Closed Hydraulic Circuits
A comprehensive technical overview of closed-loop hydraulic systems, their components, operation principles, and advantages in industrial applications.
Definition and Basic Principles
A closed hydraulic circuit is defined as a hydraulic system where the fluid returning from the actuators (oil-using equipment) is directed back to the inlet of the hydraulic pump. This configuration creates a continuous loop where the hydraulic fluid circulates without being exposed to a large reservoir in the main flow path.
In this arrangement, the hydraulic pump and hydraulic motor form the primary components of the closed loop, working in tandem to transmit power efficiently. The key distinction from open circuits is that the hydraulic fluid remains primarily within the circuit, with only minimal make-up required to compensate for leakage.
Depending on the direction of the load (or starting torque), closed circuits can be divided into high-pressure and low-pressure sides. The high-pressure side is protected by a relief valve, which diverts excessive load pressure to the low-pressure side, ensuring the hydraulic fluid remains within the circuit.
Only the continuous leakage flow generated by the hydraulic pump and hydraulic motor (depending on operating data) needs to be replenished from an external source. This makeup flow is typically provided by an auxiliary pump, which is often directly mounted to the main hydraulic pump via a flange connection.
Key Components
- Main hydraulic pump
- Hydraulic motor
- Auxiliary makeup pump
- Relief valves (high & low pressure)
- Check valves
- Small reservoir/tank
- Filter and cooler
Closed Circuit Configuration (Figure 1-7)
Figure 1-7: Schematic representation of a closed hydraulic circuit showing the main components and flow paths
Operational Characteristics
The closed hydraulic circuit operates on a continuous loop principle where the hydraulic fluid is constantly recirculated between the hydraulic pump and hydraulic motor. This design minimizes fluid loss and exposure to external contaminants.
Through a check valve, sufficient fluid (make-up flow) is continuously drawn from a small tank and delivered to the low-pressure side of the closed circuit. Any excess flow generated by the make-up pump (relative to the open circuit) is returned to the tank through a make-up relief valve.
By replenishing the fluid on the low-pressure side of the circuit, the operating characteristics of the main hydraulic pump are improved. This ensures consistent performance and prevents cavitation that could damage the hydraulic pump and hydraulic motor.
Pressure Management in Closed Circuits
Pressure distribution between high-pressure and low-pressure sides during typical operation
High-Pressure Side Operation
The high-pressure side of the closed circuit carries the working pressure required to drive the hydraulic motor and overcome the load. This pressure is determined by the system requirements and the resistance of the load being moved.
A relief valve is strategically placed on the high-pressure side to protect the system from overpressure conditions. When pressure exceeds the safe operating limit, the relief valve opens, diverting fluid to the low-pressure side, thus preventing damage to the hydraulic pump, hydraulic motor, and other components.
Low-Pressure Side Operation
The low-pressure side maintains a relatively constant pressure level, typically maintained by the make-up pump system. This side returns fluid from the hydraulic motor back to the hydraulic pump inlet, completing the closed loop.
The make-up pump ensures that any fluid lost due to leakage in the hydraulic pump or hydraulic motor is replaced, maintaining proper circuit volume and preventing cavitation. The low-pressure side also incorporates filtration and cooling components to maintain fluid quality.
Typical Characteristics of Closed Circuits
Compact Structure
The system structure is relatively compact with less opportunity for contact with air, preventing air from entering the system and ensuring smooth transmission.
Smooth Transitions
Speed change and reversal of the working mechanism are achieved by adjusting the variable mechanism of the hydraulic pump or hydraulic motor, avoiding hydraulic shock and energy loss.
Small Directional Valves
Directional valves are small in size and used only for pilot control, reducing overall system complexity and pressure drops.
Compact Components
Filters and coolers are smaller in size compared to open circuits due to the reduced fluid volume and controlled environment.
Smaller Reservoir
The tank volume is smaller, sized only to match the flow rate of the make-up pump and system volume requirements.
Higher Speed Limits
Through make-up oil, higher speed limits can be achieved without compromising the performance of the hydraulic pump or hydraulic motor.
Flexible Placement
The compact design allows for flexible placement and easy installation in various industrial environments. This flexibility makes closed circuits suitable for applications where space is limited or specific orientation is required.
Controlled Reversing
Closed circuits feature neutral control modes that allow for complete reverse rotation of the hydraulic motor. This is achieved through the variable displacement capabilities of the hydraulic pump, enabling smooth direction changes without additional valves.
Brake Power Feedback
A significant advantage of closed hydraulic circuits is their ability to implement brake power feedback through the drive hydraulic motor. When decelerating or stopping a load, the hydraulic motor can act as a pump, converting kinetic energy back into hydraulic energy which can be dissipated or even recovered. This energy recovery capability improves overall system efficiency and reduces wear on mechanical braking components. The hydraulic pump plays a crucial role in this process by controlling the pressure and flow during the feedback phase, ensuring smooth and controlled deceleration.
