VISCOSITY - measurement of a fluid’s resistance to flow

The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. For convenience, the centipoise (cp, one-hundredth of a poise) is the unit customarily used.

Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature. The measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke (cSt, one-hundredth of a stoke). Kinematic viscosity can be related to absolute viscosity by the equation:

cSt = cp - fluid density

Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported.

KINEMATIC VISCOSITY - the measure of a fluid’s resistance to flow under gravity

It is the absolute viscosity of a fluid divided by its density at the same temperature of measurement, as determined by test method ASTM D 445.

To determine kinetic viscosity, a fixed volume of the test fluid is allowed to flow through a calibrated capillary tube (viscometer) that is held at a closely controlled temperature. The kinematic viscosity, in centistokes (cSt), is the product of the measured flow time in seconds and the calibration constant of the viscometer.

VISCOSITY INDEX (VI) - empirical, unitless number indicating the effect of temperature change on the kinematic viscosity of an oil

Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its tendency to change viscosity with temperature.

The VI of an oil - with known viscosity at 40°C and at 100°C - is determined by comparing the oil with two standard oils having and arbitrary VI of 0 and 100, respectively, and both having the same viscosity at 100°C as the test oil. ASTM D 2270 is the typical test method used. There is an alternative calculation, also in ASTM D 2270, for oils with VI’s above 100.

The VI of paraffinic oils (see paraffin) is inherently high, but is low in naphthenic oils (see naphthene), and even lower in aromatic oils (often below 0). While synthetic lubricants inherently have higher VI’s than petroleum oils, adding a viscosity index improver can increase the VI of any petroleum oil. High-VI lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures.

APPARENT VISCOSITY - a measure of the viscosity of a non-Newtonian fluid (typically greases) under specified temperature & shear

Viscosity is expressed in units of centipoise (cP).

 

DENSITY / SPECIFIC GRAVITY - the ratio of the mass of a given volume of product and the mass of an equal volume of water, at the same temperature

The standard reference temperature is 15.6°C (60°F). Specific gravity is determined by test method ASTM D 1298: the higher the specific gravity, the heavier the product. Specific gravity of a liquid can be determined by means of a hydrometer, a graduated float weighted at one end, which provides a direct reading to which it sinks in the liquid. A related measurement is density, an absolute unit defined as mass per unit volume - usually expressed as kilograms per cubic meter (kg/m3).

NEUTRALIZATION NUMBER - an indication of the acidity of alkalinity of an oil

The number is the weight in milligrams of the amount of acid (hydrochloric acid {HCL}) or base (potassium hydroxide {KOH}) required to neutralize one gram of the oil, in accordance with test method ASTM D 664 (potentiometric method) or ASTM D 974 (colorimetric method). Strong acid number is the weight in milligrams of base required to titrate a one-gram sample up to a pH of 4; total acid number is the weight in milligrams of base required to neutralize all acidic constituents. Strong base number is the quantity of acid, expressed in terms of the equivalent number of milligrams of KOH, required to titrate a one-gram sample to a pH of 11; total base number is the milligrams of acid, expressed in equivalent milligrams of KOH, to neutralize all basic constituents. If the neutralization number indicates increased acidity (i.e., high acid number) of a used oil, this may indicate that oil oxidation, additive depletion, or a change in the oil’s operating environment has occurred.

 

FIRE POINT - temperature at which the vapor concentration of combustible liquid is sufficient to sustain combustion

Determined by test method ASTM D 92.

FLASH POINT - temperature at which a product’s vapor can be ignited momentarily by a flame

Determined by test method ASTM D 92.

DROPPING POINT - temperature at which the grease passes from a semisolid to a liquid state

This change in state is typical of greases containing conventional soap thickeners. Greases containing thickeners other than conventional soaps may, without change in state, separate oil.

POUR POINT - the lowest temperature at which the oil can be poured

Determined by ASTM D 97. Also, using FTM 203, the stable pour point is the lowest temperature at which an oil will remain fluid after being subjected to a cyclic temperature variation for six days.

 

OXIDATION STABILITY - ability of a lubricant to resist natural degradation upon contact with oxygen

OIL SEPARATION - percentage of oil loss, by weight, in grease over time and at elevated temperature

Oil Separation is an essential quality of greases for most applications.

EVAPORATION - loss of a portion of a lubricant due to volatilization.

 

PENETRATION – consistency

expressed as the distance in millimeters that a standard needle or cone penetrates vertically into a sample of the material under known conditions of loading, time, and temperature.

WORKED PENETRATION - the penetration of a sample of lubricating grease immediately after it has been brought to 77°F and then subjected to 60 stokes in a standard grease worker

This procedure and the standard grease worker are described in ASTM Method D 217. NLGI Grade is determined by this test.

NLGI (NATIONAL LUBRICATING GREASE INSTITUTE)

Trade association whose main interest is grease and grease technology. NLGI is best known for its system of rating greases by penetration (consistency).

FOUR BALL WEAR TEST - machine used to evaluate a lubricant’s antiwear qualities, frictional characteristics, or load carrying capabilities.

There are four steel 1⁄2-inch balls. Three of the balls are clamped together in a cup filled with lubricant while the fourth ball is rotated against them. Two test procedures are based on this same principle – the Four Ball EP Test (ASTM D-2596) and Four Ball Wear Test (ASTM D-2266).