Excavators, cranes and bulldozers lumber slowly at construction sites and surface mining operations. Inside earthmoving equipment, pumps circulate hydraulic fluid through pressurized networks of hoses, valves and cylinders to deliver power used to dig earth, grab objects, lift and lower loads, and turn, move and stop these massive machines. All of this work takes large amounts of energy, and the construction and mining industries are not exempt from the global push toward greater efficiency.
Hydraulic fluid accounted for roughly half of the 6.4 million metric tons of global demand for general industrial oils and greases in 2017, according to Parsippany, New Jersey-based Kline & Co. Such fluids could have a significant impact on industrial energy demands if they are formulated for increased efficiency.
Joe Purnhagen, regional marketing manager for Lubrizol Corp. in Wickliffe, Ohio, and Tim Smith, strategic technology manager at Lubrizol in Hazelwood, United Kingdom, confirmed that energy efficiency is a major component of sustainability priorities of heavy equipment OEMs and end users. Given this, the additive maker has focused on developing hydraulic fluids with improved energy efficiency.
Efficiency is particularly important for the next generation of mobile equipment that is currently in the development stage. OEMs are focusing on battery-powered machines and computerized electronic controls, which both place a premium value on extended equipment range, they told LubesnGreases.
Another OEM trend is more compact equipment that places greater demands on the performance of hydraulic oils. Fluids with improved energy efficiency can even influence the design process for future generations of hydraulic equipment.
Lubrizol compared conventional and experimental hydraulic fluids in field trials using mobile earthmoving equipment. Unexpected improvements in energy efficiency were observed from hydraulic fluid formulated with a new polymer. Standard performance tests only led to more questions about how the polymer was affecting the efficiency of the experimental fluids. A custom-built laboratory test rig finally coaxed some answers and new insights about fluid flow inside the hidden networks of hydraulic systems.
Smith provided details about these field trials and laboratory studies to a standing-room only crowd at the Society of Tribologists and Lubrication Engineers annual meeting in Nashville, Tennessee, in May.
Researchers formulated a base fluid from API Group II base oil, a commercial anti-wear additive package and a pour-point depressant. Additional additives were used to thicken the base fluid and formulate hydraulic fluids with ISO viscosity grade 46. All formulations met industry requirements for original equipment manufacturer hydraulic fluid specifications Parker Denison HF-0 and Eaton Brochure E-FDGN-TB002-E.
Reference hydraulic fluid was prepared by blending API Group II 600 Neutral oil with the base fluid. This reference fluid was monograde, meaning it was suitable for use in a relatively narrow range of temperatures.
Multigrade lubricants can be used over wider temperature ranges than monogrades because they are formulated with polymers that expand to partially offset the naturally occurring decrease in oil viscosity as temperature rises. Viscosity index is a measure of viscosity change with temperature. Viscosity modifiers or viscosity index improvers are typically polymeric additives that increase V.I.
One experimental fluid was formulated with a high-performance liquid hydrocarbon polymer with brand name Lucant. This viscosity modifier is specifically designed for hydraulic equipment and is distinct from similar types of polymer viscosity modifiers such as less shear-stable olefin copolymers used in engine oils, Smith said.
The second experimental fluid contained a shear-stable polyalkylmethacrylate designed for use in hydraulic fluids. This polymer typifies the viscosity modifier technology used most commonly in todays conventional multigrade hydraulic fluids.
Smith noted that there was some difference between viscosities for the three fluids at different temperatures, which was a consequence of their different viscosity index values. In this study, the PMA viscosity modifier produced a somewhat higher V.I. than Lucant, which might be beneficial in some applications.
In the Field and on the Bench
The three hydraulic fluids were compared in a series of field trials with a variety of excavators and other heavy-duty machines.
Smith explained that each piece of mobile equipment was extensively instrumented to collect data on all aspects of the machines performance and any external factors that could be influential. Researchers measured the fuel consumed by the engine and the work done by the hydraulics, then calculated brake-specific fuel consumption. Repeatable duty cycles were developed for each machine to replicate real-world conditions.
For example, a backhoe loader was used to lift a concrete weight in a set pattern covering a large range of movement. This was repeated hundreds of times, generating millions of data points and giving us great confidence in the results, Smith said. This allowed detailed comparison of the relative energy efficiency of our three hydraulic fluids.
The team analyzed energy efficiency for the experimental hydraulic fluids relative to the reference fluid. The fluid with Lucant improved energy efficiency over the reference fluid for four duty cycles with the backhoe loader, while the fluid with PMA showed no benefit over the reference fluid, Smith stated. The magnitude of these effects depended on the individual machine, operator and duty cycle.
