The following information should be used for sizing calculations ONLY!
Any formal calculations MUST use values from relevant company or engineering standards.
A male fastener, such as a bolt or stud, subjected to an axial force can fail through either tensile failure of the fasteners core or through shear force of the fasteners thread (thread stripping).
A female fastener such as a nut or tapped hole, can only fail through shear failure in the thread (thread stripping), provided that enough edge distance is given to present break out.
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In bolted joints, we do not design the bolt itself instead we select a fastener that meets the criteria for the bolted assembly. With this in mind we typically recommend a safety factor of 2!
A simplified method for sizing fasteners is shown below:
Tensile Stress in Bolt
F = Force applied to each bolt, A_T = Thread tensile area
Thread tensile areas are calculated below:
Typically UNF threads are preferred to UNC threads due to the higher tensile stress areas for equivalent thread sizes
Shear Stress in male/female thread
F = Applied Force, A_s = Shear area of thread
Shear area of male/female thread
D = nominal diameter of bolt, L_e = Length of thread engagement, p = thread pitch
How much pre-load should be applied to a bolted joint?
This is a critical question in all bolted assemblies. Obviously the answer will depend upon the application but in most cases the simple answer is as much as possible without compromising the components being bolted. For most applications we would aim for a pre-load equivalent to 70-95% of the maximum proof strength of the bolt.
A_T = Thread tensile area
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To increase the pre-load we can either increase the size of the bolt or we can increase the material properties of the bolt
Installation Torques
Once we have sized the fastening thread we need for our application, and we know the desired amount of pre-load, how do we calculate the required tightening torque on the fastener during the assembly to achieve the desired amount of pre-load?
Torque
F = Force (pre-load), D = Nominal diameter of bolt, K = torque coefficient (typically = 0.2)
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When a bolt is lubricated before installation, the amount of torque required to achieve the correct tension is reduced! If a torque specified for a dry installation is applied to a lubricated installation, the bolt may overload and break. A typical lubrication factor is 0.4
Bolt Elongation
Another useful check for bolt sizing can be calculating bolt elongation or stretching under the applied pre-load.
Change in bolt length
F = Applied Force, L = effective length of bolt, E = Young's modulus of bolt material, A_T = Thread tensile area
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Bolt elongation is the most accurate way of measuring the amount of applied pre-load to the fastener once assembled ~+/-5%. For reference, measuring pre-load with a torque wrench ~+/-25% this is mostly due to friction throughout the assembly: in the threads, under the bolt head or the use of lubrication.
Useful link for finding alternative material properties!
All information on this website is provided in good faith, ‘The Engineering Notebook’ nor its parent company ‘RYM Engineering Ltd’ make no representation or warranty of any kind for information held on this website. Under no circumstances shall we have any liability for loss or damage of any kind as a result of the use of information from this website. The use of this website and any information on this website is solely at your own risk.
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