Everything mechanical and electrical engineers need to know about the operation of hydraulic power units: components, principles, diagrams, and selection criteria.

Hydraulic power units are the core of thousands of industrial applications, from heavy load handling to precise control of machine tools. But how exactly does a hydraulic power unit work? And what are the physical principles that allow these systems to generate enormous forces in confined spaces?

In this comprehensive technical guide, we will explore in detail the operation of hydraulic power units, starting from the fundamental principles of hydraulic power transmission to the advanced circuit diagrams used in modern applications. Whether you are a novice mechanical engineer or an experienced designer, you will find in-depth information for your hydraulic design.

What is a hydraulic power unit?

Technical definition

A hydraulic power unit (or hydraulic power unit in English) is an autonomous system that generates, controls, and distributes hydraulic energy through the pressurization of a fluid, typically mineral or biodegradable oil. The unit converts mechanical energy (provided by an electric or combustion engine) into fluid-dynamic energy, which is then transferred to actuators (hydraulic cylinders or motors) to perform mechanical work.

Fundamental physical principle: Pascal’s law

The operation of every hydraulic power unit is based on Pascal’s Principle (1653):

“The pressure exerted on an incompressible fluid in a closed container is transmitted uniformly in all directions.”

Fundamental formula:

P = F / A

Where:

P = Pressure (bar or Pa)

F = Force (N)

A = Area (cm² or m²)

Practical example:
If we apply a force of 100N on a piston with an area of 1 cm², we generate a pressure of 100 bar. This pressure, transmitted to a cylinder with a piston of 10 cm², produces a force of 1000N (10x force multiplication).

This principle of force multiplication is what makes hydraulic systems so powerful and efficient for applications requiring high forces in small spaces.

Why “hydraulic fluid power” and not simply “hydraulics”?

The term hydraulics derives from the Greek oleo (oil) + dynamis (force). This term is preferred when the fluid used is oil (mineral or synthetic) instead of water, to emphasize:

• Lubricating capacity of the oil (protects components)
• Incompressibility superior to water
• Thermal stability over a wider temperature range
• Anti-wear and anti-corrosion properties

In modern industrial systems, over 95% of power units use specialized hydraulic oils (including biodegradable fluids like Matrol BI) instead of water.

Operating principle: hydraulic cycle

The operation of a hydraulic power unit follows a continuous cycle of energy conversion, transmission, and control. Let’s look at the phases in detail.

Phase 1: Mechanical energy conversion → hydraulic

Input: An electric motor (AC or DC) or a combustion engine provides rotary mechanical energy to the pump shaft.

Process: The hydraulic pump, directly coupled or via a coupling to the motor shaft, performs two fundamental actions:

1. Suction: Creates a vacuum that draws oil from the tank
2. Pressurization: Compresses the fluid bringing it to the required operating pressure (typically 50-350 bar in standard applications, up to 700 bar in special systems)

Output: Pressurized oil with potential energy ready to perform mechanical work.

Hydraulic power formula:

P_hydraulic = (Q × ΔP) / 600

Where:

P = Hydraulic power (kW)

Q = Flow rate (l/min)

ΔP = Pressure difference (bar)

600 = Conversion constant

Example:

Flow rate 20 l/min × Pressure 200 bar = (20 × 200) / 600 = 6.67 kW

Phase 2: Controlled distribution

Involved components: Directional, pressure, and flow valves.

The pressurized oil is distributed to the actuators through a system of control valves that determine:

• Flow direction (directional valves): where the oil goes
• Maximum pressure (safety valves): system protection limit
• Flow rate (regulating valves): speed of actuators

Basic circuit example:

[Tank] → [Pump] → [Safety valve] ⊥

↓

[4/3 Directional valve]

↓

[Hydraulic cylinder]

↓

[Return to tank]

Phase 3: Hydraulic conversion → mechanical

The actuators convert hydraulic energy into mechanical work:

Hydraulic cylinders: Linear movement (push/pull)

• Developed force: F = P × A_piston
• Stroke: determined by cylinder length
• Applications: lifting, pressing, clamping

