One way to do this is by improving the gear-grinding process. Grinding is important because it eliminates surface flaws, maximizes load capacity, and minimizes running noise. However, standard grinding of high-performance gears is a “wet” process” that is time-consuming and requires multiple steps. Wet grinding typically uses a large amount of lubricant. Energy consumption for lubrification and cooling represents almost two-thirds of the total energy consumption of a typical grinding machining center—a cost that gear manufacturers would love to eliminate.
To make dry grinding (no lubricants) more viable and cost-effective, a research team led by Erica Liverani, a senior assistant professor in the Department of Industrial Engineering at the University of Bologna in Italy, compared the high-performance gears made with conventional wet grinding with two other processes: superfinishing and dry grinding. By adjusting a variety of parameters, Liverani’s team developed a dry grinding system that outperformed the other two methods.
“This shows that the complete elimination of lubricant in gear production is possible, leading to a more sustainable process without compromising gear performance,” she said.
Liverani and six coauthors published a paper on the topic, “Dry Grinding: A More Sustainable Manufacturing Process for the Production of Automotive Gears,” in the October 2024 issue of the Journal of Manufacturing Science and Engineering.
Ready, set—grind
The material and manufacturing processes that were used prior to grinding were the same in all cases, followed by grinding performed by the same manufacturer. “The dimensional accuracy, roughness, microstructure, residual stress state, and wear resistance of gear flanks were then analyzed to compare the overall performance of each grinding process,” Liverani said.
Three grinding processes were selected based on current manufacturing standards and new requirements for carbon neutrality:
Wet grinding (WG) served as a benchmark
Dry hard finishing (DHG) performed without the use of lubricants
Superfinishing (SF), a wet process that increases micro-pitting
After grinding, all gears were analyzed with an optical microscope and scanning electron microscope for microstructure evaluation, an optical profiler for the acquisition of the surface topography and roughness, a Klingelnberg measuring system for evaluation of geometric errors, and an X-ray diffraction analysis for residual stress measurement. Finally, wear tests were performed to evaluate the influence of grinding on gear durability.
Microscope images of gear teeth ground through different methods: a) wet grinding, b) and c) dry grinding, and d) and e) super finishing. Scuffing and micro-pitting are highlighted with white and black arrows, respectively. Image: Alessandro Fortunato, et al. in the Journal of Manufacturing Science and Engineering.
Cycle parameters
In terms of dimensional accuracy, roughness, and microstructure, “the goal was to identify the best combination of parameters for achieving the best level of surface integrity,” Liverani said. “The grinding cycle parameters and grinding wheel specifications were changed depending on the grinding process being used.”
The results show that dry grinding can produce gears with acceptable geometric accuracy, no microstructure defects, and greater wear resistance than gears finished with conventional wet grinding or superfinishing. Findings include:
- No significant variations were found in the geometric accuracy of gears due to the grinding processes over the tested parameter range.
- Wet grinding and superfinishing achieved lower roughness, which could be detrimental for wear resistance.
- No significant variations were found in the microstructures of gears due to the grinding processes.
- Correct selection of process parameters allowed ground surfaces to be obtained without burns or a white layer.
- Dry-ground gears show slight tensile residual stresses in the flank surface, but a higher hardness was measured below the same surface with respect to wet grinding conditions.
A schematic and photos of the experimental setup (a and b) and images of the gears ground in the experiment (c and d). Images: Alessandro Fortunato, et al. in the Journal of Manufacturing Science and Engineering.
Stronger wear resistance
The flank profile of gears subject to dry grinding shows slight tensile values as a result of a high thermal gradient, which also causes microstructural defects if grinding parameters are not optimized.
“Nonetheless, in the tested conditions of this study, high hardness and beneficial topography assume a higher weight with respect to residual stresses, and wear tests revealed that dry ground gears have the longest durability,” Liverani said. “Based on these results, dry grinding could be an alternative to conventional wet grinding that satisfies the mechanical and geometric requirements of car manufacturers, while improving wear resistance, reducing waste, and increasing the sustainability of gear production in the strategic automotive market.”
Perhaps the greatest discovery was that the wear resistance of gears finished with dry grinding was considerably greater than that of gears finished with wet grinding or superfinishing. “This is a fundamental advantage for electric vehicles, where noise must be minimized,” Liverani said. “The elimination of lubricant makes this solution more sustainable, improving wear resistance without compromising geometric accuracy.”
Mark Crawford is a technology writer in Corrales, N.M.
