Adding the right kind of viscosity index improvers to hydraulic fluids can significantly boost the energy efficiency of pumps in both industrial and earth-moving applications, new research illustrates.
Formulators frequently use viscosity index improvers to increase the service temperature in a range of lubricants, from hydraulic fluids to engine oils. Attendees at the Society of Tribologists and Lubrication Engineers annual meeting in Las Vegas got a fresh perspective on these additives from Frank-Olaf Maehling, oil additives global product manager for hydraulic fluids at Evonik Resource Efficiency GmbH in Darmsdadt, Germany.
A team of Evonik researchers, including Maehlings co-authors Michael Alibert and Thomas Schimmel, recently conducted field trials assessing the effects of hydraulic fluids on performance of heavy-duty off-road construction equipment and stationary industrial equipment. They learned that hydraulic fluids with both high viscosity index and high levels of shear stability can enhance efficiency, Maehling reported at the May meeting. In field trials with excavators, up to 18 percent diesel fuel savings were obtained using shear-stable, very high V.I. fluids. In plastics molding machines and other stationary equipment, this combination of high V.I. and shear stability improved hydraulic system efficiency up to 10 percent.
Market Trends
According to data Maehling cited from BCC Research, the hydraulic market is split approximately 60/40 between mobile machinery and industrial machinery, respectively. The market for hydraulic pumps, which amounted to $7.5 billion in 2013, was dominated by earth-moving equipment (approximately 25 percent), followed by mining, agriculture, material handling, and oil, gas, and shipping applications. Global revenue for hydraulic pumps in the earth-moving industry was almost $2 billion in 2014 and is expected to grow to $2.5 billion by 2019, a compound annual growth rate of 5.2 percent. Similar growth (4.7 percent CAGR) is expected for hydraulic pumps in the plastics industry.
Lubricant manufacturers and original equipment manufacturers are developing higher efficiency hydraulic systems in response to changes in pump technology – higher pressures, smaller volumes and higher power densities – as well as environmental concerns and other trends.
Formulating Balance
Overall hydraulic pump efficiency depends on two mechanisms. Volumetric efficiency is the ratio of fluid flow rate delivered by a pump to the flow rate supplied to the pump. Loss of hydraulic fluid due to leakage decreases volumetric efficiency. Hydromechanical efficiency is the ratio of theoretical to actual input torque, and depends upon frictional losses.
Figure 1 shows that maximizing overall pump efficiency involves compromise. Overall efficiency is the product of volumetric and mechanical efficiencies. Heavier or higher viscosity hydraulic fluids have greater volumetric efficiency (less leakage) but less mechanical efficiency (more friction) than lighter or lower viscosity fluids.
The appropriate viscosity grade for hydraulic fluid depends on pump design. Distance between bearing surfaces under loads and operating conditions help to define a pumps viscosity window. Lubricant blenders can choose from an arsenal of base oils and additives to formulate hydraulic fluids to not only fit inside the viscosity window of an application, but also provide other performance advantages.
Additive Boost
Viscosity index improvers are an important class of lubricant additives. V.I. improvers are polymers, or long molecules, that resemble loose coils of string. When V.I. improvers are formulated into oils, they tend to expand as temperature increases and counter the naturally occurring decrease in base oil viscosity. At low temperatures where base oils become more viscous, V.I. improvers tend to contract and make a smaller contribution to viscosity. Thus, V.I. improvers can be used to formulate hydraulic fluids using relatively low viscosity base oil to provide better volumetric efficiency (less leakage) at high temperatures and better mechanical efficiency (less friction) at low temperatures.
The viscosity index rates the response of a lubricant to temperature. V.I. improvers increase the V.I. of a formulation by reducing its viscosity change in response to temperature variation. As polymers, however, V.I. improvers may break down under mechanical shear. Shear stability refers to the ability of V.I. improver molecules to withstand applied shear instead of stretching and breaking into fragments.
Maehling described field tests with hydraulic fluids formulated with certain V.I. improvers in a Parker vane pump driven by a 15 kilowatt electric motor operating at 1500 rpm. Flow rate, pressure, torque and rotational speed were used to calculate overall efficiency.
Fluids were tested at pump inlet temperatures between 25 and 80 degrees Celsius (77 to 176 F) at a pressure of 250 bars (3626 psi). For three ISO Viscosity Grade 46 hydraulic fluids with viscosity index 150 formulated with V.I. improvers of different shear stability in mineral oils, overall pump efficiency trended with relative shear stability of the V.I. improver; and efficiency increased up to 6 percent. A similar trend up to 10 percent efficiency boost was observed with V.I. 200 formulations.
To further test this correlation of V.I. improver and shear stability with equipment efficiency, researchers from Evonik performed field trials with excavators from several OEMs. They monitored fuel consumption and hydraulic fluid temperature during grading, digging, travelling, idling and other modes of operation.
For example, a hydraulic excavator with two axial hydraulic piston pumps (380 bar/5511 psi) was monitored while digging in a gravel quarry. A number of parameters were varied: two different operators, two engine modes (70 and 100 percent load), two initial oil temperatures (50 and 70 C), and two scoop sizes (0.75 and 1.75 metric tons). Maehling reported that diesel fuel consumption was reduced by up to 18 percent using shear-stable, very high V.I. (160 to 280) hydraulic fluids.
In a variety of other loaders and excavators with gear and piston pumps, the researchers saw the gains repeated: Replacing reference hydraulic fluids with formulations having V.I.s of 170 to 200 improved efficiency by between 12 and 35 percent.
Moving Indoors
Additional field trials were carried out with indoor stationary equipment, using a variety of injection molding machines. Electric power consumption was measured and used to calculate efficiency.
After replacing the reference fluid (V.I. 100) with test fluid (V.I. 280) in a Krauss Maffei KM 80 CX SP 380 plastic molding machine with two Bosch A 10 piston pumps, Maehling reported that efficiency improved by 9.5 percent.
In another example, hydraulic fluid was tested in a Boy 35 E injection molding machine with a Bosch PGH4 internal gear pump and a clamping force of 350 kilonewtons. The new hydraulic fluid formulation (ISO VG 32, V.I. 185) reduced energy consumption between 7 and 10 percent, depending upon temperature.
A total of six new fluids (ISO VG 32, V.I. 185) were compared in an Engel Victory 330/120 molding machine. Drive energy savings, relative to a standard ISO VG 46 reference fluid, ranged from 1 to 6 percent. Energy savings of 4.2 percent were obtained when Evonik substituted its ISO VG 32 (V.I. 184) for an ISO VG 46 low V.I. fluid.
Maehling concluded that new hydraulic fluids formulated with high V.I. and high shear stability significantly increased efficiency in a variety of mobile and stationary equipment, as well as pump test rigs.
While shear stability is clearly important for fluid service life, only time and further investigations will reveal more about its subtle role in efficiency.
Mary Moon, Ph.D., is a physical chemist with hands-on R&D and management experience in the lubricating oil and grease and specialty chemicals industries. She volunteers as treasurer of the Philadelphia section of STLE. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.