Hydraulic Power Unit Specification and RFQ Checklist

A hydraulic power unit RFQ checklist should define the required operating points, duty cycle, available power supply, fluid conditions, interfaces, control behavior, and acceptance evidence. Pressure, flow, and motor size alone do not give suppliers enough information to quote equivalent packages. The request should also identify what is included, what remains the machine builder’s responsibility, and which assumptions still need confirmation.

Use the specification as a shared engineering record. Describe the application first, ask suppliers to respond against the same requirements, and keep deviations visible. This guide provides operating-point, specification, quotation, and acceptance tables that can be adapted to a project. They support a technical review; they do not replace the design work needed to approve a complete machine.

Contents

1. Define the RFQ stage and package boundary
2. Use an operating-point table instead of two maxima
3. Describe duty cycle, supply, and control behavior
4. Specify fluid, reservoir, and environmental conditions
5. Complete a reusable HPU specification sheet
6. Request design evidence and acceptance records
7. Compare quotations with a deviation register
8. Control clarifications and release the agreed scope
9. Frequently asked questions

Define the RFQ stage and package boundary

State whether the request is for a budget estimate, a technical proposal, a replacement unit, or a purchase-ready package. A budget estimate may use declared assumptions. A production quotation needs enough confirmed information to define what the supplier will actually deliver. Label unresolved items rather than hiding them behind a selected catalog model.

Describe the machine, its functions, the number of units required, expected production quantity, installation location, and intended service. Include drawings or an action sequence where they explain the task better than a short description. Identify who can answer application questions and who can approve technical changes.

Define the package boundary. Does it include only a motor and pump, or also the reservoir, manifold, valves, filtration, cooler, sensors, cabinet, starter, drive, software, hoses, and commissioning? List separately any components provided by the customer. A supplied control valve does not necessarily include its electrical controller or the machine’s operating logic.

For replacements, record the existing model, circuit revision, settings, dimensions, and known operating conditions. Explain whether the new unit must duplicate the existing function or correct a documented limitation. A photograph and nameplate are useful evidence but rarely define every interface.

The hydraulic power unit fundamentals article explains the relationship between the package and the wider hydraulic system.

Use an operating-point table instead of two maxima

Write pressure and flow as paired requirements at defined locations. Identify whether pressure is required at the HPU outlet or at an actuator after line and valve losses. State the corresponding return or drain conditions where they matter. Flow at no load may differ from flow delivered at working pressure.

Parker’s application data sheet explicitly separates no-load flow from flow at operating pressure. That distinction is useful in an RFQ because it prevents a free-flow catalog value from being presented as loaded delivery.

An original illustrative duty table might look like this:

Phase	Required outlet pressure	Required outlet flow	Duration and overlap
A: loaded motion	180 bar	24 L/min	Project to define; separate from B
B: faster, lighter motion	60 bar	40 L/min	Project to define; separate from A
Hold	To be defined by the holding circuit	To be established, including leakage needs	Project to define
Idle	To be defined by the control arrangement	To be established	Project to define

These are invented example values, not recommended settings. Treating pressure relative to a near-zero inlet pressure for this example, hydraulic power is p × Q ÷ 600 in kW. Phase A represents 7.2 kW and phase B represents 4 kW. Combining the separate maxima as 180 bar at 40 L/min would describe a different, 12 kW hydraulic operating point.

The supplier must evaluate the actual sequence, including transitions and possible overlaps. If A and B can occur together, their combined demand needs its own analysis. Ask for achievable pressure-flow capability at the stated fluid condition and speed. Use hydraulic power unit selection when developing those requirements.

Describe duty cycle, supply, and control behavior

“Intermittent duty” is incomplete. Specify how long each function operates, how long it rests, how often the cycle repeats, and how many cycles or hours occur during the expected working period. Explain whether the pump continues running during a hold or idle phase.

Identify cold starts, frequent motor starts, continuous production periods, and unusual but required events. Distinguish the normal cycle from exceptional operation. A supplier should state any duration limit or recovery requirement rather than assume that a short peak rating is continuous capability.

