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Mixed-Power Machines: Partitioning Hydraulic, Pneumatic, and Electric Axes

Most heavy handling machines are not all-hydraulic, all-pneumatic, or all-electric: a coil tilter may lift on hydraulics, clamp on air, and traverse on a servo, and each of those choices is made axis by axis. 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 how to partition a machine's axes between the three energy types, how a shared hydraulic power unit, an air supply, and a servo drive line-up are sized and routed together, and how one PLC and one lockout procedure cover all three. Partitioning is decided at the design and engineering links of the chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, and it fixes the energy sources that controls, tuning, and maintenance have to manage for the life of the machine.

What is a mixed-power machine, and why partition axes by energy type?​

A mixed-power machine uses more than one actuation energy type on the same frame, under one control system. Typical examples across heavy industry include:

  • Steel and aluminum: a coil upender that tilts on hydraulic cylinders, grips on pneumatic or hydraulic clamps, and is fed by a VFD-driven conveyor.
  • Lumber: a log turner or lumber stacker combining hydraulic kickers and lifts, pneumatic stops and gates, and electric chain and roll drives.
  • Aerospace: a positioner or assembly fixture with a servo-indexed rotation axis, a hydraulic or electric lift, and pneumatic locating pins.
  • Research and observatory operations: a heavy instrument handling cart that combines a precise electric positioning axis with a high-force lift.

The case for partitioning comes from the fact that no single energy type is best on every measure. Kazama summarizes the published comparisons as concluding that hydraulics show the highest power density, that each system has its own appropriate areas of application, and that selection should not be limited to one system type. The experimental comparison by Pustavrh and co-authors reaches the same point from the other direction: the hydraulic actuator rated best for force and displacement, the pneumatic best for velocity, acceleration, and weight, and the electric best for power consumption and smoothness. The authors offer their eight-category rating chart specifically as a tool for choosing an actuator when certain constraints are present (Kazama 2019; Pustavrh et al. 2023).

Which axes belong on hydraulics?​

Hydraulics belong on the axes where force governs and the actuator has to be compact. In the Pustavrh bench, a small rig limited to 1.1 kW of input power and tested at 0 and 50 kg, a 25 mm bore cylinder at a 200 bar relief setting has an ideal extend force of 9,817 N (490.9 mm² × 20 N/mm²), about four times what the 63 mm bore pneumatic cylinder delivers at 8 bar. Scaled to a heavy axis by F = pA arithmetic on assumed inputs rather than any measurement, an assumed 100 mm bore cylinder at 200 bar has an ideal extend force of 7,854 mm² × 20 N/mm² = 157,080 N, about 157 kN (35,300 lbf), from an actuator that fits inside a tilting frame or under a lift table.

Hydraulic axes on a mixed machine typically include:

  • Heavy lifts and tilts: upenders, tilt tables, and lift tables, where the load acts through a large moment arm.
  • High-force clamps and kickers: log kickers, coil grips, and clamps that must hold against process forces.
  • Axes in heavy, dirty environments: the authors describe the hydraulic system as better suited to heavy, dirty industries.

The penalties come with the power unit. The complete hydraulic bench weighed 52,029.3 g against 11,636.9 g for the electric system, and the authors call the large footprint the hydraulic system requires, and the noise it makes, a very big disadvantage. The pump also has to run before any valve receives a signal; switching it on at the command would make the axis too slow to respond. On a mixed machine, that pushes designers to group every high-force axis onto one power unit rather than scatter small power units around the frame (Pustavrh et al. 2023).

Which axes belong on pneumatics?​

Pneumatics belong on light, end-to-end motions where speed matters and mid-stroke position does not. The pneumatic cylinder in the Pustavrh comparison rated highest of the three for velocity (8.38 of 10), acceleration (9.47), and weight (8.68), and the pneumatic system weighed only 15.3 % of the hydraulic system. The authors also note that pneumatics are common in automation applications that need a clean, dry environment.

