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Electric drive refers to an electric motor that directly or only through mechanical transmission drives the load. This is the real competitor that could potentially take away the “rice bowl” of hydraulic drive.
The energy required for mobile hydraulics comes from either an internal combustion engine or a power source. When using an internal combustion engine, if hydraulic drive is adopted, mechanical energy needs to be converted into hydraulic energy through a hydraulic pump. If electric drive is used, the mechanical energy provided by the internal combustion engine must first be converted into electrical energy via a generator before the electric driver can work. Therefore, from the perspective of energy conversion, both require one conversion step – a tie!
However, if the mobile equipment uses batteries as the energy source, electrical energy is readily available. Thus, electric drive is more direct than hydraulic drive! At the same time, there is no longer a need for auxiliary equipment essential to hydraulics, such as oil tanks and hydraulic pumps. Designers will naturally prioritize electric drive. If hydraulic drive is still adopted, it would be superfluous – only because the electric driver cannot meet the requirements.
All electric drives generate electromagnetic force through electromagnetic fields to drive the load in rotation (electric motor) or linear motion (linear electric motor). Electromagnetic force is proportional to the number of coil turns and the current intensity, and is also limited by the magnetic saturation strength of the magnetic material. Therefore, the driving force per unit mass of electromagnetic drive – force density – has long been only about 1/10 of that of hydraulic actuators. But that was the past; now the situation is changing.
In the late 1970s, Japanese scientists discovered that the rare earth element neodymium can increase the magnetic saturation strength of magnetic materials by 4 times, meaning the driving torque of electric motors can be increased by 4 times. Due to various difficulties, it took several decades for neodymium-containing magnetic materials to enter industrial applications. Now, NdFeB magnets are mass-produced industrially and can even be purchased online. In 2023, Tesla announced that it would adopt a new magnetic material with magnetic force twice that of NdFeB magnets.
Research has also found that if magnet blocks are placed perpendicular to each other rather than along the magnetic poles, the magnetism on one side weakens while the other side is significantly enhanced.
Tesla early on used the technology of perpendicular placement of magnet blocks to manufacture motors. Later research found that embedding specially shaped magnets in the motor rotor can further enhance driving force.
At the 2019 bauma Munich construction machinery exhibition, Parker Hannifin specially set up a “Future Laboratory” to introduce electric and electric drive technologies. Among the exhibits was a 16-pole AC servo motor called GVM, using dual three-phase coils: excitation + control, with a rated speed of 3060 r/min and a maximum speed of up to 5000 r/min. Its volume is roughly equivalent to an ordinary 4-pole 11 kW AC asynchronous motor, but the rated power reaches 186 kW and the rated torque reaches 581 N·m. In 2023, Parker announced that customers can now purchase GVM motors in a modular way: three diameter options, length increasable in 25 mm increments, voltages in six levels: 24V, 48V, 96V, 350V, 650V, and 800V, with power ranging from 2 to 250 kW. In addition, different wire gauges and turn numbers are available for the stator structure. This allows customers to optimize the generated torque based on the power curve of effective voltage and current to meet various application needs.
At the 2022 Hannover Messe, Bosch Rexroth exhibited an EMS motor: with a shaft height of only 13 cm, but output power up to 123 kW. Its power density is now comparable to Bosch Rexroth’s own long-developed and sold high-pressure heavy-duty hydraulic motor A4FM. Now, the entire series with shaft height of 20 cm and maximum power up to 553 kW is accepting orders. These facts negate one of the important reasons why hydraulic technology once relied on to stand in the market – power density!
Load motion is nothing more than linear motion (limited stroke) or rotation (unlimited stroke). In the past, Chinese and foreign hydraulic textbooks generally stated that the key advantage of hydraulic transmission over other transmission methods (electric drive) is high power density, i.e., high power-to-weight ratio. In fact, hydraulic transmission is stronger than electric drive only in force density. In the past, motors were limited by rotor dynamic balance and bearings at high speeds and could only increase torque by increasing rotor radius to meet power requirements, which led to relatively low power density. Actually, for rotation, power = torque × speed = force × radius × speed. When force is insufficient, power requirements can still be met by increasing speed. Therefore, the statement that hydraulic power density is high is theoretically not rigorous.
In 2014, the author was informed that high-speed motors in Germany, with speeds of 22,500 r/min, had achieved power densities of 2–3 kW/kg, and prototypes under development had reached levels equivalent to high-pressure motors, 5–6 kW/kg. Combined with the recent advances in motor technology mentioned above, some German hydraulic textbooks have now been rewritten!
The reducer GFT8000 used with walking machinery hydraulic motors can also be regarded as one of Bosch Rexroth’s classic flagship products. At the 2019 bauma Munich exhibition, Bosch Rexroth launched the eGFT8000 electric drive reducer for walking machinery with a rated power of 60 kW, and Eaton also exhibited an electric drive axle for loaders. In these components, hydraulic motors have been replaced by electric motors!
Therefore, in terms of rotation, electric drive has begun to encroach on the hydraulic application market.
Of course, power density and force density are not the only reasons why hydraulic technology relies on to stand in the market. Because actual applications are diverse, and the requirements for rotation in various applications differ. The high power density of electric drive has so far only been achieved at high speeds. In the vast majority of application scenarios, such high speeds are not required. Therefore, the high speed of electric motors must basically be reduced through mechanical transmission mechanisms such as gear pairs, planetary gears, worm gears, etc. As a result, in these scenarios, the competition between hydraulic drive and electric drive also involves the comparison of hydraulic transmission and mechanical transmission performance, such as speed ratio – maximum to minimum speed ratio, stepless speed regulation capability, etc.
