Hydraulics

What is hydraulics?

Hydraulics is a drive technology in which a pressurised fluid, usually mineral oil, transmits mechanical force and motion from a pump to cylinders or hydraulic motors. Within industrial automation you choose hydraulics when you need high forces, robust linear motion or accurate positioning under heavy load: presses, injection moulding machines, rolling mills, cranes, locks and storm surge barriers. Modern hydraulic installations are no longer purely mechanical. They are driven by a PLC or motion controller, measured with sensors and connected to SCADA and predictive maintenance, which also brings them within the scope of cybersecurity in OT.

🧠 How does hydraulics work?

Hydraulics is based on Pascal’s law: pressure applied to a confined fluid is transmitted equally in all directions. Because oil is barely compressible, that pressure reaches the actuator almost without delay.

The force of a cylinder follows from F = p × A (force = pressure × piston area). A worked example:

• A cylinder with a 100 mm bore has a piston area of about 7,854 mm².
• At 200 bar (20 N/mm²) that cylinder delivers 7,854 × 20 ≈ 157,000 N, so roughly 157 kN or about 16 tonnes of pushing force.
• An electric or pneumatic drive with the same force is considerably larger and heavier.

That high power density is the main advantage of hydraulics. Cylinder speed is determined by the flow rate (litres per minute) and force by the pressure. In practice you therefore control speed with flow and force with pressure.

🔩 What are the components of a hydraulic system?

A hydraulic system, often built around a hydraulic power unit (HPU), consists of the following parts:

Pumps differ widely in pressure range and controllability:

Cylinders are single-acting (pressurised in one direction, returned by a spring or by gravity), double-acting (pressurised in both directions) or telescopic for a long stroke in a short installed length. A differential cylinder is a double-acting cylinder with unequal areas on the piston and rod side, so force and speed differ per direction.

Valves determine how the system behaves:

Hydraulic oil does more than transmit energy: it also lubricates, cools, protects against corrosion and carries away contaminants. Common viscosity grades are ISO VG 32, 46 and 68. Cleanliness is expressed with the ISO 4406 code: three numbers for the count of particles per millilitre larger than 4, 6 and 14 µm. Each step up roughly doubles the particle count, so 18/16/13 is clearly dirtier than 16/14/11.

🏭 Where is hydraulics used?

A well-known Dutch example is the Oosterschelde storm surge barrier (Oosterscheldekering, 1986): its 62 steel gates, each about 42 metres wide, are each moved by two hydraulic cylinders mounted on the piers. Rijkswaterstaat is renewing these hydraulic installations over the coming years. Many locks and movable bridges operated by Rijkswaterstaat and the Dutch water boards are also hydraulically actuated and remotely controlled. This places hydraulics squarely within OT security in the water sector.

🔄 How does closed-loop hydraulic control work?

Many modern hydraulic systems operate as a closed control loop:

1. A sensor measures position, pressure or force.
2. The PLC or motion controller compares the measured value with the setpoint and computes a correction.
3. A proportional or servo valve adjusts the flow.
4. The cylinder moves and the new position is measured again.

This creates a Feedback Loop for synchronising several cylinders, precision positioning, and pressure and force control. Usually this is a PID control loop with feedforward; advanced systems use adaptive control or Model Predictive Control. In motion control, servo-hydraulics competes with electric servomotors. Hydraulics wins where forces are very high and shock loads occur.

⚡ What is the difference between hydraulic, pneumatic and electric drives?

Hydraulics remains dominant wherever extremely high forces, compact design or robustness under harsh conditions are required.

🧠 How is hydraulics integrated into the OT network?

A modern HPU contains embedded controllers, smart sensors and a network connection. Hydraulic subsystems often act as distributed IO or as an intelligent motion node within a larger architecture.

Useful measurements include pressure and pressure peaks, oil temperature, flow, vibration, oil cleanliness, pump efficiency and leakage flow. With condition monitoring, Industrial AI, machine learning and anomaly detection you can recognise pump wear, cavitation, internal leakage, clogged filters and sticking valves before they cause downtime. Linked to a CMMS, this becomes condition-based maintenance.

Within OT convergence, hydraulics is thus shifting from mechanical engineering to data-driven assets in cyber-physical systems, enabling remote diagnostics, a digital twin and central monitoring. The challenges include lifecycle management of controls that age faster than the mechanics, integration of legacy power units and dependence on a single vendor.

🔐 How do you protect hydraulic systems against cyberattacks?

Hydraulic systems work with enormous forces, so a digital attack can have immediate physical consequences. Risks arise from vendor remote access, insecure firmware, poorly segmented networks and weak authentication on controllers and HMIs.

Mitigating measures:

• Network segmentation - place HPU controllers in their own zone behind a firewall, following the zones and conduits model of IEC 62443.
• Mechanical safety as the last barrier - a pressure relief valve works without software and cannot be disabled over the network. Design critical protections so that a compromised PLC cannot bypass them.
• Hardening and access control - remove default passwords, disable unused services and apply zero trust to vendor access.
• Monitoring - watch write operations to controllers and unusual setpoints.

