Introduction to the Basic Principles of Hydraulics

What is Hydraulic Machinery?

Hydraulic machinery is equipment and tools that increase mechanical strength through the principle of fluid mechanics. Take heavy equipment as an example, where hydraulic fluid is delivered to actuators in the equipment at high pressure using hydraulic pumps. The hydraulic pump is driven by an engine or an electric motor. Control the hydraulic oil to obtain the required pressure or flow by manipulating various hydraulic control valves. The hydraulic components are connected by hydraulic pipes. Like the pneumatic system, the hydraulic system is based on Pascal's law. Any pressure applied to a fluid in a closed system at any point will be transmitted to all sides with the same pressure. Hydraulic systems use incompressible fluids as the working medium. Hydraulic mechanical equipment is widely used because of its large transmission power and transmission with fine pipes and flexible hoses, so its power density is high, the actuators with applicable power are wide and flexible, and the pressure area is appropriately changed to increase the force huge. Compared with the mechanical system composed of gears and shafts, the fluid resistance of the hydraulic system flowing through the pipeline will cause a certain power loss.

A Hydraulic is a device that uses high-pressure hydraulic oil to do work after being controlled by some mechanical parts, and its purpose is to control the movement of the load. In many high-pressure and large-capacity machinery, hydraulic pressure is usually the only choice, which is a must for those who learn hydraulics.

The most basic theory of the hydraulic system is the Pascal principle. In a closed container, the fluid pressure is equal at each point, and the direction is perpendicular to the surface. Using this principle, a small force at A can push a heavy object at B.

What are the Components of the Hydraulic Device?

The hydraulic system is composed of an oil tank, hydraulic pump, control valve, driver, and some auxiliary parts. The function of each component of the hydraulic system is to be responsible for energy conversion. Among them, only oil is an exception. It does not perform any energy conversion but only acts as a medium for energy conversion.

1. Hydraulic pump:
   The hydraulic pump is driven by a motor or an engine. From an energy point of view, its function is to convert the mechanical energy generated by the motor or engine into fluid energy. From a mechanical point of view, the pump only uses the atmospheric pressure difference or potential energy difference, the liquid is sucked from one end and discharged from the other end.
2. Actuator:
   The function of the actuator is to convert fluid energy into mechanical energy to push the load movement. It can be divided into the hydraulic cylinder and hydraulic motor. The hydraulic cylinder makes the load move linearly, and the hydraulic motor makes the load rotate.
3. Control valve:
   In the hydraulic system, the pressure of the oil is used to control the output of the driver, the flow of oil is used to control the speed of the driver, and the direction of the flow of oil is used to control the direction of movement of the driver. Therefore, the pressure control valve, flow control valve, the directional control valve is the most basic and indispensable in the hydraulic system.
4. Tank:
   The hydraulic oil used in the hydraulic system must be recycled and reused, so the oil tank is used to store the hydraulic oil on the one hand, and the mounting seat of the motor and pump on the other hand.
5. Hydraulic accessories:
   Hydraulic accessories are used to enhance the function of the hydraulic system, such as a filter to remove impurities in the oil, an oil cooler to prevent the oil temperature from being too high, a pressure accumulator, and various piping components.
6. The Definition of Pressure and the Unit of Use:
   The definition of hydraulic pressure is the same as the definition using the unit of air pressure, the difference is that hydraulic pressure has no vacuum pressure.

Advantages and Disadvantages of Hydraulic Transmission:

The efficiency of hydraulic system energy conversion: According to the law of immortality of energy, there can be different forms of mutual conversion, but there must be a loss in each conversion, and the lost energy will be dissipated in the form of heat. The energy utilization rate of the hydraulic system is necessarily not high, generally below 50%, or even lower, usually about 30~40%.

The maneuverability of the hydraulic system: The hydraulic system is used as the driving device of the system, and the special feature of hydraulic pressure is important for machinery that requires large output and high positioning accuracy. The energy efficiency of the hydraulic system is bad, but the industry is using more and more.

• Small size, large output: The hydraulic pressure is generally around 70 kg/cm2, and can also be as high as 500 kg/cm2.
• There is no danger of overloading: A pressure relief valve is installed in the hydraulic system. When the pressure exceeds the set pressure, the valve opens, and the hydraulic oil flows to the oil tank through the pressure relief valve.
• Output adjustment is easy: The output adjustment of the hydraulic device is simple, if the set pressure of the pressure control valve can be easily achieved.
• Speed adjustment is easy: The speed adjustment of the hydraulic system is simple, if the set flow rate of the pressure control valve is adjusted, it can be easily achieved, and step-less speed change is possible.
• Smooth movement and easy reversal: Due to the high incompressibility of the liquid, the hydraulic oil is not allowed to flow into or out of the drive, and the speed of the load will stop immediately. And the inertia generated by the movement of the load will be absorbed by the hydraulic oil, so there is no need to install any braking device at all. And because the mechanism of the liquid driver is simple, its inertia is small, so the movement is smooth and the reversal is easy.
• Easy to automate: Hydraulic equipment is equipped with solenoid valves, electrical components, sensors, programmable controllers, and microprocessors, which can be assembled into various automatic machines.
• High durability: Most of the components of hydraulic equipment are almost immersed in hydraulic oil, and most of the hydraulic oil contains a rust inhibitor, its resistance to rust and wear resistance is good, and the durability of the equipment is high.

