Directional Control Circuits

Introduction to Directional Control Circuits

In hydraulic systems, the circuits that control the start, stop, and direction change of actuators are called directional control circuits. These fundamental circuits are crucial for regulating the movement of hydraulic components in various industrial applications. Understanding what is hydrostatic drive is essential here, as it forms the basis for many of these control mechanisms.

Directional control circuits primarily include reversing circuits and locking circuits. Each type serves specific functions in hydraulic systems, ensuring precise control over actuator movement. What is hydrostatic drive, you might ask? It refers to a hydraulic drive system where power is transmitted through hydraulic fluid under pressure, enabling efficient power transfer and control—exactly what these circuits manage.

These circuits utilize various components such as directional control valves, check valves, and pressure control valves to achieve their objectives. The design and selection of appropriate directional control circuits depend on factors like required precision, load conditions, and operational speed. What is hydrostatic drive's role in this? It's the underlying principle that allows these circuits to transmit power efficiently while maintaining control over speed and direction.

I. Reversing Circuits

The reversal of moving parts is generally achieved using various directional control valves. In closed circuits with volumetric speed control, the direction of oil flow can also be controlled using a bidirectional variable displacement pump to reverse the direction of a hydraulic cylinder (or hydraulic motor). Understanding what is hydrostatic drive helps in comprehending how these pumps can effectively control both speed and direction in such systems.

Single-acting cylinders that return by gravity or springs can be reversed using 2/3 directional control valves. Double-acting cylinders are typically reversed using 2/4 (or 2/5) and 3/4 (or 3/5) directional control valves. Different control methods for reversing circuits can be selected according to different applications. What is hydrostatic drive's contribution here? It provides the efficient power transmission that allows these valves to control heavy loads with precision.

Solenoid-Operated Reversing Circuits

Solenoid-operated directional valve circuits are most widely used, especially in hydraulic systems of transfer machines with high automation requirements. These reversing circuits have appeared many times in the前述内容介绍过的许多回路中,这里不再赘述. What is hydrostatic drive's role in these automated systems? It enables efficient power transmission that can be quickly and precisely controlled by solenoid valves, allowing for rapid cycle times and accurate positioning.

For applications with large flow rates and high requirements for smooth reversing, solenoid-operated directional valve circuits can no longer meet the above requirements. Often, manual or mechanical directional valves are used as pilot valves with hydraulically operated directional valves as main valves, or electro-hydraulic directional valve reversing circuits are employed. These more complex systems rely on the principles of what is hydrostatic drive to manage higher power levels while maintaining control precision.

Reversing Circuits for Reciprocating Linear Motion

The function of reversing circuits for reciprocating linear motion is to enable hydraulic cylinders and connected machine components to change direction quickly, smoothly, and accurately at the end of their stroke. Simple reversing circuits only require standard ordinary directional valves, but in machines with high reversing requirements (such as various grinding machines), the directional valves in reversing circuits need special design. These types of reversing circuits can be divided into time-controlled braking and stroke-controlled braking according to different reversing requirements. Understanding what is hydrostatic drive helps in optimizing these circuits for specific applications, balancing power, speed, and precision.

Time-Controlled Braking Reversing Circuits

Figure 7-34 shows a relatively simple time-controlled braking reversing circuit. In this circuit, the main oil path is only controlled by the directional valve 3. During the reversing process, when the pilot valve 2 in the figure is in the left position, the hydraulic oil in the control oil path通向换向阀3右端 through a one-way valve, and the oil at the left end of the directional valve flows back to the oil tank through the throttle valve. The spool of the directional valve moves to the left, and the cone on the valve gradually closes the oil return passage, the piston speed gradually decreases, and after the spool of the directional valve 3 moves over a distance, the passage is closed, causing the piston to stop moving. What is hydrostatic drive's role in this process? It ensures that the hydraulic pressure remains consistent during the braking phase, allowing for predictable deceleration based on the throttle valve settings.

When the opening sizes of throttle valves 7 and 1 are set, the time required for the directional valve spool to move over the distance (the time experienced to brake the piston) is fixed. Therefore, this braking method is called time-controlled braking. What is hydrostatic drive's contribution to this system? It provides the stable pressure source needed to ensure that the time-based braking remains consistent across multiple cycles.

The main advantage of the time-controlled braking reversing circuit is that its braking time can be adjusted according to the speed and inertia of the moving parts of the machine through the opening of throttle valves 7 and 1, in order to control reversing impact and improve work efficiency. This adjustability is one of the key benefits of what is hydrostatic drive technology—adapting to varying operational conditions while maintaining control.

Its main disadvantage is that the overshoot during the reversing process is affected by the speed of the moving parts and some other factors, and the reversing accuracy is not high. Therefore, this reversing circuit is mainly used in occasions where the moving speed of the working parts is high but the reversing accuracy is not high, such as in the hydraulic system of surface grinders. Understanding what is hydrostatic drive helps in selecting the appropriate circuit type for specific application requirements, balancing speed and precision.

