Basic Knowledge: Explanations of Hydraulic Terms

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1. Pascal’s principle (principle of static pressure transmission): In a closed container, the pressure applied to a stationary liquid is transmitted equally and simultaneously to all points within the liquid. 2. System pressure: The discharge pressure of the hydraulic pump in the system. 3. Kinetic viscosity: The ratio of dynamic viscosity μ to the density ρ of the liquid. 4. Hydrodynamic force: The force exerted by a flowing liquid on a solid wall, causing a change in its flow velocity. 5. Laminar flow: The viscous force plays a dominant role; the liquid particles are constrained by viscosity and cannot move freely, resulting in a well-structured flow pattern. 6. Turbulent flow: Inertia forces play a dominant role; at high flow speeds, the viscosity between liquid particles can no longer restrain them, resulting in a completely chaotic flow state. 7. Pressure loss along the flow path: The loss incurred due to viscous friction as the liquid flows through a pipe. 8. Local pressure loss: When a liquid flows through bends, joints, sections with sudden changes in cross-section, and valve openings in pipes, the speed and direction of the fluid flow change rapidly, resulting in vortices and intense turbulence; this leads to pressure loss. 9. Hydraulic clamping phenomenon: When a liquid flows through a conical annular gap, if the valve core is eccentric within the valve body’s bore, it may experience a hydraulic lateral force. When the hydraulic lateral force is large enough, the spool will press tightly against the wall of the valve port, resulting in a jamming phenomenon. 10. Hydraulic shock: In hydraulic systems, when the liquid pressure suddenly increases due to certain reasons, resulting in very high pressure peaks, this phenomenon is known as hydraulic shock. 11. Cavitation phenomenon ； Cavitation: In hydraulic systems, when the pressure at a certain point is lower than the air separation pressure of the hydraulic fluid at that temperature, the air originally dissolved in the liquid separates out, resulting in the formation of numerous bubbles within the liquid. This phenomenon is known as cavitation. When bubbles enter a high-pressure area along with the liquid flow, they burst rapidly or shrink sharply under the high pressure, and then re-condense into liquid. The space previously occupied by the bubbles creates a local vacuum, and the liquid particles surrounding it fill this space at extremely high speeds. As these particles collide with each other, local high pressures are generated, resulting in pressure waves. If this local hydraulic shock acts on the metal surface of the part, it causes corrosion on that metal surface. This type of corrosion caused by cavities is called cavitation. 12. Displacement: The volume of oil that should theoretically be discharged by the hydraulic pump with each rotation ； The volume of fluid required for the output shaft to rotate one full revolution in a hydraulic motor with no leaks. 13. Self-priming pump: A hydraulic pump whose suction chamber volume can increase automatically.
14. Variable displacement pump: A hydraulic pump whose displacement can be changed.
15. Constant power variable displacement pump: A variable displacement pump in which the product of the outlet pressure p and the output flow rate q is approximately constant.
16. Oil trapping phenomenon: During operation of a hydraulic pump, a closed volume is formed between the suction and pressure chambers; as the drive shaft rotates, the size of this volume changes, leading to pressure surges and cavitation – this phenomenon is known as oil trapping.
17. Differential connection: In a single-rod hydraulic cylinder, a connection method in which pressure oil is supplied to both the left and right chambers simultaneously is called a differential connection.
18. Reciprocating speed ratio: The ratio of the piston’s movement speed v2 when oil enters the smaller chamber and exits the larger chamber, to the piston’s movement speed v1 when oil enters the larger chamber and exits the smaller chamber.
19. Neutral position function of a spool valve: The way in which various oil ports are connected when a three-position spool valve is in its neutral position; this reflects the control capabilities of the directional control valve.
20. Pressure-flow characteristics of a relief valve: After the pre-compression amount of the relief valve’s regulating spring is set, the property by which the inlet pressure of the valve fluctuates as the relief flow rate changes is known as the pressure-flow characteristics or opening/closing characteristics.
21. Throttling speed control circuit: A hydraulic system that uses a fixed-displacement pump to supply fluid, and employs a flow control valve to adjust the flow rate to the actuator, thereby achieving speed control.
22. Volume control speed control circuit: A hydraulic system that uses a variable-displacement pump to supply fluid, and adjusts the flow rate to the actuator by changing the pump’s displacement, thus achieving speed control.
23. Power-adaptive circuit (load-sensitive speed control circuit): In a hydraulic system, a circuit in which both the output pressure and flow rate of the variable-displacement pump meet the requirements of the load is called a power-adaptive circuit.
24. Effective cross-sectional area of a pneumatic element: When gas flows through a throttle orifice, due to the viscosity of the fluid, the flow rate is reduced further compared to the actual area of the orifice; this minimum area is known as the effective cross-sectional area.
25. Mach number: The ratio of the gas flow velocity v to the local sound speed c is called the Mach number.
26. Non-sequential logic system: A system whose output depends only on the combination of input variables, regardless of the order in which those variables are applied.
27. Sequential logic system: A system whose output depends not only on the combination of input signals but also on a specific order. Also known as sequential control or program control