Hydraulic Systems: Principles, Components, and Applications

Posted on Dec 18, 2024 in Technology

Hydraulic Systems: Advantages and Disadvantages

Hydraulic systems offer several advantages over pneumatic systems:

  • Easy Speed Control: It is possible to regulate the flow of oil-water to control velocity.
  • Transmission of Large Power: Hydraulic systems can transmit significant power.
  • Position Control: Achieved thanks to the incompressibility of the fluid used.
  • Reversibility of Drives: The fluid can move in either direction.
  • System Protection: Possible through the use of safety valves.
  • Start and Stop Under Load: Hydraulic systems can start and stop while under load.

However, there are also disadvantages:

  • They are slow.
  • They are expensive.
  • They are complicated.
  • They can only transmit energy, not store it.

Characteristics of Hydraulic Fluids

Hydraulic fluids must meet several requirements for proper use in a central hydraulic system. These include:

  • Lubricating properties
  • High thermal and chemical stability
  • Very low compressibility

Basic Principles of Hydraulic Systems

The fundamental principle of hydraulic systems is Pascal’s Law, which states that pressure applied to a point in an incompressible fluid at rest is transmitted equally in all directions throughout the fluid mass. The mathematical expression is: P1 = P2 => F1/A1 = F2/A2. Thus, a large force can be achieved from a smaller force by increasing the area of the piston opposite to the one where the force is applied.

Hydraulic Circuit Components

A hydraulic circuit is a set of elements arranged so that oil performs work or a series of actions to drive machinery or mechanisms. The main components are:

  1. Motor
  2. Pump
  3. Transport Elements
  4. Regulation and Control Devices
  5. Actuators/Work Elements

Hydraulic Pumps

Hydraulic pumps convert mechanical energy into hydraulic energy, causing fluid movement along the installation’s pipes. There are two main types:

  • Hydrodynamic Pumps: Used in hydraulic circuits only for force and work by centrifugal force that drives the fluid in a specific direction.
  • Hydrostatic Pumps: Provide the same amount of fluid in each cycle or revolution. The most important are:
    • External Gear Pumps: Produce flow by transporting fluid between the teeth of two fully assembled gears. One gear is driven by the pump shaft (motor), which turns another gear. This is the most commonly used type due to its simplicity and economy, despite being the noisiest and having a low yield.
    • Rotary Vane Pumps: Another type of hydrostatic pump with fixed or variable flow. Its operating principle is based on a rotor with rotating vanes in a mobile housing or ring. Pumping chambers are formed between the vanes, rotor, and housing.
    • Rotary Piston Pumps: Use the principle of oscillating pumps to produce flow. Instead of a single cylinder, these pumps have many piston-cylinder sets. Part of the mechanism rotates around a motor axis, generating piston movement within the cylinder. This causes a vacuum inside the cylinder during expansion and expels the fluid during compression, producing flow. There are two classes: axial and radial piston. They usually have the best pump efficiency.

Hydraulic Motors

These elements transform hydraulic energy into rotary mechanical energy. They operate in reverse to pumps. The most used are piston, gear, and vane motors. Piston motors are preferred for their excellent performance. Gear motors are also used for their low cost.

Regulation and Control Elements

The fluid provided to the pump at a certain pressure must be controlled, regulated, and distributed according to needs. Valves are used to regulate and control pressure and flow parameters, and to direct or block fluid passage to actuate the work elements. Valves can be classified as distribution, flow regulators, and pressure regulators.

Distributor Valves

These components open and close, directing fluid through the installation’s conduits and enabling the control of work elements.

Unidirectional Valves

Allow fluid passage in only one direction. There are two types: piloted and non-return valves. Non-return valves are the simplest, working with a spring and a cone. When the force favors the spring, the fluid does not pass. If reversed, the fluid overcomes the spring force, and the fluid flows. Piloted valves allow passage in the opposite direction only under certain conditions.

2/2 Distributor Valves

These are the simplest valves. They can be normally open or normally closed. Fluid passes only when the valve is activated, serving as a simple on/off switch.

4/2 Distributor Valves

Used for double-acting hydraulic cylinders, allowing fluid passage in both directions. When at rest, fluid flows between A and B, and C and D. When actuated, connections change: A connects to D, and B to C.

4/3 Distributor Valves

Also used for double-acting cylinders but have an intermediate position for various possibilities, such as blocking the work element in intermediate positions. Investment can be axial or radial.

Flow Control Valves

Actuators can vary speed by changing the fluid flow. They are classified into fixed and variable flow control valves.

Variable Flow Control Valves

The simplest is the needle valve, where flow increases as the regulatory command opens. These valves are not compensated; the retention is transformed into heat energy, which is lost. Flow also depends on pressure; increasing pressure increases flow. Compensated flow control valves are commonly used for more stable actuator speeds, maintaining constant flow regardless of pressure. These valves may also have a non-return feature for one-way regulation.

Pressure Control Valves

Their objective is to reduce the regulated pressure to a lower value. There are two types: direct action and piloted.

Pressure Limiting Valves

These are protection and safety devices. When fluid pressure exceeds a limit, the valve closes, preventing flow and avoiding accidents.

Hydraulic Cylinders

These actuators convert hydraulic energy into linear motion or force. There are single-acting and double-acting cylinders.

  • Single-acting cylinders: Allow force in one direction and retrieve position through a spring.
  • Double-acting cylinders: Allow movement in both directions, depending on which chamber the fluid enters.

There are also special cylinders, such as double-rod cylinders, blind cylinders, and telescopic cylinders.