The role that density plays in the performance of hydraulic oil is key. Density is the measurement of a substance’s mass as it relates to a known volume. Water’s density has been measured at 62.4 pounds per cubic foot, but this number varies when the temperature of water changes. Where lubricants are concerned, they tend to be less dense than water, which is why water will tend to settle to the sump’s bottom and is the first thing to be drained from the lube system when a valve is opened or a plug pulled.
Density also plays a key role in some viscosity calculations. Converting absolute viscosity to kinetic viscosity also requires a known density value, but this value can cause miscalculations to occur if the fluid’s density happens to change.
Density And Specific Gravity
Specific gravity (SG), the relationship of fluid density to water, is yet another important determination of lubricant performance. Should the fluid be lighter than water, it will have a specific density of less than one, whilst a fluid that’s heavier than water will have a specific density of more than one.
Specific gravity is critical to many lubricant attributes. Fluid becomes thicker as specific density of the lubricant increases, which will lengthen the time it takes for particles to settle out. In the case of rust, its SG is between 2.44 and 3.6, so as oil thickens, any rust particles in areas with fluid dwell time, such as reservoirs, will take far longer to settle out. This longer settling out time can result in hydraulic system failure due to corrosion, silt lock and cavitation.
However, it’s also important to realise that not every fluid is lighter than water. For example, most fluids containing phosphate-ester are heavier than water, and as a result will sink below it.
Other Problems Caused By Changes In Density
Hydraulic systems can experience several issues when deviations in density occur. A system may pump poorly because of fluid inertia, or its pumping elements can experience increased stress. As well, changes in density can cause a requirement for increased pumping power, as well as increase the probability of cavitation in downstream openings and at pump suction points.
Better Control Of Contamination
Higher density fluids can allow for better control of contaminants. This is because they help to suspend, transport and remove particulates. Particles can be easily removed by filters because they are held in suspension for longer in dense fluids. This ultimately makes cleaning the hydraulic system far easier.
Although higher density fluids allow for better contamination control, they can also increase the erosive potential of a fluid. In a situation where velocity or turbulence is high, piping can begin to erode, as can valves and other surfaces in the path of the fluid.
Fluid density can be significantly impacted by air and water as well as oxidation. In fact, the further that oxidation is allowed to progress, the higher density the oil will have.
Density is important to how lubricants function and how a machine performs. Specifically, density is crucial to pump efficiency. It’s important to note that most hydraulic systems are designed for specific-density fluid. Therefore, as fluid density changes, so too will the efficiency of the pump.
Not considering the role of density when calculating the performance of your hydraulic oil can lead to premature replacement of O-rings and other components, and unexpected system downtime or shutdowns. However, when density and how it impacts your hydraulic equipment is understood, you can use this knowledge to lower your facility’s running costs.