In case of multipurpose engine oils with VI improvement additives, besides temperature and pressure, viscosity also varies with the shear rate.
If the lubrication gap and the lubricating film is magnified several thousand times, the oil particles located at the non-moving part (e.g. bearing cup, cylinder lining) have zero speed. The oil particles at the moving parts will have the speed of the respective component (e.g. the peripheral speed of the bearing journal of the crankshaft or the piston speed). This process can be theoretically presented with the velocity triangle.
Definition: Shear rate
The term shear rate refers to the speed at the moving parts [m/s] divided by the thickness of the lubricating film [m].
The term shear rate refers to the speed at the moving parts [m/s] divided by the thickness of the lubricating film [m]. The resulting shear rate [s-1] of combustion engines, in the area of the main bearing, piston and cylinder lining, is approx. 10-5 s-1 at idle speed and up to 10-6 s-1 at maximum rpm.