Hydraulic Systems: Fluids, Actuators, and Control Valves Explained

Posted on Sep 5, 2025 in Technology

Hydraulic Fluids in Power Transmission Systems

Functions of Hydraulic Fluid

Hydraulic fluids are primarily used in hydrostatic power systems to transmit power. In addition to power transmission, these fluids also serve to lubricate contact surfaces, cool various elements, and clean the system.

Key Requirements for Hydraulic Fluids

  • Satisfactory flow properties throughout the entire range of operating temperatures.
  • A high viscosity index that ensures moderate viscosity variation in relation to temperature fluctuations.
  • Good lubricating properties to reduce wear and increase the service life of the system.
  • Low vapor pressure to avoid cavitation.
  • Compatibility with system materials, as the fluid should not react chemically with any of the used materials or deteriorate their physical properties.
  • Chemical stability to increase the service life of the liquid and avoid performance deterioration.
  • Corrosion protection, achieved by adding effective corrosion inhibitors.
  • Rapid de-aeration and air separation.
  • Good thermal conductivity to rapidly dissipate heat generated due to friction between elements and hydraulic losses.
  • Fire resistance, which is essential in some applications.
  • Electrically insulating properties, significant in a number of modern designs.
  • Environmental acceptability.
  • Optimal viscosity.
  • Appropriate oil density.
  • Controlled oil compressibility.
  • Specific vapor pressure.
  • Effective lubrication and anti-wear characteristics.
  • Material compatibility.
  • Chemical stability.
  • Oxidation stability.
  • Resistance to foaming.
  • High cleanliness.
  • Suitable thermal properties.
  • Balanced acidity.
  • Low toxicity.

Viscosity and Compressibility Impact on Performance

  • Viscosity: Influences hydraulic losses in transmission lines, resistance to fluid flow in narrow conduits, and viscous friction forces and damping effects.
  • Compressibility: Affects hydraulic capacitance.

Types of Hydraulic Fluids: Advantages and Disadvantages

Mineral Oils

  • Advantages: Inexpensive, widely available, great range of viscosity, good lubricity, non-corrosive, compatible with most sealing materials, chemically stable for reasonable operating temperatures.
  • Disadvantages: Non-compatible with butyl rubber sealings, chemical breakdown at high temperatures, flammability, increase of viscosity at high pressures.

Oil-in-Water Emulsion

  • Advantages: Fire resistant, highly incompressible, good cooling properties.
  • Disadvantages: Poor lubricity and low viscosity.

Water-in-Oil Emulsion

  • Advantages: Fire resistant.
  • Disadvantages: Reduced lubrication properties, reduced fire-resistant properties at high temperatures.

Water Glycol Fluids

  • Advantages: Very low flammability, very stable with respect to shear, good anti-freeze properties.
  • Disadvantages: Cannot be used at high temperatures, they attack most paints.

Synthetic Oils

  • Advantages: Compared to mineral oils, they offer better thermal stability, oxidation stability, viscosity-temperature properties, low-temperature fluidity, operational temperature limits, and fire resistance.
  • Disadvantages: Compared to mineral oils, they are worse in hydrolytic stability, corrosion protection, toxicity, compatibility with elastomers and construction materials, solubility of additives, frictional characteristics, cost, and availability.

Fluid Properties Enhanced by Additives

  • Chemical stability
  • Oxidation stability
  • Foaming resistance
  • Lubrication and wear protection
  • Viscosity index improvement
  • Cleanliness

Hydraulic Actuators: Cylinders, Motors, and Rotary Types

Function of Hydraulic Actuators

Hydraulic actuators convert hydraulic power into mechanical power to drive loads. The mechanical power delivered to the load is managed by controlling fluid pressure and flow rate using various hydraulic control valves.

Classification of Hydraulic Actuators

Hydraulic actuators are classified primarily based on their motion type.

Types of Actuators in Hydraulic Systems

The different types of actuators used in hydraulic systems include:

  • Hydraulic Cylinders: Perform linear motion.
  • Hydraulic Motors: Perform continuous rotary motion.
  • Hydraulic Rotary Actuators: Perform a limited angular displacement.

Main Parts of Hydraulic Cylinders

A hydraulic cylinder mainly consists of a piston, a piston rod, a cylinder barrel, a cylinder head, and a cylinder cap.

Sizing Hydraulic Cylinders: Specifications and Calculations

The size of a hydraulic cylinder is determined by considering the required force and the available hydraulic operating system pressure. Dividing the force by the pressure yields the area, which then helps determine the rod size. System losses and inefficiencies must also be considered.

For high-speed or high-inertia operations, a cushioning system is essential. The cushioning should produce a controlled deceleration of the cylinder near one or both end positions by creating a decelerating pressure force. For ideal operation, the piston will be fully stopped at the end of the cushioning stroke. When the cylinder is installed horizontally, the decelerating (cushioning) force can be calculated.

Furthermore, the maximum axial load acting on the hydraulic cylinder must not exceed the limit at which buckling occurs. The limiting load for buckling can be calculated. Finally, the minimum length of the hydraulic cylinder can be calculated.

