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Hydraulic technology has been developing for over 200 years now. Although relatively mature, it is still evolving.
Hydraulic professionals must always remember: it is social demand that gave birth to machinery capable of meeting those demands, and it is these machines that adopted hydraulic transmission to meet social needs—to drive loads and overcome load forces in motion. Therefore, the unwavering goal of improving and developing hydraulic technology should always be to meet social demands.
The following aspects are basically timeless and never-fully-satisfied social demands for hydraulics:
Energy efficiency.
Long service life (pressure resistance, impact resistance).
High reliability.
Miniaturization, weight reduction, compactness, integration.
Green environmental protection and safety.
Ease of use.
The current development of hydraulic technology uses various means to meet these demands.
Energy conservation in hydraulic systems has long received attention for the following reasons: 1) Extending oil service life. For hydraulic systems, wasted energy ultimately converts to heat, causing oil heating (1MPa pressure loss increases oil temperature by 0.57℃). This reduces oil viscosity on one hand, increasing leakage, and on the other hand causes molecular chain breakage in oil molecules, changes in additive chemical composition, reduced wear resistance, and accelerated aging. According to research, above 80℃, for every 10℃ increase in oil temperature, oil service life is reduced by half. 2) Extending equipment service life. Reduced oil viscosity decreases lubricating film thickness, increasing mechanical wear. Polymer materials widely used as seals in hydraulic systems age easily at high temperatures; therefore, overheated oil reduces seal component service life. 3) Cooling. If fans and radiators are used to reduce oil temperature, this brings additional energy consumption. 4) Reducing operating costs. According to statistics, the operating costs of mechanical equipment, especially power costs (electricity and fuel), currently approach or exceed equipment purchase costs. For example, in the United States in 2010, mobile machinery energy consumption costs were $56 billion, fixed machinery energy consumption costs were $42 billion, while the entire hydraulic components market was only $26 billion. Therefore, from a comprehensive cost perspective, energy conservation must also be considered.
Since entering the 21st century, energy conservation has occupied almost the most important position in social demands. 5) Fossil fuels are finite. As fossil fuels on Earth are consumed by humanity, extraction difficulty will increasingly rise, leading to continuously rising prices. Therefore, even if they won’t be completely exhausted for a while, the energy era marked by coal and oil will eventually pass. The distribution of fossil fuels on Earth is also uneven: even if there are still some on Earth, it doesn’t mean every country and region has their own, especially when the political situation is unstable, this situation is more serious. 6) Protecting public health. Using fossil fuels emits pollutants harmful to human health: nitrogen oxides (NOₓ), hydrocarbons (HC), particulate matter (PM2.5), etc. 7) Avoiding legal restrictions. Many countries now have mandatory regulations on energy conservation and emission reduction. Compared with other regions of the world, energy conservation and emission reduction regulations in Europe, America, and Japan are much stricter. According to these regulations, emission limits starting from 2014 are only 10% of those before 2012. Moreover, the higher the installed engine power, the stricter the emission limits. Therefore, existing host equipment must undergo major improvements to be allowed for sale in these countries. This requires host manufacturers to demand that hydraulic systems maximize energy efficiency and reduce energy consumption as much as possible, thereby reducing installed power. 8) Responding to climate change and protecting human living environment. Currently, climate change caused by the greenhouse effect from CO₂ emissions from burning fossil fuels has become a severe challenge facing all humanity. Countries around the world are formulating and implementing a series of strategies, measures, and actions to address climate change, proposing more aggressive carbon emission reduction targets. In March 2022, China released the “14th Five-Year Plan for Modern Energy System,” proposing to comprehensively promote large-scale development and high-quality development of wind and solar power generation. By 2025, the proportion of non-fossil energy consumption should increase to about 20%, and the proportion of non-fossil energy power generation should reach about 39%. Although natural water, wind, and solar energy themselves are free, their supply is not stable, and obtaining the stable and convenient form humans need—electricity—also requires massive investment and research and development. Therefore, to protect the human living environment and public health, energy conservation is now something every citizen should consider.
For example, it is estimated that a 300kW excavator consumes approximately 2000 tons of diesel and emits 6000 tons of CO₂ during its entire life cycle. If consumption can be reduced by 5%, it can save 100 tons of diesel and reduce emissions by 300 tons of CO₂. Similar equipment in China alone has annual production exceeding 100,000 units, with equipment in use numbering in the millions. Overall, the reduction in emissions that can be achieved is extremely considerable.
Therefore, as hydraulic system designers, when improving existing systems and designing new systems, carefully considering energy conservation first is an unshirkable responsibility.
For example, at the 2022 Hannover Messe, Bosch Rexroth used a considerable amount of space to exhibit their energy consumption management software, which can display and analyze energy consumption status, efficiency, heat generation, etc., refined from regions to factories, then to workshops, and even to individual equipment.
Electronic control has been used in stationary hydraulics for nearly a century. For mobile hydraulics, Europe and America also started from the 1990s (see Section 2.1 for details). The current research focus has shifted to how to fully utilize computer intelligence, combined with hydraulic characteristics, to achieve optimal comprehensive benefits.
As is well known, Industrial Revolution 1.0 was mechanization, greatly expanding human power beyond human and animal power. Industrial Revolution 2.0 was assembly line production, which simplified production processes through decomposition, thereby improving production efficiency and reducing production costs (however, products were uniform without individuality). Industrial Revolution 3.0 was automation, which greatly reduced human participation on assembly lines, improved product performance consistency, and laid the foundation for diversified production. Industrial Revolution 4.0 is personalization, using computer and network intelligence to flexibly, efficiently, and cost-effectively produce diverse products to meet individual needs. German academia and industry generally believe that German hydraulic technology has basically met the requirements of Industrial Revolution 4.0, so the research focus is on “Prediction 4.0” to achieve predictive maintenance. All of this is based on various types of sensors, software, algorithms, and strategies related to electronic control. In January 2024, led by the German Mechanical Engineering Industry Association, the “Fluid 4.0” working group was established with funding from the EU and the German Ministry of Economics to promote digitalization of fluid components and system control.
