Decoding Fluid Power: The Essential Types of Hydraulic Systems

Hydraulic systems serve as the strength for the world's heaviest work, from construction cranes, and aircraft landing gear, to the lift in your nearby auto repair shop. These systems move force through pressurized fluid. Not all hydraulic systems are alike, though! They smartly come in different configurations designed to fit certain application functions.

Differentiating between systems depends on how the hydraulic fluid travels through the system.

1. Open-Loop Hydraulic Systems: The Basic Circuit 

The open-loop system is the most common and simplest of the hydraulic systems. This type is often utilized for application functions that need lower pressures, variable flow and easy maintenance.

How It Works:

• In an open loop system, the hydraulic fluid starts in the reservoir (the tank) and ends in the reservoir.
• The pump draws fluid directly from the reservoir.
• It pressurizes the fluid, then a directional controls valve directs it to the actuator (e.g., a cylinder or motor).
• When the work is finished, the hydraulic fluid flows directly back to the reservoir to be ready for its next cycle.

Key Characteristics:

• Continuous Flow: The pump generally operates at a constant rate to provide steady flow. When fluid is not being actuated, the control valve directs the fluid towards the reservoir at low pressure.  
• Heat Control: The bigger the reservoir, the more time the fluid has to rest and cool naturally, leading to better heat dissipation.  
• Used in: Industrial presses, rudimentary machine tools, agricultural devices, and steering systems.  

2. Closed-Loop Hydraulic Systems: The High-Performance Circuit  

The closed-loop system is a more complicated design system designed for high-performance applications requiring precision control and fast, constant response times like hydrostatic transmissions.  

How it Operates:  

• In a closed-loop system, the working fluid flows in a continuous, tight circuit between the pump and the motor/actuator while bypassing the main reservoir.  
• The main pump pumps fluid to the hydraulic motor.  
• The fluid is now sent to the motor, and then immediately sent back to the inlet of the main pump after the motor is turned a certain degree.   
• There is a charge pump that adds cooled, filtered fluid from a small reservoir (charge tank) to the low-pressure side of the loop. This ensures that the main pump always has fluid and minimal internal pressure.

Key Features: 

• Accuracy and Speed: This is ideal for hydrostatic drives where steps can be changed very quickly and accurately (controls for a skid-steer loader, for example). Often, even the direction can be reversed by simply changing the pump displacement. 
• Compactness: The working fluid is routed, in a continuous cycle,  which reduces the physical size of the main reservoir. 
• Applications: For hydrostatic drive transmissions (fork trucks, heavier loaders), advanced mobile equipment, or aerospace control systems. 

3. Open-Center compared with Closed-Center Valves (A Key Sub-Classification) 

While the open circuit and closed circuit distinction refers to the complete fluid path, we also classify systems based on the design of the directional control valve (DCV) in its neutral or centered position.

Choosing the right type of hydraulic system is a fundamental engineering decision, balancing cost, efficiency, size, and the critical need for precision control in any demanding task.