Electronic Hydraulic Linear Axes in Annular Machine Tools

• 1
  What makes an axis electronic hydraulic
• 2
  How force and speed trade off in the same axis
• 3
  Why the valve curve has to be linearized
• 4
  Where the accuracy actually comes from
• 5
  When electronic hydraulic linear axes beat a ball screw
• 6
  Hydraulic axis versus ball screw axis

A hydraulic cylinder on its own is a dumb actuator. Add a proportional or servo valve with an onboard controller, a position encoder on the slide, and a closed-loop position loop, and the same cylinder becomes an axis that can be commanded in millimeters. That is the whole idea behind electronic hydraulic linear axes.

The valve sits between the pump and the cylinder. It receives a command signal, usually ±10 V or a fieldbus setpoint, and meters flow to one side of the piston while opening the other side to tank. The onboard electronics close a spool position loop inside the valve at roughly 1 kHz. The machine control closes the outer position loop at 1–4 kHz, depending on the drive.

Two feedback loops in series sound redundant. They are not. The inner spool loop cancels valve deadband, hysteresis and oil temperature drift before those errors ever reach the slide. The outer loop then only has to correct the mechanics: friction, cutting force and column deflection.

On a ring machine this matters because the axes carry the tool head around a fixed or indexing workpiece. The slide is short, often 150–400 mm of stroke, but the moving mass is high and the force demand is uneven. A ball screw would need a large motor to hold that force at standstill.

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  EncoderLinear scale on the slide, 0.1 μm resolution or finer.
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  ValveServo or high-response proportional, onboard spool loop.
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  ControllerOuter position loop, 1–4 kHz, with feedforward.
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  Power unitPressure-compensated pump, 100–210 bar working range.

A hydraulic axis makes force from pressure and area, and speed from flow and area. Those two are linked by the pump, so you cannot have maximum force and maximum speed at the same instant. The valve splits the available flow between the two cylinder chambers, and the pressure difference across the piston creates the force.

In a typical ring machine axis, the bore is 40–80 mm. At 140 bar that gives roughly 18–70 kN of thrust. Increase the bore and force rises with the square of diameter, but the same valve now accelerates more oil, so bandwidth drops. This is the first trade a machine builder has to settle.

Acceleration is the other half. A slide that reaches 1 g needs the valve to fill the chamber fast enough to build pressure before the tool touches the part. Undersized valves show up as a soft ramp at the start of every cut, which leaves a witness mark on the first 0.2–0.5 mm of the pass.

The practical fix is usually not a bigger valve. It is a smaller moving mass: lighter tool head, shorter stroke, or an accumulator close to the manifold so the valve does not have to draw flow through 3 m of hose.

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  High force, low speedLarge bore, low pump flow, heavy cut or forming pass.
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  Low force, high speedSmall bore, high flow, rapid approach and retract.
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  Both at onceNot possible from one pump without an accumulator.

A spool valve does not deliver flow in proportion to command. Near center the overlap makes the response flat, then it rises steeply, then it saturates. If the controller uses the raw curve, the axis feels dead at small commands and violent at large ones. The gain that works for a 0.05 mm move will overshoot on a 5 mm move.

Linearization maps the real flow curve, measured at several pressures and oil temperatures, into a correction table stored in the valve electronics. The command the controller sends is then proportional to the flow the valve actually delivers. One gain setting can cover the full stroke.

The measurement matters more than the table. A curve taken at 30 °C will be wrong at 55 °C, because oil viscosity changes leakage across the spool clearance. Good valves store two or three temperature-compensated curves and interpolate between them.

The payoff is repeatability across speed. A ring machine that runs at 200 mm/min for a finishing pass and 8,000 mm/min for rapid moves should show the same following error behavior in both. Without linearization, the finishing pass is the one that suffers.

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  DeadbandSpool overlap; causes lost motion at reversal.
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  HysteresisSame command gives different flow on up and down stroke.
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  SaturationBeyond rated stroke, extra command changes nothing.

People assume a hydraulic axis is less accurate than a ball screw. In a ring machine, the reverse is often true, because the errors come from different places. A ball screw loses accuracy through screw growth, nut wear and the compliance of a long shaft under thrust. A hydraulic axis has none of those.

What it has instead is oil stiffness. The trapped volume between valve and piston acts like a spring. A 50 mm bore with 100 mm of trapped oil on each side gives a hydraulic spring rate in the range of 2,000–6,000 N/mm at 140 bar. Double the trapped length and the stiffness halves.

So the mounting detail matters. Put the valve on the cylinder block, not on a panel 2 m away. Every extra centimeter of hose adds trapped volume and softens the axis. This is the single most common installation mistake on retrofit machines.

Thermal drift is the second limit. Oil at 40 °C versus 55 °C changes both leakage and bulk modulus. A machine that holds ±0.01 mm cold may drift to ±0.03 mm after four hours of continuous running if the power unit has no cooling. A chiller and a stable reservoir temperature are part of the axis, not an accessory.

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  Short trapped volumeValve on the cylinder; stiffer axis, higher gain.
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  Cooled oilHold reservoir at ±2 °C for stable positioning.
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  Rigid mountCylinder flange bolted to a machined pad, not shimmed.

Choose hydraulic when the axis needs high thrust at low speed, or when it has to hold a force while stationary for a long time. Forming, ring rolling, broaching and heavy clamping all fall into that group. A ball screw holding 30 kN at standstill needs a large motor and a brake, and it wears while it holds.

Choose a ball screw or a linear motor when the axis is light, fast and mostly unloaded, or when the machine has to run in a clean room without an oil power unit. Small pick-and-place heads, optical stages and light milling on aluminum are better served by electric drives.

There is a middle ground where both work: medium force, medium speed, moderate duty. Here the decision usually comes down to infrastructure. If the plant already has a hydraulic power unit and maintenance staff who understand it, the hydraulic axis is cheaper to own. If not, the electric axis avoids a new discipline.

One more criterion is controllability during the cut. Hydraulic axes absorb shock loads through the oil, so a chipped insert or an interrupted cut produces less position disturbance than it would on a stiff ball screw. That damping is a real advantage on cast or forged surfaces.

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  Pick hydraulicHigh holding force, shock loads, forming, hot or dirty shop.
• 2
  Pick electricLight load, high speed, clean room, no power unit.
• 3
  Either worksMedium duty; decide on existing plant infrastructure.