Grease Density and Specific Gravity
To engineer a grease dispensing system, it is often neccesary to convert a unit of
weight (grams, oz) to a unit of volume (cubic centimeters, cubic inches). In order
to make this conversion the density or specific gravity of the grease must be known.
Density
The density
of a substance is the total mass (m)(weight) of that substance divided by
the total volume (V) occupied by that substance (mass per unit volume). Common units
of measure for density are pounds per cubic foot (lbs/ft3), kilograms per cubic meter (kg/m3), or
grams per cubic centimeter (gm/cc3).
= m/V
Specific Gravity
Specific gravity (S.G.) is a measure of the relative density of a substance
as compared to the density of water at a standard temperature. Physicists use
39.2°F (4°C) as the standard, but grease data sheets ordinarily use 77°F (25°C).
In the International System of Units (SI Units), the density of water is 1.00 g/cm3
at the standard temperature.
Therefore, the specific gravity of grease has exactly the same value as its density
in units of g/cm3.
Any liquid with a specific gravity less than 1.00 will float in water.
Any liquid with a specific gravity greater than 1.00 will sink in water. For example,
Motor oil (Texaco Havoline 10W40) has a specific gravity of 0.88. The specific gravity of
grease depends on the SG of the base oil used, as well as the density of the thickeners
and additives. Greases based on petroleum base oils usually have a specific gravity
of 0.85 to 0.95, greases based on synthetic oils often have an SG of 1.5 or more.
Conversion
When designing grease dispensing systems, the specific
gravity of the grease is used to convert weight to volume.
Any stated density units must be converted into
grams/cm3. The following conversion factors may be used:
grams/cubic centimeter = pounds/cubic foot x 0.016018463
grams/cubic centimeter = kilograms/cubic meter x 0.001
Practical Application
The grease requirement for a part is often specified in grams, while grease dispensers
are usually calibrated in cubic centimeters. Thus, we need to convert the units of
weight (grams) to the units of volume (cc). To do this, simple divide the weight
in grams by the specific gravity.
Volume(cc) = Weight(grams) / Specific Gravity
