What a hydraulic power unit bundles

A hydraulic power unit is the self contained source of fluid power for a machine. It does not move a load by itself. It converts an electric motor or engine into pressurised fluid that downstream cylinders, motors or presses turn back into force. The unit packs the prime mover, pump, reservoir, valve manifold, filtration, cooling and instrumentation into one skid, so buying one is a system exercise rather than a single part lookup.

The same logic applies when you ask a contract manufacturer to build one to your drawings. You are not ordering a welded tank with a motor bolted on. You are asking a supplier to take responsibility for a welded and machined structure, a machined manifold, an assembled pumping set and a tested package. The contract manufacturing services at Realjet cover fabricated and machined machinery built around customer drawings, and a hydraulic power unit sits squarely in that scope.

Drawing and BOM readiness before RFQ

Send a complete package before asking for a firm quote. At minimum the supplier needs the hydraulic schematic or P&ID, the reservoir general arrangement, the manifold block drawings with port callouts, the bill of materials with approved material grades, and the electrical interface for the motor and controls.

A partial drawing forces the supplier to assume pump type, displacement, port sizes and relief setting. Those assumptions then drive welding, machining and test scope, and they rarely survive first contact with the real duty cycle. A welding procedure and NDT review early in the RFQ keeps the fabrication scope honest instead of guessed.

Reservoir fabrication and welding scope

The reservoir stores fluid, helps cool it, releases entrained air and lets contaminants settle. It is usually a fabricated steel tank with baffles, sight glass or level switch, filler and breather, and drain. State the grade, plate thickness, internal baffle layout and any stainless or coated zones.

Welding should be specified, not left open. Name the code (AWS D1.1 is common for structural and pressure adjacent steel fabrication), the WPS and PQR basis, the weld joint types and the inspection level. Seams that hold oil under pressure deserve a defined weld map and, where the duty calls for it, a leak test on the welded shell. Material certificates to EN 10204 (3.1 where the spec requires traceable chemistry and mechanical properties) should travel with the steel.

Machined manifold and interface tolerances

The valve manifold is where most of the precision lives. It is a machined block with drilled passages and port faces for the relief, directional and pressure control valves. The faces must be flat and the ports must match the valve standards, or the unit leaks the first time it is pressurised.

Give the supplier the manifold drawing with datum references, port thread or face seal standard, surface finish on sealing faces and the bore tolerances. General tolerances such as ISO 2768 should be stated so the shop does not have to infer them. Machined mounting feet, pump flange and motor interface also need their datums and fits called out, the same way any machined interface built to drawing does.

Pump, motor and valve assembly

The prime mover and pump set the pressure ceiling and flow. Gear, vane or piston pumps, fixed or variable displacement, each change the bracket, coupling and alignment work. State the motor specification (voltage, phase, power, duty), the pump displacement and the relief pressure, and the supplier will plan the skid layout, coupling guard and wiring route.

Valves mount on the manifold and must be named by type and standard. ISO 1219-1 covers the schematic symbols; the supplier builds to your circuit, not to a generic one. If the unit needs accumulators, coolers, heaters or digital sensors, list them as separate line items with their own acceptance notes so they are not treated as optional.

Pressure test, filtration and inspection evidence

A delivered hydraulic power unit should be cleaned, filled, pressurised and function tested before it leaves the shop. ISO 4413, the international standard for hydraulic fluid power systems and their components, sets the general safety and construction rules a buyer can cite when requiring the unit to be safe to install and operate (ISO 4413:2010). ISO 4406 covers the fluid cleanliness code, which drives the filter rating and the flush target.

Define the test: flush to a cleanliness class, proof pressure on the welded group, pressure and leakage test on the assembled unit, and a function check of every valve. Ask for a test report with the as found readings, not a pass or fail stamp. The report is the evidence that the package behaves the way the drawing promised.

What to send with the RFQ

A useful hydraulic power unit enquiry includes:

  • hydraulic schematic or P&ID, with revision and status;
  • reservoir general arrangement, material grade, plate thickness and baffle layout;
  • manifold block drawings with datum, port standard and sealing face finish;
  • bill of materials with approved material grades and certificate level;
  • welding code, WPS and PQR basis, joint types and inspection level;
  • motor and pump specification, relief pressure and duty cycle;
  • valve list by type and standard, plus accumulators, cooling or sensors;
  • fluid type, target cleanliness class and filter rating;
  • required pressure test, flush target and inspection documents;
  • preferred packaging, marking and delivery scope to site.

Ask each bidder to list every assumption and deviation. The proposal should separate fabrication and machining from valves, motor, pump and instrumentation supply, and from installation and commissioning, so you can compare like for like.