Alıntı:
Shear stability is a measure of the amount of viscosity an oil may lose during operation. Oil experiences very high stresses in certain areas of the engine such as in the oil pump, cam shaft area , piston rings, and any other areas where two mating surface areas squeeze the oil film out momentarily . Most multigrade engine oils contain special types of additives, called Viscosity Index improvers, which are composed of very large, viscosity-controlling molecules. As the oil passes through the engine, these molecules are permanently sheared or torn apart over time, causing the additive to lose its viscosity-contributing advantages which reduces the oils ability to maintain its higher number. ie.. 10w30 ,
Synthetic oils do not rely as much on special Viscosity Index Improver additives and will experience little permanent viscosity loss.
When the oil film is sheared or squeezed out, then your protection is now reliant on barrier lubricant additives. Oil has 3 states of lubrication, hydrodynamic, mixed film, and boundary(barrier). The best is hydrodynamic which is nothing more than a flow of oil separating two mating surfaces. After a period of time shearing, the oil will lose it’s ability to hold up to the same flow as before since the VI Improvers are breaking down causing the oil to thin down in grade. Once this happens, there is less film strength between the mating surfaces so it doesn’t take as much for the oil to shear, therefore creating more heat which attacks the base oil even more and then starts to cause the oil to thicken up due to the excessive levels of heat and the broken down VI Improvers become a contaminant which added to the existing oil will continue to thicken and ultimately cause sludge if not changed in a reasonable time..
The shear stability of an oil is measured by using both ASTM test methods D445 and D5275.
Yağın belirli bir süre sonra inceleceği doğru, tamam kabul. Peki bu süre ne kadar kısa yada uzun ? Bu konuda ben herşeyi biliyorum diyip ahkam kesecek birinin az üstte numaraları verilmiş testlerin sonuçlarını da yazabilmesini beklerim ben. Yazamıyorsa bu kadarlık google searchı ninemde yapar. Ayrıca zaten hepi topu 8 10bin kullanılacak olan yağ, 15000-20000 km sonra vizkozitesini kaybetse ne olur kaybetmese ne olur, her halükarda çöp zaten o.