Lubrication science
Viscosity index improvers: how they work and how they wear out
Quick answer
Polymer additives that thicken an oil less when cold than when hot, and the shear loss that can undo them.
How a viscosity index improver works
A viscosity index improver (VII) is a long-chain polymer dissolved in the oil. When the oil is cold the polymer coil stays tight and adds little thickness. When the oil is hot the coil opens and thickens the oil, which offsets some of the thinning.
Types
| Polymer | Strengths | Notes |
|---|---|---|
| Olefin copolymer (OCP) | Good thickening, lower cost, widely used in engine oils | Medium shear stability |
| Polymethacrylate (PMA) | Very good low-temperature flow and shear stability | Higher cost. Also used as a pour point depressant |
| Hydrogenated styrene-diene | Good oxidation stability and shear stability | Used in premium and long-drain oils |
| Blends | Balanced performance | Chosen for the base oil and the duty |
Shear loss
In gears and bearings the polymer chains are cut by high shear. Permanent shear means a lower viscosity that does not come back, so the oil film gets thinner than the grade suggests. This is why a multigrade oil in a hard-working gearbox must be chosen with a shear-stability rating, and why a falling viscosity in an oil analysis can point to polymer shear and not only to dilution.
Where you find them
- Multigrade engine oils and automatic transmission fluids.
- High-VI hydraulic oils for wide temperature swings.
- Mostly not in industrial gear oils, which are usually straight-grade.
Takeaway. A VII widens the temperature range but can shear down in service. Check shear stability for gears and hydraulics, and read falling viscosity with that in mind.
Put it to work in the tool