A new topic on special fasteners and tightening procedures has been added to our free General Service Manual.
Tightening fasteners correctly is key in mechanical work. Most folks are familiar with torque specifications as the most common form of estimating fastener tension by how much force it takes to rotate the component. The fastener tension can vary at an identical torque spec under different conditions, and it is the tension that matters.
The friction against a fastener’s threads and/or flanges is a big factor in how much axial tension applied with a torque value. A bolt that has its threads and flange lubricated with oil has less friction as it is tightened compared to a dry bolt. This means that an oil lubricated bolt can apply more axial tension at the same torque value as a dry bolt. In the opposite direction, a dirty bolt with contaminated (or damaged) threads will have more friction than a clean dry bolt. The dirty bolt will have less axial tension than the clean dry bolt at the same torque value. This is why general torque specification are given for clean threads without lubrication.