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When an Electro-Hydraulic Actuator (EHA) Beats a Hydraulic Power Unit

An electro-hydraulic actuator (EHA) puts the electric motor, pump, cylinder, and load-holding valves in one sealed unit at the axis, while a central hydraulic power unit (HPU) feeds many valve-controlled cylinders from one pump and tank through hoses and pipes. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article sets out what published sources say about each option's energy use, regeneration, heat, duty rating, stiffness, load holding, controls, and lockout, and it marks where that evidence stops: no neutral source read for it gives a power, duty-cycle, stroke, or cost threshold for choosing one over the other, and the closing decision framework is labelled engineering reasoning. The choice is made at the design link of the chain design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and it moves work between the frame, the drives, and the controls.

What is an electro-hydraulic actuator, and how is it different from a cylinder fed by an HPU?​

In conventional hydraulic systems, Ketelsen and co-authors write, a centralized hydraulic power unit delivers power to all actuators through a common pressure rail, and each actuator is controlled by a throttling valve, "leading to significant losses." In a pump-controlled drive the motion of the cylinder is controlled directly by the pump flow, which requires at least one pump per actuator. The review describes the EHA as the development "characterized by having a pressurized sealed oil reservoir (accumulator) instead of a vented tank," which emerged in the aircraft industry during the 1990s. Bosch Rexroth's white paper on the same class of product states that "from a technical perspective, self-contained actuators are electro-hydrostatic actuators." It says they combine an electric motor, a pump, and a hydraulic cylinder with important control functions and shut-off devices in a compact, hermetically sealed system, and that because they contain their own pump there is no need for separate hydraulic components and hoses.

The names vary by maker. Parker's Oildyne catalog calls its sealed unit "a fully self-contained electro-hydraulic actuator," while NASA's flight-test report and the Rexroth paper use "electro-hydrostatic actuator." As engineering practice, a specification should say "self-contained, pump-controlled actuator" rather than rely on the acronym alone, because the word electrohydraulic also names the valves that control HPU-fed cylinders: Moog's valve primer, for example, is titled Electrohydraulic Valves... A Technical Look. Valve selection for HPU-fed axes is covered in the live article on proportional versus servo valves and is not repeated here (Ketelsen et al. 2019, §1, §2 and §7; Thienen and Kamschitzki, Bosch Rexroth, pp. 1 to 2; Parker Hannifin HY22-3101E 7/13, p. 2; Navarro, NASA/TM-97-206224, 1997, Abstract; Moog CDL6566 Rev D, title page).

Where does a central HPU lose energy that an EHA can avoid?​

The loss the review names for conventional systems is throttling. Each valve-controlled actuator on a common pressure rail is metered by a throttling valve, and to enhance energy efficiency, Ketelsen and co-authors write, "the resistive losses associated with throttling must be reduced." The Oak Ridge National Laboratory (ORNL) study of the U.S. fluid power industry adds that throttling losses introduce both energy losses and the generation of heat. ORNL's system-efficiency estimates, and the narrow industrial data behind them, are set out in the hydraulic efficiency article.

Two standby effects also bear on a central unit that runs between moves:

  • Zero-stroke pump losses. The review cites manufacturers' published data showing that, for axial piston pumps, energy consumption at zero delivery or zero stroke against large pressure differences may be over 10% of the input energy required at full flow.
  • Idle draw of the power unit. The live comparison of hydraulic, pneumatic, and electric actuation reports bench measurements of a hydraulic unit drawing power before its valve shifts; that page covers them.

An EHA is not loss-free at standstill either. The review states that at piston standstill energy is still consumed by both the variable-displacement and the variable-speed approaches because of inherent losses. For variable-speed drives, those losses come from the torque the fixed-displacement pump generates, which causes electric losses even though the load is kept stationary (Ketelsen et al. 2019, §1 and §6; Love et al., ORNL/TM-2011/14, 2012, p. 9).

How much energy can an EHA save, and what do the published figures measure?​

The savings found for this article are reported per application and against a stated baseline, except the vendor figure, which gives no test basis:

  • The review's summary. Ketelsen and co-authors conclude that, compared with valve-controlled systems, significant energy savings are obtainable, and that "some references report energy savings up to 75% for specific applications."
  • Individual studies inside the review. A simulation showed a 60% saving potential, compared with a conventional valve-controlled system, for a two-link knuckle boom crane over a representative load cycle. A closed-circuit drive using variable-displacement pumps cut the energy consumption of a hydraulic broaching machine by 55%, compared with a conventional valve system, for a given load cycle.
  • The vendor claim. Bosch Rexroth states, of self-contained actuators, that "compared to the conventional design with a central power unit, the energy requirement can be reduced by up to 80 percent depending on the application." The white paper gives no test basis for the figure. It also reports that, in a pilot project involving a drawing press, the Technical University of Dresden demonstrated that the energy consumption of the drawing cushion could be reduced by up to 40 percent.