On a heavy machine, that makes pneumatics a good fit for:

  • Locating pins, stops, and gates that move between two positions against hard stops.
  • Light clamps and hold-downs where the clamp force needed is modest. An assumed 80 mm bore air cylinder at 6 bar has an ideal extend force of 5,027 mm² × 0.6 N/mm² = 3,016 N.
  • Ejectors, diverters, and brake or lock releases that act quickly and do not carry the main load.

Two properties rule pneumatics out of load-bearing axes. The first is compressibility: the compressibility factor the authors cite for air, 1.4 × 10⁶ Pa, is about three orders of magnitude below oil's 1.7 × 10⁹ Pa, and under load the pneumatic carriage went into damped oscillation when the valve returned to center. The second is energy: the pneumatic system drew 943.3 W at 6 bar and rated worst of the three for power consumption, at 0.92 of 10, because the compressor runs regardless of whether the axis moves. Kazama's comparison of transmission lines adds a third limit: pneumatic tubes carry roughly watts to kilowatts, against kilowatts to megawatts for hydraulic hoses (Pustavrh et al. 2023; Kazama 2019).

Which axes belong on electric servo?​

Electric servo belongs on axes that must stop at an exact position, repeat that position every cycle, or sit idle for long periods between moves. The electric actuator in the Pustavrh comparison gave the most consistent response and the lowest power consumption of the three, because the servo motor turns the screw with controlled torque in a closed loop and uses only as much power as it needs at any given moment. Its peak draw at a 15 m/s² acceleration limit was 233.6 W unloaded and 313.9 W with the 50 kg load, compared with 740.5 to 1,529.9 W drawn by the idling hydraulic unit. Extend and retract speeds are also equal on an electric actuator, because there is no change in piston area between directions.

Typical servo axes on a mixed machine include:

  • Index and rotation axes on positioners, where a weldment or airframe section has to stop at a programmed angle.
  • Traverse and docking axes on transfer cars and gantries that align to a machine tool or furnace door.
  • Axes with long dwell times, where a constantly running hydraulic power unit or compressor would waste idle energy.

The limit is scalability. The electric actuator on the bench topped out at about 0.3 m/s, and the authors point out that making an electric axis faster requires upgrading the whole package, including a larger servo motor, cylinder, and controller and higher power. On a servo axis carrying a vertical or tilting load, the brake matters as much as the drive: on the Kinetix 5700, the motor brake option is a spring-set holding brake that releases when voltage is applied to the brake coil, with a customer-supplied 24 V supply driving the brake output, so removing that voltage lets the spring set the brake (Pustavrh et al. 2023; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700).

How is a shared hydraulic power unit sized for several axes?​

Once axes are grouped, the hydraulic power unit is sized to the flow that moves simultaneously, not to the sum of every cylinder on the machine. Flow for each axis is Q = A × v, and hydraulic power is P = pQ. A worked example, using stated assumptions rather than any vendor rating:

  • Lift axis: 100 mm bore cylinder extending at an assumed 0.1 m/s. Q = 7.854 × 10⁻³ m² × 0.1 m/s = 7.854 × 10⁻⁴ m³/s = 47.1 L/min.
  • Clamp axis: 80 mm bore cylinder extending at an assumed 0.05 m/s at the same time. Q = 5.027 × 10⁻³ m² × 0.05 m/s = 2.51 × 10⁻⁴ m³/s = 15.1 L/min.
  • Simultaneous demand: 47.1 + 15.1 = 62.2 L/min, or 1.037 × 10⁻³ m³/s.
  • Hydraulic power at an assumed 200 bar: P = 20 × 10⁶ Pa × 1.037 × 10⁻³ m³/s ≈ 20.7 kW of fluid power, before pump, motor, and valve losses.

The valve on each axis has to pass its share. Proportional valves such as the Bosch Rexroth 4WRE and 4WREE family are ordered by nominal flow at a stated valve pressure differential, in more than one nominal size, so the 47.1 L/min lift in the example is matched to a spool and size from the data sheet edition in hand, at the pressure drop the circuit can afford across the valve. If the clamp is moved to pneumatics instead, it leaves the hydraulic flow budget entirely, and the lift and any other heavy axis get the whole pump.

