Base Pump Symbol
The base hydraulic pump symbol consists of:
- Circle: Standard component boundary
- Solid triangle: Indicates hydraulic (filled) versus pneumatic (outline)
- Directional arrow: Shows fluid flow direction from the pump
This symbol represents a fixed displacement, unidirectional hydraulic pump. Fluid flows in one direction only, and displacement cannot be changed.
Figure 1: Base Hydraulic Pump Symbol
( Circle + Directional Arrow + Solid Triangle )
Key Point: Gear pumps, vane pumps, and piston pumps all use the same schematic symbol. Physical configuration does not change the symbol per ISO 1219.
Bi-Rotational Pump Symbol
The bi-rotational pump symbol adds a second directional triangle on the opposite side. This indicates the pump can expel fluid from either port depending on rotation direction.
This pump type is rare outside of advanced mobile machinery, especially in fixed displacement versions. A series of check valves typically allows either port to function as tank or pressure line based on rotation direction.
Variable Displacement, Pressure Compensated Pump
Basic Symbol
The variable displacement pressure compensated pump symbol adds:
- Variable arrow bisecting the entire symbol — indicates adjustable displacement
- Small arrow to the left — indicates automatic displacement variation with pressure compensation
Detailed Symbol Breakdown
The detailed ISO 1219 symbol reveals additional information:
Shaft Connection
The shaft extends to the right, connecting to a prime mover (combustion engine or electric motor symbol).
Rotation Direction
The semicircular arrow shows clockwise rotation. Rotation is always observed from the shaft end viewpoint.
Variable Displacement Arrow
The diagonal arrow bisecting the pump indicates adjustable displacement based on system demands.
Pressure Compensator
Located on the left side. Includes rectangle with spring, variability arrow, and triangular pilot input.
Pressure Compensator Operation
The orange/red dashed pilot line from pump outlet supplies pilot energy. Spring setting fights pilot pressure to infinitely adjust flow rate. When downstream pressure equals compensator setting, flow matches demand exactly.
Load Sensing Pump Symbol
The load sensing pump symbol contains multiple components labeled (a) through (j). This represents the most detailed pump symbol in hydraulic schematics.
Component (a) – Pump Body
Main pump circle with diagonal variable arrow. Attached to bias and control pistons.
Component (b) – Bias Piston
Forces pump to full displacement when possible. Spring pushes piston forward.
Component (c) – Control Piston
Larger bore than bias piston. Controls maximum displacement via stroke adjustment.
Component (d) – Pressure Compensator
Relief valve governing control piston. Shifts when pilot energy enters to reduce displacement.
Component (g) – Load Sense Compensator
Takes pilot signal from pump outlet and work line after metering orifice. Operates at lower pressure than (d).
Component (j) – Metering Orifice
Variable flow control in system pressure line. Creates backpressure when flow is reduced.
Operation Modes
Standby Mode
When the pump fires up with all downstream directional valves closed:
- Spring inside bias piston (b) fully strokes pump to maximum displacement
- Pressure rises in work and pilot lines as fluid fills plumbing
- Pressure at compensator (d) forces pressure compensator to shift right
- Pilot energy enters through line (i) filling control piston (c)
- Control piston (larger bore) wins the fight against bias piston
- Pump displacement reduces to minimum — only flow for leakage compensation
- Pump is on standby
Active Mode
When a downstream directional valve opens:
- Flow path creates pressure drop below compensator (d) setting
- Pressure compensator (d) succumbs to spring pressure and snaps back to neutral
- Drain lines open to tank
- Orifices (e) and (f) dampen compensator motion, preventing oscillation
- Pump flow balances between bias and control pistons
- Flow matches downstream demand at exactly the pressure compensator setting
Load Sense Compensator Operation
The load sense compensator (g) receives pilot signals from two sources:
- Pump outlet pressure
- Work line after the metering orifice (j)
It compares the combined effort of spring value and load-sense pilot signal. The pressure setting of (g) is lower than (d) — typically creating 300 psi pressure drop across valve (j).
Example: If load pressure is 1,000 psi, pump holds pressure at 1,300 psi (1,000 psi load + 300 psi across metering valve).
Pump Symbol Comparison
| Symbol Element | Fixed Displacement | Variable Displacement | Pressure Compensated | Load Sensing |
|---|
| Circle with triangle | Yes | Yes | Yes | Yes |
| Variable arrow | No | Yes (diagonal) | Yes (diagonal) | Yes (diagonal) |
| Pressure compensator symbol | No | No | Yes (left side) | Yes |
| Load sense compensator | No | No | No | Yes |
| Bias/control pistons | No | No | No | Yes |
| Pilot lines | No | No | Yes (dashed) | Yes (dashed) |
Frequently Asked Questions
Q1: Why do gear pumps, vane pumps, and piston pumps all use the same schematic symbol?
Per ISO 1219, schematic symbols represent function, not physical construction. All fixed displacement pumps produce the same unidirectional flow regardless of internal mechanism. The symbol shows what the component does, not how it does it mechanically.
Q2: What does the variable arrow across the pump symbol indicate?
The variable arrow indicates the pump has adjustable displacement. Combined with other symbols (pressure compensator, load sense compensator), it shows how displacement changes automatically based on system demands.
Q3: How does a pressure compensated pump maintain constant pressure?
The pressure compensator spring setting fights against pilot pressure from the system. When downstream pressure rises to the compensator setting, the compensator shifts to reduce pump displacement. This maintains system pressure at the set value while reducing flow to match only leakage requirements.
Q4: What is the purpose of the load sense compensator?
The load sense compensator optimizes pump output for multiple actuators. It measures pressure drop across a metering orifice and adjusts pump flow to provide exactly the pressure needed (work pressure plus a small differential for flow). This prevents excess flow and reduces energy consumption.
Q5: Why does the load sensing pump have two compensators?
The pressure compensator (d) sets maximum system pressure. The load sense compensator (g) operates at a lower pressure setting to create flow across the metering valve while maintaining work circuit pressure. Together they provide efficient supply-and-demand hydraulic control.
Conclusion
Hydraulic pump symbology follows ISO 1219 standards where symbols represent function, not physical construction. The base pump symbol with solid triangle indicates a unidirectional hydraulic pump. Variable displacement pumps add a diagonal arrow. Pressure compensation adds the compensator symbol on the left side. Load sensing pumps add the secondary compensator and metering components.
Understanding load sensing pump operation reveals the elegance of hydraulic design: using fluid pressure columns to create an efficient supply-and-demand scenario. The pump provides exactly the flow and pressure each downstream actuator requires, nothing more.
