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Hydraulic Fluid Trends


Hydraulic fluids are one of the largest-volume lubricant products sold worldwide, according to market research consultants Kline & Co. in Little Falls, N.J. Accounting for 11 percent of global lubricants demand, theyre second only to automotive engine oils in market size. Yet, hydraulic fluid technology until recently was relatively static. Many equipment designers and users treated hydraulic fluids as an afterthought, expecting these oils to be offered at commodity prices and to do their job with minimal additives.

That attitude is changing, attendees at the Society of Tribologists and Lubrication Engineers annual meeting learned in May. In a number of presentations, formulators described how they are cracking open new opportunities to differentiate hydraulic fluid performance, by cracking down on wasted energy, wasted fluid and wasted maintenance dollars.

During a special forum sponsored by Richmond, Va.-based Afton Chemical and moderated by J. Philip Rohrer, the companys marketing manager for general industrial solutions, the Orlando meeting heard from two members of the Center for Compact and Efficient Fluid Power. CCEFP is a research partnership with the ambitious goal of transforming fluid power to dramatically reduce energy consumption and create whole new fields of use for fluid power, explained Mike Gust, the consortiums industrial liaison director. The alliance of seven universities and more than 50 companies has undertaken dozens of projects, with the unifying goal of reducing the size and energy drain of hydraulic systems.

It is possible to make a dramatic – plus 50 percent – improvement in fluid power efficiency, declared Gust, and partners are working to overcome numerous technological barriers: controls and energy management; pumps and motors; individual component improvements; machine feel and human interface; noise, harshness and vibration. The fluid is a critical factor in their efforts, he emphasized, because the fluid is the only system component that touches all the rest.

CCEFPs research so far has focused on off-road equipment such as excavators and draglines. Excavators are a $26-billion-a-year market, Gust noted. Our major question is, just how efficient can a mobile piece of equipment be? When you look at energy consumption per gallon of fuel, only about 15 percent of it results in useful work.

The group calculates that the largest culprit by far in this waste is metering valves, which hog 44 percent of the systems power. Pumps (22 percent), charging and cooling (14 percent), and actuator friction (6 percent) are other major areas of loss.

Cutting the Losses

Todays hydraulic equipment can operate at 90 percent efficiency – but that only occurs at maximum power, Gust said. Unfortunately, most equipment time is spent in other modes, such as partial stroke, where efficiency quickly evaporates. Improving performance at low displacement is critical to cutting the losses.

Because users also are demanding that units be smaller, the big question is how to downsize them. Motors today are about 30 percent larger than they need to be, stated Gust, but if they shrink, the stresses on fluids could be enormous.

Other fluid-related challenges the CCEFP is investigating include:

Leakage. Reducing the leakage rate is essential to improve hydraulic motor starting efficiency.

Compactness. Some believe it may be possible to eliminate system coolers – but the thermal effect on the fluid could be profound. Smaller systems will be expected to do the same amount of work, so fluids could see wider pressure ranges (from 10 to 12,000 psi) and higher internal bearing loads. Theyll need better heat transfer capacities to manage this.

Environmental impact. We want to get away from the dirty image of hydraulics, said Gust. We foresee components that are sealed for life, with 10 years or 25,000 hours of life. This could mean having a time-release charge of fresh fluid into the system, or a method to readditize the fluid. Fluids also should help dampen equipment noise, which also will add to their acceptability.

Aftons Rohrer next introduced another CCEFP member: Paul Michael, a research chemist at the Fluid Power Institute at the Milwaukee School of Engineering. While most researchers look at hydraulic pumps, we are looking at the other end of the system – at the hydraulic motor, he said. The hydraulic motors torque efficiency helps determine the whole systems performance; it sets the system pressure and size. But unlike the pump, the motor operates at low speed and high torque. Its a stop-start action, with lots of changes in direction.

Hydraulic motors put very different demands on the fluids than pumps do, Milwaukees researchers have found. They tested three commercial ashless antiwear fluids, all ISO 46 viscosity, in various hydraulic motor types (axial, radial and geroler designs) to see where and in which regimes the fluid may make a difference. The tested formulations included a standard 100 viscosity index fluid; an API Group II product boosted to 200 v.i. with viscosity index improvers; and a third based on biodegradable synthetic esters with a natural v.i. of 200 (no v.i. Added).

The team has been finding that fluids can improve energy efficiency in axial piston motors during startup and low-speed operation – and that the motors definitely respond to viscosity effects in the fluid, Michael said. Next, we’ll ask if friction modifiers and boundary lubrication additives can make a difference.

Undertreat and Underperform?

