Closed Hydraulic Circuit

A closed hydraulic circuit is a circuit configuration in hydraulics in which the hydraulic fluid circulates directly between the pump and the actuator without passing through a pressureless reservoir. The motor’s return line is connected to the suction side of the pump, creating a self-contained fluid circuit. Closed hydraulic circuits are primarily used where high dynamics, compact design, and stepless reversibility are required.

Basics and Functioning of the Closed Hydraulic Circuit

The closed hydraulic circuit differs fundamentally from the open circuit in the way the fluid is routed. Instead of draining the oil into a reservoir after each work cycle, the low-pressure line returns the fluid directly to the suction side of the pump. The system operates as a self-contained circuit in which the hydraulic oil flows continuously back and forth between the pump and the motor. The direction of rotation and speed of the load can be controlled solely by adjusting the pump, without the need for directional control valves.

Pressure Build-up and Fluid Flow

The pump, typically a variable-displacement pump of the axial piston type, delivers the oil to the motor under high pressure. The motor converts the hydraulic energy into mechanical rotational energy and returns the oil on the low-pressure side to the pump. Since there is no large tank serving as a buffer in this circuit, the system pressure must be actively regulated on both lines. Typical operating pressures in closed hydraulic circuits range between 300 and 450 bar; in special applications, pressures of up to 500 bar can be reached.

Role of the make-up pump

A closed hydraulic circuit cannot operate without supplementary components, as unavoidable leaks at the pump and motor draw fluid volume out of the circuit. The make-up pump, usually designed as a constant-displacement pump, compensates for this volume difference. It delivers fresh oil from a small reservoir to the low-pressure side of the circuit, thereby maintaining the minimum pressure. Typical make-up pressures range from 10 to 30 bar. This preload pressure prevents cavitation on the suction side of the main pump and ensures that the circuit remains completely filled with fluid at all times.

Components of a closed hydraulic circuit

In addition to the pump and motor, the closed hydraulic circuit requires several auxiliary components that ensure and protect operation. Each of these components fulfills a specific function that is covered in the open circuit by the tank or simpler valve designs.

Bleed valve

The purge valve diverts a portion of the heated oil from the low-pressure side of the circuit and directs it through a cooler to the reservoir. Since the oil in the closed circuit does not circulate through a tank, natural heat dissipation is absent. Without a flush valve, the oil temperature would continuously rise, which alters the viscosity and shortens the service life of the components. The replenished fresh oil simultaneously replaces the drained hot oil, resulting in a continuous exchange.

Pressure relief valves

Pressure relief valves on both sides of the circuit protect the system from excessive pressure spikes. Especially during rapid reversals or sudden load changes, short-term pressure spikes can occur that overload the components. The valves open when the set pressure limit is reached and divert oil from the high-pressure side to the low-pressure side. In many designs, these valves are integrated directly into the pump or motor.

Check valves

Check valves on the replenishment line ensure that the replenishment oil always flows into the line that is currently on the low-pressure side. Since the high- and low-pressure sides switch when the direction changes, the check valves must automatically follow this change. At the same time, they prevent pressure oil from flowing back from the main circuit into the make-up line.

Closed hydraulic circuit compared to open circuit

A comparison between closed and open circuits highlights the respective strengths and weaknesses of both concepts. The choice of circuit configuration depends on the requirements of the specific application.

The open circuit offers advantages in simpler applications with multiple consumers, as a central tank ensures the oil supply to all circuits. The closed hydraulic circuit excels where a single drive requires high dynamics and energy efficiency.

Applications of the closed hydraulic circuit

Closed hydraulic circuits are found in both mobile hydraulics and stationary industrial systems. The common requirements of these applications are high power density, stepless speed and torque control, and the ability to reverse quickly.

Mobile hydraulics

In mobile hydraulics, closed hydraulic circuits dominate in the travel drives of wheel loaders, excavators, and telehandlers. Winch drives on cranes and ships, as well as slewing drives on turntables, also operate using this circuit design. The compact design without a large tank is a decisive advantage on vehicles where installation space and weight are limited. Stepless travel control via pump adjustment enables precise maneuvering and braking via the hydraulic system, which reduces wear on mechanical brakes.