Hydraulic motors: Rotary movement

• Developed torque: T = (P × Displacement) / (20π)
• Speed: proportional to flow rate
• Applications: driving wheels, blades, drills

Phase 4: Return and cooling

The exhausted oil (after performing work) returns to the tank where:

1. It cools (via heat exchangers or natural dispersion from the walls)
2. It settles (heavy particles settle to the bottom)
3. It deaerates (air bubbles rise to the surface)
4. It is filtered (before being re-suctioned by the pump)

Optimal oil temperature: 40-60°C
Maximum allowable temperature: 80°C (beyond which properties degrade)
Cooling system: Necessary if temperature exceeds 70°C consistently

Main components: anatomy of a power unit

Every hydraulic power unit is composed of fundamental components that work in synergy. Let’s analyze them in detail.

Electric motor (or combustion engine)

Function: Provide the primary mechanical energy to the system.

Common types:

Motor power sizing:

P_motor = P_hydraulic / η_pump / η_motor

Typical η_pump: 0.85-0.92

Typical η_motor: 0.88-0.95

Example:

For 6.67 kW hydraulic, approximately 8-9 kW electric is needed

Trend 2025: Increasing adoption of high-efficiency brushless DC motors (>92%) with integrated electronic control, in line with Industry 5.0 directives and energy efficiency.

Hydraulic pump: the heart of the system

Function: Convert rotary mechanical energy into fluid-dynamic energy (flow + pressure).

Main types:

External gear pumps

• Principle: Two meshing gears transport oil between teeth and pump body
• Pressures: Up to 370 bar
• Flow rates: 0.5 – 250 l/min
• Advantages: Simplicity, reliability, low cost
• Disadvantages: Lower efficiency (82-88%), noise
• Applications: Forklifts, agricultural machinery, general systems

Axial piston pumps

• Principle: Axially arranged pistons compress oil through an inclined plate
• Pressures: Up to 400-450 bar (450-700 bar in special versions)
• Flow rates: 10 – 1000 l/min
• Advantages: High efficiency (93-96%), long life, low noise
• Disadvantages: High cost, complexity
• Applications: Machine tools, presses, high-performance systems

Vane pumps

• Principle: Sliding vanes in a centrifugal rotor transport oil
• Pressures: Up to 200-250 bar
• Flow rates: 5 – 250 l/min
• Advantages: Quietness, balance, compactness
• Disadvantages: Sensitivity to contamination
• Applications: Machine tools, robotics, noise-sensitive applications

Fixed vs variable displacement pumps:

• Fixed: Flow rate proportional to rotation speed, always constant
  • Pro: Simple, economical, reliable
  • Con: Energy waste if the load is variable
• Variable: Displacement (and thus flow rate) automatically adjustable based on load
  • Pro: 30-60% energy savings, reduced heat generation
  • Con: Higher cost (+40-80%), greater complexity
  • Ideal applications: Systems with variable work cycles (presses, injectors, test benches)

Hydronit insight: Hydronit’s Compact and Micro power units predominantly use high-efficiency gear pumps (88-90%) with optimized tooth profiles for noise reduction -5 dB compared to market standards.

Tank: not just a container

Main function: Oil storage, but performs multiple critical roles.

Tank sizing:

Minimum capacity = 2.5 × Pump flow rate (l/min)

Example: Pump 20 l/min → Tank ≥ 50 liters

Ratio 3-4x preferable for applications with intense cycles

Tank functions:

1. 1. Deaeration: Residence time allows air bubbles to rise
      • Minimum 3-5 minutes of recirculation time
      • Wide surface favors air release
   2. Thermal dissipation: Metal walls dissipate heat
      • External surface ≥ 0.02 m²/kW dissipated (for natural cooling)
      • For powers >10 kW, a dedicated exchanger is often necessary
   3. Decantation: Heavy particles settle to the bottom
      • Periodic sludge accumulation drainage
      • Suction filters protect the pump

• Level and temperature control:

1. • Visual level indicator or electronic sensor
   • Analog thermometer or PT100 probe

Construction materials:

• Painted carbon steel: Standard, economical, 60% market
• Stainless steel 304/316: Marine, food, pharma applications
• Aluminum: Mobile applications, weight saving -40%
• Plastic (HDPE): Special applications, corrosion resistance

Tank accessories:

• Breather cap with air filter (prevents contaminant ingress)
• Level indicator with alarm
• Thermometer
• Oil fill/drain port
• Magnets for capturing ferrous particles

Hydronit innovation: The smart power unit series tanks in 6082-T6 aluminum integrate IoT level and temperature sensors with real-time data transmission.