Technology is a combination of technologies. Hydraulics is making progress by leveraging new design methods, processes, and materials invented in other technological fields. For example: 1) Carbon fiber winding, increasing pressure resistance of hydraulic cylinders and accumulators, reducing weight. 2) Additive manufacturing (3D printing) technology can make hydraulic component production flexible, fast, and lightweight.
Electro-Hydraulic Actuator (EHA): motor-pump-tank-hydraulic cylinder integration, comprehensively utilizing the latest technologies in electronic control, volumetric speed control circuits, integrated blocks, motors, variable frequency speed control, etc., highly integrated. Just connect to power, input position or speed commands, and it can work, so it is also called an autonomous cylinder. This specifically refers to differential cylinders, because differential cylinders account for more than 85% of the number of hydraulic actuators in actual applications.
The working principle of electro-hydraulic actuators was proposed decades ago, and volumetric speed control circuits for differential cylinders were also invented long ago. Starting at the end of the 20th century, especially with breakthroughs in a series of technical difficulties such as high power density servo motors, they were actually applied on aircraft. For example, oil pipes are a weakness of military aircraft. In the F35 fighter, the centralized oil supply hydraulic system was abandoned in favor of electro-hydraulic actuators with distributed oil supply.
Now, because electro-hydraulic actuators are integrated and energy-efficient, they are also beginning to be used in other fields. At the Hannover Messe, as early as 2011, the German company VOITH exhibited civilian models. Since 2013, multiple hydraulic companies have exhibited their own variants. Since then, Bosch Rexroth has exhibited new products at every Hannover Messe Drive & Automation exhibition.
The electro-hydraulic actuators exhibited in 2017 could achieve thrust of 2000~2500kN, while electric motor actuators of similar size could only achieve thrust of 290kN.
At the 2022 and 2023 Hannover Messe, Bosch Rexroth exhibited multiple series of electro-hydraulic actuators available for order. They announced that such products have now entered a profitable period.
Due to high integration, eliminating on-site installation and pipe connection work, electro-hydraulic actuators can achieve plug-and-play similar to ordinary electric motors, greatly simplifying user workload. Due to solid and reliable components that can operate fault-free for long periods, users don’t need to worry about maintenance, thus realizing Bosch Rexroth’s motto “Let Hydraulics disappear.” The “disappear” here should not be understood as “vanish,” but rather as “go invisible, move behind the scenes.” This is truly the killer weapon that will reduce employment in the hydraulic industry and compress the hydraulic job market—”hydraulics killed hydraulics”!
Hydraulic technology has been developing for over 200 years and has had many, many R&D personnel conducting in-depth exploration in all aspects; it is quite “mature.” Major, structural, fundamental functional innovations have become relatively rare and more difficult. On one hand, new product performance needs to be superior to products already on the market that have been produced, used, and repeatedly improved over long periods, while on the other hand, prices must be acceptable to the market (those performance advantages must be prominent enough to show that the application is worth that higher price). This requires long-term refinement and improvement, maintaining an R&D team from conception to design, sample production, and testing, undisturbed by daily production, requiring large investments—truly not easy.
For example, Hydac launched an electroproportional with emergency manual multi-way valve LX-6 with constant pressure differential components in 2018.
From the test curves, it can be seen that its constant flow characteristics are quite good: flow is minimally affected by pressure differential changes, with almost no visible hysteresis. Even more remarkable is that when using valve spools with maximum controllable flow up to 170L/min, at flows of 1L/min it still has quite flat pressure differential-flow characteristics.
But this valve was developed by more than 10 R&D personnel over 6 years, with labor costs alone being considerable.
Bucher launched the AX-type axial piston fixed displacement pump motor in 2018: due to the adoption of double-row pistons, axial forces are basically balanced, so instantaneous pressures up to 500bar can be withstood, hydraulic mechanical efficiency reaches up to 99%, total efficiency up to 95%, low flow pulsation, minimum stable speed of 1r/min.
In fact, Dutch engineer Peter Achten Innas proposed its principle starting in 2003 under the name “floating cup pump,” and samples were already exhibited in 2009. But it still requires years of testing and improvement before becoming a commercial product entering the market.
In Germany, universities and research institutions (including pre-R&D departments of large companies) focus on exploring various possibilities! For example: currently all spool valves on the market have cylindrical valve spools, while at a certain German research institute, when developing ceramic spool valves that can be used with clean water, to avoid the inconvenience of high-precision machining of ceramic holes, they are exploring the possibility of using rectangular spool valve spools.
For the vast majority of German enterprises, in the market economy environment, meeting market demand is the development direction. To put it bluntly, it is to use various new technologies, new materials, and new processes to make products more economical than competitors’, better meeting customers’ personalized needs and social needs, thereby being able to legally and reasonably obtain profits—this is their main focus and pursuit goal. They care about whether they will infringe on others’ patents and intellectual property rights, and whether anyone has infringed on their patents and intellectual property rights. Of course, many enterprises also pay attention to environmental protection and energy conservation, assuming certain social responsibilities. They don’t pursue advancement for advancement’s sake, nor do they care much about whether they rank among the world’s top.
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