Hydraulic Systems: Principles, Fluids, and Components

Pascal's Principle

Pressure exerted at a point in a liquid mass is transmitted entirely and equally in all directions.

Oleo-hydraulic Fluids

Mineral oils are petroleum-based fluids commonly used in oleo-hydraulic circuits. Their high lubricating power is essential for the proper conservation of mechanical elements and components. However, their flammable nature discourages their use in facilities at risk of fire. In such cases, fire-resistant fluids like phosphate esters and water-in-oil emulsions are preferred.

Oleo-hydraulic Circuit Elements

A typical hydraulic unit consists of a tank, filter, pump, and pressure regulator valves. Distributor valves control the operation of work items. Work items convert hydraulic pressure energy into mechanical work. There are two main types:

• Cylinders: Transform pressure energy into rectilinear (linear) motion.
• Hydraulic Motors: Convert pressure energy into circular (rotary) motion.

Auxiliary elements, such as additional valves and filters, ensure the oil is maintained in optimum conditions of flow, pressure, and cleanliness to achieve high system performance.

Hydraulic Pumps

Pumps draw in oil and propel it at a certain pressure and flow rate into pipes and other components. They are characterized by three main parameters:

• Nominal Pressure: The maximum operating pressure for which the pump is designed (measured in bar).
• Flow Rate: Represents the volume of oil transmitted by the pump in a given time (e.g., liters per minute).
• Nominal Rotational Speed: The recommended operating speed of the pump (measured in revolutions per minute, rpm).

Gear Pump

A gear pump consists of a cast iron casing with two openings or ducts. Inside, two gears mesh perfectly. One gear receives rotational movement from an electric motor, and the other is driven by the first. As the gears rotate, a vacuum is created at the inlet, causing oil to be drawn in (aspiration) and then expelled under pressure.

Vane Pump

A vane pump features a rotor driven by an electric motor. The rotor can rotate eccentrically inside an oval-shaped ring. Within the rotor, there are slots containing vanes that maintain permanent contact with the inner surface of the ring. During rotation, a vacuum is created at the pump's inlet, drawing in oil. The oil is then trapped in a pumping chamber, which closes and subsequently expels the oil under pressure.

Hydraulic Distributor Valves

The symbology for hydraulic distributor valves is similar to that used for pneumatic valves. Connections for the ports indicate the input and output of oil, with flow direction often shown by arrows. Control mechanisms are typically located on the right and left sides. Inlet ports are usually indicated by capital letters:

• P: Pressure inlet
• R, S, T: Exhaust or return oil ports
• A, B, C: Work ports (for connecting to actuators)

The primary differences between oleo-hydraulic and pneumatic valves are the type of fluid used (oil instead of compressed air) and the significantly higher operating pressures in hydraulic systems.

Hydraulic Work Items

Oleo-hydraulic Cylinders

Single-Acting Cylinders

In a single-acting cylinder, the forward movement is achieved by oil pressure acting on one side of the piston, overcoming the force of a return spring. The retraction (rollback) movement is accomplished by the action of this spring. Oil enters and exits through the same port.

Double-Acting Cylinders

In a double-acting cylinder, both the forward and backward movements are achieved by oil pressure acting on opposite sides of the piston. These cylinders have separate inlet and outlet ports for oil.

Basic Parameters of Cylinders

Key parameters for cylinders include:

• Force: The force exerted by the piston rod.
• Stroke: The total distance the piston travels.
• Speed: The speed of the piston's movement.
• Damping: The mechanism to absorb kinetic energy at the end of the stroke. This can involve elastic rings made of elastic material or utilizing the residual oleo-hydraulic oil trapped in the opposite chamber of the cylinder.
• Oil Volume Consumed: The amount of oil required for a full stroke.

Technical Characteristics of Cylinders

Hydraulic cylinders share basic elements with pneumatic cylinders, such as the barrel (tube), piston, piston rod, and ports for fluid entry and exit. Additionally, they include:

• Watertight Seals: Essential for preventing fluid leakage.
• Wiper Seals: Located near the piston rod to clean it and prevent contaminants from entering the cylinder.
• Air Purge Device: A mechanism to eliminate air pockets that can cause pressure loss and erratic movement within the cylinder.