Sequencing lowers the peak further: if the clamp closes before the lift starts, the simultaneous demand drops to 47.1 L/min. The Pustavrh authors also note that hydraulic accumulators help prevent sudden releases of high stored pressure, and an accumulator can cover short simultaneous peaks, at the cost of more stored energy to control at lockout (Bosch Rexroth RE 29061/10.05; Pustavrh et al. 2023).

How do hoses, tubes, and cables affect routing on a moving machine?​

On a transfer car, a traveling gantry, or a rotating positioner, every energy type has to cross a moving joint, and the lines differ sharply in power per kilogram. Kazama models transmitted power as P = pQ = πDᵢ²pU/4 for hoses and tubes and P = EI for cables, and finds that the transmittable power follows a power law of the line's mass per unit length, P ∝ m′ⁿ. The exponents were about 1.4 for hydraulic hoses, 1.2 for pneumatic tubes, and 0.8 for electrical cables, calculated at U = 5 m/s for hoses, U = 15 m/s for tubes, and 600 V for cables.

For partitioning, three findings matter:

  • Hydraulic hoses carry the most power per unit mass at high power, but a hydraulic circuit also needs return lines, and the oil inside the hoses (900 kg/m³ in Kazama's model) adds mass that reduces transmittable power per unit mass.
  • Pneumatic tubes carry the least, notably below hoses by about two orders of magnitude under the study's conditions.
  • Electrical cables are comparable to hoses and ahead of them in the middle power range, with less scatter in power versus mass and size than hoses and tubes.

That is a practical argument for putting traveling axes on electric servo wherever force allows, and for mounting a hydraulic power unit on board a traveling machine, so that only a power cable crosses the moving joint, rather than running pressure and return hoses on a reel. By the same measure, pneumatic lines are the least mass-efficient way to deliver power to the far end of a long machine (Kazama 2019).

How is one PLC made to coordinate hydraulic, pneumatic, and servo axes?​

A mixed machine needs one sequence and one set of interlocks, even though each energy type presents a different interface to the controller:

  • Proportional hydraulic axes. A proportional valve with electrical position feedback, such as the 4WREE with integrated electronics, takes a voltage or current command from an analog output; the valve closes its own spool-position loop, and a cylinder position transducer closes the axis loop in the PLC or motion controller. Rexroth's data sheet lists a zero-point displacement with changes in fluid and operating temperature, so the outer loop has to absorb thermal drift.
  • Pneumatic axes. Discrete outputs to solenoid valves and discrete inputs from end-of-stroke sensors, with the PLC confirming each position before the next step.
  • Servo axes. Networked servo drives such as the Kinetix 5700, which close position, velocity, and current loops on motor feedback and include safe torque-off.

In a Logix 5000 controller, code is organized into continuous, periodic, and event tasks, so the fast motion and interlock logic that coordinates a hydraulic lift with a servo traverse can run on a fixed period rather than whenever the processor has time. The Pustavrh experiment is a reminder of why the hydraulic and pneumatic axes need that closed loop: driven open-loop with a step command, both fluid systems showed pressure shocks, speed overshoot, and oscillation that the authors conclude a feedback controller on the valve opening is needed to remove. UTEC Industrial programs Allen-Bradley ControlLogix and CompactLogix controllers over EtherNet/IP to run hydraulic, pneumatic, and servo axes from one sequence (Bosch Rexroth RE 29061/10.05; Rockwell Automation 1756-RM094N-EN-P-2025; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700; Pustavrh et al. 2023).

