Open Hydraulic Circuit

An open hydraulic circuit describes a hydraulic circuit structure in which the hydraulic fluid flows back from the pump via directional control valves and consumers to the tank, where it is collected in an atmospherically ventilated reservoir. This type of circuit is particularly common in mobile hydraulics, as it offers a simple design and enables effective heat dissipation.

Design and operating principle

The open hydraulic circuit consists of several essential components that are arranged in a defined sequence. The hydraulic pump draws hydraulic oil from an open tank and feeds it into the system at a constant or adjustable volume flow. From there, the fluid flows through pressure lines to the directional control valves, which act as control elements and direct the oil flow to the individual consumers.

The consumers can be hydraulic cylinders for linear movements or hydraulic motors for rotary drives. After passing through the consumers, the oil flows back into the tank via return lines. This closed path of the hydraulic fluid is: pump, consumer, tank, pump. The tank is open to the atmosphere, which is why it is called an “open circuit”.

A pressure relief valve (PRV) protects the system from impermissibly high pressures. It is connected in parallel to the pump as a safety valve and opens when the set maximum pressure is reached in order to divert excess oil directly into the tank. There must be no shut-off by a valve between the pump and the pressure relief valve.

Neutral position and oil flow

In the neutral position of the directional control valves, when no movement of the consumers is to take place, the passage from the pressure side (P) to the tank side (T) in the valve is open. The hydraulic oil flows back into the tank through the hydraulic valves with virtually no pressure. The pressure in the system only increases when a directional control valve is actuated and the oil is diverted to a consumer that builds up counterpressure.

This mode of operation differs fundamentally from other systems: the oil circulates continuously without permanently high pressure. Only the demand from a consumer generates the necessary working pressure.

Types of pumps in open circuits

Hydraulic systems with a pressure source generally operate in an open circuit. Various types of pumps are used as pressure sources. Constant pumps such as gear pumps deliver a constant volume flow regardless of the load condition. They are characterized by their simple design, robustness, and low cost.

Internal gear pumps also offer a low noise level, which makes them attractive for machine tools and mobile applications. Gear ring pumps based on the gerotor principle enable very low drive speeds of 10 to 250 rpm and are therefore suitable for direct drives without intermediate gearboxes.

Alternatively, variable displacement pumps can be used to adjust the volume flow to the actual demand. In conjunction with load-sensing systems, efficiency can be significantly increased, as only the amount of oil currently required is delivered.

Parallel and multiple movements

An important feature of the open circuit is its behavior during multiple movements. If several consumers are operated simultaneously, the volume flow is distributed according to the resistances. The oil follows the path of least resistance, which can lead to different speeds of the consumers.

When several consumers are connected in parallel, the speed of each cylinder or motor depends on the load ratio. A consumer with low load resistance receives more volume flow than one with high resistance. Additional measures are therefore necessary for precise multiple movements, such as flow divider valves or individual flow regulators for each consumer.

The total available flow rate of the pump limits the number and speed of functions that can be operated simultaneously. The more consumers operate in parallel, the slower the individual actuators move, as the total volume flow is divided.

Advantages of the open circuit

The open hydraulic circuit offers several technical and economic advantages. The simple design significantly reduces investment and maintenance costs. The number of components required is manageable, which minimizes the susceptibility to errors.

Heat dissipation is a key advantage. Since the hydraulic oil continuously flows through the tank, the heat generated can be dissipated into the environment. The tank acts as a heat exchanger and cooling reservoir. If necessary, external oil coolers can be easily integrated into the return line.

The atmospherically ventilated tank allows for easy compensation of volume changes due to temperature and pressure fluctuations. Contaminants can settle in the tank, and oil quality can be maintained by return filters. Open access facilitates maintenance and oil level checks.

Oil leaks from cylinders and valves are not a problem, as fresh oil is continuously supplied from the tank. No feed pump is required to compensate for leaks.

Disadvantages and limitations

The open circuit also has limitations. Continuous oil circulation generates flow losses that must be dissipated as heat. With constant pumps, the entire delivery capacity is pumped back into the tank without pressure via the directional control valve in the neutral position, resulting in poor efficiency.

