Nearly four million tons of hydraulic fluids are sold annually, and with the continuing trend to system downsizing, performance needs are growing ever more demanding. Industry insiders say that smaller equipment means smaller oil sumps, higher flow rates, increased pressure and higher operating temperatures. All of these have led lubricant additive companies to look more closely at fluid formulations.
Downsizings Effects
The most compelling problem faced by equipment manufacturers using high-performance hydraulic systems is that systems are getting smaller and smaller while reservoir residence times have decreased, Lubrizol Product Manager, Francesc Alomar i Belmonte said at Junes Lubmat conference in Bilbao, Spain. Also, reservoir size and shape often are not optimum to maximize fluid life, and oil flow rates are very high compared to oil volumes. This, combined with the fact that oil pressures have increased, results in systems operating at higher power densities and higher oil temperatures.
End users are also calling for improved fluid performance, BASF Technical Service Manager Europe Matthias Hof said in January at the International Colloquium on Tribology in Ostfildern, Germany. Specifically, they are asking for longer drain intervals, higher efficiency, and improved and longer equipment protection. All these demands place higher stress on hydraulic fluids. Filterability also has been a concern, especially in the presence of water and chemical contaminants.
Fluid suppliers are responding by developing oils with improved wear protection that help extend component life. They have also improved hydrolytic stability so that fluids maintain performance when contaminated with water. Better oxidation and thermal stability help boost service life even under high operating temperatures. Finally, improved corrosion inhibition provides better protection of metal surfaces.
These developments have many consequences for todays hydraulic systems, said Alomar, who is based in Madrid. For example, foaming and cavitation increase because the fluid does not spend sufficient time in the reservoir for air to release and foam to collapse. In addition, he added, fluid life is reduced because of increased oxidation, and valve response suffers due to sludge and varnish buildup. There is also an increased need to replace blocked filters, and valve and pump wear have increased.
Base Stock Reform
Hof, located in Ludwigshafen, Germany, pointed out that there is an ongoing trend to use more highly refined base stocks that may require significant reformulation. In the past, Group I oils governed the market, he said, while other base stocks filled only certain specific niche applications.
Today, Group I oils are still the primary base stocks, but their use and availability are shrinking fast. Group II and III oils are emerging, and synthetics are more widely used. The changing base stock landscape demands reformulation mainly because different base stocks have different additive demands, Hof said. And additization strategy must be adjusted to account for differences in base stock stability, lubricity, solvency and potential corrosiveness.
Hof added that improving stability will be a challenge because no new antioxidant technology is currently being introduced. Improving stability requires state-of-the-art antioxidant technology not commonly used in hydraulics. Thus, the trick is to balance treat costs and performance. Oxidation stability is measured with the TOST (turbine oil stability test) Life, TOST Sludge, and Cincinnati Milacron tests. Most market claims are based on TOST Life. And, independent of standards, oil blenders aim for a minimum TOST Life of 5,000 hours or sometimes higher.
Oil stability as measured in the TOST Life test can be optimized by carefully adjusting antioxidant load, said Hof. He showed how slight modifications in antioxidant treat rate can have major effects on TOST Life results with the latest generation of high-performance antioxidants.
TOST Sludge results, however, showed little response to antioxidant level and depend more on the composition of the antioxidant system. Using the antioxidant system mentioned above, adopted from non-hydraulic applications, produces low sludge levels across the entire spectrum of TOST life results. And in the Cincinnati Milacron test, said Hof, oil stability is highly dependent on carefully matching antioxidant level to the base stock.
Corrosion protection is obviously a critical property for hydraulic fluids and is measured in the steel finger, copper coupon, and Cincinnati Milacron tests. Currently, there is very little differentiation among fluids with regard to corrosion protection, said Hof, although some new anticorrosion additive technologies are being introduced. One driver here is the trend toward overall treat rate optimization as well as elimination of the known antagonism with antiwear additives in the hydraulic package. He added that anticorrosion additives must be evaluated carefully to ensure that they do not negatively impact compatibility properties such as demulsibility, air release, and foaming.
Wear protection is evaluated with the VKA, FZG and pump tests. The four-ball rig can be used as a very simple and basic screening tool, and more meaningful test data are required to show further differentiation. Market claims are made using FZG and pump performance. Additional OEM approvals may be required, depending on regional demands.
Again, said Hof, the treat rate of antiwear additives must be carefully matched to the base stock for optimum performance. Overall, a package combining high-performance antioxidants, properly adjusted antiwear additives and corrosion inhibitor chemistry that do not interfere with wear protection can be used at substantially reduced treat rates in the finished hydraulic fluid. Hof added, Such a package will provide a significant performance gain and differentiation in the most critical properties.
Fighting Varnish
The combination of smaller sump sizes and more severe operating conditions has increased the generation of sludge and varnish, said Alomar. We conducted an end user web-based survey that investigated a wide variety of applications to understand their concerns regarding hydraulic fluids, problems they were facing and the potential impact on their business. Survey participants included manufacturers of plastic products and other materials, government agencies, construction companies, waste disposal companies, agricultural and forestry enterprises, operators of marine machinery, oil and gas companies and aerospace operators.
The findings identified key needs for improving productivity, extending equipment life and reducing downtime, said Alomar. We also asked what benefits they would consider most important if a product were available that would significantly reduce lubricant contamination. Their responses included extended equipment and fluid life, reduced oxidation, less downtime and valve replacement, improved efficiency and fewer leaks.
Valve replacement proved to be an important issue, Alomar noted, because slow valve response means reduced production. Most operators react only when slow actuation is obvious, added Alomar. In the worst case, the valve does not operate at all, resulting in unscheduled downtime. So valve failure is expensive. It can also be a safety issue.
Participants were then asked what operational problems caused them to evaluate the performance of hydraulic valves in their equipment. Their top responses were delayed system response, loss of pressure, decreased product output, increased cycle times and reduced flow.
Alomar then described a Lubrizol study on the effects of sludge and varnish on the operation of common hydraulic systems used in plastic injection molding machines, gas turbine generators and off-highway equipment.
A typical plastic injection molding machine contains 4 valves. A shop comprising 50 machines may have to replace 10 percent, or 20 valves, annually at a cost 1,600 per valve or 32,000 per year. If it takes 1 hour to replace each valve at labor cost of 40 per hour, 800 per year would be spent on labor. This results in lost production of 50 hours.
If it takes 115 seconds to produce a part costing 2.5, lost production amounts to 3,912 per year. This all adds up to a total cost of 36,712 per year that could be saved if the machines are protected from sludge and varnish.
Similar studies of gas turbine power generation equipment showed potential cost savings of up to 100,000 per year. For an off-highway excavator fleet, estimated savings were 53,000 per year.
Cleaning Up
Alomar concluded by describing two additive technologies: one that prevents the deposition of varnish on critical components and one that can remove varnish once it is deposited. These technologies significantly reduce equipment downtime compared to manual cleaning, said Alomar. They help eliminate valve sticking, thereby increasing productivity, reducing downtime, and saving money by cleaning valves rather than replacing them.
Using clean hydraulic technology in a 50-machine shop results in a potential savings of 160,419 annually due to reduced labor costs and increased production, Alomar concluded. He added that the chemistry also eliminates health and safety concerns by eliminating the need to use solvent-based cleaners.