The research chemist seemed especially excited by the possibilities offered by more heavily formulated hydraulic fluids, and suggested that the lessons of automotive engine oils could be transferred to hydraulic fluid power. Over the three decades that I have been in the field of tribology, 15 new API classifications have been introduced to keep up with advancements in gasoline and diesel engine technologies, Michael commented later. While there have been significant improvements in fluid technology over the years, the lions share of the hydraulic fluid market remains zinc-based mineral oil with less than 1.5 percent additive. By contrast, fuel-efficient engine oils contain more than 10 times as much additive.

There are numerous reasons for this, he conceded, but the bottom line is that engines demand a lot more from the lubricant than a base oil with a little additive can deliver. If fluid power is going to achieve the transformation that the industry seeks, it will be necessary to raise the bar for hydraulic fluids.

At this point, it is pretty clear that advanced fluids can improve hydraulic pump efficiency by at least 5 percent, Michael added. Double-digit improvements are really quite plausible if you consider the entire system.

The time is right for hydraulic fluids to graduate from a maintenance item to an enabling technology, he urged in closing. Research projects are under way to reduce these efficiency losses, enthused Minnesotas Gust. We may be on the tip of a revolution in fluid power.

Steven Herzog, OEM liaison manager at Evonik RohMax in Horsham, Pa., echoed this optimism in another presentation at the STLE meeting. An early sponsor of CCEFP, his company also created an independent yardstick for hydraulic fluid efficiency. Herzog reminded listeners that for many years, viscosity and wear control were the principal measures of hydraulic fluid performance. The ISO 3448 viscosity classification system, dating back to the mid-70s, was based simply on minimum and maximum fluid viscosities at 40 degrees C.

That system was challenged by modern equipment, and by user demands for more power. And fluid choices became wider with the introduction of the Maximum Efficiency Hydraulic Fluid standard, which was one of the first performance specifications to provide a way to measure fuel efficiency improvements that can be attributed to the fluid. Meanwhile, the National Fluid Power Association also introduced guidelines for hydraulic fluid selection, which added further emphasis to the importance of kinematic viscosity and v.i. Improvers.

Research has shown that the viscosity of both fresh and sheared hydraulic oils has a large impact on fuel efficiency, Herzog said, and shear stability of the v.i. improver is a critical factor in sustaining the performance over time. The trick, of course, if finding that formulating sweet spot that satisfies NFPA, MEHF and end-user needs.

Varnish: Here and Now

Another STLE session directed attention to a more immediate problem faced by hydraulic equipment operators: the nasty varnish forming in their systems. Brian Filippini of Wickliffe, Ohio-based Lubrizol Corp., noted that users today are seeking both low initial cost and also lowest ownership cost over their hydraulic equipments life. They want system cleanliness, Filippini pointed out, and to get it theyve grown accustomed to applying ISO standards for fluid cleanliness (ISO 9406) and filterability (ISO 13357). These however focus on particles and dirt in the fluid, not on varnish. A broader approach, focusing on both fluid cleanliness and varnish prevention, is needed for longer equipment life, he said.

Varnish in antiwear hydraulic fluids – the leading type used in industry – can be created by the decomposition of base oils and additives, such as spent antioxidants and antiwear agents. Varnish is a tenacious brown film on the internal parts of the hydraulic system, coating the pump, lines and sump, Filippini described. Users have increasingly reported varnish-forming tendencies in the past few years, he added, especially in fluids formulated with API Group II and higher base oils. These base oils are more oxidatively stable, which should help prevent varnish, yet they also have less solvency than conventional Group I base oils. So while their oxidative stability prevents oxidized hydrocarbons from forming (good), they lack the solvency to dissolve or solubilize the polar precursors to varnish (bad).

And varnish doesnt take long to show up in a hydraulic system. In a standard 1,000hour Vickers 35 VQ-25 vane pump test, the researchers were able to detect varnish precursors beginning to form only halfway through the test. After 650 hours, a shadow could be seen on the sumps walls, and by the tests end, full varnish had formed. Once it takes hold, varnish is very hard to remove from a hydraulic system. You need to scrub with a solvent, and scrub it physically to get that varnish to come off, Filippini said.

Rather than cleaning up varnish after it appears, Filippini suggested that fluids be formulated to prevent it from forming. You need to put the fluid components together in a proper way and balance them, Filippini said. Some think you can just add a dispersant to keep the polars suspended, but no, its more complicated than that. Most dispersants tend to emulsify, retaining rust-inducing moisture in the system, so their use in hydraulic fluids must be cautiously controlled. Dispersants also compete with the fluids surface-active corrosion inhibitors.

New additive packages have been developed, he added, and show good results in both Group I and Group II base oils in Parker-Dennisons rigorous T6H20C hybrid pump test. The key areas to watch include thermal stability, oxidative life, hydrolytic stability, wear protection and foaming.

With the right formulation, he concluded, users may be able to stop varnish from forming – and save their elbow grease for something other than scrubbing equipment.

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