Introduction to Hydraulics:
Hydraulics refers to the transmission of energy (forces and movements) through a fluid. The word “hydraulics” comes from Greek (hydro = water, aulos = pipe). Hydraulics is a drive, control, and regulation technology encountered in automotive engineering, mechanical engineering, drive and control technology, aviation, and agriculture. Hydraulics can be divided into hydrokinetic and hydrostatic drive:
- Hydrokinetic: high fluid velocities and relatively low pressures, such as the torque converter in automatic transmissions;
- Hydrostatic: low fluid velocities and high pressures, as seen in power steering.
In practice, alongside hydraulics, we also find pneumatics, electronics, and mechanical drive technology. Each technology has its own advantages and disadvantages for the application for which it is used. The advantages and disadvantages of hydraulics compared to other technologies are:
Advantages:
- High power density; large forces and torques can be transmitted with small component sizes;
- Stepless adjustable speed, force, and torque;
- Hydraulic energy can be stored and reused;
- High precision and constant positioning are possible.
Disadvantages:
- Relatively expensive technology;
- Sensitive to dirt;
- Possibility of leakage (both internal and external).
In a hydraulic system, fluid displacement occurs. The fluid flow can be set in motion by means of a pump or a piston. All hydraulic systems are based on Pascal’s law:
“Pressure applied to a fluid at rest is transmitted undiminished in all directions throughout the container.”
This principle is illustrated in the following image, where a piston applies a force (F1) to the piston area. The force creates pressure in the fluid-filled (closed) system, causing the piston to be pushed upward with force F2.
The pressure depends on the force and the area of the piston. On the page “pressure in the hydraulic system” this is explained through animations and calculations.
Hydraulic Schematics:
The hydraulic schematics, composed of symbols, are assembled by the manufacturer to allow reading how components are connected during maintenance and/or repair tasks. The flowchart also indicates what types of components are in the system. An overview with the symbols can be found on the page with the hydraulic symbols list.
The following image illustrates the most commonly used components in a hydraulic system. The components are represented with a color and number.
An electric motor drives the hydraulic pump (1), which moves the hydraulic oil to the control valve (4).
The relief valve (2) protects the system against excessive pressures. The system pressure can be read from the pressure gauge (3).
The manually operated control valve has four connections:
P (pump), T (tank), and the connections A and B for the cylinder.
The control valve can be set in three positions:
- resting (current position);
- to the right;
- to the left.
Depending on the position of the control valve, the cylinder is supplied with hydraulic oil and the piston will move.
The following images illustrate the different positions of the control valve with which the cylinder can be displaced.
1. Control Valve in Neutral Position:
In the following diagram, the hydraulic pump is again driven by an electric motor. The pump draws hydraulic oil from the reservoir and supplies the oil under increased pressure to the relief valve, the pressure gauge, and the control valve.
The control valve is in the middle position, whereby connections P and T are interconnected, and the hydraulic oil enters the control valve via P and exits via T.
The hydraulic oil flows from connection T through the return filter to the reservoir. A pressure protector is located in the return filter housing, which opens against spring force when fluid pressure rises.
The pressure increase can occur when the filter becomes clogged with dirt particles.
Because the hydraulic oil in this control valve position circulates, hardly any pressure builds up. There is only resistance to the oil in the control valve, the pipes, and the return filter to a certain extent. However, this resistance is so low that the oil is circulated without pressure.
2. Control Valve in Left Position:
The control valve is placed in the left position. The connections P and A, as well as T and B, are interconnected in this position. The hydraulic oil moves through the lines to the left side of the cylinder. The pressure builds on the left side of the piston and is now activated.
Because the return (B) of the cylinder is now connected to the T-connection of the control valve, the oil on the right side in the cylinder can flow to the reservoir via the return filter.
The cylinder makes an outward movement until the end stop is reached. This is depicted in the following situation.
3. Piston in Extreme Position:
The piston is fully extended in this situation, so the end stop is reached. The relief protection prevents the pressure from rising too high. Without this protection, the pressure would rise uncontrollably, leading to malfunctions.a0
The pressure control valve (shown to the left of the hydraulic pump in the image) opens when the preset pressure is reached. The relief valve connects the supply line from the hydraulic pump to the return. A constant circulation now takes place through this relief valve until the pressure decreases.
4. Control Valve in Right Position:
The control valve is now operated in the right position (opposite). Compared to situation 2, the lines are crossed: P is now connected to B, causing pressure build-up on the right side of the piston. Connection A is connected to T (return). The piston moves to the left in this control valve position.
The pressure will rise again when the end stop of the piston is reached to the pressure at which the relief valve opens. The control valve should then be set back to the center position.
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