Analysis of the effect of temperature on the performance of waste cooking oils used as lubricants on threaded joints

To date, considerable research has been devoted to replacing mineral oils with renewable alternatives to reduce the environmental impact of industry. However, the main limitation of vegetable-based lubricants lies in their degradability and poor resistance to high temperatures. A particularly intriguing opportunity is the reuse of Waste Cooking Oils (WCO), whose disposal poses significant environmental challenges. This study investigates, for the first time, the use of WCO as a lubricant during the fastening of threaded joints. Specifically, the nut factor obtained by lubricating a zinc-plated steel screw M10 class 8.8 coupled with a zinc coated nut and tightened on an aluminum substrate is experimentally analyzed. Additionally, to examine the effect of temperature, the joints are subjected to thermal cycles at high temperatures. The difference between the maximum torque measured during tightening and that measured during untightening serves as a descriptor of the lubricant's efficiency loss due to thermal degradation. In order to have a comparison with a mineral oil the tests have been repeated also with the ISO VG 46. The experimental data have been analyzed by means of the ANOVA tools to assess the effectiveness of the input parameters (oil type and aging conditions) on the response in terms of both nut factor at tightening and breakaway torque of the fastener after tightening completion and thermal cycles at high temperatures. Guidelines useful to the design engineer are reported in the conclusions.