How to Select a Hydraulic Pump, Valve, and Cylinder as a System

Select a hydraulic pump, valve, and cylinder by matching them to the same load and motion cases. Begin with cylinder force, stroke, and speed; calculate the pressure and flow in both directions; then check the valve's flow paths and the pump's delivery and control range. Repeat the checks until the proposed combination satisfies the complete cycle.

Matching nominal pressure ratings is only part of the work. A valve can have adequate pressure capacity but insufficient return-flow capacity, while a pump can provide enough peak flow but behave poorly with the chosen neutral arrangement. A useful selection record therefore connects operating points, pressure losses, control behavior, power, and interfaces. It also identifies the engineering checks still needed before manufacture or installation.

Contents

1. Start with simultaneous load and motion cases
2. Use cylinder geometry to establish both directions
3. Calculate supply and return flow separately
4. Build the pressure budget at each working point
5. Read the valve curves and control limits
6. Choose a compatible pump and neutral state
7. Check power, losses, and duty cycle
8. Review interfaces and protective functions
9. Verify the proposed combination before release
10. Frequently asked questions

Start with simultaneous load and motion cases

Write a movement table before choosing component sizes. For each action, define load direction, required cylinder force, stroke, speed, acceleration or stopping conditions, duration, and whether other functions operate simultaneously. Include holding, unloading, restart, and foreseeable changes in external load.

Case	Required information	Why it affects matching
Loaded extension	Push force and speed at the relevant position	Sets cap-side demand
Loaded retraction	Pull force and return speed	Sets annular-side demand and cap-side discharge
Holding	Load, permitted movement, and duration	Defines holding and standby requirements
Simultaneous motion	Functions that genuinely overlap	Defines shared supply demand
Cold and warm operation	Specified fluid and temperature conditions	Changes losses, inlet conditions, and control

Do not combine maximum force from one phase with maximum speed from a different phase unless they must occur together. Conversely, averaging a short but necessary high-power movement can hide a requirement the drive cannot meet.

Record whether an input is confirmed, estimated, or awaiting a supplier proposal. A provisional load should remain visibly provisional when it is carried into the pump and valve calculations.

The basic hydraulic system design guide describes the wider design process. This article concentrates on the compatibility of the three selected components.

Use cylinder geometry to establish both directions

For a conventional single-rod cylinder, cap area Ac = πD²/4 and annular area Aa = π(D² − d²)/4, where D is bore and d is rod diameter. Pressure forces must be calculated using the appropriate area on each side.

As an original illustration, assume a 63 mm bore and a 36 mm rod. The resulting areas are approximately 3,117.245 mm² and 2,099.369 mm². These are illustrative dimensions, not a selected product or a strength assessment.

For an extension-force requirement of 30 kN, assume the rod chamber is at 10 bar gauge pressure. Ignore friction and acceleration for this first calculation. The required cap pressure is:

pc(bar) = [10 × F(N) + pr(bar) × Aa(mm²)] / Ac(mm²)
pc ≈ 102.974 bar.

The opposing chamber pressure increases the required cap pressure. For retraction, reverse the force balance and use the full cap area for the opposing pressure. Do not use a single pressure-difference value multiplied by one area for both directions.

Parker's cylinder selection guide links push/pull requirements to bore, rod, attachment, and rod-strength considerations. Those mechanical checks remain necessary even when the hydraulic force calculation works. Parker HMI selection guide

See pressure, force, and area for the force relationships and hydraulic cylinder sizing for the broader selection checks.

Calculate supply and return flow separately

For approximately incompressible flow with leakage neglected:

Q(L/min) = A(mm²) × v(m/s) × 0.06.

Use the supplied chamber area for pump demand and the exhausting chamber area for return demand. Continuing the same example:

Motion	Assumed speed	Supply flow	Cylinder discharge flow
Extension	0.10 m/s	18.703 L/min into cap side	12.596 L/min from rod side
Retraction	0.15 m/s	18.894 L/min into rod side	28.055 L/min from cap side

A pump capable of roughly 19 L/min does not mean every valve path only needs to carry 19 L/min. During the illustrated retraction, the cap-side return path carries about 28 L/min.