How are safety functions partitioned across three energy types?​

Each energy type needs its own way of reaching a safe state, and the safe state is not always "power off." Rockwell Automation certifies the Kinetix 5700 hardwired and integrated safe torque-off to Performance Level e per ISO 13849-1 and SIL CL 3 per IEC 61508, IEC 61800-5-2, and IEC 62061, but explicitly for applications in which removing motion-producing power is considered the safe state. The manual adds that, in the event of a malfunction, the most likely stop category is Stop Category 0, and that the timing and distance of a coast to stop must be considered in the machine design. On a servo axis carrying a suspended or tilting load, removing torque alone lets gravity act, so the holding brake and its control become part of the safety function.

On the fluid-power axes, the equivalent questions are whether a valve blocks the cylinder or vents it, and whether stored pressure can move the load after the stop. ISO 4413:2010 and ISO 4414:2010 are the general safety standards for hydraulic and pneumatic systems respectively; this library identifies them at title level until the purchased texts are registered.

Tying the three together, safety functions run only in the safety task of a safety controller such as a GuardLogix 5580, rated up to SIL 3 and PL e, Cat. 4, with a safety partner and up to SIL 2 and PL d, Cat. 3, without one. GuardLogix controllers can exchange safety data with safety I/O devices, and with other GuardLogix controllers, over EtherNet/IP, which is one way for a single emergency stop or guard door to reach the servo drives and safety I/O on every axis of the machine. ISO 13849-1:2023 is the standard for the design of those safety-related control parts, and ISO 12100:2010 covers the risk assessment that identifies the hazards on each axis (Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700; Rockwell Automation 1756-RM012J-EN-P-2025; ISO 4413:2010; ISO 4414:2010; ISO 13849-1:2023; ISO 12100:2010).

What does lockout look like on a machine with three energy sources?​

A mixed-power machine has more than one energy source under OSHA's lockout/tagout standard, which defines an energy source as any source of electrical, mechanical, hydraulic, pneumatic, chemical, thermal, or other energy. That has direct consequences:

  • No single-source exception. The note to 1910.147(c)(4)(i) lets an employer skip documenting a procedure only when, among other conditions, the machine has no potential for stored or residual energy and a single energy source that can be readily identified and isolated, and a single lockout device achieves a locked-out condition. A machine with a power unit, an air supply, and servo drives fails that test, so a written procedure is required.
  • Procedure content. Under 1910.147(c)(4)(ii)(B) and (D), the procedure must contain specific steps for shutting down, isolating, blocking, and securing the machine, and specific requirements for testing to verify the effectiveness of the energy control measures.
  • Isolation points for every source. Push buttons, selector switches, and other control-circuit-type devices are not energy-isolating devices; a disconnect switch, a line valve, and a block are. Each energy type needs its own lockable point, and under 1910.147(c)(2)(iii) new machines must have energy-isolating devices designed to accept a lockout device.
  • Stored energy. After lockout devices are applied, 1910.147(d)(5)(i) requires stored or residual energy to be relieved, disconnected, restrained, and otherwise rendered safe. That covers trapped hydraulic pressure and accumulator charge, residual air downstream of the lockout valve, and loads held by brakes or cylinders. Where stored energy can reaccumulate, 1910.147(d)(5)(ii) requires verification of isolation to continue until the work is done.
  • Verification and review. Under 1910.147(d)(6), isolation and de-energization must be verified before work starts, and under 1910.147(c)(6)(i) the energy control procedure must be inspected at least annually.

Designing the isolation points, bleed valves, and mechanical blocks into the machine at the partitioning stage is far cheaper than retrofitting them after the first maintenance audit (OSHA 29 CFR 1910.147-1989).

Where does axis partitioning sit in the design-to-monitoring chain?​

Partitioning is a design decision with fabrication consequences. Hydraulic axes put concentrated cylinder reactions into welded clevis lugs and trunnion blocks, and those welds see a load cycle every stroke; weld fatigue and stress relief before final machining decide whether the pin bores stay in line. Electric axes move the reaction into screw, rack, or gearbox mounts, where machined flatness and alignment govern. The power unit, manifold, and compressor need space on or beside the frame, and the Pustavrh authors note that the hydraulic system needs a power unit and takes up more space, while the electric system needs minimal floor space and only a power connection.

