A gear pump performance curve shows what a pump will actually deliver at a stated speed, pressure difference, oil viscosity and temperature. For a fixed-displacement gear pump, the flow-versus-pressure line is usually almost flat, with a slight downward slope as differential pressure rises. That small drop matters: it represents internal leakage, or slip. At the same time, the power needed at the shaft rises with pressure.
For an engineer or buyer, the curve answers a more useful question than"What is the maximum pressure?" It shows the flow available at the real operating point, expected drive power and selection margin.
The practical goal is not to find the pump with the largest catalog number. It is to select a hydraulic pumps that delivers repeatable flow without excessive heat, noise, power consumption or premature wear throughout the machine's normal duty cycle.
What a Gear Pump Curve Should Tell You
A useful curve identifies the pump model, displacement, speed, differential pressure, actual flow and absorbed power. It should also state the test fluid, viscosity, temperature and inlet condition. Without these conditions, similar-looking curves may not be comparable.
Gear pumps are positive-displacement pumps. Each shaft revolution carries a predictable volume of oil from inlet to outlet. The basic theoretical relationship is:
Theoretical flow (L/min) = displacement (cm³/rev) × speed (rpm) ÷ 1,000
Actual flow is lower because no real pump is perfectly sealed:
Actual flow = theoretical flow × volumetric efficiency
Theoretical flow is useful initially, but the machine moves according to delivered flow, not catalog displacement alone.
The Three Curves That Matter Most
Flow vs. Differential Pressure
At a fixed speed, a gear pump's theoretical flow does not change with pressure. Actual flow does. As the difference between outlet and inlet pressure increases, more oil can leak through the small running clearances around the gears, side plates and bearings. The result is a nearly horizontal line that slopes slightly downward.
A steep drop may indicate low-viscosity oil, high temperature, excessive clearance or wear. Installing a larger pump without finding the cause can create heat and waste energy.
Input Power vs. Pressure
Hydraulic output power can be estimated from: Hydraulic power (kW) = pressure difference (bar) × actual flow (L/min) ÷ 600
The shaft must supply more than this because of mechanical and volumetric losses: Required input power = hydraulic power ÷ overall efficiency
Thus, rising pressure can leave flow almost unchanged while increasing the load on the motor, coupling and shaft. Relief settings and motor selection must be checked together.
Efficiency vs. Operating Point
Volumetric efficiency compares actual flow with theoretical flow. Mechanical efficiency reflects friction and torque losses. Overall efficiency combines both effects.
Efficiency often improves as a pump moves away from very low speed because leakage becomes a smaller share of total flow. However, "higher viscosity is always better" is the wrong conclusion. More viscosity can reduce slip, but excessive viscosity increases inlet loss, viscous drag and starting torque. The best region depends on pump clearances, oil grade, temperature, speed and pressure.
Gear Pump Curves Are Not Centrifugal Pump Curves
A centrifugal pump's operating point is strongly determined by the intersection of its pump and system curves. Its flow normally falls substantially as head rises.
A fixed-displacement gear pump behaves differently. Speed and displacement establish the basic flow. System resistance creates the pressure that the pump must withstand. Higher resistance normally changes delivered flow only by the amount of additional slip, but it increases torque and power demand significantly.
A gear pump should not run against a closed discharge without a correctly sized relief path. Continued displacement can cause rapid pressure rise, heat, seal damage or mechanical failure.
How to Read a Gear Pump Performance Curve
Use this sequence instead of starting with the largest pressure number on the datasheet.
- Confirm the exact pump size. Curves for two pumps in the same series may have different displacement, speed and pressure limits.
- Check the test conditions. Match oil viscosity and temperature as closely as possible to the application. Confirm whether pressure is outlet pressure or differential pressure.
- Find the operating speed. Do not assume the motor nameplate speed equals pump speed when a gearbox, PTO or belt drive is used.
- Locate the required differential pressure. Use normal continuous pressure first, then check short-duration peaks separately.
- Read actual flow and power. Use the plotted value rather than substituting theoretical flow into every calculation.
- Check the operating envelope. Verify inlet pressure, minimum and maximum speed, continuous pressure, intermittent pressure, fluid temperature and duty cycle.
When several speed or viscosity lines are published, stay on one condition line. Mixing flow from one curve with efficiency from another creates an untested operating point.
