5.3: Fixed-Displacement Pumps - Operation and Schematic Symbol

Fluids 2: Basic Hydraulics · LibreTexts

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    Understanding Hydraulic Pumps

    In a hydraulic system, the pump is responsible for generating fluid flow. Most industrial hydraulic pumps use a positive displacement design, meaning they deliver a consistent volume of fluid with each revolution of the pump shaft. If the pump's shaft is driven at a constant speed by an electric motor, the pump provides a nearly constant flow rate, regardless of pressure fluctuations in the system.

    Fixed vs. Variable Displacement Pumps

    Positive displacement pumps fall into two main categories:

    1. Variable displacement pumps – These pumps can adjust their flow output based on system demand.
    2. Fixed-displacement pumps – These pumps deliver a constant flow rate, meaning the output remains fixed as long as the pump speed does not change.

    It is important to understand that a fixed-displacement pump does not generate pressure—only flow. System pressure develops only when there is resistance to flow. Resistance may come from:

    • A load on an actuator (such as a hydraulic cylinder or motor).
    • Fluid friction within the system, caused by narrow passages, bends, or control valves.

    How a Fixed-Displacement Pump Works

    A typical hydraulic pump consists of a housing with an inlet and an outlet port, along with internal rotating components connected to a shaft. This shaft extends outside the housing and is generally driven by an electric motor.

    Fluid Flow Process

    Diagram of the fluid flow process of a fixed-displacement pump showing fluid being drawn from a reservoir, through the pump, to the system.
    Figure \(\PageIndex{1}\): Diagram of the fluid flow process of a fixed-displacement pump.
    1. Oil is drawn from the reservoir. As the pump operates, it creates a partial vacuum at its inlet by increasing the volume inside the pump. This vacuum allows atmospheric pressure to push hydraulic oil from the reservoir through the suction line into the pump's inlet port.
    2. Oil is forced into the pressure line. The pump then moves the oil through the system by reducing the volume inside the pump and forcing fluid out through the outlet port.
    3. System pressure builds only when flow meets resistance – The pump itself does not create pressure; it simply moves fluid. Pressure forms only if there is an obstruction or a load preventing free flow.
    A syringe drawing in fluid from a cylinder and a syringe pushing fluid out into a cylinder.
    Figure \(\PageIndex{2}\): Fixed-displacement pumps work like a syringe. As the syringe draws fluid in, it increases the volume of fluid in the syringe. As the syringe pushes fluid out, it decreases the volume of fluid in the syringe.

    A simple way to visualize this process is by using a syringe:

    • Pulling the plunger back increases the volume inside the syringe, drawing liquid in (like the pump's suction process).
    • Pushing the plunger forward decreases the volume, forcing the liquid out (similar to the pump’s discharge process).

    Observing Flow in a Fixed-Displacement Pump

    When the pump is running, a flow meter placed either before or after the pump will typically show a steady flow rate as long as the pump speed remains constant. However, as system pressure increases due to flow restrictions, a slight decrease in flow may be observed.

    This flow reduction occurs because:

    • Higher pressure reduces pump efficiency. Some hydraulic fluid may leak internally from the outlet back to the inlet due to clearances inside the pump.
    • All pumps have inherent inefficiencies. While fixed-displacement pumps are generally reliable, their performance can decline slightly at higher pressures.

    Understanding these principles helps technicians troubleshoot performance issues and optimize hydraulic system efficiency.

    Schematic symbol for a fixed-displacement pump with symbols for a coupling, electric motor, reservoir, and filter.
    Figure \(\PageIndex{3}\): The schematic symbol for a fixed-displacement hydraulic pump is shown with symbols for a coupling, electric motor, reservoir, and filter.

    This page titled 5.3: Fixed-Displacement Pumps - Operation and Schematic Symbol was last modified on Tue, 29 Apr 2025 17:49:42 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Jessy Haid.