In the current trend of increasing energy costs, which appears to be underpinned by big-picture issues such as peak oil and carbon emission reduction, there is a growing awareness of the need to make hydraulic systems more efficient.
The three obvious ways of doing this are:
1. Use components with higher native efficiency: a piston pump instead of a gear pump, for example.
2. Minimize the use of energy-loss devices, such as flow controls, pressure reducing valves and even proportional valves.
3. Design the circuit so that energy is not consumed when work is not being done; unloading the pump between cycles or employing load-sensing control, for example.
But there's also another way to increase the overall efficiency, and therefore the energy cost of operating a hydraulic machine: specify a better oil.
Hydraulic oil is a lubricant and a power transfer device. To be most efficient in its role as a power transfer device, hydraulic oil needs high bulk modulus (high resistance to reduction in volume under pressure) and high viscosity index (low rate of change in viscosity with temperature).
As an analogy, consider the tension on a vee belt. If it is out of adjustment, the belt will slip. The result is a higher percentage of input power wasted to heat. This means less power is available at the output to do useful work. In other words, the drive becomes less efficient.
A similar situation can occur with hydraulic oil. Change in its bulk modulus and/or viscosity can affect the efficiency with which power is transferred in the hydraulic system.
The perfect hydraulic fluid for transmission of power would be infinitely stiff (incompressible) and exhibit a constant viscosity of around 25 centistokes regardless of its temperature. Such a fluid does not exist.
Bulk modulus is an inherent property of the base oil and can't be improved with additives. But viscosity index (VI) can be improved by using high VI base stocks such as synthetics and/or by adding polymers called Viscosity Index Improvers to the formulation.
Viscosity Index Improvers were first used to make multi-grade engine oils in the 1940s. These days, this common and well-tested technology is used to make high VI oils for other applications, including automotive transmission fluids and manual transmission gear oils. However, the VI improvers used in oils for the above applications are not typically shear stable when used in modern hydraulic systems.
But recent advances in VI improver technology means that mineral hydraulic oils with a shear-stable viscosity index in the 150 to 200 range are now commercially available.
While this may be good to know, how can these high VI oils improve the overall efficiency of a hydraulic system? Well, within the allowable extremes of viscosity required to maintain adequate lubricating film thickness for hydraulic components, there's a narrower viscosity range where power losses are minimized, and therefore, power transfer is maximized.
By maintaining the oil's viscosity in this optimum range, machine cycle times are faster (productivity is increased) and power consumption (diesel or electricity) is reduced.
So using a higher VI oil means the hydraulic system will remain in its power transmission "sweet spot" across a wider operating temperature range.
You could think of this as similar to installing an automatic-tensioner on the vee belt drive discussed earlier - to maintain optimum power transfer conditions.
Of course, there are many variables which can influence the performance of a hydraulic machine. But the efficiency gain which may be achieved by switching to a high VI oil is something every savvy machine designer and hydraulic equipment user should be aware of.
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