Check the relevant valve metering edge, port, hose, fitting, filter, and any return-connected cooler at that discharge condition. A high return pressure also feeds back into the cylinder force balance.

Include specified leakage and any genuinely simultaneous users when establishing actual pump delivery. Pilot supply, cooling, or auxiliary branches should be counted according to the real architecture, rather than added as an arbitrary percentage.

If the design changes bore, rod, or motion speed, repeat both flow calculations. Improving one requirement can move the constraint somewhere else in the circuit.

Build the pressure budget at each working point

Starting at the working cylinder chamber, work upstream to the pump. Add the pressure drops in the active supply path at the required flow, including the valve's necessary operating differential and other relevant restrictions.

In the example, a hypothetical 15 bar supply-path drop would put required pump pressure at about 117.974 bar for the stated extension condition. The rod-side 10 bar has already been included in the force equation; do not add it a second time as if it were another independent supply loss.

That 15 bar is only an assumption demonstrating the accounting. Replace it with evidence from candidate curves, expected fluid conditions, and the implemented control strategy. There is no universal pressure allowance that works for every valve and circuit.

Calculate separate budgets for extension, retraction, combined functions, and relevant transient or holding states. Identify whether each value is at the pump outlet, valve inlet, cylinder chamber, tank port, or case drain.

A component's maximum pressure rating is a limit, not the pressure it will always generate or require. Similarly, a relief setting is not a substitute for knowing the pressure needed for useful motion.

The hydraulic power unit selection guide connects those working points to the supply assembly.

Read the valve curves and control limits

A valve's nominal flow is meaningful only with its stated pressure drop, spool opening, fluid conditions, and rating convention. Some data describe one metering edge; other data describe a complete flow path. Compare the same quantities before deciding that the valve is large enough.

Danfoss's proportional-valve technical information includes system calculations, flow-gain curves, and power-capacity envelopes. It illustrates why selection requires a working point and a usable operating region, rather than a flow number alone. Danfoss valve technical information

For an unchanged idealized orifice and fluid density, flow varies approximately with the square root of pressure drop. That relationship can help explain trends, but actual manufacturer curves and limits should govern selection.

Check both directions and both metering paths for an unequal-area cylinder. An oversized proportional valve may place ordinary low-speed operation near a small part of its command range. Lower pressure loss does not automatically mean better controllability.

Also examine minimum controllable motion, deadband, hysteresis, response, command type, and whether feedback is required. A valve with an integral amplifier still needs compatible electrical signals and an appropriate system control design.

Confirm the pressure capability of each relevant port. The tank port, external drain, and pilot circuit can have limits different from the main pressure port. Model-code options may change those limits.

Choose a compatible pump and neutral state

Select the supply concept together with the valve and machine states.

Supply concept	Compatibility question
Fixed displacement	Where does delivered flow go when the actuator stops?
Pressure compensated variable displacement	How does normal regulation interact with valve demand and protection?
Load sensing	Are signal routing, demand selection, and pressure margin compatible?
Suitable variable-speed drive	Can the selected pump operate across the required speed and duty range?

Parker's power-unit manual describes different fixed and variable pump arrangements and their unloading behavior. A neutral-state assumption from one configuration should not be transferred to another. Parker power-unit manual

A fixed pump paired with a blocked flow path needs an appropriate provision for its delivery. A variable pump is not automatically compatible with any proportional valve simply because both are adjustable.

For load-sensing arrangements, identify how the controlling load signal is selected and what happens at pump saturation. When several functions request more flow than the pump can deliver, motion priorities depend on the actual valve architecture.

Define restart behavior, control loss, and the required response when a command disappears. Holding the load and reducing standby losses are separate functions, even if the same manifold contains parts supporting both.