Control valves: the hydraulic brain

Valves are the components that determine where, when, and how pressurized oil performs work.

Safety valves (pressure limiters)

Function: Protect the system from dangerous overpressures.

Operating principle:

• Set to max system pressure (e.g., 400 bar)
• If pressure exceeds threshold → valve opens → discharges oil to tank
• Antagonist spring closes valve when pressure returns

Types:

• Direct: Spring directly opposes oil pressure (up to ~100 bar)
• Pilot-operated: Pilot stage commands main stage (100-400 bar)
• Proportional: Gradual opening proportional to pressure

Setting:

P_setting = (P_max_system × 1.1) ÷ 1.15

Safety margin 10-15% above max working pressure

Directional valves

Function: Direct flow to actuators, determining movement direction.

Standard ISO 1219 nomenclature:

• 4/3: 4 ways (ports), 3 positions
• 4/2: 4 ways, 2 positions
• 3/2: 3 ways, 2 positions

Example 4/3 valve:

Positions:

1. Closed center: flow blocked, cylinder stopped
2. Right: oil goes to cylinder chamber A → rod advances
3. Left: oil goes to cylinder chamber B → rod retracts

Valve controls:

• Manual: Lever, button (simple machines)
• Electric: Solenoid 12V/24V/230V (automation)
• Hydraulic pilot: Oil pressure commands (complex systems)
• Proportional: Gradual opening with 0-20mA / 4-20mA current

1. C) Flow control valves

Function: Control actuator speed by limiting flow rate.

Types:

• Fixed orifice: Calibrated orifice (simple, non-adjustable)
• Adjustable orifice: Adjustment screw (manual setup)
• Pressure compensated: Maintains constant flow even if load varies
• Proportional: Electronic flow control

Cylinder speed formula:

v = Q / (A × 60)

v = Speed (m/s)

Q = Flow rate (l/min)

A = Piston area (cm²)

Example:

10 l/min / (50 cm² × 60) = 0.0033 m/s = 20 cm/min

Filters: guardians of cleanliness

Function: Remove contaminant particles from oil before they damage components.

ISO 4406 filtration classes:

Example: ISO 18/16/13

18 = Particles > 4 μm

16 = Particles > 6 μm

13 = Particles > 14 μm

Objectives:

– Standard systems: ISO 20/18/15

– Servo-controls: ISO 17/15/12

– Critical systems: ISO 16/14/11

Filter placement:

• Suction filter (in the tank):

1. 1. • Coarse 100-150 μm
      • Protects pump from coarse contaminants

• Delivery filter (after pump):

1. 1. • Fine 10-25 μm
      • Protects valves and actuators

• Return filter (before the tank):

1. • Medium 25-40 μm
   • Captures particles generated by component wear

Clogging indicator: Essential for preventive maintenance

• Visual (3 colors: green/yellow/red)
• Electrical (switch contact for PLC alarm)

Accumulators: hydraulic batteries

Function: Store hydraulic energy for quick release.

Principle: Compressible gas (nitrogen) separated from oil by membrane/piston.

