Hydraulic Power Unit Product

Overview

A hydraulic power unit is the engine room of a hydraulic system: it holds the oil, pressurizes it, cleans it, cools it, and meters it out to cylinders and motors elsewhere on the machine. The unit described here is a typical mid-size industrial HPU — 30 kW, 210 bar, 250 L tank — of the kind that drives presses, injection-moulding clamps, steel-mill equipment, test rigs, and dock machinery. Everything mounts on or beside the Tank Body: the motor-pump group on the Tank Lid, the Valve Manifold above it, the cooler and control cabinet alongside. ISO 4413 governs the safety design.

Pressure generation

The heart is the Axial Piston Pump, a swashplate axial piston pump. Nine pistons in a rotating Cylinder Barrel ride with their bronze slippers on the inclined face of the Swashplate; barrel rotation strokes each piston once per revolution, and the stationary Valve Plate times each bore to suction or pressure. What makes this pump the standard HPU choice is the Pressure Compensator: a small spool valve that tilts the swashplate back toward zero stroke as system pressure approaches the 210 bar set point. The pump then delivers exactly the flow the load consumes — at standby it idles at near-zero displacement drawing only a few kilowatts, where a fixed gear pump would be converting its full corner power into heat across a relief valve.

The pump hangs from the 30 kW Electric Motor through a Bell Housing and elastomer Pump Coupling; the whole group sits on rubber Damping Rails because the nine-piston pulsation (about 220 Hz at 1,475 rpm) would otherwise turn the tank lid into a loudspeaker. The oversized Suction Hose keeps inlet vacuum under 0.2 bar — piston pumps cavitate, and then erode their valve plates, on starved suction.

Reservoir and fluid conditioning

The 250 L tank is sized near three times pump flow per minute, the classic rule giving oil roughly three minutes per circuit to shed entrained air and drop contamination. The Baffle Plate forces return oil on a long path before it can reach the suction; the Filler Breather filters every litre of air the tank inhales as oil level cycles. A Level Switch stops the motor before a leak can uncover the suction line, with the Level Gauge and a PT100 Temperature Sensor giving local and remote indication.

Contamination control decides component life — some 70–80% of hydraulic failures trace to particles. Three stages hold the charge at ISO 4406 17/15/12: a 125 µm Suction Strainer for gross protection, the 10 µm Return-Line Filter catching system wear debris, and a 3 µm Off-Line Filter polishing continuously on the cooler loop. Each carries a Clogging Indicator so elements are changed on differential pressure, not guesswork.

Heat is the other slow killer. Throttling losses and pump inefficiency end up in the oil; the off-line loop — Coolant Pump, Radiator matrix, Blower Motor fan — rejects about 8 kW, while the wax-element Thermostatic Valve bypasses the cooler until the oil reaches its 45 °C working temperature, since cold ISO VG 46 oil is too viscous for fine valve clearances.

Control and distribution

Valves mount on the cross-drilled Manifold Block rather than discrete pipework, eliminating most leak joints. The pilot-operated System Relief Valve, set 10–15% above compensator pressure, is pure safety backup — in normal operation the compensator keeps it shut. Two CETOP 3 Directional Control Valves do the routing: each shifts a ground DCV Spool with 24 VDC wet-pin solenoids against centering springs, connecting pressure and tank to the actuator A and B ports. A Pressure Sensor feeds the PLC; Test Point Set accept diagnostic gauges under pressure.

The Bladder Accumulator — 10 L, nitrogen precharged to 90 bar — covers demand spikes beyond pump flow, damps ripple, and can hold an emergency reserve. As stored-energy equipment it falls under the Pressure Equipment Directive, and its Accumulator Safety Block (isolate, relieve, bleed) is mandatory so the accumulator can be made safe before any maintenance.

Operation

The Control PLC in the Control Cabinet sequences a star-delta start, proves oil level and filter status, enables the directional valves, and trips on low level, 70 °C oil, or clogged filters. Routine service is unglamorous and decisive: oil analysis every 2,000 hours, elements on indicator, nitrogen precharge check quarterly through the Charging Valve. An HPU kept clean and cool routinely runs 50,000 hours before the pump needs its first overhaul.