What is Hydraulic Pump Flow?

Hydraulic pump flow, typically measured in Gallons Per Minute (GPM) or Liters Per Minute (LPM), is the volume of fluid a pump can move within a specific timeframe. In fluid power systems, flow determines the speed at which actuators (like cylinders or motors) operate. Understanding your pump's flow rate is essential for sizing valves, hoses, and reservoirs correctly to prevent overheating and system inefficiencies.

The Hydraulic Pump Flow Formula

To calculate the theoretical flow of a hydraulic pump, you need to know the pump's displacement (the volume of oil moved per revolution) and the rotational speed of the drive source (usually an electric motor or engine). The standard formula for US units is:

Flow (GPM) = (Displacement in in³/rev × Speed in RPM) / 231

The constant 231 represents the number of cubic inches in one US gallon. For the actual flow, you must account for volumetric efficiency, as internal leakage within the pump reduces the output slightly.

How to Use the Calculator

Using this tool is straightforward. First, enter the Pump Displacement, which is usually found on the pump's manufacturer nameplate or data sheet. Second, enter the Pump Speed in RPM; for many industrial applications, this is 1750 or 3450 RPM for electric motors. Finally, enter the Volumetric Efficiency. If you are unsure, 85% to 95% is standard for most gear, vane, or piston pumps in good condition. Click calculate to see the theoretical and actual performance values.

Why Volumetric Efficiency Matters

In a perfect world, a pump would deliver 100% of its displacement. However, due to internal clearances between gears, pistons, and wear plates, some oil bypasses the outlet and returns to the inlet or case drain. This is known as internal slip. As a pump wears out, this slip increases, leading to lower GPM and slower machine cycle times. Monitoring the difference between theoretical and actual flow is a primary method for diagnosing hydraulic pump health.

Frequently Asked Questions

Does pressure affect flow? Ideally, no, but in reality, higher pressures increase internal leakage, which slightly decreases the volumetric efficiency and the actual flow output.

How do I convert GPM to LPM? Simply multiply the GPM result by 3.785. Our calculator provides this conversion automatically for your convenience.