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An Ideal Hydraulic Fluid? Impossible!

Okay, I realize that impossible is a strong word. So let’s just say that with today’s technology, and that of the foreseeable future, developing an ideal hydraulic fluid is impossible. Why? Because many properties of a hydraulic fluid are diametrically opposed for most applications. A selection of a hydraulic fluid is always (there’s another strong word) a compromise.

Alan Hitchcox
Alan Hitchcox
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For example, take viscosity. Viscosity is a measure of a fluid’s resistance to flow. Because a high viscosity fluid can provide high volumetric efficiency and low external leakage, it would seem ideal for any application. However, because a pump must work harder to move a high-viscosity fluid, it produces low mechanical efficiency. So hydraulic components are designed to operate with fluid within a range of viscosities, which affords a good compromise of efficiency and leakage resistance. This also makes it important to specify components that are all designed to use fluid of the same viscosity, if at all possible. Again, compromises usually must be made.

Bulk modulus is another. Bulk modulus refers to a fluid’s resistance to compressibility. In most cases (especially those with closed-loop mot ion cont rol ) , a fluid with a high bulk modulus is desired to make the hydraulic system “stiff.” However, fluid with a high bulk modulus transmits shock and vibration more readily than one with low bulk modulus. So for systems where high fluid-bourne shock (sometimes called water hammer) and vibration occur, choosing a fluid with a lower bulk modulus might be a better choice.

The most dramatic illustration of my point is when water is used as a hydraulic fluid instead of oil. Water has a much lower viscosity, a higher bulk modulus, higher vapor pressure, and a higher density than oil Water is also inexpensive, non-flammable, and can be completely “environmentally friendly.” Some of these characteristics are desirable, some are not, and most depend on the application. Not surprisingly, environmental friendliness has become an important characteristic, as evidenced by the growing number of fluids with lower toxicity and higher biodegradability.

So there is no ideal hydraulic fluid. All we can have is the optimum compromise of characteristics for a given application. In the future, we’ll probably have smart hydraulic fluids, where properties can be changed to suit an application. We’ve even written about some of these, which are dubbed electro- and magneto-rheological fluids. But for use in hydraulic systems, these are primarily reserved for research labs. And if they were commercially available, the high price tag of smart fluids would make them less than ideal. Fortunately, though, a wide variety of hydraulic fluids is available to provide a great combination of characteristics for just about any application — and new formulations continue to enter the marketplace, providing an even wider selection.