Hydraulic Cylinder Cushioning Systems

A cushioning arrangement reduces the piston speed to a limiting value before it reaches its end position. The cushion dissipates the kinetic energy of the moving parts. This arrangement is necessary in high-speed and/or high-inertia applications because the sudden stopping of the piston can result in a severe impact force (proportional to the mass and the square of the velocity of the moving parts). This impact affects both the cylinder and the driven mechanism, and a cushioning arrangement is used to prevent it.

Common Rotary Actuator Types

  • Rotary Actuator with Rack and Pinion Drive
  • Parallel Piston Rotary Actuator
  • Vane-Type Rotary Actuators

Rotary Actuators vs. Hydraulic Motors

Hydraulic motors perform continuous rotary motion, while rotary actuators perform a limited angular displacement.

Types of Hydraulic Motors and Their Specifications

Common types of hydraulic motors used in hydraulic systems include:

  • Bent-Axis Axial Piston Motors
  • Swash Plate Axial Piston Motors
  • Vane Motors
  • Gear Motors

The motor speed depends on the flow rate, while the supply pressure depends mainly on the motor loading torque. In the case of an ideal motor with no leakage and no friction, specific relations are used.

Hydraulic Control Valves: Types and Functions

Power Control in Hydraulic Systems

The control of hydraulic power in hydraulic power systems is achieved through control valves. System function dictates control requirements. The parameters of mechanical power delivered to the load are managed hydraulically by controlling pressure, flow rate, or the direction of flow.

Classification of Hydraulic Control Valves

Control valves are classified into the following main categories:

  • Ordinary Switching Valves
  • Proportional Valves
  • Servovalves
  • Digital Valves

Focusing on ordinary switching valves, these include:

  • Pressure Control Valves (PCVs):
    • Relief valves (direct- and pilot-operated)
    • Pressure-reducing valves (direct- and pilot-operated)
    • Sequence valves (direct- and pilot-operated)
    • Accumulator charging valves
  • Directional Control Valves (DCVs): (direct- and pilot-operated)
  • Flow Control Valves (FCVs):
    • Throttle valve
    • Series pressure-compensated FCV
    • Parallel pressure-compensated flow control valves
    • Flow dividers
  • Check Valves:
    • Direct-operated check valves
    • Pilot-operated check valves (hydraulically or mechanically piloted)

Directional Control Valves (DCVs)

Function of Directional Control Valves

Directional control valves (DCVs) are used to start, stop, or change the direction of fluid flow. The control positions determine how lines are interconnected, and consequently, the directions of fluid flow.

Specifying DCV Types and Sizes

These valves are specified by the number of connected lines (ways) and the number of control positions. For example, a 4/3 DCV has four ways and three positions. There are two main types of directional control valves: spool type and poppet type.

Direct-Operated 4/3 DCV: Functionality

A 4/3 DCV has three positions and four ports. When the valve is normally in the neutral position, no flow occurs. When solenoid a is activated, the cross-arrow position is engaged, allowing flow in one direction. When solenoid b is activated, the parallel-arrow position is engaged, allowing flow in the opposite direction. The valve typically returns to the neutral position by springs.

Pilot-Operated DCVs: Necessity and Operation

Pilot-operated directional control valves are used when the system’s flow rate is very high. In such cases, using a direct-operated valve would require a very high force from the solenoid, leading to high power consumption. Pilot-operated valves avoid this high consumption.

A pilot-operated directional control valve consists of two stages: a pilot valve and a main valve. The pilot valve is a direct-operated directional control valve, often controlled electrically. It is supplied by high-pressure oil from the main valve’s high-pressure port (P). The oil is also drained through the main valve’s return line (T). With the pilot valve spool in the neutral position, the control chambers (C and D) are drained to the tank, and the main spool is in its neutral position. When solenoid (a) is energized, the pilot valve spool moves to the right. High-pressure oil then reaches chamber (D), causing the main spool to move to the left. Similarly, energizing solenoid (b) results in the main spool moving to the right. The supply and return lines of the pilot stage are usually connected to the main stage supply and return lines. Optionally, the valve may include an arrangement for switching any of them to external ports.

Control Methods for Directional Control Valves

DCVs can be controlled in various ways, including mechanically (by a hand lever, cam and roller, or rotary knob), hydraulically, pneumatically, or using solenoids.

Pressure Losses in Directional Spool Valves

Flow rates through spool valve restrictions, Q, and the pressure drop across them, ΔP, are related by specific expressions. The loss of power, ΔN, due to oil flow through the valve restriction is also given by specific relations. It is crucial to reduce pressure and power losses in valves; therefore, the restriction area, Av, should be increased as much as possible. Manufacturers of DCVs provide characteristic charts showing the pressure-flow relation for their valves. Generally, pressure losses are reduced by increasing the spool diameter and stroke. However, these requirements contradict the need for minimum dimensions and weight. Moreover, an increase in diameter increases mass, decreases the valve’s natural frequency, and increases response time. Thus, for small flow rates, spool dimensions can be minimized, while increased flow rates necessitate larger dimensions.