The Bosch Rexroth sizing assistant describes selection using pressure-flow requirements and time-dependent load cycles. The RFQ should supply the project’s own cycle; a generic example in a configurator is not proof that the application has been represented.

For AC supply, state the available voltage, frequency, phases, starting arrangement, and relevant site restrictions. For DC or variable-speed drives, state the actual supply range and required operating conditions. Identify who provides the drive, electrical protection, control power, connectors, and wiring.

Describe required operating states: start, loaded motion, hold, return, idle, stop, and relevant fault responses. Specify commanded behavior and feedback separately. A pressure switch, for example, supplies a signal; the machine logic determines what action follows it. The functional explanation in how a hydraulic power unit works helps turn these states into an unambiguous sequence.

Specify fluid, reservoir, and environmental conditions

Identify the intended fluid, expected operating temperature range, cold-start conditions, and any compatibility restrictions. If these are not yet known, ask the supplier to propose a documented basis for engineering approval. Do not select a seal material or fluid grade solely by copying another application.

Reservoir requirements include usable fluid volume, actuator volume changes, return-flow peaks, service access, and mounting orientation. Nameplate tank capacity and usable oil volume are different quantities. The selected arrangement must accommodate the machine’s actual fluid exchange and installation conditions.

Bosch Rexroth’s HPU specification article highlights actuator sequence, return-flow behavior, and the installation environment as design inputs. Its broader lesson for a buyer is to describe the application boundary, rather than prescribe a tank size without its basis.

For filtration, state the required cleanliness outcome, where it will be checked, the applicable component requirements, and who establishes the acceptance method. A filter element’s nominal description alone does not establish system cleanliness.

Define ambient conditions, elevation where relevant, corrosion exposure, vibration, noise assessment conditions, and available cooling utilities. Ask the supplier to identify the expected heat load and cooling assumptions. Specify the conditions for any noise limit instead of writing only “quiet operation.” For thermal background, see hydraulic system overheating.

Complete a reusable HPU specification sheet

The following is an original working template. Replace “To define” with a value, a drawing reference, or a clearly assigned open item. Give each revision a date and identify the document that takes precedence if attachments disagree.

Field	Buyer requirement or reference	Supplier response needed
Project, machine, quantity, RFQ revision	To define	Confirm basis and quantity
Package boundary and customer-supplied items	To define	Included items and exclusions
Functions and action sequence	To define	Proposed operating concept
Paired pressure-flow points and measurement locations	To define	Capability at each point
Cycle durations, overlap, starts, and operating hours	To define	Duty limits and assumptions
Actuator data and circuit revision	To define	Missing information or concerns
Electrical source and control-power conditions	To define	Motor, drive, and interface proposal
Command, feedback, and fault-response responsibilities	To define	Interface and responsibility matrix
Fluid, temperature, and cleanliness requirements	To define	Compatibility and conditioning proposal
Reservoir usable volume and fluid exchange	To define	Capacity basis and level limits
Cooling utilities and environmental conditions	To define	Heat-removal assumptions
Ports, drains, connections, and mounting envelope	To define	Drawings and connection schedule
Inspection, tests, criteria, and records	To define	Test scope and stated deviations
Documentation, spares, support, and delivery scope	To define	Included deliverables and options

Attach an operating-point table rather than compressing a changing cycle into one cell. For cylinder-driven applications, the cylinder specification and RFQ checklist can help define the actuator inputs.

Mark every requirement as confirmed, provisional, or awaiting information. Add an owner and target resolution point to open items. “Supplier to propose” is useful only when the required outcome and approval responsibility are also stated.

Request design evidence and acceptance records

Ask the supplier to explain how the proposed package meets the operating points and duty cycle. Useful evidence can include the circuit, component selections, pressure-flow capability, motor loading, reservoir-volume basis, thermal assumptions, installation drawings, and a list of configuration-dependent limits.

Agree which documents require review before manufacture and which records will be supplied with the completed unit. If a standard applies, state its identifier, edition, scope, and the project responsibility for checking it. The public scope of ISO 4413:2010 concerns general rules and safety requirements for hydraulic systems and components. Citing its title does not demonstrate compliance with the full standard or approve a machine.

