February 13, 2015

The Thick and Thin of Oil Viscosity

Oil viscosity – the measurement of resistance to flow in oil – is a definite factor in the performance of your equipment. Viscosity determines the performance and protective functions of machine lubricants by providing a seal between components, cushioning them against impact, maintaining a boundary layer between sliding or rolling surfaces.

Any oil-viscosity measurement is directly dependent on operating temperature, which will change in conjunction with machine temperature. In order to consistently measure viscosity, oil is tested at one of two standard temperatures. Lubricants in an engine are tested at 100 deg. C, and non-engine applications are measured at 40 deg. C. Although not exact, these two points align with the operating temperatures of their respective systems.

When oil viscosity test results are abnormal, chances are something else is wrong in the system or engine. It is not typical that oil viscosity is the first property to fall out of range without some contributing cause. In other words, abnormal viscosity results often correlate with and confirm other abnormalities, helping maintenance managers identify these issues before they lead to machine failure.

Let's examine what factors can contribute to a change in oil viscosity:

  • Fluid dilution – this can include the dilution of water, fuel, or coolant, or the addition of incorrect oil.
  • Contamination – Soot, sludge, or additives could have infiltrated the system.
  • Oxidation – When oxygen from water, air, or contaminants attaches to the oil, the oil thickens and forms deposits (sludge).
  • Shearing – Oil moving through a pump or motor system or exposed to high heat causes the molecular units to break or cut (shear), which lowers the viscosity and thins the oil.
  • Operating temperature – The actual temperature may be colder or warmer than that of typical or recommended operating conditions.

Frequent oil sampling is your best bet for reducing excessive wear and keeping downtime to a minimum. Check operator manuals or consult your dealer to set up a proper maintenance schedule.