The internal friction of a fluid. Lubrication work mostly uses kinematic viscosity in centistokes (cSt, equal to mm2/s), measured by timing flow through a glass capillary at 40 C and 100 C under ASTM D445. Too thin and the film collapses into boundary contact; too thick and the machine runs hot, wastes energy and starves on a cold start. A shift of about 10 percent from new oil is the classic oil-analysis alarm for a wrong top-up, dilution, oxidation or polymer shear.
Viscosity came back at 54 on a 68 oil, somebody's been topping off with the 46 hydraulic.
When a sample comes back off-grade, check the top-up log before blaming the oil. Most viscosity excursions are a mixed product, not degradation.
Also heard as
- vis
- KV40
- KV100
Related terms
Viscosity index
Number showing how little an oil's viscosity changes with temperature; higher means flatter.
ISO VG
ISO viscosity grade, the number on an industrial oil that names its kinematic viscosity in cSt at 40 C.
Oil analysis
Lab testing of in-service oil samples to judge oil condition, contamination and machine wear.
Elastohydrodynamic lubrication
Film regime in rolling contacts where huge pressure stiffens the oil and elastically flattens the metal.