Designing a Fluid Power System
Line sizing, pressure drops, efficiency, energy cost and a complete worked design.
The design steps
- Define the work: force, stroke, speed, cycle time, duty cycle and environment for each actuator.
- Choose the actuators (Cylinders and Actuators): bore from force and working pressure, rod from buckling, then flow from speed.
- Draw the circuit (Circuits and Accumulators and Valves and Flow Control): directions, speeds, load holding, sequence, safety.
- Size the pump and motor (Hydraulic Pumps and Motors): flow and pressure; consider the idle strategy.
- Size the lines, tank, filters and cooler (Fluids, Filtration and Reservoirs).
- Check pressures, temperatures and safety, then build, test and commission.
Sizing the lines
Lines are sized for velocity: about 0.5 to 1.2 m/s on the suction line (to keep the pump from cavitating), 2.5 to 5 m/s on the pressure line (up to 6 m/s at high pressure) and 1.5 to 4 m/s on the return. The required inside diameter is d = √(4 Q ÷ (π v)). Oil in a small line is usually laminar (Re below about 2 300), where the pressure drop is Δp = 32 μ L v ÷ d² (Hagen–Poiseuille), and the drop climbs with the viscosity, so a cold start is the worst case.
Worked example: lines for 30.2 L/min at 45 °C
| Line | Ideal ID | Standard ID | Velocity | Reynolds | Drop per meter |
|---|---|---|---|---|---|
| Suction | 28.3 mm | 32 mm | 0.63 m/s | 543 | 0.006 bar/m |
| Pressure | 12.6 mm | 16 mm | 2.50 m/s | 1 085 | 0.100 bar/m |
| Return | 17.9 mm | 20 mm | 1.60 m/s | 868 | 0.041 bar/m |
- Oil of VG 46 at 45 °C has ν = 36.9 cSt, μ = 0.032 Pa·s; all three lines are laminar.
- The pressure line (16 mm) loses 0.100 bar per meter; for a 6 m run that is 0.60 bar: negligible against 160 bar, but the pump line loss is a heating load.
- A larger suction line of 32 mm (0.63 m/s) keeps the pump inlet near atmospheric pressure.
Pressure drops and settings
Add the drops to the load pressure to find the pump pressure, and set the relief above it. For the press at 160 bar load: line 0.6 bar, the directional valve about 5 bar, the return filter and fittings 4 bar: 9.6 bar in all, so the pump delivers about 170 bar at the pump outlet and the relief is set at about 190 bar (10 to 15 percent above the working pressure); every part must be rated above that.
Energy: three ways to idle
Presses and many machines work only part of their cycle. What the pump does in the rest decides the energy and the heat. Three designs for the 10 s press cycle (4.0 s pressing at 160 bar, 2.7 s retracting at 30 bar, 3.3 s idle):
| Design | Idle behavior | Energy per cycle | Per year (4 000 h) | Cost per year |
|---|---|---|---|---|
| A. Fixed pump, all flow over the relief valve | full pressure, full power | 92.0 kJ | 36 808 kWh | 4 417 |
| B. Fixed pump with an unloading valve | flow to tank at 5 bar | 42.5 kJ | 16 986 kWh | 2 038 |
| C. Servo (speed-controlled) pump | pump nearly stopped | 41.7 kJ | 16 675 kWh | 2 001 |
Design B costs only an unloading valve and saves 54 percent of the energy; C saves 54.7 percent (a little more, since nothing flows when idle) and runs much quieter, with the speed under electronic control. (Costs at 0.12 per kWh; the arithmetic is the motor power times the time in each phase.) The heat saved is the same fraction: the cooler of Fluids, Filtration and Reservoirs may not be needed.
Worked example: the press design
Worked example: the complete press
- Cylinder: 80/45 mm, 400 mm stroke: 80.4 kN at 160 bar; rod buckling margin more than 3 (see Cylinders and Actuators).
- Pump: 21.2 cm³/rev (standard 22), at 1 500 rpm; motor 9.2 kW load → 11 kW motor at 84 percent loading in the pressing phase.
- Valves: NG10 4/3 tandem valve (5 bar at 30 L/min), relief at about 190 bar, unloading valve for the idle.
- Lines: 16 mm pressure, 25 mm suction, 20 mm return, as the table above.
- Reservoir: 100 L, return filter 10 µm (β ≥ 200), breather, level and temperature gauge; a cooler only if the oil exceeds 55 °C (design B generates little heat).
- Safety: guard with interlock, a pressure relief for the pressing end, a dump valve to release stored energy.
Hoses, tubes and fittings
Hoses are rated for a working pressure with a burst factor of 4:1. The normal working pressure of 160 bar plus a 25 percent allowance for pressure spikes is 200 bar, so choose a hose of at least 250 bar working pressure (burst at least 800 bar), with the right temperature rating, bend radius and length (hoses change length by 2 percent under pressure). Steel tube is cheaper and more rigid; the wall for a 25 mm tube at 250 bar and an allowable stress of 150 MPa is t = p d ÷ (2 σ) = 2.08 mm (thin-wall approximation, plus a corrosion allowance and the code’s factor). Use ISO 8434 (flareless) or SAE fittings; avoid tapered threads in new designs and use O-ring face seals for leak-free joints.
Checks before you build
- Does every actuator have a clear pressure, flow and speed? Is the highest pressure anywhere (including meter-out intensification) below the lowest component rating?
- Are there relief valves in every possible trapped volume (thermal expansion of trapped oil raises the pressure by about 10 bar per °C)?
- Is the load held safely when the power fails (check valve, counterbalance, brake)?
- Can the system be isolated, vented and discharged (accumulators, trapped pressure)?
- Are filters, breathers, test points and the cooler placed so that they can be reached?
- Is the cleanliness code and the filter rating right for the most sensitive part?
Related
- Previous: Pneumatic Systems. Next: Troubleshooting and Safety.
- Pipe Flow and Pressure Drop Calculator (for water lines) and Pipe Friction and the Moody Diagram.
- Hydraulics and Pneumatics: the whole guide.
FAQ
Why is the suction line so large?
The pump can only pull oil in by atmospheric pressure, so any restriction lowers the inlet pressure and causes cavitation. A short, large, straight suction line avoids it.
How do I choose the relief setting?
About 10 to 15 percent above the highest working pressure including the line drops, and below the rating of the weakest component.
Should I use a load-sensing pump?
If several actuators share one pump and their loads differ, yes: it saves most of the energy a fixed pump would waste at the relief valve.
Is a hydraulic power unit better bought or built?
Buy a standard power pack unless the loads are unusual; the savings from custom design are rarely more than the cost of testing it.
References
- Esposito, A., Fluid Power with Applications, Pearson; Rabie, M. G., Fluid Power Engineering, McGraw-Hill.
- ISO 4413 (hydraulic fluid power: general rules and safety requirements); ISO 8434 (tube fittings); ISO 18752 and SAE J517 (hose); ISO 4414 (pneumatics).
- Manufacturers’ design guides: Bosch Rexroth, Parker, Eaton, Hydac, Danfoss.
Last reviewed: October 2026. Energy costs and velocity limits are typical.