Proper setup of time-controlled braking circuits requires careful calculation of the optimal braking time based on the specific application parameters. Engineers must consider the mass of moving components, operating speed, and required cycle time when configuring these systems. What is hydrostatic drive's role in this calculation? It provides the power density and control characteristics that form the basis for determining appropriate braking parameters.

Stroke-Controlled Braking Reversing Circuits

Figure 7-35 shows a stroke-controlled braking reversing circuit. The structure and working conditions of this circuit are mainly different from those of time-controlled braking in that the main oil path here is not only controlled by the directional valve 3 but also by the pilot valve 2. When the pilot valve 2 in the图示位 is moving to the left during the reversing process, the right braking cone of the pilot valve spool will gradually close the oil return passage of the right cavity of the hydraulic cylinder, so that the piston speed gradually decreases to pre-brake the piston. What is hydrostatic drive's advantage in this configuration? It allows for consistent power delivery during the braking stroke, ensuring predictable deceleration regardless of load variations.

When the oil return passage is closed very small and the piston speed becomes slow, the control oil path of the directional valve 3 starts to switch, and the spool of the directional valve moves to the left, cutting off the main oil path, stopping the piston, and immediately starting it in the opposite direction. Here, regardless of the original speed of the moving parts, the pilot valve always moves a fixed stroke, first pre-braking the working parts, and then reversing it by the directional valve. Therefore, this braking method is called stroke-controlled braking. Understanding what is hydrostatic drive helps in appreciating how these systems can maintain consistent braking strokes even under varying operating pressures.

The stroke-controlled braking reversing circuit has high reversing accuracy and small overshoot; however, because the braking stroke of the pilot valve is constant, the length of braking time and the magnitude of reversing impact will be affected by the speed of the moving parts. So, this reversing circuit is suitable for occasions where the moving speed of the working parts of the machine is not high but the reversing accuracy is high, such as in the hydraulic systems of internal and external cylindrical grinders. What is hydrostatic drive's role in these precision applications? It provides the smooth power delivery necessary for accurate positioning, a key advantage over mechanical drive systems.

The selection between time-controlled and stroke-controlled braking systems depends on the specific requirements of the application. While stroke-controlled systems offer superior accuracy, they come at the cost of complexity and potentially longer cycle times. What is hydrostatic drive's influence on this decision? It's the foundational technology that enables both approaches, with different circuit designs optimizing its characteristics for specific performance criteria.

II. Locking Circuits

In order to enable the working parts to stay at any position and prevent movement under force when stopped, locking circuits can be used. These circuits are essential in applications where precise positioning must be maintained without external power, leveraging the principles of what is hydrostatic drive to hold loads securely using hydraulic pressure.

A 3-position directional valve with O-type or M-type function can be used. When the spool is in the neutral position, both the inlet and outlet of the hydraulic cylinder are closed, which can lock the piston. However, this locking circuit has poor locking effect due to the influence of spool leakage. What is hydrostatic drive's limitation in this simple configuration? It cannot completely prevent leakage through valve spools, which eventually causes drift in positioned loads.

Pilot-Operated Check Valve Locking Circuits

Figure 7-36 shows a bidirectional locking circuit using pilot-operated check valves. Pilot-operated check valves (also known as hydraulic locks) are connected in series in both the inlet and return oil paths of the hydraulic cylinder, allowing the piston to be locked at any position in the stroke. Its locking accuracy is only affected by a small amount of internal leakage in the hydraulic cylinder, so the locking accuracy is high. This type of typical circuit is often used in papermaking machinery. Understanding what is hydrostatic drive helps in designing these circuits to minimize leakage paths and maximize locking efficiency.

For locking circuits using pilot-operated check valves, the neutral function of the directional valve should release the control oil of the pilot-operated check valve (the directional valve adopts H-type or Y-type). At this time, the pilot-operated check valve closes immediately, and the piston stops moving. What is hydrostatic drive's role in this immediate response? It ensures that pressure is rapidly relieved from the pilot circuit, allowing the check valves to seal securely.

If an O-type function is used, when the directional valve is in the neutral position, the control chamber hydraulic oil of the pilot-operated check valve is blocked and cannot be closed immediately until the control chamber is depressurized by the internal leakage of the directional valve, the pilot-operated check valve can be closed, which affects its locking accuracy. This is why understanding what is hydrostatic drive's characteristics—including pressure retention and leakage properties—is crucial when designing effective locking circuits.

Locking circuits are critical in many industrial applications, from machine tools to material handling equipment. They ensure safety by preventing unintended movement and improve precision by maintaining positions during work processes. What is hydrostatic drive's contribution to these circuits? It provides the pressure necessary to maintain locked positions while allowing for smooth, controlled movement when needed—offering a unique combination of security and flexibility that mechanical locking systems cannot match.

When designing locking circuits, engineers must consider factors such as load characteristics, required holding time, operating environment, and safety requirements. Properly designed locking circuits, leveraging the principles of what is hydrostatic drive, can significantly enhance system performance, safety, and efficiency in numerous industrial applications.