Tiboni compared five drive architectures on one variable-speed, variable-force hydraulic blanking press, sizing each and estimating energy over a load cycle common to all of them. Every alternative to the standard circuit, including a high–low pump circuit, a variable-displacement pump, and a variable-speed drive on a fixed-displacement pump, cut average active power by between 50.5% and 64.3%, and the press as built with the variable-speed arrangement drew a measured average active electric power of 14.85 kW. As engineering reasoning, the baseline matters as much as the actuator, and an EHA proposal should be compared with the best central-HPU architecture for the same load cycle, not only with a throttled baseline (Ketelsen et al. 2019, §4.2, §4.4 and §7; Thienen and Kamschitzki, Bosch Rexroth, p. 3; Tiboni 2023, §3.2, §4 and §5).

Where does the energy go when an EHA lowers a heavy load?​

Ketelsen and co-authors state that, generally, pump-controlled systems can recover kinetic or potential energy, for example during load lowering, whereas in valve-controlled systems energy recovery is generally not possible, due to throttling across the control valve. Recovered energy has to go somewhere, and the sources describe three destinations:

  • Storage and dissipation in the actuator electronics. On NASA's flight-test EHA, regenerative energy was stored in capacitors, and any excess was dissipated through an external resistor bank.
  • Sharing between actuators. If a system contains multiple actuators able to share power, for example through a common DC bus, the review says pump-controlled systems may offer increased energy efficiency at the system level as well as the actuator level.
  • The drive's DC bus. The Kinetix 5700 user manual's drive system overview lists a capacitor module for energy storage and for extending the DC bus to another inverter cluster, and a regenerative power supply, a sinusoidal PWM converter that can control the increase of DC-bus voltage and perform continuous power generation for one or more servo drives in multi-axis DC common-bus configurations.

ORNL describes the hydraulic-side counterpart: hybrid systems that size the primary power source for the average power demand and use accumulators to store energy during negative power flow, for example while a load is being lowered, illustrated with a delivery truck. No industrial quantification of that approach was found for this article. As engineering reasoning, a heavy lowering axis such as a coil downender in an aluminum mill, a log deck lift, or a mirror-handling lift at an observatory should be specified with a stated destination for its regenerated power, because a drive that cannot absorb it has to dissipate it (Ketelsen et al. 2019, §1; Navarro, NASA/TM-97-206224, 1997, p. 3; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700, p. 16 Table 3; Love et al., ORNL/TM-2011/14, 2012, p. 10).

When does heat decide between an EHA and an HPU?​

Ketelsen and co-authors note that a compact, sealed drive holds a significantly reduced oil amount compared with conventional systems, which means filtration and cooling have to be handled differently, without the conventional additional subsystem connected to the oil tank to manage the fluid condition. Avoiding the cooling and filtration elements entirely would require the system's energy losses to be balanced only by passive heat transfer to the surroundings. The studies the review reports give these results:

  • Hot. In one cited study, a compact pump-controlled system reached oil temperatures above 70 °C at indoor working conditions when no oil cooler was used, which the review says may be beyond limits for standard hydraulic components and fluids and may reduce durability and reliability.
  • Cold. Another cited study ran a pump-controlled system with oil below 0 °C, conditions sometimes experienced by mobile machinery, and found drastically decreased efficiencies at the lower temperatures.
  • Contamination. For one architecture, a cited test found a high particle load from self-contamination after 960 hours, leading to the observation that operating a compact pump-controlled drive "is currently not recommended without an oil filter, at least when using a standard HLP 46 hydraulic fluid."
  • Low speed. As one cited work points out, "most pump types are not able to continuously run at low rotational speeds (<300 RPM)," and running at low speed may reduce reliability and durability.

Two product documents give their own thermal figures. The NASA aileron EHA was "designed to require no active cooling" and carried a temperature transducer that measured motor winding temperature, and Parker's small DC unit has an operating temperature range of −34 °C (−30 °F) to +65 °C (150 °F). The review assesses thermal issues and drive controllability as the main challenges for the market penetration of the technology. As engineering reasoning, an EHA mounted near a furnace door, a hot rolling line, or a dryer shell gives up the cooler and oil volume a remote HPU can provide, and the oil temperature it reaches at the real duty and ambient should be a supplier-confirmed figure rather than an assumption (Ketelsen et al. 2019, §5 and §7; Navarro, NASA/TM-97-206224, 1997, p. 3; Parker Hannifin HY22-3101E 7/13, p. 3).

