Definition
Shear stability describes a viscosity modifier’s resistance to permanent viscosity loss caused by mechanical shearing of polymer chains. Long-chain polymers used as VI improvers can break under the high shear rates in engine components (valve train, oil pump, turbocharger bearings), causing irreversible thinning of the oil.
Shear stability is characterized by the Permanent Shear Stability Index (PSSI) — higher PSSI means greater viscosity loss. It is measured by:
- Kurt Orbahn test (ASTM D6278 / KRL) — 30 cycles through a diesel fuel injector; viscosity measured before and after
- Sonic shear (ASTM D2603) — ultrasonic irradiation; less correlated to field experience
Modern PCMO and HDDO specifications set maximum viscosity loss limits (e.g., ILSAC GF-6: maximum 2.9% KV100 loss after KRL test).