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What is the Proper Torque to
Use on a Given Bolt
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by Joe Greenslade
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"What torque should
I use to tighten my bolts?" is a question suppliers of bolts
are frequently asked by end user customers. Many times I have been
asked if a chart is published on the recommended tightening torque
for various bolt grades and sizes. I do not know of any. This article
provides such a chart for "Initial Target Tightening Torque.
It See Figure 1.
The formula for generating these
values is explained below.
The widely recognized engineering formula, T= K x D x P (to be explained
later in this article), was used to provide the chart's values, but
it must be understood that every bolted joint is unique and the optimum
tightening torque should be determined for each application by careful
experimentation. A properly tightened bolt is one
that is stretched such that it acts like a very ridged spring pulling
mating surfaces together. The rotation of a bolt (torque) at some
point causes it to stretch (tension). Several factors affect how much
tension occurs when a given amount of tightening torque is applied.
The first factor is the bolt's diameter. It takes more force to tighten
a 3/4-10 bolt than to tighten a 318-16 bolt because it is larger in
diameter. The second factor is the bolt's grade. It takes more force
to stretch an SAE Grade 8 bolt than it does to stretch an SAE Grade
5 bolt because of the greater material strength. The third factor
is the coefficient of friction, frequently referred to as the "nut
factor." The value of this factor indicates that harder, smoother,
and/or slicker bolting surfaces, such as threads and bearing surfaces,
require less rotational force (torque) to stretch (tension) a bolt
than do softer, rougher, and stickier surfaces.
The basic formula T = K x D
x P stated earlier takes these
factors into account and provides users with a starting point for
establishing an initial target tightening torque.
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• T
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Target tighten torque (the result
of this formula is in inch pounds, dividing by 12 yields foot
pounds |
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• K
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Coefficient of friction (nut factor),
always an estimation in this formula |
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• D
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Bolts nominal diameter in inches |
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• P
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Bolt's desired tensile load in pounds (generally
75% of yield strength) |
The reason all applications should be evaluated to determine the optimum
tightening torque is that the K factor in this formula is always an
estimate. The most commonly used bolting K factors arc 0.20 for plain
finished bolts, 0.22 for zinc plated bolts, and 0.10 for waxed or
highly lubricated bolts.
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Thread
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Tensile Stress Area
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SAE Grade 2
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SAE Grade 5
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SAE Grade 8
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Size
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TSA
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75%
Yield Strength (PSI) - 43000
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75% Yield Strength (PSI) - 69000
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75%
Yield Strength (PSI) =
98000
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Plain
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Zinc Plated
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Waxed
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Plain
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Zinc Plated
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Waxed
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Plain
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Zinc Plated
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Waxed
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Square Inches
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A.
lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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Ft.Lb.
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114-20.
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0.0318
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6
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6
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3
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9
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10
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5
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13
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14
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6
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1/4-28.
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0.0364
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7
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7
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3
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10
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12
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5
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15
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16
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7
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5116-18.
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0.0524
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12
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13
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6
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19
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21
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9
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27
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29
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13
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5116-24.
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0.0580
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13
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14
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6
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21
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23
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10
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30
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33
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15
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318-16.
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0.0775
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21
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23
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10
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33
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37
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17
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47
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52
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24
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318-24.
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0.0878
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24
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26
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12
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38
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42
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19
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54
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59
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27
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7/16-14.
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0.1063
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33
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37
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17
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53
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59
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27
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76
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83
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38
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7/16-24.
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0.1187
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37
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41
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19
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60
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66
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30
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85
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93
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42
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112-13.
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0.1419
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51
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56
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25
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82
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90
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41
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116
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127
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58
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112-20.
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0,1599
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57
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63
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29
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92
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101
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46
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131
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144
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65
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9116-12.
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0.1820
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73
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81
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37
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118
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129
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59
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167
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184
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84
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9116-18.
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0.2030
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82
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90
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41
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131
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144
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66
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186
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205
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93
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5J8-11.
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0.2260
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101
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111
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51
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162
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179
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81
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231
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254
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115
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5J8-14.
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0.2560
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115
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126
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57
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184
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202
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92
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261
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287
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131
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3/4-10.
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0.3340
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180
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197
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90
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288
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317
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144
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409
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450
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205
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3/4-16.
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0.3730
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200
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221
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100
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322
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354
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161
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457
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503
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228
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The only way to properly
determine the optimum tightening torque for a given application is
to simulate the exact application. This should be done with a tension
indicating device of some type on the bolt in the application. The
bolt is tightened until the desired P (load) is indicated by the tension
indicating device. The tightening torque required to achieve the desired
tension is the actual tightening torque that should be used for that
given application. It is extremely important to realize that this tightening
value is valid only so long as all of the aspects of the application
remain constant Bolt suppliers sometimes have customers say that their
bolts are no good because they have started breaking while being installed.
Thorough investigation commonly reveals that the customer has started
lubricating the bolts to make assembly easier, but maintained to same
torque as was used when the were plain finished
The table in this article shows that by using this formula
a 1/2-13 Grade 5 plain bolt should be tightened to 82 foot pounds,
but the same bolt that is waxed only requires 41 foot pounds to tighten
the same tension. A perfect 1/2-13 Grade 5 waxed bolt will
break if it is tightened to 81 foot pounds because the K factor is
drastically lower. The bolts are fine, but the application changed.
Suppliers need to understand this and be able to educate their customers
to resolve this common customer complaint about breaking bolts.
The chart is provided for quick reference by fastener suppliers and
users for selecting an initial target tightening torque. This chart
was derived by using the formula shown earlier. An example of the
calculation is as follows: |
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Product: 3/4-10 Grade
5 zinc plated bolt
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Formula: T= K x D x
P
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• K=
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0.22 (zinc plated) |
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• D=
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.750 (3/4-10 nominal diameter |
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• P=
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23.046 pounds |
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| Hopefully the chart will help suppliers with an initial
answer to the customer's question, "What torque should I use
to tighten my bolts?" Keep in mind this is only an estimated
value. It may provide satisfactory performance, but it also may not.
Every application should be evaluated on its own to determine the
optimum torque value for each application. Major bolt suppliers should
have tension indicating equipment necessary to help their customers
determine the appropriate tightening values for their specific applications.
Keep in mind that if the lubricant on a bolt and nut combination is
changed, the tightening torque value must be altered to achieve the
desired amount of bolt tension. |
Joe Greenslade is President of Greenslade and Company, Inc. located
in Rockford, Illinois. His firm specializes in providing manufacturing
tooling and inspection equipment
to suppliers of screws, bolts, rivets, and nuts
throughout the world.
Joe is an inventor, author, and lecturer. He holds eleven US Patents.
Has written over 80 technical articles for industrial trade journals,
and has spoken frequently at trade association meetings and technical
conferences on issues related to industrial quality for the past ten
years.
He is an Associate Member of the Industrial Fastener Institute and
a member of the American Society of Mechanical Engineers B1 Thread
Specification Committee. In 1992, Joe was recognized for his technical
and innovative contributions to the fastener industry when, at age
44, he became the youngest person to be inducted into the National
Industrial Fastener Show "Hall of
Fame. " |
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