).

    Values of “K” can be determined experimentally, see Table 2. The range of values for any lot of fasteners tested was plus or minus 14%, however different fasteners lots increased the variation to plus or minus 20%. The variation in friction (and therefore “K”) is wide since it is the result of extremely high pressures acting on surfaces which vary in roughness, oxide levels, plating finish and thickness, and lubrication types and levels.

    Note:
    These values were obtained using 16 TPI, 3/8 UNC nuts and bolts, where the bolt was captive and the nut was turned. Both the threads and the nut face were lubricated. An unlubricated thrust face, either nut or bolt head, can almost double the “K” value. The dry solvent cleaned bolt would never achieve the clamp load for which it was designed, irrespective of the amount of torque applied, while the bolt lubricated with anti-seize compound (which is not an uncommon practice with the mining industry and heavy engineering) is stretched well into its elastic limit and is a disaster waiting to happen. The application of the modern anaerobic thread-locking compound Loctite 243 substantially reduces the torque tension scatter envelope over identical “”as received” lightly oiled fasteners.

    The “K” factor variation ranging from 0.11 to 0.17 found in seemingly identical fasteners here results in a substantial clamp load variation. At exactly the same torque level of 24 foot pounds, variations between 4500 pounds and 6700 pounds are experienced. This is not exactly a recipe for reliable engineering assembly. The same fasteners treated with Loctite 243 would exhibit a variation between 4700 pounds and 5400 pounds, which is close to the design clamp load for such a fastener when tightened to 75% of its proof load.

     

    TABLE 2 – TYPICAL "K" VALUES
     

    LIGHTLY OILED

    LIGHTLY OILED
    + LOCTITE 243

    DEGREASED

    DEGREASED
    + LOCTITE 243

    STEEL FASTENER

    0.15

    0.14

    0.2

    0.2

    PHOSPHATED STEEL

    0.13

    0.11

    0.24

    0.14

    CADMIUM PLATED STEEL

    0.14

    0.13

     

     

    STAINLESS STEEL 404

    0.22

    0.17

     

     

    ZINC PLATED STEEL

    0.18

    0.16

     

    0.15

     

    Clearly, Loctite anaerobic thread-lockers perform a task which is more important than maintaining bolt tension, they provide a reliable means of controlling friction forces so that, once again your torque wrench allows you to achieve the correct tension.

    Torque Tension - Effect of Lubrication

    For more information on Loctite and their products, please visit their website – http://www.loctite.com.au

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