Viscosity can be broadly defined as :
- Viscosity is a measure of a fluid’s resistance to flow (Kinematic viscosity)
- Viscosity is a measure of a fluid’s resistance to shear (Dynamic viscosity)
High viscosity means high resistance to flow while low viscosity means low resistance to flow. Viscosity varies inversely with temperature.
There are mostly two methods for measuring viscosity : Shear method and Time method.
DYNAMIC VISCOSITY or ABSOLUTE VISCOSITY :
Expressed in typical units : Centipoise (cP), Poise(P), milli pascal seconds (mPa.s).
In this method the viscosity is determined by directly measuring shear rate and shear stress.
A practical example is to insert a rod in two beakers, one containing hydraulic oil and another containing thick gear oil. When both the oils are stirred it will be observed that a greater force is required to stir the thick gear oil and less force is required to stir the hydraulic oil. Thus the shear rate, the shear stress of the oil is measured by this method.
KINEMETIC VISCOSITY :
Expressed in typical units : Centistokes (cSt) = mm2/sec, Stokes (St) = cm2/sec, Saybolt Universal Seconds (SSU or SUS).
In this method the viscosity is determined by its resistance to shear and flow due to gravity.
A practical example is to fill two beakers, one containing hydraulic oil and another containing thick gear oil. If the oil from both are tipped and allowed to fall, the hydraulic oil falls faster than the thick gear oil which easily defines that the hydraulic oil has a lower kinematic viscosity than the thick gear oil.
THE EFFECTS OF LOWER AND HIGER VISCOSITY RELATING TO OILS ARE :
Lower viscosity or a significant reduction in viscosity can result in :
- Loss of lubricity causing excessive wear
- Higher mechanical friction causing high heat generation which may cause high energy consumption
- Reduction of oil film will cause friction between moving parts in the system causing particle contamination
Likewise, too high a viscosity can cause :
- High heat generation and production which results in oil oxidation, sludge and varnish formation
- Inadequate oil flow to bearing and pumps resulting in gaseous cavitation to machinery
- Inadequate oil flow also resulting in lubrication starvation to the machhinery
- Oil whip in journal bearings
- High friction causing excess energy consumption
- Poor air to oil ratio causing demulsibility
- Poor cold start pumpability
In order to care for the machinery it would be prudent to investigate the cause and correct the problem. The reason for the viscosity change may be due to molecular or may be due to contamination with other liquids like :
TESTING OF VISCOSITY IN OUR LABORATORY :
Equipment used : Anton Paar SVM 3000 Stabinger Viscometer : Measures Dynamic and Kinematic Viscosities of oils and fuels at 40°C and 100°C (100°F and 212°F) and also automatically calculates the Viscosity Index (VI). It is the most accurate and high precision viscometer in the world which can measure upto 30 samples in an hour.
Method of Testing :
Viscosity : As per ASTM D445 - ISO 3104 - IP 71 - BS 2000 - DIN 51550 or ASTM D7042
Viscosity Index (VI) : As per ASTM D2270