Closed vs. Open Circuit Comparison
| Characteristic | Closed Circuit | Open Circuit |
|---|---|---|
| Fluid Path | Direct loop between hydraulic pump and hydraulic motor | Fluid returns to large reservoir after use |
| Reservoir Size | Small, only for make-up fluid | Large, holds entire system volume |
| Air Exposure | Minimal, reduced aeration risk | Significant, higher aeration risk |
| Contamination Risk | Lower, closed environment | Higher, more exposure |
| Response Time | Faster, less fluid to move | Slower, more fluid volume |
| Energy Efficiency | Higher, especially with variable displacement | Lower, more pressure losses |
| Complexity | Higher, requires make-up system | Lower, simpler design |
| Cost | Higher initial cost | Lower initial cost |
| Maintenance | Specialized, but less frequent | More routine maintenance |
Common Applications
Closed hydraulic circuits are widely used in various industrial applications where efficiency, compactness, and precise control are essential. The unique characteristics of these systems make them particularly suitable for applications requiring reversible motion, variable speed control, and high power density.
Mobile Hydraulics
Construction equipment, agricultural machinery, and material handling vehicles often utilize closed circuits for their main drives, where the hydraulic pump and hydraulic motor work together to provide efficient power transmission.
Industrial Machinery
Metal forming machines, injection molding equipment, and rolling mills benefit from the precise control and energy efficiency of closed circuits featuring advanced hydraulic pump and hydraulic motor configurations.
Marine Applications
Deck machinery, winches, and steering systems on ships often employ closed circuits for their reliability and resistance to harsh environments.
Efficiency Comparison in Real-World Applications
Energy efficiency comparison between closed and open circuits across different operating conditions
Key Component Details
Hydraulic Pump in Closed Circuits
Variable displacement hydraulic pump used in closed circuit applications
The hydraulic pump is the heart of any closed circuit system, responsible for generating flow and pressure. In most closed circuit applications, variable displacement pumps are used to allow for speed and direction control without the need for large directional valves.
These specialized hydraulic pumps are designed to operate continuously in a closed loop, with internal passages that allow for efficient fluid recirculation. They often incorporate integrated relief valves and make-up flow check valves to simplify system design.
The ability to reverse flow direction by changing the pump's displacement direction eliminates the need for complex valve arrangements, making the hydraulic pump a versatile component in closed circuit systems.
Hydraulic Motor in Closed Circuits
High-efficiency hydraulic motor designed for closed loop operation
The hydraulic motor converts hydraulic energy back into mechanical energy to drive the load. In closed circuits, the hydraulic motor works in direct partnership with the hydraulic pump, receiving pressurized fluid from one port and returning it to the other.
These motors are designed to handle the continuous flow and pressure conditions of closed circuits, often featuring high torque capabilities and efficient operation across a wide speed range.
Many modern hydraulic motor designs for closed circuits incorporate features that allow them to operate as pumps during braking or deceleration, enabling energy recovery and improving overall system efficiency when paired with an appropriately designed hydraulic pump.
Auxiliary Systems and Components
While the hydraulic pump and hydraulic motor form the core of the closed circuit, several auxiliary components are essential for proper operation. The auxiliary pump, typically mounted directly to the main hydraulic pump, provides make-up flow to compensate for leakage in the hydraulic pump and hydraulic motor.
Make-up System
This system, consisting of an auxiliary pump, check valves, and a relief valve, maintains proper fluid volume in the circuit, preventing cavitation and ensuring the hydraulic pump and hydraulic motor receive adequate lubrication.
Filtration and Cooling
Compact filters and coolers maintain fluid cleanliness and temperature within optimal ranges. The closed nature reduces contamination risks, allowing for smaller filtration components than open circuits.
Control Valves
Relief valves, check valves, and small directional control valves manage pressure, prevent backflow, and provide pilot control functions, ensuring safe and efficient operation of the hydraulic pump and hydraulic motor.
Conclusion
Closed hydraulic circuits represent a sophisticated approach to fluid power transmission, offering numerous advantages over open circuits in specific applications. By creating a continuous loop between the hydraulic pump and hydraulic motor, these systems minimize fluid exposure to the environment, reduce contamination risks, and improve overall efficiency.
The compact design, smooth operation, and energy efficiency of closed circuits make them particularly well-suited for applications requiring precise control, reversible motion, and high power density. While the initial complexity and cost may be higher, the long-term benefits often justify the investment in industrial and mobile hydraulic applications.
As hydraulic technology continues to evolve, closed circuit designs incorporating advanced hydraulic pump and hydraulic motor configurations are expected to play an increasingly important role in energy-efficient fluid power systems.
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