When oil is squeezed between surfaces, like a bearing and a journal, VII polymers can align and stretch, causing a temporary drop in viscosity. Once past the bearing, the polymers return to their original shape, restoring viscosity. This temporary shear-thinning happens as the oil film thickness reduces temporarily under pressure but recovers once the stress is removed.
Under high shear conditions, VII polymers can break into smaller chains, causing permanent shear-thinning. This leads to a lasting reduction in viscosity and oil film thickness, increasing engine wear. For example, SAE 5W-30 oil can shear down to SAE 5W-20, causing damaging deposits and reduced engine life.
Shear stability measures an oil’s resistance to viscosity changes under mechanical stress. High shear stability is essential to maintain adequate oil film thickness and prevent metal-to-metal contact in high-temperature/high-shear (HT/HS) conditions. Larger VII molecules are better thickeners but more prone to breakage, affecting shear stability.
Different VII polymers have varying shear stability characteristics. The Shear Stability Index (SSI) reflects a polymer’s resistance to permanent viscosity loss; lower SSI indicates better shear stability. The “Kurt Orbahn Diesel Injector Test” measures permanent viscosity reduction after 30 cycles, indicating the extent of polymer breakage.
For instance, if an oil starts with a base viscosity of 5 cSt (at 100°C) and VII increases it to 15 cSt, a test showing a drop to 12 cSt means a 3 cSt loss. The VII’s SSI is 30% (3 cSt loss / 10 cSt contribution).
Synthetic oils have a higher native viscosity index and need less VII, making them more shear-stable. They maintain their viscosity longer, providing better protection and staying ‘in-grade’ more effectively than conventional oils.