How is an EHA's duty cycle rated, and what does that mean for continuous-duty handling?​

The only EHA duty rating read for this article belongs to a small unit. Parker's Compact EHA catalog describes a 12 V or 24 V DC actuator, with 245 W or 560 W motors and a maximum extend force of 21.35 kN (4,800 lbf), that it calls "suitable for a wide range of mobile, light industrial and domestic applications"; its listed material-handling uses are pallet lifts, lift tables, scissors tables, and a light aircraft tug. The catalog's duty charts are headed standard motor duty cycle characteristics. They plot S2 duty, "time at constant load followed by 'off' time to allow the motor to cool to ambient temperature," and S3 duty, the "percentage of 'on' time in a repetitive 10 minute cycle," each against current draw. The catalog's application data sheet asks for cycles per day, time between cycles, product life requirement, maximum allowable amperage, and operating temperature range.

For larger classes, Bosch Rexroth describes a compact self-contained actuator for "the power class up to 6.2 kW," a separated design for high-power applications, and use "with a separated system architecture" in ring rolling mills in the steel industry, but it gives no duty rating.

No source read for this article gives a general continuous-duty limit for an EHA. As engineering reasoning, a heavy continuous-duty axis, such as a transfer-car lift that cycles every few minutes around the clock, needs the supplier's thermal and duty rating at the actual cycle and ambient before an EHA is chosen, because a motor's S2 or S3 rating describes motor heating, not oil temperature (Parker Hannifin HY22-3101E 7/13, pp. 2, 3, 5 and 7; Thienen and Kamschitzki, Bosch Rexroth, pp. 2 and 5).

Can an EHA hold a heavy load in position, and is it stiff enough to position one?​

Both makers describe load holding with valves. Bosch Rexroth states that if no power is required, the motor stands still, and "any loads or forces applied are supported by the integrated isolation valves." Parker's unit has a built-in locking circuit, pressure relief, thermal and check valves to protect against overload and "to allow loads to be held safely in position," and it retains a manual release option "for emergency use only." The review adds a condition: because common fixed-displacement pump types are not recommended at low pump speeds, it may be necessary to keep oil circulating at piston standstill, which is associated with losses, and to avoid this, components such as pilot-operated check valves, overcenter valves, or counterbalance valves may be used.

Stiffness is the controllability problem the review describes. For drives built around a single variable-speed pump, the pressure of the non-load-carrying chamber generally cannot be controlled and stays close to accumulator or tank pressure. The review says this makes the overall drive stiffness low, decreasing the drive's eigenfrequency, which may lead to positioning inaccuracy and unsatisfying dynamic performance, for example at impact loads. One study it reports replaced the vented tank of a variable-speed pump-controlled drive with a pressurized accumulator, compared the drive experimentally with an electro-mechanical counterpart, and found similar energy efficiency and compactness, better resistance to impact loads and better overload protection, but inferior drive stiffness due to low-pressure operation. The review concludes that some architectures' low drive stiffness makes them inappropriate for applications requiring high control bandwidths and positioning accuracies. NASA's EHA also stalled twice, as expected, at high hinge-moment maneuvers where the external load was greater than its maximum output load (Thienen and Kamschitzki, Bosch Rexroth, p. 2; Parker Hannifin HY22-3101E 7/13, pp. 2 and 3; Ketelsen et al. 2019, §4.1, §4.2, §6 and §7; Navarro, NASA/TM-97-206224, 1997, p. 8).

What sensing, drives, and PLC control does an EHA need?​

In an EHA the motor drive, not a throttling valve, meters the flow: Rexroth's paper says self-contained actuators use a displacement control system with a variable-speed drive instead of loss-causing valves, and it names control through conventional frequency converters as a key requirement for a standardized product. The NASA aileron EHA shows a complete sensor and loop set:

  • Feedback. A linear variable differential transformer (LVDT) measured ram position, and a resolver measured motor rotor position and velocity.
  • Health. A temperature transducer measured motor winding temperature, and a pressure transducer measured reservoir pressure.
  • Loops and fault response. The power control and monitoring electronics closed the actuator position, motor velocity, and current (acceleration) loops, and provided continuous fault monitoring that transferred the actuator to a trail-damped mode when any failure was detected.

The flight program's lessons transfer to industrial EHAs only as checks, and treating them that way is engineering reasoning. The controller and actuator were calibrated as a set and were not interchangeable. An open motor phase was detected in the pre-flight built-in test but not in normal operation, and continuous open-phase detection had to be added. The power-control thyristors were sensitive to 28 V dc power transients, and a power surge filtering circuit was added.

