Pump Flow Rate Fundamentals

A hydraulic pump converts mechanical energy into hydraulic energy by displacing a fixed volume of fluid per revolution. The theoretical flow rate depends on the pump's displacement and rotational speed.

Theoretical Flow Rate

The theoretical (ideal) flow rate assumes 100% volumetric efficiency with no internal leakage:

Q_theoretical = D x N

• D = Displacement per revolution (cc/rev or in3/rev)
• N = Rotational speed (RPM)
• Q = Flow rate (cc/min or in3/min)

Actual Flow Rate

Real pumps have internal leakage that reduces output. Volumetric efficiency accounts for this:

Q_actual = Q_theoretical x eta_v

Where eta_v is the volumetric efficiency (typically 0.80 to 0.98 depending on pump type and pressure).

Unit Conversions

• GPM = (D_cc x N) / 3785.41 x eta_v
• LPM = (D_cc x N) / 1000 x eta_v
• 1 in3 = 16.387 cc
• 1 GPM = 3.785 LPM

Factors Affecting Volumetric Efficiency

• System Pressure: Higher pressure increases internal leakage
• Fluid Viscosity: Lower viscosity increases leakage
• Pump Wear: Worn components have larger clearances
• Pump Type: Piston pumps generally more efficient than gear pumps
• Speed: Very low speeds reduce efficiency

Typical Volumetric Efficiencies by Pump Type

• External Gear Pump: 80-91%
• Internal Gear Pump: 85-93%
• Vane Pump: 82-92%
• Axial Piston (Fixed): 92-98%
• Axial Piston (Variable): 90-97%
• Radial Piston: 92-98%