A deeper comparison is that of the characteristics of liquids versus solids. Mechanical transmission is all carried out using solids (metals). Compared with liquids, it has high rigidity, but this also means it is not easily deformable, so the force cannot be dispersed. No matter how finely processed, due to the working principle, the force-bearing area is always very small, which leads to high stress, limiting the bearable load, especially impact loads, and also limiting service life. Liquids have no fixed shape, so force is dispersed, and elasticity is high, thus having stronger impact resistance.
In this application, driving a manipulator mainly needs to overcome gravity and inertial force, with minimal friction and little impact. But in other applications, it may be different. For example, when a tank climbs out of mud, it needs to overcome large friction at extremely low speeds; when attacking (escaping) at high speed on rugged slopes, it faces large impact forces due to inertia. And for a conveyor belt thousands of meters long carrying materials (soil), starting requires overcoming enormous inertia at zero speed, i.e., large starting torque, etc.
In addition, there are comparisons in terms of working environment adaptability, purchase cost, operating cost, maintenance capability, etc. Therefore, even replacement will be gradual and should not simply infer “autumn has come” from “one leaf falling”!
Moreover, in terms of quantity, motors only account for one-tenth of hydraulic actuators. Even if the entire rotary market is taken by electric drive, hydraulics can still survive on linear motion.
To aid understanding, it can also be analogized to the transportation industry. Drive forms include hydraulic, mechanical, pneumatic, and electric drive. Transportation tools include airplanes, ships, trains, and trucks. Various transportation tools also compete in certain applications, with one rising as another falls, but none will be completely replaced.
The competition between pneumatic and electric drive has also lasted for decades. Because each has its strengths, sometimes electric drive gains the upper hand, sometimes pneumatic is more popular. At the 2019 Hannover Messe, some companies simply exhibited pneumatic and electric drive grippers of similar size and identical interfaces, allowing customers to choose according to their applications. In rotary applications, the competition between electric drive and hydraulic drive should be similar.
Although linear electric motors can also directly produce linear motion, due to very low force density, they are not suitable for high-load force occasions. Current electric drive linear motion mostly still uses electric motors to generate rotary motion, then converts it through ball screw-nut mechanisms, with thrust limited by the stress at the contact point between screw and nut. Liquids have no shape restriction, and hydraulic cylinder pistons bear force uniformly over the entire piston area, so they can withstand much greater load force and impact resistance than mechanical transmission. This puts hydraulics in a favorable competitive position in linear motion.
It is reported that even specially refined electro-mechanical actuators, also known as electric cylinders, in terms of volume, are only equivalent to hydraulic cylinders with a working pressure of 4 MPa. For large forging presses with pressing forces of over ten thousand kN, hydraulic cylinder working pressures reach up to 70 MPa. If electric drive were used, volume and weight would be dozens of times larger, so it is simply impossible to adopt.
Boston Dynamics’ robot dog uses electric drive, while the robot Atlas uses hydraulic drive: carrying batteries and a miniature hydraulic power station on its back, driven by multiple servo hydraulic cylinders for limbs, capable of jumping and flipping, extremely dynamic. On this, the company’s engineering vice president Mr. Aaron Saunders explained on the afternoon of March 20, 2018, at the International Fluid Symposium at RWTH Aachen University in Germany: Simply put, with electric drive it can only jump 2 ft, but with hydraulic drive it can jump 6 ft.
In contrast, electric robots are more like graceful ladies: they can carry 5 kg of goods and deliver express, but cannot jump and flip like the hydraulic Atlas. To date, all electric drive robots cannot jump like the hydraulic Atlas.
In April 2024, Boston Dynamics announced that the hydraulic Atlas would retire and be replaced by an electric drive Atlas. I think this is because there is currently no market demand for robots that can jump and flip, and it should not be broadly concluded that hydraulic drive will be replaced by electric drive.
In summary, the future applications of hydraulic technology will focus more on high-dynamic-performance high-pressure linear motion. As mentioned in section 1.5, differential cylinders in the form of electro-hydraulic actuators have become a hot spot in industrial development. Their technical details will be discussed in depth in section 7.13.
At the same time, hydraulic R&D personnel are continuously opening up new applications for hydraulic technology. For example, for wave (tidal) power generation.
Electric vehicle charging cables heat up when current passes through them, and heating increases wire resistance, which exacerbates heating and limits charging current. Using a hydraulic pump to inject circulating coolant into the wires can maintain the wires at normal temperature, creating favorable conditions for fast charging and greatly shortening charging time. Although this does not belong to classic hydraulic transmission, it fully utilizes standard components developed from hydraulic technology.
In 2023, Rexroth’s sales reached 7.6 billion euros (approximately 60 billion RMB), a year-on-year increase of 8.5%, while Danfoss’s sales reached 10.7 billion euros (approximately 85 billion RMB), a year-on-year increase of 4%.
In summary, hydraulic transmission will not be completely replaced by electric drive and still has development and application prospects. Of course, it must be recognized that hydraulic equipment is not a consumer good but a means of production, so the sales volume of hydraulic equipment, and thus hydraulic components, will still be influenced by the level of investment for a while.
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