Modern Machine Shop

NOV 2018

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Page 69 of 155

MMS NOVEMBER 2018 68 SURFACE TREATMENT special—but the Ra value does not give a true picture of the real surface profile. Identical Ra values can result in very different lubrication performance. An optimum peak structure (low Rpk values) generates the least wear and reduces friction losses." Stream finishing uses fine-grain media with a high density and a diameter of 0.2 to 1 mm—much smaller than media used in vibrato- ry-finishing processes. Mixed with tensides and corrosion-protective media, the material looks similar to wet sand and is accelerated in the machine's chamber. The workpiece or several workpieces are mounted on a workpiece holder and are immersed in the media at a specific angle. According to Mr. Gegenheimer, the speed of rotation of both the workpiece and the abrasive media as well as the angle in which the parts are clamped and dipped into the chamber depend on the application, requiring experienced operators. "The abrasive media's rotational speed con- trols the pressure on the workpiece, and the workpiece rotation controls the cutting speed," Mr. Gegenheimer says. "The generated high mechanical energy changes the crystal structure on the workpiece surface. Since the media is very fine-grained, it reaches areas such as holes and the roots of gears. Stream finishing enables the so-called 'third body' to be formed more quickly between two surfaces in friction. This is the boundary layer where the surfaces, which are separated by a thin oil film, transfer their kinetic energy to each other. This flexing action makes the crystalline nanostructure of the layer extremely fine with a pasty viscosity, reducing friction." Stream finishing eliminates the need for lengthy breaking-in of engines and gearboxes, which reduces oil contamination and extends oil-change intervals by as much as 100 percent. An additional benefit is the 10-percent reduction in heat generated and as much as a 50-percent reduction in noise emissions compared to conven- tionally ground parts, Mr. Gegenheimer explains. Decrease in Micropitting Another advantage is the decrease in micropit- ting with gear wheels where the lubricating film breaks through at localized points causing mixed friction and excess pressure. This wear causes f lattening at the contact point. The most import- ant individual parameter related to this is the surface roughness. Values below 0.2 micron Ra, which Otec finishing typically achieves, can sig- nificantly reduce micropitting. The process is designed for high-value parts, not for mass finishing of screws or other parts that are commonly mass finished with disc or vibration-finishing processes, which take a couple of hours. According to Otec, stream finishing takes only a minute or less. The pro- cess can be performed in two stages: a grinding process followed by polishing. "The polishing process reduces the peaks after the first pro- cess step described above even further," Mr. Gegenhenheimer says. "It rounds down the peaks to reduce friction while maintaining the surface valley for good lubrication." "As a rider, confidence is critical. You can push it that little bit harder knowing the components and engine are reliable, something overlooked by many of my customers." Stream finishing is capable of achieving surface smoothness values of 0.01 micron Ra for parts such as gears. Reduced friction means less heat and as much as 50 percent less noise emissions.

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