"What role does density play in the overall performance of lubricants, especially in regards to hydraulic oil?"

Density is a key property not only in lubricants but in all fluids. It is the measure of the mass of a substance in relation to a known volume. For example, the density of water is 62.4 pounds per cubic foot. However, this varies according to the temperature of the fluid.

Lubricants generally are less dense than water. If the density of an object is less than that of water, then that object will float. This is why if you have a moisture problem in your lube system that the water settles to the bottom of the sump and is drained out first whenever the plug is pulled or the valve is opened.

This value is also used in some calculations involving viscosity. Viscosity is the single most important property of a lubricant, so if any math is to be done using this value, the density must be known. A common conversion between absolute viscosity to kinematic viscosity uses the density value. As the density of a fluid changes for whatever reason, this can introduce errors in converting between both values of viscosity.

A common term used in conjunction with density is specific gravity, which is the relationship of density to water. Water has a specific gravity of one, so if a fluid is heavier than water, the specific gravity value will be more than one. Anything lighter than water will have a value that is less than one.

This property of a fluid is crucial for several attributes of a lubricant. For instance, as the density of a lubricant increases, the fluid becomes thicker. This leads to an increase in the amount of time it takes for particles to settle out of suspension.

The specific gravity of rust falls somewhere between 2.44 to 3.6, so as the oil thickens, particles such as this settle out much slower in reservoirs and other areas with dwell time in the fluid. In hydraulic systems, this can cause failure. These types of systems are very sensitive to any contamination. Therefore, if the particles are in suspension longer, it can result in problems such as silt lock, cavitation and corrosion.

In E.C. Fitch's Proactive Maintenance for Mechanical Systems, many other problems associated with density deviations are given, particularly in hydraulic systems. These include:

  • Greater propensity for cavitation, both at pump suction and downstream of orifices
  • Increased pumping power
  • Increased stress on pumping elements
  • Poor pumpability due to fluid inertia

According to Fitch, "High-density fluid contributes to better contamination control by aiding in the suspension, transport and removal of particulate contaminants." Since the particles are held in suspension longer, they are more easily removed by filters and other particle-removal systems, thus making it easier to clean the system.

It is important to note that some fluids used as lubricants are in fact heavier than water. Most phosphate-ester fluids have a specific gravity of more than one. In these systems, water would float on top of the oil.

Keep in mind that as density increases, so too does the erosive potential of the fluid. In high turbulence or high-velocity regions of a system, the fluid can begin to erode piping, valves or any other surface in its path.

Not only are solid particles affected by the density of a fluid, but so are contaminants such as air and water. Both of these contaminants have a marked impact on density. Oxidation influences the density of a fluid as well. As oxidation progresses, the density of the oil increases.

Overall, density plays a critical role in how a lubricant functions as well as how machines perform. Most systems are designed to pump a fluid of a specific density, so as the density begins to change, the efficiency of the pump begins to change as well. Understanding the importance of density and how it relates to your equipment is essential for the reliability and health of your machines.