Hydraulic systems need the correct viscosity fluid to balance opposing efficiency loss mechanisms. A team of researchers looked into how this plays out in metal-cutting machine tools for the first time. Nick Augusteijn takes a look at their findings.
The viscosity of lubricants has a crucial part to play in the energy efficiency of machinery. Engineers can factor in the effects of viscosity based on a body of literature and test results. But no such data are available for a very specific niche – the effects of hydraulic oil viscosity on the energy efficient for metal-cutting machine tools. Until now.
A team from academia and industry in Germany carried out the first influence analysis of hydraulic fluid viscosity on the energy demand of hydraulic systems of metal-cutting machine tools. These hydraulic units typically perform workpiece clamping, locking or conveying functions.
Neither estimates for the energy-saving potential of an oil change nor suggestions for the selection of energy-efficient oils exist for machine tools, the research team found.
The research team comprised Faried Makansi and Katharina Schmitz from RWTH Aachen’s Institute for Fluid Power Drives and Systems; Christoph Wincierz and Michael Alibert from specialty chemicals company Evonik; and Johannes Sossenheimer, Lars Petruschke and Eberhard Abele from the Institute of Production Management, Technology and Machine Tools at the Technical University of Darmstadt.
Studies to date have focused on the efficiency of components and adjustment of operational strategies to reduce energy consumption. These approaches have been found to yield a significant reduction of more than 80 percent in some cases when compared with conventional components and typical operational behaviors.
“A combination of efficient components, operation strategies and optimized fluid selection could ideally complement each other,” Makansi told Lubes’n’Greases.
Meanwhile, the significant effect of the hydraulic fluid on the efficiency and productivity of mobile excavators is well known. In recent years, a similar range of efficiency gains has been reported for injection molding equipment.
“Our studies with excavators and injection molding equipment have shown that increasing the viscosity index above 180 … leads to efficiency improvements in the two digit range,” Wincierz told Lubes’n’Greases.
Not Fully Apparent
In general terms, the selection of the various hydraulic fluids boils down to the type of base oil used, the fluid’s viscosity, the viscosity index improvers in the formulation and their shear stability – a fluid’s resistance to viscosity changes caused by mechanical stress – and the specified level of performance for the application.
“Fluid selection can influence wear and corrosion in hydraulic components, as well as the dynamic behavior and stationary equilibrium of a hydraulic system. Therefore, the hydraulic fluid can impact the longevity and maintenance requirements of hydraulic components and the performance and energy consumption of a system,” said Wincierz.
It has been shown many times that hydraulic fluids can make a difference in improving energy efficiency and productivity in excavators and injection molding equipment, which raises the question of why this study was carried out. As Wincierz pointed out, “the relation between oil viscosity and energy efficiency usually is not fully apparent, as different loss mechanisms are contrarily dependent from the viscosity.”
Wincierz referred to the example of complex hydraulic systems with variable operating conditions and a multitude of components. Pumps, valves, motors and piping can each demand a different fluid viscosity for maximal efficiency at a given operating point, and determining the energetically optimal viscosity analytically can be laborious.
“One way to bypass this issue is to select fluid properties based on experience and data on fluids that are typically advantageous for a specific group of systems with similar structure and functions of the hydraulics,” said Wincierz.
Energy efficiency is defined as ratio of energy output to energy input. For hydraulic drives and actuators, this overall efficiency is typically split into two factors: volumetric efficiency and hydro-mechanical efficiency. Taking, for example, a pump with known displacement volume and rotational speed, it is possible to determine the theoretically obtainable fluid flow rate. In practice, actual fluid flow is less than this theoretical flow. It is the ratio of actual to theoretical flow that gives volumetric efficiency. The difference between these two values is known as the leakage flow.
Continuing with the pump example, the theoretical input torque required to create a specific pressure at the output can be calculated from the displacement volume and the pressure. In practice, the actual required torque is larger because of mechanical and flow-related frictional losses. The ratio of theoretical to actual required torque is known as hydro-mechanical efficiency.
These two efficiency measures are dependent on fluid viscosity. However, at low viscosity, the fluid has a low flow resistance and therefore leads to a high hydro-mechanical efficiency. Concurrently, fluid flow through gaps is eased, which causes a high leakage and a low volumetric efficiency. Conversely, increasing viscosity leads to a decreased hydro-mechanical efficiency and increased volumetric efficiency. Hence, for optimal overall efficiency, fluid viscosity should be selected so as to balance these two loss mechanisms.
Finding a Balance
Because no data was available on energetically advantageous fluid selection for hydraulics in metal-cutting machine tools and on the energy saving potentials for these machines due to an optimized fluid selection, team members from Evonik and the Technical University of Darmstadt set up the study’s test conditions.
For their reference fluid, the researchers used a common mineral-oil based monograde HM46 oil with kinematic viscosity of 46 centistoke at 40 degrees Celsius. HM-designated oils have corrosion protection, aging resistance and wear reduction additives and are designed to meet the requirements of standard hydraulic components.
The reference fluid was compared with high V.I. oils HV32, HV46 and HV68. HV designated oils also have increased aging resistance and improved temperature-viscosity performance. All three were formulated with Evonik’s proprietary technology to lower the susceptibility of the oil viscosity to changes in temperature. Additionally, all three had a nominal V.I. of 185 whereas the reference oil had a V.I. of 100. The experiment was conducted using an Emag turning center built in 2014, an older Traub CNC lathe and a pump test bench.
In the pump bench test, there was only a very small difference between the fluids tested, while in the metal-cutting equipment there were huge differences.
The study found that energy savings can be made by switching from regular HM46 oil to HV68 oil, indicating that the leakage mechanism is dominant in the tested equipment. Energy demand from the Emag turning center’s hydraulic system was reduced by 8 percent per workpiece as a result of the change. This translated into an estimated energy saving of 194 kilowatt hours per year, based on the machine operating three shifts per day for 240 days per year. The energy demand of the Traub CNC lathe dropped by 27 percent, leading to estimated attainable savings of 380 kWh per year.
A typical machine tool uses about 20 percent of its energy on the hydraulic system, with the rest taken by cooling and auxiliary systems, among other things.
The fluid effect on efficiency ranged between 8 and 27 percent in this study, depending on the machine. This translates as a total energy consumption reduction of the equipment in the range of 1 to 5 percent, Wincierz explained.
“Sustainability is a strong driver for improving the design of machines and for retro-fitting existing equipment in factories. A fluid change is always an option, while changing components might be difficult,” Wincierz said.
The upshot of the study was a recommendation that companies running machine tools or other hydraulic units that largely perform static functions should consider higher V.I. oils. The upfront cost maybe higher, but there are savings to be made on the energy bills and environmental Brownie points to be scored.