Define the factory acceptance test, or FAT, around measurable requirements:

Requirement	Conditions to define	Evidence to retain
Loaded delivery	Pressure, flow, fluid state, speed, and location	Paired measurements
Duty capability	Sequence, duration, repetition, and utilities	Performance and temperature records
Control functions	Commands, settings, signals, and expected response	Functional record
Interfaces and dimensions	Approved drawings and supplied configuration	Inspection results
Leakage and protective functions	Applicable approved method and limits	Test results and review
Documents and configuration	Required versions and deliverable list	Completed handover register

A brief pressure hold cannot establish full-flow power capability. A no-load run cannot establish the required continuous loaded cycle. Record actual conditions, instrument suitability, and the agreed acceptance rule. The test bench capability requirements article explains why test capacity must match the requested evidence.

Separate factory checks from site acceptance or commissioning. Identify what can only be verified after connection to the actual machine and site utilities.

Compare quotations with a deviation register

Compare suppliers against the same RFQ revision. A lower total can reflect excluded controls, omitted testing, a smaller usable reservoir volume, different duty assumptions, or customer-supplied installation work. Resolve those differences before treating the totals as equivalent.

Use explicit response categories: included as specified, offered with deviation, optional at additional cost, excluded, or awaiting clarification. Avoid a single “complies” checkbox where the proposed configuration contains a limitation.

Requirement	Supplier response	Deviation or evidence	Buyer action
Operating point A	To be completed	Curve or test basis	Technical review
Continuous cycle	To be completed	Duration and cooling assumptions	Confirm duty
Controls and cabinet	To be completed	Included functions and interfaces	Resolve boundary
Acceptance testing	To be completed	Method, criteria, and records	Agree test plan
Documentation and support	To be completed	Deliverables and exclusions	Confirm handover

Ask suppliers to identify substitutions by model and function. “Equivalent” is a proposal for review, not evidence that pressure rating, seal compatibility, control response, or service requirements are unchanged.

Keep commercial comparison separate from unresolved engineering questions. Record lead time, delivery scope, optional prices, commissioning, spares, and support conditions alongside the technical deviations. The test bench buyer’s checklist offers a related approach to comparing scope and acceptance evidence.

Control clarifications and release the agreed scope

Maintain one clarification register with the question, agreed answer, responsible party, affected document, and resolution status. Update the requirement when an answer changes the design basis. An email answer that never reaches the controlled specification can leave two conflicting versions in circulation.

Before purchase or manufacture release, confirm the agreed circuit concept, operating points, interfaces, approved deviations, and verification plan. Make the approval stage explicit: permission to quote, acceptance of a proposal, drawing approval, and release for manufacture are different decisions.

Check consistency between the quotation, technical specification, drawings, component list, and test plan. If a supplier changes a pump control, motor, valve function, or cooling arrangement, review the affected performance and interfaces rather than only the part price.

At handover, obtain the final configuration, settings record, circuit and electrical drawings as applicable, installation and maintenance instructions, test records, spare-parts information, and open-issue list. Identify which adjustments require authorized technical control.

Keep a record of what was actually accepted and under which conditions. This provides a useful basis for later troubleshooting or replacement. It also prevents a budget assumption from silently becoming an approved operating limit.

Frequently asked questions

Can an RFQ be issued before all parameters are known?

Yes, for an explicitly defined proposal or budget stage. List the unknowns, responsible owners, and assumptions permitted for the quotation. Resolve design-critical items before the relevant release decision.

Is the existing motor power enough to specify a replacement HPU?

No. The replacement must match the operating points, duty, control behavior, interfaces, fluid, and installation requirements. Existing problems should be documented rather than reproduced unknowingly.

Should the buyer specify a fixed reservoir multiplier?

Only if a justified project requirement applies. Otherwise describe fluid exchange, duty, environment, and installation constraints, and request the supplier’s sizing basis. A multiplier alone cannot describe all of those conditions.

Does an accepted quotation mean the machine is technically approved?

No. The quotation defines a proposed supply scope. Engineering approval, verification, installation, and machine acceptance remain separate responsibilities. Complete the known fields, assign the open questions, and request a line-by-line response before closing the RFQ review.