In an Allen-Bradley system, the variable-speed drive can be a frequency converter or a servo drive. The Kinetix 5700 user manual's drive system overview describes its -ERS3 single- and dual-axis inverters as supporting DSL and Hiperface encoder feedback, with the -ERS4 versions given the same encoder feedback support, and its motor control appendix gives position-, velocity-, and torque-loop axis configurations with a current regulator loop. Logix 5000 controller tasks can be configured as continuous, periodic, or event, and a periodic task performs a function at a specific time interval. UTEC Industrial, a Rockwell Automation Recognized System Integrator, integrates VFD and servo drives with ControlLogix and CompactLogix controllers over EtherNet/IP. Loop design, force measurement, and tuning for hydraulic axes are covered in the live article on closed-loop hydraulic position and force control (Thienen and Kamschitzki, Bosch Rexroth, pp. 3, 4 and 5; Navarro, NASA/TM-97-206224, 1997, pp. 3 and 8; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700, p. 16 Table 3 and App. D; Rockwell Automation 1756-RM094N-EN-P-2025, Ch. 5 pp. 39 and 41).

What stored energy and safety functions does an EHA leave at lockout?​

A sealed actuator still stores energy. NASA's EHA contained a hydraulic fluid accumulator packaged with its fluid components, and the Ketelsen review characterizes the EHA by its pressurized sealed oil reservoir (accumulator). As engineering reasoning, the isolation or locking valves that hold a raised load also trap the pressure that supports it. OSHA's lockout/tagout standard requires that, following the application of lockout or tagout devices, "all potentially hazardous stored or residual energy shall be relieved, disconnected, restrained, and otherwise rendered safe." If there is a possibility of reaccumulation of stored energy to a hazardous level, verification of isolation continues until the servicing or maintenance is completed, or until the possibility of such accumulation no longer exists.

Accumulator makers' instructions, written for their own bladder accumulators, point the same way. Parker's bladder accumulator manual says to "always assume the accumulator is under pressure until it is confirmed that it isn't," and Rexroth's operating instructions for one of its bladder-type accumulators note "that the pre-filling pressure still exists on the gas side after hydraulic relief." On the electrical side, the Kinetix 5700 manual states that disabling the power transistor output "does not provide physical isolation of the electrical output that is required for some applications." Where a safety function uses the actuator's position or pressure signals, ISO 13849-1:2023 specifies a methodology and provides related requirements, recommendations and guidance for the design and integration of safety-related parts of control systems (SRP/CS) that perform safety functions; it applies to SRP/CS for high demand and continuous modes of operation and does not apply to low demand mode of operation. Accumulator discharge hardware and gravity-load blocking are covered in the live article on stored energy and LOTO (Navarro, NASA/TM-97-206224, 1997, p. 3; Ketelsen et al. 2019, §2; OSHA 29 CFR 1910.147-1989, (d)(5)(i) and (ii); Parker Hannifin HY10-2300-M1, p. 3; Bosch Rexroth RE 50170-B/10.08, p. 28; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700, p. 250; ISO 13849-1:2023).

Where does the EHA-versus-HPU choice sit in the design-to-monitoring chain?​

The choice is made at the design link, and the sources describe what it removes from the machine. Ketelsen and co-authors note that when power is distributed electrically, with an individual pump and electric motor on each cylinder and the vented oil tank replaced by a sealed accumulator, only an electrical and mechanical machine interface is present, and the hoses and pipes related to a centralized HPU are avoided. Rexroth lists installation without piping, hoses, and fluid tanks, and commissioning that requires only electrical and control-technology knowledge, among the requirements for a standardized product.

As engineering reasoning, that relocation has consequences at each later link:

  • Engineering and fabrication. An EHA puts motor, pump, and reservoir mass at the axis, on the mount and frame that already carry the cylinder reaction, where a remote HPU leaves only the cylinder on the axis.
  • Machining, weld fatigue, and stress relief. The clevis or trunnion bores and the welded lugs that carry them see a load cycle every stroke with either architecture, and stress relief before final machining bears on whether those bores stay aligned.
  • Drives, controls, and tuning. Tuning work moves from valve amplifiers into the motor drive and the PLC.

For monitoring, Rexroth places monitoring with integrated sensor systems, and straightforward exchange of data with higher-level IT systems, among the priorities in industry and heavy industry, and it says integrated pressure sensors let process forces be measured and internal system parameters be monitored. UTEC Industrial stress-relieves and machines the welded frames that carry actuators like these before assembly (Ketelsen et al. 2019, §1; Thienen and Kamschitzki, Bosch Rexroth, pp. 2, 4 and 5).

