calculation of internal thread tensile strength Hi All! I've got a coupling device that has two internal threads of different pitch, length, diameter. The smaller thread takes a lifting bolt, the larger attaches to a piece of machinery. I've been asked to work out the safe working load (SWL) limit of the coupling. I have the lifting bolt and thankfully it is stamped by the manufacturer and so I've been able to easily calculate it's maximum lifting capacity. What I need help with is working out the maximum load that can be applied on the internal threads of the coupling device in pure tension. To make it harder I don't yet know the material of the device or the thread and pitch of the largest internal thread, but I'm working on it! Can anyone suggest a good method for the calculation? It does not need to account for shear forces, pre-load or torque. The coupling is hand tightened and lifted purely along it's axial line.