Hydraulic System Graphic Symbols

Introduction to Hydraulic Schematics

Figure 1-1 shows a semi-structural schematic diagram of a hydraulic transmission system. This type of schematic diagram is highly intuitive and easy to understand but features relatively complex graphics. Drawing becomes particularly inconvenient when there are many components. To simplify the creation of such schematics, each component in the system can be represented using symbols. These symbols indicate only the function of the components, not their structure or parameters. The national standard for hydraulic component function symbols is specified in GB/T786.1-2009, which provides the foundation for creating a clear and consistent hydrostatic transmission diagram.

To facilitate understanding of hydraulic system diagrams represented by function symbols, the following introduces the graphic symbols of hydraulic components appearing in Figure 1-1 (as shown in Figure 1-2) within the context of a hydrostatic transmission diagram. These symbols form the visual language that allows engineers and technicians to interpret and work with hydraulic systems effectively.

Hydraulic Pump Symbols

The graphic symbol for a hydraulic pump in a hydrostatic transmission diagram consists of a circle with a solid triangle and a rotational direction indicator outside the circle. The tip of the triangle points outward, indicating the direction of oil flow. In the diagram, a unidirectional arrow for rotation indicates one-way rotation, while a bidirectional arrow indicates bidirectional rotation.

In a hydrostatic transmission diagram, a pump without a diagonal arrow穿过 the circle represents a fixed-displacement pump, while one with an arrow represents a variable-displacement pump. This distinction is crucial for understanding the operation and capabilities of the hydraulic system being represented.

The simplicity of these symbols in a hydrostatic transmission diagram belies their importance—they immediately communicate critical information about pump type, flow direction, and operational capabilities to anyone familiar with the standard.

Pump Symbol Variations

Fixed-displacement Pump

Variable-displacement Pump

Unidirectional Rotation

Bidirectional Rotation

Directional Control Valve Symbols

The graphic symbol for a directional control valve in a hydrostatic transmission diagram represents components that change the direction of oil flow. The valve spool must shift positions, typically between 2-3 positions, and valve bodies have different numbers of passages. Based on the number of positions the spool can move to and the number of passages in the valve body, directional control valves are classified as x-position x-port valves.

Understanding these symbols is essential for interpreting the flow control capabilities in any hydrostatic transmission diagram. Each variation provides specific information about how the valve will direct fluid within the system under different operating conditions.

Directional Valve Symbol Interpretation

• Valve positions are represented by squares - the number of squares indicates the number of positions
• Arrows within squares show oil communication paths
• "T" symbols indicate blocked passages
• Symbols outside squares indicate actuation methods

Common Directional Valve Configurations

Valve Type	Symbol	Description
2-position, 2-port		Simple on/off valve commonly used in basic hydrostatic transmission diagram applications
3-position, 4-port		Versatile valve with neutral position, widely used in directional control for a hydrostatic transmission diagram
4-position, 3-port		Specialized valve with multiple operating modes for complex hydrostatic transmission diagram requirements

Pressure Valve Symbols

In a hydrostatic transmission diagram, the square in a pressure valve graphic symbol represents the valve spool. Arrows within the square indicate the oil passages, while the lines on both sides represent the inlet and outlet oil pipes. The dashed lines in the diagram represent control oil paths.

Pressure valves operate on the principle of balancing the hydraulic pressure from the control oil path with the spring force on the other side, a concept clearly communicated through their symbols in a hydrostatic transmission diagram. This balance determines when the valve opens or closes to control system pressure.

Different types of pressure valves—including relief valves, pressure reducing valves, and sequence valves—each have distinct symbols in a hydrostatic transmission diagram, allowing for quick identification of their function within the system.

Pressure Valve Symbol Examples

Pressure Relief Valve

Protects system from overpressure in a hydrostatic transmission diagram

Pressure Valve Operation Principles

All pressure valves in a hydrostatic transmission diagram operate based on force balance principles, but each type serves a specific function:

• Relief Valves: These valves in a hydrostatic transmission diagram open when system pressure exceeds a preset level, diverting fluid back to the reservoir to prevent overpressure damage.
• Pressure Reducing Valves: In a hydrostatic transmission diagram, these maintain a constant reduced pressure in a branch circuit regardless of pressure fluctuations in the main system.
• Sequence Valves: These valves in a hydrostatic transmission diagram control the order of operation of hydraulic actuators by opening only when a preset pressure is reached in the primary circuit.
• Unloading Valves: In a hydrostatic transmission diagram, these reduce pump pressure to a low value when the system doesn't require flow, improving efficiency.

Flow Control Valve Symbols

The flow control valve symbol in a hydrostatic transmission diagram features a gap formed by two arcs, representing the orifice through which oil flows. Oil flow is reduced as it passes through this orifice, creating the flow control function essential in many hydraulic systems.

The arrow in the flow control valve symbol in a hydrostatic transmission diagram indicates that the size of the orifice can be changed, identifying it as an adjustable flow control valve. This adjustability means the flow rate through the valve can be regulated, a critical function in controlling actuator speed in hydraulic systems.

In a hydrostatic transmission diagram, different styles of flow control valve symbols distinguish between fixed and adjustable types, as well as between pressure-compensated and non-compensated varieties, each with specific applications based on system requirements.

Flow Control Valve Variations

Fixed Orifice

Provides constant flow restriction in a hydrostatic transmission diagram

Adjustable Flow Control

Allows manual flow rate adjustment in a hydrostatic transmission diagram

Pressure Compensated

Maintains constant flow despite pressure changes in a hydrostatic transmission diagram

Standards and Conventions

In hydraulic system diagrams, including any hydrostatic transmission diagram, there is an important convention: the graphic symbols of hydraulic components should be represented in their static or neutral position. This standardization ensures consistency and clarity across all hydraulic schematics, allowing professionals worldwide to interpret any hydrostatic transmission diagram regardless of its origin.

Following this convention, the semi-structural diagram shown in Figure 1-1 can be converted into the functional symbol-based hydraulic transmission system schematic shown in Figure 1-2. This conversion demonstrates the effectiveness of using standardized symbols in creating a clear, concise hydrostatic transmission diagram that retains all essential functional information while eliminating unnecessary structural details.

Hydraulic Component Symbol Reference

This comprehensive table provides a quick reference for common hydraulic component symbols used in a hydrostatic transmission diagram, following the GB/T786.1-2009 standard.

Component Category	Component Name	Symbol	Symbol Meaning in Hydrostatic Transmission Diagram
Pumps & Motors	Fixed-displacement Pump		Constant flow output in a hydrostatic transmission diagram
	Variable-displacement Pump		Adjustable flow output in a hydrostatic transmission diagram
	Hydraulic Motor		Converts hydraulic energy to mechanical rotation in a hydrostatic transmission diagram
Valves	2/2 Directional Valve		Two-position, two-port flow control in a hydrostatic transmission diagram
	Pressure Relief Valve		Protects system from overpressure in a hydrostatic transmission diagram
	Adjustable Flow Control		Regulates fluid flow rate in a hydrostatic transmission diagram
Actuators & Accessories	Double-acting Cylinder		Produces force in both directions in a hydrostatic transmission diagram
	Reservoir		Stores hydraulic fluid in a hydrostatic transmission diagram
	Filter		Removes contaminants from fluid in a hydrostatic transmission diagram