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Basic hydraulic circuit design: classifications of circuits.

Effective communication is important in any endeavor. In designing fluid power systems and circuits, it is vital that there be a common understanding of the language and terminology of hydraulics. This discussion will focus on that understanding.

Fluid power systems can be divided into two major groups: open loop and closed loop. Note that the terms system and circuits are used interchangeably. In a closed-loop system, a feedback mechanism continually monitors system output, generating a signal proportional to this output and comparing it to an input or command signal. If the two match, there is no adjustment and the system continues to operate as programmed. If there is a difference between the input command signal and the feedback signal, the output is adjusted automatically to match command requirements.

There is no feedback mechanism in an open-loop system. The performance characteristics of the circuit are determined entirely by the characteristics of the individual components and their interaction in the circuit. A typical open-loop circuit is illustrated in Fig. 1. Most industrial circuits fall in this category.


An electrohydraulic servo system is a feedback system in which the output is a mechanical position or function thereof as shown in Fig. 2.


Open-loop systems can be grouped by the functions they perform, their control methods, the type of open-loop circuit and their applications.

Classification of open-loop circuits by function is related to the basic areas of control used in a fluid power system. Directional controls regulate the direction of the distribution of energy; flow controls regulate the rate at which energy is transferred by adjusting the flow rate in a circuit or branch of circuits, and pressure controls regulate energy transfer by adjusting the pressure level or by using a specific pressure level as a signal to initiate a secondary action.

Classification by control functions entails directional control, flow control and pressure control. Valve controls, under the directional control class, make use of many types of directional control valves to regulate the distribution of energy throughout the circuit. These valves switch flow streams entering and leaving the valve. Pump control, under directional control is limited to reversal of direction of flow from a variable-displacement, reversible pump. Finally, fluid motor control is similar to pump control in that it uses reversible, variable-displacement motors.

The flow control method includes valve, pump and fluid motor controls. Valve controls use one of several types of pressure-compensated or non-compensated flow control valves. The position of the flow control valve in the circuit determines the appropriate type to use. A meter-in flow control valve is between the source of energy, the pump and the actuator. See Fig. 3(a).


A meter-out flow control valve is in the return line from the actuator and controls energy transfer by limiting the rate of flow out of the actuator. See Fig. 3(b).A bleed-off flow control valve is in parallel with the actuator. It limits the rate of energy transfer to the actuator by controlling the amount bypassed through the parallel circuit. See Fig. 3(c).

Pump control involves the use of one of two methods, depending on the type of pump used. Multiple pumps provide a step variation in flow as shown in Fig. 4(a).Variable-displacement pumps deliver infinitely from zero to maximum variable flows, as indicated in Fig. 4(b).


Fluid motor controls use techniques similar to pump controls. This involves the use of multiple motors (Fig. 5(a)) for step variation or variable-displacement motors (Fig. 5(b)) for infinite variation in output speeds.


Pressure control is achieved through valve controls, pump controls or rotary actuator control, which is not generally used.

Valve controls use one or more of six types of pressure control valves: relief, unloading, sequence reducing, counterbalance and decompression valves. Relief valves limit the maximum energy level of the system by limiting maximum operating pressure (Fig. 6).


Unloading valves regulate the pressure level by bypassing return fluid to the tank at a low energy level. The valves shift when system pressure reaches a preset level (Fig. 7). Sequence valves react to a pressure signal to divert energy from a primary circuit to a secondary circuit (Fig. 8).


Reducing valves react to a pressure signal to throttle flow to a secondary circuit, thus delivering energy at a lower level to the secondary than to the primary circuit (Fig. 9). Counterbalance valves control the potential energy differential across an actuator by maintaining a preset backpressure in the return line (Fig. 10) . Their purpose is to prevent a load from drifting. Decompression valves provide controlled release of energy stored in high-pressure systems because of elasticity in the system (Fig. 11).


Pump control of pressure fluid in open-loop circuits is generally achieved with pressure-compensated variable-displacement pumps. Energy transfer is controlled by varying flow from the pump in response to a pressure-level signal across the compensator (Fig. 12).


There are two basic types of open-loop circuits, constant flow and demand flow. In a typical constant flow circuit, shown in Fig. 13, the directional control valve bypasses fluid to the tank when the valve is in the center or neutral position shown, thus unloading the pump to the reservoir. This circuit has a fixed-displacement pump protected by a relief valve, a design frequently used in fluid power systems.


Energy transfer starts from a low, essentially zero, level when the valve is in neutral and builds as the operator shifts the valve. As the valve continues to shift, the fluid stream flows into the actuator, linear or rotary, and thus begins to act against a load resistance.

Internal leakage is normally minimal when the valve is in center position, unless the actuator is supporting a load in an elevated position. Generally, constant flow circuits are the most economical, provided they meet performance requirements.

Demand flow circuits, shown in Fig. 14, use a fixed-displacement pump, an unloading valve and an accumulator, and have certain characteristics:

* All ports are blocked when the directional control valve is in center or neutral position (shown).

* If a fixed-displacement pump is used, an accumulator is ordinarily included and an unloading valve is required.

Energy transfer starts from the maximum pressure settling of the system. The energy is available to the actuator as soon as the valve is shifted. Internal leakage is of greater concern here than in constant flow circuits because the valve is holding against full system pressure at all times.

Another version of a demand flow circuit (Fig. 15) uses a pressure-compensated, variable-displacement pump instead of the combination of fixed-displacement pump, accumulator and unloading valve (Fig 14).The characteristics of this demand flow circuit are the same as those in the aforementioned combination.


Mobile equipment and marine applications use a variety of circuit functions, depending on machine type. In mobile equipment the circuits use both linear and rotary positioning and velocity control. Attachment, lift and steering mechanisms use linear actuators with directional, flow and pressure control. Lift mechanisms also use pressure control, as sometimes do attachment mechanisms. Hydrostatic transmissions use rotary pumps and motors with directional, flow and sometimes pressure control.

In marine applications the circuits utilize position, flow and pressure control in a variety of functions. Winches and windlasses use rotary drives with directional and flow and sometimes pressure control. Steering mechanisms and hydroplane control mechanisms use linear actuators with directional and flow control, while cargo handling mechanisms use linear and rotary actuators with directional and flow control.

Russ Henke, PE, CFPE

Some information and illustrations for this article are from "Fluid Power Systems & Circuits," by Russell W. Henke, published by Penton.
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Author:Henke, Russ
Publication:Diesel Progress North American Edition
Geographic Code:1USA
Date:Sep 1, 2005
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