When does an EHA beat an HPU, and when does the central unit still win?​

No neutral source read for this article gives a power, duty-cycle, stroke, or cost threshold at which an EHA beats an HPU. The 2019 review states that pump-controlled cylinder drives have primarily been of interest in the academic community, that commercial products have recently entered the market, and that "the real industrial breakthrough is however yet to come." It also cautions that the hydraulic drive is part of a bigger system, and that optimizing energy consumption on an individual drive level may not be sufficient. Tiboni writes that her energy comparison could be complemented by an analysis of the initial investment costs each architecture requires. The framework below is engineering reasoning drawn from the sources above, not a published rule:

  • Conditions that favor an EHA. A few axes that are far apart or ride on a moving carriage, where hose runs to a central unit would be long; long dwells between moves, where a stopped motor and valve-held load avoid central-unit standby losses; loads lowered under gravity, where a drive can recover energy; and duty and ambient conditions that fall inside the supplier's confirmed rating.
  • Conditions that favor a central HPU. Many axes sharing flow that rarely peaks at once; forces or powers beyond the packaged ranges suppliers publish; axes that need high stiffness and control bandwidth; heat or continuous duty that needs a cooler and a large oil volume; and a plant that already maintains hydraulic power units.
  • Lifecycle cost. The U.S. Department of Energy's motor sourcebook gives an example of a hypothetical 100 hp, 94.5%-efficient motor running 6,300 hours per year for 18 years at $0.075/kWh, in which electricity costs represent approximately 95% of lifetime operating costs and purchase price and repair costs only 5% of the total. That example assumes 6,300 operating hours a year; as engineering reasoning, an intermittent heavy handling axis has to be costed on its own hours and cycle.

A specification that compares the two should state, for each axis, the force and speed in both directions, cycles per day and time between cycles, the life requirement, the operating temperature range, the load-holding and lowering cases, the positioning accuracy, and where regenerated power goes. For its own unit, Parker's application data sheet asks for the operating force and rate in extend and retract, cycles per day, time between cycles, product life requirement, maximum allowable amperage, and operating temperature range (Ketelsen et al. 2019, §2, §3 and §7; Tiboni 2023, §5; U.S. DOE DOE/GO-102014-4356, 2014, p. 25; Parker Hannifin HY22-3101E 7/13, p. 7).

Related Articles

References​

  • Ketelsen, S., Padovani, D., Andersen, T. O., Ebbesen, M. K., and Schmidt, L. (2019). "Classification and Review of Pump-Controlled Differential Cylinder Drives." Energies, 12(7), 1293.
  • Thienen, S., and Kamschitzki, W. High Power Density, Electric Control: How Self-Contained Actuators Optimize Machines and Systems (white paper). Bosch Rexroth AG (undated web documentation, accessed September 2026).
  • Parker Hannifin Corporation, Oildyne Division. Compact EHA: Electro-Hydraulic Actuators for High Power Density Applications (Catalog HY22-3101E 7/13). Parker Hannifin, 2013.
  • Navarro, R. Performance of an Electro-Hydrostatic Actuator on the F-18 Systems Research Aircraft (NASA/TM-97-206224). NASA Dryden Flight Research Center, 1997.
  • Moog CDL6566 Rev D: Electrohydraulic Valves... A Technical Look. Moog Inc., Industrial Controls Division, 2002.
  • Love, L. J., Lanke, E., and Alles, P. Estimating the Impact (Energy, Emissions and Economics) of the U.S. Fluid Power Industry (ORNL/TM-2011/14). Oak Ridge National Laboratory, 2012.
  • Tiboni, M. (2023). "Power Drive Architectures for Industrial Hydraulic Axes: Energy-Efficiency-Based Comparative Analysis." Applied Sciences, 13(18), 10066.
  • Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.
  • Rockwell Automation 1756-RM094N-EN-P-2025: Logix 5000 Controllers Design Considerations. Rockwell Automation, 2025.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.
  • Parker Hannifin HY10-2300-M1: Bladder Accumulators Maintenance and Installation Manual. Parker Hannifin Corporation, 2025.
  • Bosch Rexroth RE 50170-B/10.08: Bladder-type accumulator Type HAB..-4X: Operating instructions. Bosch Rexroth AG, 2008.
  • ISO 13849-1:2023: Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. International Organization for Standardization, 2023.
  • U.S. DOE Advanced Manufacturing Office. Improving Motor and Drive System Performance: A Sourcebook for Industry, DOE/GO-102014-4356. U.S. Department of Energy, 2014.

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