Applications:

• Emergency energy reserve: Activation in case of blackout
• Pressure peak absorption: Damping water hammer
• Leak compensation: Maintain pressure without active pump
• Energy for fast cycles: Instant release for rapid operations

For motor power supply

AC power units (Alternating Current)

Single-phase 230V:

• Max power: 2.2 – 3 kW
• Applications: Small machines, workshops, garages
• Advantages: Plug & play on domestic network
• Disadvantages: Limited power

Three-phase 230V:

• Max power: 7.5 kW
• Applications: Small machines, workshops, garages
• Advantages: Plug & play on domestic network
• Disadvantages: Limited power

Three-phase 400V:

• Power: 0.5-100+ kW
• Applications: Industry, fixed systems
• Advantages: High efficiency, high power, robustness
• Disadvantages: Requires dedicated three-phase line

1. DC power units (Direct Current)

12V / 24V DC:

• Power: 0.15-4 kW typical
• Applications: Vehicles, carts, RV, marine
• Advantages: Battery power supply, mobility, low voltage safety
• Disadvantages: Limited power, requires sized batteries

48V DC (growing trend):

• Power: 0.15-3 kW
• Applications: Electric vehicles and electrified systems
• Advantages: Better efficiency vs 12/24V, medium power
• Disadvantages: Less widespread standard

1. Combustion power units (Gasoline/Diesel)

• Power: 5 kW
• Applications: Construction sites, agriculture, rescue, areas without power grid
• Advantages: Autonomy, total mobility
• Disadvantages: Emissions, noise, thermal engine maintenance

Learn more: Combustion vs Electric Power Units

For size and compactness

Micro power units, PPM

• Volume: < 6 liters
• Power: 0.15-3 kW
• Weight: 4-10 kg
• Applications: Medical devices, robotics, aerospace, compact actuators
• Hydronit example: MICRO Series – Micro hydraulic power units

Mini / compact power units

Volume: 1.5-30 liters

• Power: 0.15-7.5 kW
• Weight: 8-40 kg
• Applications: Forklifts, platforms, RV, marine, mobile machines
• Hydronit example: COMPACT Series (PPC) – Modular Compact Power Units

Standard power units

Volume: 80-500 liters

• Power: 7-75 kW
• Weight: 150-800 kg
• Applications: Presses, machine tools, fixed industrial systems

Smart power units

• Control: Programmable dedicated PLC/Controller
• Sensors: pressure, temperature, flow, position, current
• Communication: CAN, Ethernet, Modbus, IO-Link, wireless
• Diagnostics: Predictive with ML algorithms
• Efficiency: Load-sensing, automatic standby

Applications: Industry 4.0, fleet management, safety-critical applications

• Hydronit example: Smart Power Unit (SPU) – First programmable digital power unit

Applications of hydraulic power units

Hydraulic power units find application in virtually every industrial sector. Let’s look at the main ones.

Mobile sector

Forklifts and handling:

• Fork lifting (multi-stage telescopic cylinders)
• Mast tilting
• Clamp/accessory movement
• Requirements: Compactness, 12/24V DC, energy efficiency

Aerial platforms (MEWP):

• Lifting/lowering (main cylinders)
• Tower rotation
• Stabilizers
• Requirements: SIL2 redundancy, EN280, precise control

Agricultural machines:

• Tool lifting (plow, harrow, seeder)
• Steering (load-sensing systems)
• Distribution arms
• Requirements: Robustness, IP65, extreme temperatures

Industrial sector

Hydraulic presses:

• Metal, plastic, composite pressing
• Requirements: High pressures (250-400 bar), precise force control

Machine tools:

• Piece clamping
• Tool advancement
• Automatic tool change
• Requirements: Precision, repeatability, oil cleanliness (ISO 16/14/11)

Handling systems:

• Lifting tables
• Rotary platforms
• Locking systems
• Requirements: Multi-actuator synchronization, operator safety

Marine and offshore sector

Nautical applications:

• Hydraulic steering
• Onboard cranes
• Gangways/ramps
• Anti-roll stabilizers
• Requirements: RINA/Lloyd’s certifications, stainless steel 316L, IP68, saltwater resistance

Oil & Gas:

• BOP (Blowout Preventer)
• Subsea valves
• ROV (Remotely Operated Vehicles)
• Requirements: Extremely high pressures (500-700 bar), critical reliability, ATEX Zone 1

Renewable energy sector

Solar tracking:

• Single-axis/dual-axis solar trackers
• Requirements: Energy efficiency, outdoor IP66, minimal maintenance

Wind turbines:

• Orientation system
• Hydraulic brakes
• Requirements: Reliability, temperatures -40 to +60°C, remote diagnostics

How to choose a hydraulic power unit: selection criteria

Designing or selecting the right power unit requires a methodical analysis of various parameters. Here is the complete checklist.