But we were mystified by the improvement of energy efficiency from hydraulic fluids formulated with Lucant versus PMA because PMA was representative of a large, general commercial family of VMs used widely in energy efficient lubricants, he continued. So we did some laboratory studies to investigate.
Smith and his team developed a lab-scale test rig to replicate a hydrostatic drive system typically used in mobile hydraulic equipment. A high-pressure piston pump circulated test fluid through a loop of pressurized lines containing a heat exchanger for temperature control, flow meters, valves and a motor. They measured the power input to drive the pump and the power output to the motor. Then they calculated efficiency across the total system as the ratio of power output to power input.
The experimental fluid formulated with the Lucant viscosity modifier consistently improved energy efficiency between 2 percent and 5 percent relative to the reference fluid over temperatures ranging from 30 degrees Celsius to 90 C. The PMA polymer had little effect on efficiency in this specific test.
While these results were consistent with field trials, they shed little light on the behavior of the Lucant polymer.
Next, the Lubrizol team used a Mini-Traction Machine to measure traction in a lubricated ball-on-disc contact. The experimental fluid with Lucant had a lower traction coefficient than the experimental fluid with PMA, which was itself lower than the reference fluid at typical hydraulic operating temperatures.
MTM data did not fully explain the results from the field trials and the lab-scale test rig. Low traction or internal friction among molecules in a thin fluid film is a known contributor to lubricant efficiency. However, hydraulic fluids predominantly undergo bulk fluid flow in field applications.
Fluid Flow Visualization
According to Smith, engineers and tribologists have a good understanding of how to measure and control lubricant rheology (fluid flow behavior) in confined spaces such as tiny gaps between meshing gear teeth. Less is known about controlling the bulk flow of hydraulic fluids. Multigrade fluids present a special challenge because they contain polymers and are non-Newtonian.
Oil, water and other Newtonian fluids are relatively simple to analyze and understand. These liquids have constant viscosity and a linear flow curve or plot of shear stress versus shear rate.
Honey, ketchup and mixtures of oil and polymer are examples of non-Newtonian fluids. Viscosity depends on the speed at which fluid flows, and flow curves can be non-linear or not pass through the origin of the graph. The viscosity usually drops at the speed that corresponds to the rate at which flexible viscosity modifier molecule chains can stretch and relax.
This difference is important to engineers because it is much more difficult to design equipment and model flows of non-Newtonian versus Newtonian fluids. It is also more challenging to predict whether non-Newtonian fluid will flow smoothly and efficiently (laminar flow) or waste energy in turbulence and swirling (vortical flows).
Smith and his team used clear acrylic plastic to make models of a 180-degree bend in a hydraulic line and inserted it into a custom-built test rig. They added microscopic polyamide (plastic) beads to each test fluid and photographed the beads as fluid flowed through the acrylic model, in a technique known as particle image velocimetry.
Photographs were taken at 30,000 frames per second, and computer software was used to calculate the speed and direction of fluid flow from changes in the positions of beads. The software also produced detailed images of fluid flow.
In the 180-degree bend, fluid travelled both along the pipe (primary flow) and across the pipe (secondary flow). Secondary flows mixed fast-moving fluid in the center channel with slow-moving fluid near the pipe walls. Mixing caused loss of fluid momentum, wasted energy and pressure drop. Bends in lines, valves and filters all cause similar mixing and energy losses in hydraulic equipment in the field.
Smith observed less mixing due to secondary flows in the experimental fluid formulated with Lucant than in the reference fluid or the experimental fluid formulated with PMA.
From these studies, it appears that Lucant viscosity modifier has the ability to decrease or prevent mixing due to secondary flows in hydraulic fluids. Flexibility influences the ability of the polymer chains to suppress these unwanted flows. When the polymer chains stretch and relax near the inner wall of a curved pipe, they produce elastic forces acting against fluid inertia, the primary cause for secondary flows in hydraulic systems.
Formulators already appreciate the importance of hydraulic fluid viscosity and V.I. However, Smith discovered a new opportunity for increasing hydraulic efficiency by formulating to optimize the flow characteristics of the fluid. While efficiency gains are dependent on specific equipment, operating conditions and duty cycle, Lubrizol anticipates up to 6 percent improvement when using a hydraulic fluid formulated with Lucant viscosity modifier.
This discovery is a major change in how the lubricant industry thinks about energy efficient hydraulic fluids. According to conventional wisdom, energy efficiency comes from all viscosity modifiers used to improve V.I. in multigrade lubes. Lubrizol believes that more mechanisms leading to efficiency may be found in addition to the behavior described here. These findings may also be relevant to formulation of engine oils and design of engines as well as hydraulics. At this time, there are no plans to update specifications or write new standards for hydraulic fluids, although there may be new developments in the future.
Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at firstname.lastname@example.org or (+1) 267-567-7234.