Hydraulic Pump Flow Rate Calculation

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Hydraulic Pump Flow Rate Calculator

Standard pumps usually range from 85% to 95%.

Calculation Results

Theoretical Flow Rate: 0.00 GPM

Actual Flow Rate (w/ Efficiency): 0.00 GPM

(Liters Per Minute: 0.00 LPM)

function calculateFlow() { var disp = parseFloat(document.getElementById('displacement').value); var speed = parseFloat(document.getElementById('speed').value); var eff = parseFloat(document.getElementById('efficiency').value); if (isNaN(disp) || isNaN(speed) || isNaN(eff) || disp <= 0 || speed <= 0 || eff <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Formula: Flow (GPM) = (Displacement in³/rev * Speed RPM) / 231 var theoreticalGPM = (disp * speed) / 231; var actualGPM = theoreticalGPM * (eff / 100); var actualLPM = actualGPM * 3.78541; document.getElementById('theoretical-flow').innerText = theoreticalGPM.toFixed(2); document.getElementById('actual-flow').innerText = actualGPM.toFixed(2); document.getElementById('flow-lpm').innerText = actualLPM.toFixed(2); document.getElementById('results-area').style.display = 'block'; }

Understanding Hydraulic Pump Flow Rate

In hydraulic systems, the flow rate determines the speed at which actuators (cylinders or motors) move. Calculating the pump flow rate is essential for sizing system components, determining required horsepower, and troubleshooting performance issues.

The Flow Rate Formula

The standard formula used to calculate the output of a hydraulic pump in U.S. Customary units is:

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

Where:

  • Displacement: The volume of fluid the pump moves per single revolution.
  • Speed: The rotational speed of the pump's drive shaft (usually from an electric motor or engine).
  • 231: The constant representing the number of cubic inches in one U.S. gallon.

Theoretical vs. Actual Flow

In a perfect world, a pump would deliver exactly its theoretical flow. However, internal leakage (slippage) occurs as pressure increases. This is why we use Volumetric Efficiency.

If a pump has a displacement of 2.0 in³/rev and runs at 1800 RPM, its theoretical flow is 15.58 GPM. If the pump is 90% efficient, the actual flow delivered to the system is 14.02 GPM.

Common Volumetric Efficiencies by Pump Type

Pump Type Typical Volumetric Efficiency
External Gear Pump 80% – 90%
Vane Pump 85% – 92%
Piston Pump (Axial) 92% – 98%

Example Calculation

Let's say you have a tractor hydraulic pump with a displacement of 1.55 in³/rev. The engine runs the pump at 2400 RPM. You assume a standard efficiency of 85% due to the age of the machine.

  1. Theoretical Flow: (1.55 × 2400) / 231 = 16.10 GPM
  2. Actual Flow: 16.10 × 0.85 = 13.69 GPM

This result helps you realize that if you need 15 GPM to run a specific attachment, this pump/engine configuration will fall slightly short under load.

Why Does Flow Rate Matter?

  • Cylinder Speed: The flow rate (GPM) determines how fast a hydraulic cylinder will extend or retract.
  • Heat Generation: Undersized pumps running at excessive speeds or oversized pumps bypassing fluid through relief valves both generate excessive heat.
  • Horsepower Requirements: Flow rate is a direct component of the hydraulic horsepower formula: HP = (GPM × PSI) / 1714.

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