Application analysis

Step 1: Determine required force and speed

For hydraulic cylinders:

Required force (N) = Load (kg) × g × Safety coefficient

= Load × 9.81 × 1.2-1.5

Required piston diameter:

d = √(4F / π×P)

Where:

F = Force (N)

P = Working pressure (Pa)

Example:

Lift 1000 kg at 200 bar

F = 1000 × 9.81 × 1.3 = 12,753 N

d = √(4 × 12,753 / 3.14 × 200×10⁵) = 0.028 m = 28 mm

→ Standard cylinder Ø32mm

Rod speed:

v = Q / (A × 60)

v = speed (m/s)

Q = flow rate (l/min)

A = piston area (cm²)

Example:

Desired speed: 10 cm/s = 0.1 m/s

Area Ø32mm = 8 cm²

Required Q = 0.1 × 8 × 60 = 48 l/min

Step 2: Calculate system flow and pressure

Total required flow:

Q_tot = Σ(Q_single × Duty_cycle)

Consider:

– Number of actuators simultaneously

– Duty cycle (% active time)

– Safety margin 10-15%

Working pressure:

P_working = (F / A) × 1.1

10% margin for pipeline/valve pressure losses

Safety setting pressure:

P_safety = P_working × 1.25-1.4

Component selection

Pump selection

Criterion 1 – Application type:

• Constant load, high duty cycle (>60%) → Gear or piston pump
• Variable load → Variable displacement pump (40-60% energy saving)
• Pressures >300 bar → Axial piston pump

Criterion 2 – Noise level:

• Sensitive environment (laboratories, offices) → Vane or piston pumps (<60 dB)
• Standard industrial environment → Optimized gears (<70 dB)

Criterion 3 – Budget:

• Entry-level → Gear pumps
• High-performance → Piston pumps

Tank sizing

Rule of thumb:

V_tank = (2.5 ÷ 4) × Q_pump

Example:

Pump 30 l/min → Tank 75-120 liters

Choose 100 liters standard

Consider:

– High duty cycle (>70%) → 4x ratio (more cooling time needed)

– Low duty cycle (<30%) → 2.5x ratio acceptable

Motor selection

P_motor = (Q × ΔP) / (600 × η_pump × η_motor)

η_pump: 0.85-0.92

η_motor: 0.88-0.95

Example:

30 l/min × 250 bar / (600 × 0.88 × 0.90) = 15.7 kW

→ Choose standard commercial motor 18.5 kW

Motor voltage:

• Fixed application with industrial network → 400V three-phase (max efficiency)
• Mobile application commercial vehicle → 24V DC
• RV/caravan application → 12V DC
• Hybrid/electrified application → 48V DC (growing trend)

Environmental and regulatory considerations

Ambient Temperature

IP Protection (Ingress Protection)

• IP20: Protected workshop interior
• IP54: Standard industrial environment (non-harmful dust)
• IP65: Outdoor, agriculture (water jet protection)
• IP66/67: Marine, carwash (powerful jet protection)

IP68: Temporary immersion (subsea, drainage)

Need support for your hydraulic power unit project?

We hope this technical guide has provided valuable insights for your hydraulic power unit application. Whether you’re designing a new system or optimizing an existing one, selecting the right components and configuration is crucial for achieving optimal performance, reliability, and efficiency.

If you need concrete assistance with your hydraulic project, our engineering team is ready to help. With over two decades of experience in designing custom hydraulic solutions for industrial applications worldwide, we can provide technical consultation tailored to your specific requirements.

Contact our team to discuss your project challenges and explore how our intelligent power units can meet your application needs. Alternatively, browse our complete catalog to discover our full range of hydraulic power units, components, and customization options.