After fabrication, each energy type adds its own tuning and monitoring items:

  • Hydraulic: fluid cleanliness within the contamination class the 4WRE proportional valve data sheet sets, since Rexroth notes that effective filtration both prevents faults and extends component life, plus fluid temperature and viscosity within the data-sheet ranges.
  • Pneumatic: supply pressure at each consumer, since the bench showed that a lack of air flow caused a pneumatic rod to oscillate.
  • Electric: servo tuning on each axis; the Kinetix 5700 commissioning procedure includes a tuning step, and autotuned loop bandwidths can need adjustment once the motor and load are connected.

UTEC Industrial fabricates, stress-relieves, and machines the welded frames that carry these cylinders, screws, and drives before assembly and factory acceptance testing (Pustavrh et al. 2023; Bosch Rexroth RE 29061/10.05; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700).

What should a mixed-power machine specification define for each axis?​

A specification that names a machine's capacity but leaves actuation to the vendor forces the vendor to guess at the partitioning. A complete request defines, for each axis:

  • Load and motion: force or torque, stroke or angle, speed, acceleration, and whether the axis must hold a mid-stroke position under load.
  • Duty: cycles per hour, dwell time between moves, and which axes move at the same time, since simultaneity sets the hydraulic flow budget and compressor demand.
  • Environment: heat, dust, chips, washdown, and whether hydraulic fluid leakage is acceptable near the process; the Pustavrh authors note that a pipe burst can release large amounts of hydraulic fluid into the environment, one reason industry is increasingly moving to partial or complete electrification.
  • Safe state: whether the axis must stop and hold, stop and vent, or stop and coast, and the brake or blocking valve needed to reach it.
  • Energy isolation: an energy-isolating device designed to accept a lockout device for every energy source, as 1910.147(c)(2)(iii) requires on new machines, plus the means, such as bleed-down valves and mechanical blocks, to relieve or restrain stored energy as 1910.147(d)(5)(i) requires.
  • Controls interface: PLC platform, network, analog or networked valve and drive interfaces, and the feedback device on each axis.

With those inputs, the partition follows from the numbers rather than from habit: force and dirt toward hydraulics, light two-position motion toward pneumatics, and precise, long-dwell, or traveling axes toward electric servo (Pustavrh et al. 2023; OSHA 29 CFR 1910.147-1989).

Related Articles

References​

  • Kazama, T. (2019). "Comparison of power density of transmission elements in hydraulic, pneumatic, and electric drive systems." Mechanical Engineering Letters, 5, 19-00139. DOI 10.1299/mel.19-00139
  • Pustavrh, J., Hočevar, M., Podržaj, P., Trajkovski, A., Majdič, F. (2023). "Comparison of hydraulic, pneumatic and electric linear actuation systems." Scientific Reports, 13(1), 20938. DOI 10.1038/s41598-023-47602-x
  • Bosch Rexroth RE 29061/10.05 (2005): 4/2 and 4/3 Proportional Directional Valves, Direct Operated, with Electrical Position Feedback, Types 4WRE and 4WREE. Bosch Rexroth, 2005.
  • ISO 4413:2010: Hydraulic fluid power — General rules and safety requirements for systems and their components. International Organization for Standardization, 2010.
  • ISO 4414:2010: Pneumatic fluid power — General rules and safety requirements for systems and their components. International Organization for Standardization, 2010.
  • ISO 13849-1:2023: Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. International Organization for Standardization, 2023.
  • ISO 12100:2010: Safety of machinery — General principles for design — Risk assessment and risk reduction. ISO, 2010.
  • OSHA 29 CFR 1910.147-1989: The Control of Hazardous Energy (Lockout/Tagout). Occupational Safety and Health Administration, 1989.
  • 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.
  • Rockwell Automation 1756-RM012J-EN-P-2025: GuardLogix 5580 and Compact GuardLogix 5380 Controllers Safety Reference Manual. Rockwell Automation, 2025.

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