In critical environments such as energy and water management, add redundancy, fail-safe design, accumulators for emergency operation, emergency pumps and mechanical bypasses.

⚠️ Which safety risks and standards apply to hydraulics?

Industrial and mobile hydraulics usually operates between 100 and 350 bar; hydraulic tools and jacks reach 700 bar. Key risks are hose bursts, unintended motion, overheating, fire when oil leaks onto hot surfaces, and injection injury: a fine jet of oil from a leak can penetrate the skin. The UK HSE reports serious cases above 100 bar and anecdotal evidence that it can happen from about 7 bar. An injection injury is always a medical emergency, even when the wound looks minor.

A second underestimated risk is stored energy. An accumulator or trapped oil remains under pressure after the pump stops, and a raised load can drop. Maintenance therefore requires lock-out tag-out, including depressurising the system and discharging accumulators. Safety functions are implemented with a safety relay, a safety PLC, emergency stop circuits, pressure monitoring and mechanical interlocking.

For hoses, the German DGUV rule 113-020 is a widely used practical reference: a maximum service life of six years, including no more than two years of storage, unless the manufacturer or a competent inspection justifies otherwise.

📏 How do you approach a hydraulic installation in practice?

Typical values help with design, operation and interpreting measurement data:

A practical approach when digitalising an existing HPU:

1. Inventory the power units, controllers, firmware versions and existing sensors in your asset register.
2. Choose measurement points - pressure, temperature, oil cleanliness and vibration give the most value for failure prevention.
3. Expose data read-only via OPC UA or an edge gateway, without changing the control logic.
4. Segment the power unit according to IEC 62443 and manage vendor access centrally.
5. Define alarm limits and maintenance rules in SCADA and the CMMS, and check them against real failures after three to six months.

If your organisation falls under the Dutch Cybersecurity Act (Cyberbeveiligingswet), for example as a drinking water company, water board or energy company, the hydraulic controls of locks, barriers and turbines belong in your risk analysis.

🔋 How do you make hydraulics more energy efficient?

Conventional hydraulics loses a lot of energy to heat, internal leakage, throttling losses and pumps that run continuously at full pressure. Improvements include:

• Variable-displacement pumps - the pump delivers only the flow that is needed.
• Load sensing - pump pressure follows the load instead of a fixed maximum pressure.
• Variable-speed pumps - a variable frequency drive or servomotor lets the pump stand still or run slowly when no motion is required.
• Accumulators and energy recovery - storing braking energy or the energy of lowering loads.

Particularly on injection moulding machines and presses with long idle periods in the cycle, a variable-speed pump delivers substantial savings in energy and cooling.

❓ Frequently asked questions

What is the difference between hydraulics and pneumatics?

Hydraulics uses a practically incompressible fluid, usually oil, while pneumatics uses compressed air. As a result hydraulics delivers much higher forces and more accurate positioning, whereas pneumatics is faster, cleaner and cheaper for light tasks. Hydraulics typically operates at 100 to 350 bar, pneumatics usually at 6 to 10 bar.

How much force does a hydraulic cylinder deliver?

The force of a hydraulic cylinder equals pressure times piston area (F = p × A). A cylinder with a 100 mm bore delivers about 157 kN, or roughly 16 tonnes, at 200 bar. At the same pressure, doubling the bore gives four times the force.

Why is oil contamination so harmful in hydraulics?

Particles in hydraulic oil wear pumps and make valves stick or seize. Servo and proportional valves have very small clearances and are therefore especially sensitive. The cleanliness of hydraulic oil is measured with the ISO 4406 code and kept under control with filters and condition monitoring.

What is a hydraulic injection injury?

A hydraulic injection injury occurs when oil under high pressure is forced through the skin from a leak or hose. The wound often looks small, but the oil destroys tissue and can lead to amputation. Always seek medical help immediately after a hydraulic injection injury, and never search for a leak with your hand.

Can a hydraulic system be hacked?

A hydraulic system with a networked PLC, HMI or HPU controller can be attacked through that control system. An attacker could then change setpoints or pressure limits and cause damage or unsafe motion. Network segmentation according to IEC 62443 and mechanical pressure protection that works without software limit that risk.

Which standard covers the safety of hydraulic systems?

The main standard is EN ISO 4413:2010, which sets general rules and safety requirements for hydraulic systems and their components. For the control of safety functions, ISO 13849-1 or IEC 62061 also apply. In the EU, the Machinery Regulation replaces the Machinery Directive from 20 January 2027.

📌 In summary

Hydraulics uses pressurised oil to transmit very large forces in a compact package, making it indispensable in presses, injection moulding machines, mobile equipment, locks and storm surge barriers. Modern hydraulics is a cyber-physical system: PLCs, sensors and networks enable predictive maintenance and energy savings, but also call for segmentation according to IEC 62443, mechanical pressure protection as the last barrier and safe maintenance in line with EN ISO 4413 and lock-out tag-out.