A double-acting force pump alternates intake and delivery on opposite sides of a piston. Both travel directions displace liquid, while the rod reduces the available area on one side. The calculator estimates displacement and flow for a single rod passing through one cylinder end, using an entered volumetric efficiency.
Force Pump (Form 2) Interactive Calculator
Vary bore, rod size, stroke, speed, and volumetric efficiency to see double-acting pump flow and displacement update live.
Equation Used
The double-acting displacement per revolution is the sum of both piston faces times stroke, minus the piston-rod area on the rod side. Volumetric efficiency eta_v reduces theoretical flow to estimated delivered flow.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Double-acting piston pump with one piston rod passing through one side.
- One forward and one return delivery stroke occur per revolution.
- Liquid is treated as incompressible; leakage and valve losses are included only through volumetric efficiency.
- Use a rod diameter smaller than the bore. Inputs use mm and rpm, with internal SI conversion. Animation pace is illustrative rather than real-time RPM.
Each piston face has an intake and delivery path
As the piston moves toward one end, that chamber decreases in volume and discharges. The opposite chamber expands and draws in liquid. On reversal their roles swap. Each check valve retains its own intake or delivery function; the open pair changes.
A rod passing through one end occupies part of that chamber’s area. Its stroke displacement is therefore smaller than the displacement of the full piston face.
There are two delivery strokes per complete forward-and-return cycle. Flow still varies during the cycle and can approach zero at reversal. An air vessel or phased cylinders can reduce pulsation, but their behavior is outside this calculator.
Double-acting reciprocating pumping
The principle allows both sides of a piston to contribute to delivery. Drive arrangements and the required fluid-compatible seals and valves depend on the application.
“Double acting” describes use of both piston faces; “duplex” describes two pumping cylinders or units. They are not interchangeable terms. This page’s “form 2” identifies the illustration and does not establish a commercial pressure or speed class.
Sum the full and rod-side displacement
With bore D and rod diameter d in metres, piston area is A = πD²/4 and rod area is Ar = πd²/4. For stroke L, displacement per full cycle is V = AL + (A − Ar)L = (2A − Ar)L.
The model assumes one crank revolution makes one complete forward-and-return cycle. Multiply V by 1,000 to display litres per revolution, then by RPM N for theoretical litres per minute. Estimated flow is theoretical flow multiplied by the entered efficiency η.
Rod volume reduction is 100Ar/(2A) percent. This compares the combined two-stroke displacement with two full-bore faces; it is not the percentage reduction of the rod-side stroke alone, which would be 100Ar/A.
Use 0 ≤ d < D. If rod diameter equals or exceeds the bore, the calculator shows no flow result and waits for valid dimensions. Pressure, drive force, power, inlet conditions and cavitation are not calculated.
Example: unequal piston-face volumes
Enter a 100 mm bore, 25 mm rod, 200 mm stroke, 90 RPM and 92% volumetric efficiency. Full-face displacement is 1.571 L and rod-side displacement is 1.473 L, for a combined 3.043 L per revolution.
Theoretical flow is 273.91 L/min and estimated flow is 252.00 L/min. Rod volume reduction across the whole cycle is 3.125%; the rod-side stroke alone is 6.25% smaller than the full-face stroke.
At 30 RPM with the same entered efficiency, estimated flow is 84.00 L/min. Efficiency does not change automatically with speed. These results do not establish a pressure capability, a safe suction condition or a food-transfer duty.
Volume comparison versus pump selection
For the same bore, stroke and cycle rate, using both faces increases displacement compared with a single full-face delivery stroke. A single rod makes that increase slightly less than twofold.
Actual performance also depends on filling, valve dynamics, leakage, fluid properties and the complete system. The entered efficiency summarizes an assumed volume ratio; it does not predict how those effects change with speed or pressure.
Use the pump’s ratings and manufacturer procedures for operation and maintenance. This page does not supply packing-adjustment rules, air-vessel charging instructions, dry-running permissions or a speed-increase limit.
Frequently asked questions
Why is rod-side displacement smaller?
The rod occupies area Ar, so the working area on that side is A − Ar rather than A.
Why is Rod volume reduction 3.125% rather than 6.25% in the example?
The readout uses the combined two-stroke reference 2A. The rod-side stroke alone loses Ar/A, twice that percentage.
Does double acting make discharge perfectly steady?
No. There are two delivery strokes, but piston velocity and flow still vary and fall at reversal. Pulsation control is a separate calculation.
Does efficiency fall automatically as RPM rises?
No. The entered efficiency stays fixed until changed. Actual speed-dependent behavior requires pump data.
What happens if the rod equals or exceeds the bore?
There is no working annular area for the rod-side stroke. The calculator shows — and stops the diagram until the rod is smaller than the bore.
Can the calculator check suction lift or pressure?
No. It has neither a pressure input nor an inlet-system model. Estimated volume flow alone cannot establish those conditions.
Is duplex the same as double acting?
No. Double acting refers to both sides of one piston. Duplex refers to two pumping cylinders or units.
Pump references
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