Energy consumption is higher than in closed systems with variable displacement pumps, as the pump continues to deliver even when no consumers are activated. Load-sensing systems can mitigate this disadvantage, but increase system complexity.

The response time is longer than in closed circuits, as the pressure must first be built up when a valve is actuated. The open circuit is therefore less suitable for highly dynamic applications with rapid reversals of motion.

The tank must be sufficiently dimensioned to ensure heat dissipation, ventilation, and dirt separation. This increases the weight and space requirements of the entire system.

Areas of application

The open hydraulic circuit dominates in mobile hydraulics. Construction machines such as excavators, wheel loaders, cranes, and forklifts use this circuit structure for their working functions. Simplicity and robustness are decisive factors here, as these machines must operate reliably under harsh conditions.

In agricultural engineering, open circuits are used in tractors, combine harvesters, and self-propelled machines. Front loaders, attachments, and lifting mechanisms are typically controlled by directional control valves in open systems.

Stationary applications can be found in machine tools, presses, and production facilities where the highest demands on dynamics and energy efficiency are not required. The open circuit is an economical solution for simple lifting, clamping, and pressing operations.

Wherever several independent functions with different load cycles need to be controlled, the open circuit offers flexibility and easy expandability.

Comparison with closed circuits

In a closed hydraulic circuit, the oil is returned directly from the consumer to the pump without detouring via the tank. This circuit is mainly found in hydrostatic transmissions with variable displacement pumps and hydraulic motors, for example in travel drives.

The closed circuit enables 4-quadrant operation, i. e. , drive and braking in both directions of rotation. The direction of movement is determined by the swivel direction of the variable displacement pump. Reaction times are shorter because the system is permanently under pressure.

However, the closed circuit is more complex in terms of design. It requires a feed pump to compensate for leakage losses and for cooling. Heat dissipation is more difficult because the oil remains in the working circuit. Additional components such as flush valves and circuit safety valves increase complexity and costs.

The closed circuit offers few advantages for pure cylinder drives without energy recovery. Its strengths lie in reversible drives with frequent direction changes and high dynamic requirements.

Increased efficiency through load sensing

Modern open circuits use load sensing technology to improve energy efficiency. A variable displacement pump automatically adjusts its flow rate to the demand of the consumers. A measuring line records the highest load pressure in the system, and the pump adjusts its flow rate so that only a small pressure drop occurs across the valves.

This significantly reduces throttling losses and heat generation. Energy consumption is reduced because the pump only delivers the amount of oil that is actually needed. In the neutral position, the delivery volume is reduced to a minimum, which virtually eliminates power loss.

Load-sensing systems require special valves with LS connections and a suitably equipped variable displacement pump. The investment costs are higher, but are offset by energy savings and lower thermal stress.

Important design criteria

Several parameters must be taken into account when designing an open hydraulic circuit. The pump must be dimensioned so that it can deliver the peak volume flow of all consumers operating simultaneously. In addition, a safety margin of 10 to 20 percent should be included.

The tank size depends on the oil residence time, which should ensure sufficient heat dissipation and ventilation. As a guideline, two to three times the pump delivery rate per minute is recommended. Oil coolers may be necessary in the case of higher thermal loads.

The pipe cross-sections must be selected so that the flow velocity in pressure pipes does not exceed 4 to 6 meters per second and in return pipes 2 to 3 meters per second. Excessive velocities cause pressure losses, heating, and cavitation.

The pressure relief valve is set to approximately 10 to 20 percent above the maximum working pressure. It serves exclusively as a safety element and should not be activated during normal operation.

Summary

The open hydraulic circuit is the most widely used circuit structure in mobile hydraulics. Its strengths lie in its simple design, good heat dissipation, and uncomplicated maintenance. It is the most economical solution for applications with several independent consumers, different load cycles, and moderate dynamic requirements.

The use of modern control concepts such as load sensing can significantly reduce the traditional disadvantages in terms of energy efficiency. The combination of proven technology and intelligent control makes the open circuit attractive for future applications where reliability and ease of maintenance are paramount.