Power Units

A hydraulic power unit (HPU) generates and conditions fluid power for the machine that uses it. The HPU dictates available pressure, flow, and reliability for every downstream actuator.

Overview

HPUs range from suitcase-sized 1 hp portable units to multi-hundred-horsepower skids on stamping presses. Match the unit to a duty cycle, not to a peak demand.

Components

Electric motor (typically 1,800 rpm 4-pole TEFC).
Hydraulic pump.
Reservoir with breather, level/temperature gauge, cleanout, baffle.
Suction strainer, return-line filter.
Pressure relief / unloading valve.
Manifold for directional & flow control valves.
Heat exchanger (air or water cooled).
Accumulator for shock and ride control.

Pump Types

Gear — simple, robust, low cost; fixed displacement.
Vane — smooth, quieter; balanced or unbalanced.
Piston (axial / radial) — high pressure, variable displacement, best efficiency.
Screw — very quiet, continuous flow (lubrication systems).

Sizing

Pump flow: Q (gpm) = D (cu-in/rev) × N (rpm) / 231
Motor power: HP = Q × P / (1714 × η)
Reservoir: typically 3–5× pump flow (gpm → gal); >10× for poor cooling environments.
Verify NPSH available exceeds NPSH required by the pump.
Plan for fluid expansion (~0.7% per 10 °F rise).

Cooling & Filtration

Target steady-state fluid temp 110–140 °F (43–60 °C); never >180 °F (82 °C).
Heat load ≈ inefficiency × input power.
Air-blast vs water shell-and-tube; thermostat-controlled fan.
Filter cleanliness target: ISO 4406 16/14/11 for servo, 18/16/13 for proportional, 19/17/14 for general.
Bypass indicator + clogged-filter alarm.

Maintenance

Sample oil quarterly; trend particle count, water, TAN, viscosity.
Replace return filter per ΔP indicator; suction strainer at oil change.
Inspect hoses for wear, kinks, blisters.
Check breather; desiccant breathers in humid environments.
Bleed accumulators before service.