Stationary industrial plants

In stationary applications, closed hydraulic circuits are used in presses, injection molding machines, and rolling mills. Test benches and testing systems also utilize this concept when fast cycle times and reproducible motion profiles are required. Industrial gearboxes and fan drives benefit from high efficiency and reversible rotation without additional directional control valves. In foundries and rolling mills, compact systems with minimal oil volume enable high-performance drives in confined spaces.

Control and Regulation in a Closed Hydraulic Circuit

A closed hydraulic circuit is primarily controlled by adjusting the main pump. The swivel angle of the axial piston pump determines the flow direction and flow rate, thereby directly controlling the motor’s direction of rotation and speed. This direct coupling between pump adjustment and motor behavior enables short response times and high control quality.

Hydraulic control

In purely hydraulic control, the pump’s adjustment mechanism is actuated via control pressures. DA valves (pressure and flow valves) regulate the adjustment depending on the system pressure and the desired driving behavior. This solution is frequently used in simple drive systems where electronic control is not required.

Electrohydraulic control

Modern closed hydraulic circuits increasingly utilize electrohydraulic control. Proportional valves control the pump’s variable displacement mechanism based on electrical signals from a higher-level controller. This enables integration into automated systems that precisely approach and monitor operating points. Sensors measure pressure, temperature, and speed in real time and provide the data to the controller for adaptive control. Electrohydraulic control improves efficiency, as the pump’s operating point can always be maintained within the optimal range.

Maintenance and Servicing

Maintenance of a closed hydraulic circuit requires special attention, as the oil circulates continuously within the circuit and contaminants cannot settle in the tank as they would in an open circuit. Particles circulate in the system until they are captured by the filter or cause damage.

Oil Quality and Filtration

Oil purity is more critical in a closed hydraulic circuit than in an open system. The make-up line is the central point for filtration, as this is where all the fresh make-up oil passes through the filter. High-quality filter elements with appropriate fineness are essential. Regular oil analyses provide information on wear particles, water content, and changes in viscosity.

Temperature Management

Since the closed hydraulic circuit does not provide natural cooling via a large tank, the oil temperature must be actively monitored and controlled. The bypass valve diverts a defined flow rate of heated oil from the circuit and directs it through an oil cooler to the reservoir. The cooler must be sized to match the heat load to be dissipated. If cooling is insufficient, the oil temperature rises, which lowers viscosity, reduces the load-carrying capacity of the lubricating film, and ultimately leads to component damage.

Leakage Monitoring

The make-up flow rate is a direct indicator of the circuit’s condition. If the make-up requirement rises above the usual value of about 5 to 10 percent of the main flow rate, this indicates increased internal leakage, which may be due to wear on pistons, control discs, or seals. Continuous monitoring of the make-up flow enables condition-based maintenance and prevents unexpected failures.

Advantages and Disadvantages of the Closed Hydraulic Circuit

The decision to use a closed hydraulic circuit entails specific advantages and disadvantages that designers must weigh during the design phase.

Advantages

• High energy efficiency: The pump delivers only the flow rate actually required. At partial load and during standstill, energy consumption drops significantly compared to open systems with constant-flow pumps.
• Compact design: Eliminating the need for a large tank significantly reduces the system’s footprint and weight.
• Fast reversal: The direction of rotation is reversed solely by adjusting the pump, without having to switch directional control valves. This shortens response times.
• Precise control: Speed and torque can be adjusted continuously and with high precision, which is essential for demanding travel and positioning tasks.
• Brake energy recovery: When loads are moving downward, the motor can function as a pump and feed brake energy back into the circuit.

Disadvantages

• Greater system complexity: The required additional components, such as a makeup pump, flush valve, and check valves, increase design and commissioning efforts.
• Limited multi-consumer capability: A closed hydraulic circuit typically supplies a single consumer. Separate circuits or complex additional circuits are required for multiple consumers.
• Increased requirements for oil purity: Contaminants circulate in the system and can cause damage if not filtered out early on.
• More complex troubleshooting: The close coupling of components and the lack of visual access to the tank make diagnosis more complex than in open systems.

Standards

The safety requirements for hydraulic systems with closed hydraulic circuits are governed by DIN ISO 4413. This standard specifies requirements for pressure limitation, leakage control, labeling, and commissioning. In addition, the general requirements for hydraulic fluids regarding viscosity, temperature stability, and purity class apply. Manufacturers such as Bosch Rexroth, Danfoss, and Liebherr offer detailed design guidelines for closed hydraulic circuits that go beyond the standard requirements and include field-proven sizing rules.