This article from
Fastenal has some enlightening details explaining the torque process. The torque applied to a designed dry thread fitting places a "designed/engineered" tension on the bolt/stud. By using any lube, including the manufacturing oils remaining on some hardware, will cause an over tension of the fastener. If the fastener exceeds the "elastic" stage, the ability to stretch and return to original state, due to over torquing, it will enter the plastic stage, permanently stretching and is then unable to return to the original state, causing permanent damage to the bolt and weakening the union. Catastrophic failure can then occur through dynamic loading of the union or significantly reducing the lifespan of the bolt.
I have seen the results of this. Many years ago my wife and I were diving on a country road around a sweeping left curve. A beautiful 55 Chevy was coming from the opposite direction. Just before he passed, his left front tire separated from his car and flew in front of and over the hood our 71 Olds 442 (didn't touch us) but at the same time I heard a CRACK and my wife's terrified scream. Her side of the windshield was shattered, not a small rock like, but the entire right 1/3rd, by one little lug nut, with stud piece in it that was launched with force as it snapped off the Chevy's hub. The other driver made a miraculous recovery behind our car and onto the shoulder behind us. He was returning from having new tires put on and the tire tech used an impact wrench. Evidence was he over torqued the wheel nuts as all 5 studs nubs remaining were stretched and twisted and the last one to give up and let go with high velocity into our windshield. This was a case of an extreme over torque of a dry fitting. By lubing, the same results of an over stretch increases significantly. 125 lb/ft overcoming a dry fitting does not give the same clamping force as a lubricated 125 lb/ft. The lubricated clamping force is significantly larger and stretches the fastener accordingly.
When the dry torque method is stated, that means clean any residual oils, dirt, rust, light galling by chasing the threads, or replacing the damaged fittings for proper torque to be applied. Again, the engineers have accomplished numerous trial and error tests, with an understanding in the various metallurgy characteristic to come up with the safest and best performance procedures. There are important reasons for each fastener type, materials used, and their applications.
The safest practice is following the manufacture's recommendations; and there are a LOT of them. Thank goodness "most" fasteners are/were overengineered. There are a lot of changes being made to manufacturing processes to lighten weight, cut costs, automate assembly, etc., which has affected the robustness of the materials and demanding more precision in tightening of the fasteners. In other words, that 25% to 50% over engineered part from the past now has a mere 10% to 15% margin.
There is a lot involved in the art proper torquing.
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