Hydraulic oil is different than other lubes. Not only is it a lubricant, it’s also the means by which power is transferred throughout the hydraulic system. So, it’s a lube and a power transfer device. This dual role makes it unique.
To be an effective and reliable lubricant, hydraulic oil must possess properties similar to most other lubes. These include: foaming resistance and air release; thermal, oxidation and hydrolytic stability; anti-wear performance; filterability; demulsibility; rust and corrosion inhibition; and viscosity in respect of its influence on film thickness.
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 V-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.
As I have explained in previous columns, 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 basestocks 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 aforementioned applications are not typically shear stable when used in modern hydraulic systems.
But, recent advances in VI improver technology mean 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, what does it really mean to a hydraulic equipment owner? 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 V-belt drive we talked about earlier in order to maintain optimum power transfer conditions.
However, based on simple cost/benefit analysis, if the cost to install the auto-tensioner was $200, we wouldn’t spend this money unless we were satisfied we can recover this investment – plus an acceptable return – through savings attributable to more efficient power transfer and/or reduced maintenance costs.
The same approach should be applied when evaluating the cost and benefits of using a higher VI hydraulic oil. But unlike the relatively simple V-belt drive, savings accruing from increased hydraulic machine performance can be more difficult to quantify. The results, though, of field trials conducted by a manufacturer of shear-stable VI improvers1 have demonstrated real economic benefit to the equipment end-user. In one trial, the performance of a 40-horsepower compact excavator was evaluated using an all-season 142 VI “baseline” oil and compared to the performance of the same machine using a 200 VI “test” oil.