• Poor piping will cause leakage of hydraulic oil, which will not only pollute the workplace but cause a fire hazard.
• The viscosity of hydraulic oil is greatly affected by temperature. When the oil temperature rises, the viscosity decreases, and when the oil temperature drops, the viscosity increases. The change of viscosity will affect the flow and make the speed of the drive unstable.
• The hydraulic system converts the mechanical energy output by the engine or motor into fluid energy. After some adjustments are made through the valve, the fluid energy is converted into mechanical energy by the driver to drive the load. Because the energy is converted many times, the loss is large. Its energy efficiency is lower than that of traditional machinery.
• The energy lost during energy conversion will be dissipated in the form of heat, and the heat will be transmitted to the equipment through the hydraulic oil, resulting in abnormal heating of the system.
• To reduce the viscous friction loss when the oil flows, the flow rate of the oil must be limited to make it a steady flow, thus affecting the working efficiency of the hydraulic equipment.
• The hydraulic system uses many various control valves, joints, and pipes. To prevent leakage and loss, the machining accuracy of the components is required to be high, and professional piping technology is also required.

What is Hydraulic Circuit?

The hydraulic circuit is a system that connects the various components that transmit the liquid. The purpose of such a system is to control the fluid flow therethrough or to control the fluid pressure. Hydraulic machinery equipment uses hydraulic circuits to move heavy objects. The approach to describing fluid systems from individual components is inspired by circuit analysis. Circuits are easier to analyze when the electronic components are independent and linear. Likewise, hydraulic circuit theory is easier to analyze when considering mutually independent linear elements.

Components of a hydraulic circuit include passive (passive) devices such as pipes or transmission lines, and powered (active) devices such as power packs or pumps. Hydraulic circuit theory is particularly applicable to systems with long and small pipes and separate pumps, such as flow systems in chemical processes, or micro-scale devices.

• The open-middle circuit uses a pump to provide a continuous flow of liquid that returns to the tank through the open-middle channel of the control valve. In other words, if the control valve is in the neutral position, it provides an open return passage to the tank without pumping out the oil at high pressure. Once the control valve is actuated (change of valve position), it will direct the oil into/out of the actuator and the tank. Because the output of the pump is constant, the pressure of the oil will rise and fall with the amount of resistance encountered. If the pressure rises too high, the oil will return to the tank through the overflow valve. Multiple control valves can be integrated into series. Dosing pumps can be used for this type of circuit, which is inexpensive to operate.
• A closed circuit is a circuit that provides sufficient pressure to the control valve regardless of whether any valve operates. The flow rate of the hydraulic pump is variable, and the flow rate of the pump is small until no one operates the valve. Since the valve changes the valve position, the spool of the valve does not have to open the middle return oil passage to the oil tank. Multiple valves can be connected in parallel, and the system pressure is equal to all valves.
• Open loop: In the open-loop system, the suction port of the pump and the oil return port of the motor are both connected to the hydraulic oil tank, which is also an open/closed circuit. The mid-pass circuit uses a pump to provide a continuous flow of liquid. The fluid flows back to the tank through the mid-passage of the control valve. At this time, the control valve is in the neutral position to provide an open return channel to return to the tank and prevent the fluid from being pumped into high pressure. Additionally, once the control valve is operated, it directs fluid to/from the actuator and reservoir. Since the pump output is constant, the pressure of the oil will increase with the resistance encountered. If the pressure rises too high, the oil will return to the tank through the relief valve. Several control valves can be connected in series with each other. Dosing pumps can be used for this type of circuit, which is inexpensive to operate.
• Closed loop: In a closed-loop system, the return port of the motor is directly connected to the suction port of the pump. To maintain pressure on the low-pressure side, the circuit has a charge pump that supplies filtered cold oil to the low-pressure side. Closed loop circuits are typically used as hydrostatic transmissions in-vehicle applications. The advantage of a closed loop is that there is no directional valve at all, so the response is fast, and the loop can work at higher pressure. The rotation angle of the pump can provide positive and negative liquid flow directions. On the downside, cooling can be an issue as the exchange of fluid flow is limited, and the pump is also difficult to use for other hydraulic functions. In the circuit of the high-power closed-loop system, to increase the amount of oil for cooling and filtering, a refill valve must be installed so that the amount of oil exchanged is greater than the basic leakage of the pump and motor. The replenishment valve is usually integrated into the casing of the motor to cool the circulating oil in the casing of the motor itself. Losses from the internal circulation of the motor housing and losses from the ball bearings can be significant since the motor speed of the vehicle can reach 4000-5000 r/min, or even higher at full speed. Leakage, like an additional charge, will be provided by the charge pump. If it is set to be used in high-pressure and high motor speed drives, a large displacement charge pump is important. When driving at high speed for a long time, if a hydrostatic transmission is used, high oil temperature is usually the main problem. High oil temperatures will drastically reduce the life of the transmission. To suppress oil temperature, transportation equipment must reduce system pressure and motor displacement must be limited to the minimum reasonable value. Closed-loop systems are also used in mobile equipment to replace mechanical and hydraulic transmissions. The advantage is that the gear ratio is infinitely variable, and the transmission ratio can be controlled more flexibly according to the load and operating conditions.