Use power unit operation to review the possible operating stages without assuming one universal circuit.

Check power, losses, and duty cycle

At a pump working point, hydraulic output is approximately Phyd(kW) = Δp(bar) × Q(L/min) / 600. For an illustrative 120 bar pressure rise at 19 L/min, output is 3.8 kW.

If overall pump efficiency were assumed to be 0.85 at that point, required shaft power would be about 4.47 kW. Electrical input would additionally reflect motor and drive losses. The efficiency assumption is not a recommended value or proof that a particular motor size is sufficient.

Separate peak capability from repeated-duty and thermal capability. Include acceleration, startup, time at pressure, dwell, and the cooling opportunity between cycles.

A 15 bar pressure drop across a path carrying 19 L/min converts approximately 0.475 kW into heat. Do not simply equate all hydraulic output with heat if useful mechanical work leaves the hydraulic system. Conversely, prolonged high-pressure bypass can create substantial heat without useful actuator work.

Check losses over the complete cycle and at the stated ambient and oil conditions. Reducing one valve restriction will not resolve every thermal problem. The hydraulic system overheating guide provides a diagnostic framework.

Review interfaces and protective functions

A compatible component list should be accompanied by a hydraulic schematic and an interface record. Include port identification, thread and seal forms, mounting, fluid compatibility, filtration requirements, electrical commands, and service access.

Confirm return and drain pressure limits during the highest discharge-flow case, including cold conditions where applicable. Do not merge a sensitive drain into a convenient return connection without checking the manufacturer's requirements.

For cylinders supporting external loads, establish holding and lowering functions, mechanical load paths, and response to energy loss. A directional spool in neutral is not automatically an accepted load-holding device.

Pressure protection must cover the relevant pressure zones and foreseeable operating conditions. A main supply relief cannot be assumed to control every isolated or intensified chamber. The responsible engineer must confirm the arrangement and settings.

Record who supplies and approves valves mounted on or near the cylinder, hoses, sensors, and software. The cylinder RFQ checklist helps preserve these responsibilities during procurement.

Verify the proposed combination before release

Create a table linking each operating case to the candidate components and their supporting evidence.

Check	Evidence to retain
Cylinder force and structure	Load case, chamber pressures, geometry, and mechanical review
Valve flow and control	Direction-specific working points on applicable curves
Pump delivery	Pressure-flow-speed point and operating limits
Standby and combined functions	Schematic states and demand-priority behavior
Power and thermal duty	Peak demand, cycle losses, and cooling basis
Interfaces and protection	Controlled drawings, port limits, and assigned responsibilities

Identify the least-supported assumption. It may be low-speed valve behavior, warm leakage, return pressure, or the actual load profile. Resolve it through supplier data, a suitable calculation, or a planned test before treating the combination as confirmed.

Verification should cover representative extension and retraction loads, repeat operation, and relevant boundary conditions. Use an approved test method and suitable measurement equipment. Record what was tested, at which temperature and supply condition, and against which acceptance criteria.

The test bench buyer’s checklist helps specify the evidence required from a test arrangement. Passing a no-load motion check does not establish loaded force, control quality, or thermal suitability.

Frequently asked questions

Can all three components simply use the same pressure rating?
That can be one requirement, but each component must also meet its local port pressures, flow paths, duty, and control demands. Matching the headline number is insufficient.

Is a larger valve always better?
It may reduce some losses, but useful command range and low-speed control still matter. Select against the operating points and required response.

Can a fixed pump work with a closed-center valve?
Only with a system arrangement that properly handles delivered flow and operating states. The valve's center designation alone does not define that arrangement.

Why recalculate after changing the rod diameter?
Rod diameter changes annular area, retraction force, supply demand, and the relationship between supply and discharge flows, as well as mechanical behavior.

Prepare the movement table, chamber-pressure budget, and directional flow calculations, then attach the candidate curves for a joint system review.