Hydraulic System

Hydraulic Principle and Hydraulic System:

Basic Hydraulic Knowledge

Hydraulic pressure is a technology that transmits power through pressurized oil. It can transform a small force into a large force and is used in various devices and machines such as jacks, automobile brakes, excavators, and elevators. In this series, we will explain the principles of hydraulics and hydraulic equipment so that those who are new to hydraulics can easily understand.

Table of contents

1. The principle of Pascal

2. Overview of hydraulic system

3. Advantages and disadvantages of hydraulics

1. The principle of Pascal

Pascal's principle can be defined in a way that "A fluid of any type in a closed container, with out depending of the shape of the container, transmits the pressure per unit area from it received at a certain point in that container as it is to all other parts of the fluid." , Is the basic principle of fluid mechanics. 

Knowledge necessary to understand hydraulic pressure.

Let's consider the principle of Pascal with the closed container shown in Fig. 1. The closed container is filled with liquid and fitted with a piston of cross section A.

Pascal's principle

When the force F acts vertically on the top of the piston, the pressure on the bottom of the piston is calculated as P = F / A. Here, SI units for pressure P, force F, and area A are Pa, N, and m2, respectively. According to Pascal's principle, the pressure P is transmitted to all points of the liquid with equal magnitude, pushing the sealed container from the inside (Fig. 1). The hydraulic jack uses the principle of Pascal.

Principle of hydraulic jack

The principle that power is amplified by the hydraulic jack will be explained using 

Fig. 2. When an operating force f is manually applied to the lever, force F1 is applied to piston 1 and pressure P1 is generated on the bottom surface of the piston. This pressure P1 is transmitted to the lower surface of the piston 2 having the cross-sectional area A2 with the same strength through the oil (hydraulic oil) in the connecting pipe, and the upward force F2 is applied to the piston 2. That is, P1 = F1 / A1 = F2 / A2. Therefore, F2 = (A2 / A1) × F1 and the force can be amplified in proportion to the piston area ratio. In the amplification of force by hydraulic pressure, the point is to convert mechanical power and hydraulic power. The power of piston 1 is represented by mechanical power F1 · v1. When the piston 1 pushes the hydraulic oil, mechanical power is converted into hydraulic power p × Q. After that, when the hydraulic oil pushes the piston 2, it is reconverted into mechanical power F2 ・ v2, and the load of mass m is lifted. 

2. Overview of hydraulic system

The hydraulic system consists of three types of hydraulic equipment: hydraulic pump, hydraulic valve, and hydraulic actuator

Overview of hydraulic system

First, the mechanical power obtained from the prime mover (electric motor, engine) is converted into hydraulic power by a hydraulic pump. The oil in the hydraulic pump is under high pressure and passes through various hydraulic valves depending on the application. At this time, pressure, flow rate, and flow direction are controlled and adjusted by electrical signals or manually. When hydraulic fluid is pumped into the hydraulic actuator, hydraulic power is converted back to mechanical power. This mechanical power (load) does the necessary work.
The hydraulic system consists of main equipment such as hydraulic pumps, hydraulic valves and hydraulic actuators, as well as equipment such as motors, oil tanks, pipelines, filters and pressure gauges. Electric circuits that control hydraulic valves with electric input signals and electric / electronic / information technologies such as sensors, amplifiers, computers and software are also part of the hydraulic system. In this way, hydraulic systems are Mechatronics (a technical field that combines technologies such as mechanical, electrical, electronic, and information engineering).

A hydraulic pump driven by an electric motor sucks hydraulic oil from an oil tank and sends it as high-pressure oil to the main pipeline (the pipe leading to the cylinder). At that time, the hydraulic oil in the oil tank is filtered by a filter to maintain the cleanliness. The pressure is adjusted with the pressure control valve (relief valve), which is a type of hydraulic valve, while checking the pressure gauge. The pressure control valve also acts as a safety valve when the load condition is abnormal. The direction of the flow to the main line is operated by manually moving the lever of the directional control valve and moving the cylindrical spool.

When hydraulic fluid flows into the left side of the cylinder, the piston receives pressure and moves to the right. Then, the hydraulic oil is discharged from the right side of the cylinder. Adjust the flow rate of hydraulic oil with the flow control valve to change the moving speed of the piston. The hydraulic fluid from the flow control valve passes through the directional control valve again, through the return line, and is discharged to the oil tank under atmospheric pressure. Similarly, excess hydraulic fluid from the relief valve, hydraulic pump, etc. is returned to the oil tank through the drain line.

3. Advantages and disadvantages of hydraulics

The characteristics of hydraulics vary greatly depending on the application field, intended use, operating conditions, etc. Here are eight pros and cons.

11111111111111111111111111111111111111111111111

・ Advantages of hydraulic pressure

1: By giving a slight electric operation signal to hydraulic equipment, position (angle), speed (angular velocity), force (torque), acceleration (angular acceleration), etc. can be controlled precisely and at high speed.

2: The force (torque) and speed (angular velocity) that drive the load can be adjusted independently, steplessly and in a wide range. The direction of movement can also be changed between straight line and rotation.

3: By increasing the number of pipe lines and changing the length of the hose, you can flexibly distribute the hydraulic energy. Therefore, one hydraulic pump can operate multiple hydraulic actuators. 

4: When an overload occurs, the safety of the system can be secured by automatically letting high pressure oil escape from the relief valve. Also, since it is sturdy and durable, it can withstand external shocks and is suitable for heavy work. 5: Since the power rate of the actuator alone (power per unit time: W / s) is higher than that of electric drive or pneumatic drive, the actuator can be made smaller and lighter. In addition, the inertia of the moving part is small, so the responsiveness can be improved and the impact can be suppressed. 6: Hydraulic oil has almost no compressibility compared to air and can be raised to an extremely high pressure. Therefore, from the viewpoint of rigidity, the positioning accuracy and responsiveness of the actuator are excellent.

7: Fluid energy can be stored using an accumulator (device that stores high-pressure fluid). Instantly send high-pressure and large-capacity hydraulic oil to the hydraulic actuator to move heavy objects at high speed.

8: Hydraulic fluid has excellent lubricity and rust prevention, so maintenance of equipment and systems is easy.

Hydraulic cons

1: Leakage of hydraulic oil causes pollution of factory equipment, natural environment

 and soil. Although the amount of leakage can be reduced due to advances in sealing 

technology, it is difficult to reduce it to zero. 

2: In hydraulic equipment, hydraulic fluid flows through a narrow gap, so it is necessary to strictly control contaminants (contamination) mixed in the oil. In particular, it is essential for improving the reliability of hydraulic systems that use servo valves. 

3: Mineral hydraulic fluid is a flammable substance with a flash point of 200-250 ℃. The Fire Service Law may be applied, so it is necessary to consider fire. 

4: The hydraulic pump causes noise at frequencies that humans dislike. Fluid noise is also generated from the hydraulic valve. Noise reduction is a major issue when compared to electric drive. In addition, pressure pulsation from the hydraulic pump may travel in the hydraulic circuit and vibrate pipes, etc., damaging hydraulic equipment.

5: Hydraulic pump has energy loss due to friction and leakage. In particular, energy loss when the hydraulic system is inactive must be suppressed.

6: The hydraulic system requires a hydraulic power source (hydraulic pump, hydraulic tank, etc.). They must have a large installation space compared to electrical equipment.

7: Since the viscosity changes due to the temperature change of the hydraulic oil, flow rate control is difficult depending on the environment. The speed control accuracy of the hydraulic actuator may decrease. 8: Compared with electric wiring, hydraulic piping requires more work because it is connected with pipes and fittings.