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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 Density Symbol 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).

Grease Density = 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 39.2F (4C) as the standard, but grease data sheets ordinarily use 77F (25C). 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





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