The Engineer’s Handbook states the torque for a 5⁄16-18 NC Grade 8 cap screw is 29 foot pounds, and a 5⁄8-11 NC Grade 8 cap screw is 230 foot pounds. Why is torque important? The proper tension on threads (torque) ensures the strength of the connection and will make two independent items work as one. Proper tension also protects the connection against fatigue caused by stress cycles (vibrations) that will create joint failure.
Now, you ask, what all this has to do with my drill rig. I ask, “Do you know what the torque value for your drill pipe thread is? Is it 31⁄2 API thread, or a 27⁄8 IF thread? How do I achieve the proper torque for my drill pipe thread joints?” In order for the drill string to work as one, like any thread connection, the proper torque should be applied to the threads at each tool joint. If the tool joints are not in proper tension you are at risk of breaking a thread joint and/or losing a drill string.