A Worked Hydraulic Gear Pump Example
The Bohang AZPF external gear pump series covers geometric displacements from 4 to 28 cm³/rev. Depending on size, the series offers continuous pressure ratings up to 250 bar and intermittent ratings up to 280 bar, with multiple shaft and port configurations.
Consider an illustrative application using a 16 cm³/rev pump at 1,500 rpm and 180 bar differential pressure. The theoretical flow is: 16 × 1,500 ÷ 1,000 = 24 L/min
Assume the model-specific test curve shows 90% volumetric efficiency at the stated oil temperature and viscosity. Actual flow would be: 24 × 0.90 = 21.6 L/min
Hydraulic output power would be: 180 × 21.6 ÷ 600 = 6.48 kW
If the tested overall efficiency at that point were 82%, estimated shaft input would be: 6.48 ÷ 0.82 = 7.90 kW
The 90% and 82% figures are examples, not guarantees for every AZPF configuration. Final motor sizing must use model-specific test data and consider starting conditions, peak pressure, transmission losses and service margin. "16 cc and 180 bar" alone is insufficient for reliable selection.
How Speed, Viscosity and Temperature Shift the Curve
Speed: Flow changes approximately in direct proportion to shaft speed. Increasing speed raises output, but the selected value must remain within the pump's inlet and mechanical limits. At very high speed, insufficient inlet pressure can cause aeration or cavitation.
Viscosity: More viscous oil tends to reduce internal slip, raising volumetric efficiency. It also increases suction resistance and friction. For this reason, the viscosity at cold start and at stabilized operating temperature should both be checked.
Temperature: As hydraulic oil heats, its viscosity usually falls. A worn pump may appear acceptable when cold but lose flow after the system reaches operating temperature. Comparing cold and hot flow at the same speed and pressure is a useful diagnostic check.
Wear and contamination: Abrasive contamination enlarges critical clearances and increases leakage. Clean oil and suitable filtration protect not only service life but also the shape and repeatability of the performance curve.
Inlet condition: A pump cannot deliver its published outlet performance if it is starved at the inlet. Check absolute inlet pressure, suction-line diameter, hose collapse, filter restriction, oil level and air leaks before blaming the pressure side.
Using Curve Deviations for Troubleshooting
If measured flow is low, first confirm that speed, pressure, temperature and viscosity match the test conditions. Lower shaft speed reduces flow even when the pump is healthy.
If flow declines sharply as pressure rises, investigate internal leakage, oil viscosity and wear. If power is higher than expected, look for excessive back pressure, an incorrectly adjusted relief valve, oil that is too viscous or mechanical misalignment. Noise accompanied by unstable flow often points to inlet restriction, aeration or cavitation rather than insufficient displacement.
Record inlet pressure, outlet pressure, flow, speed, oil temperature and input power at several steady points to create a field curve for comparison.
What Data Should You Send to a Hydraulic Pump Manufacturer?
To select or customize a hydraulic gear pump, provide:
- Required flow at normal operation
- Continuous, intermittent and peak differential pressure
- Pump speed range and drive type
- Hydraulic fluid, viscosity grade and temperature range
- Inlet pressure and filtration level
- Rotation direction viewed from the shaft end
- Mounting flange, shaft and port details
- Duty cycle, noise limits and installation environment
- Existing pump model or dimensional drawing
- Required quantity and destination country
These details let an engineer check the operating point and identify where shaft, flange, port, seal or displacement customization may solve an installation problem.
Ask for a Curve You Can Trust
ISO 4409:2019 specifies methods for testing and presenting the steady-state performance and efficiency of positive-displacement hydraulic pumps and motors. For rotary pump testing, ANSI/HI 3.6-2022 is another relevant reference. A credible performance document should state the test conditions, instruments or accuracy class, measured flow and power, and whether the values are typical, minimum or guaranteed.
Bohang supplies piston, vane and gear pumps for mobile machinery and industrial systems. For an AZPF pump or customized replacement, complete operating data helps confirm delivered flow, driver size and operating margin before installation, reducing commissioning risk and downtime.
Need help checking an operating point? Send Bohang your required flow, pressure, RPM, oil viscosity, temperature, mounting dimensions and existing pump model for an application review and quotation.