Displacement is the single number that tells you how much fluid a hydraulic pump moves each revolution, yet it is often buried inside the model code or misread on a datasheet. This guide explains how to read hydraulic pump displacement in cc/rev, convert it into flow, and use it alongside pressure and speed for basic hydraulic pump sizing. The math here is indicative; final sizing must always be confirmed against the manufacturer datasheet and your real duty cycle.
What Hydraulic Pump Displacement Actually Means
Displacement (Vg) is the geometric volume of fluid a pump's internal chambers sweep through in one full shaft revolution. It is expressed in cc/rev, also written cm³/rev or, on older datasheets, ml/rev — all the same unit. A pump stamped "16 cc/rev" theoretically pushes 16 cm³ of oil per turn, independent of pressure.
Two pumps can share a displacement and behave very differently. Displacement only describes swept volume; it says nothing about maximum pressure, efficiency, noise, or behaviour under varying load. Treat it as a starting point, not a performance grade.
The other key distinction is fixed versus variable displacement. A fixed-displacement gear pump always moves the same volume per rev; to change flow you change shaft speed. A variable-displacement piston pump such as a PrimeFlow PF-A10VSO (Rexroth A10VSO equivalent) can alter its effective swept volume on the fly via a swashplate, holding flow or pressure constant across a range of demand.
Reading Displacement from the Pump Model Code
Most hydraulic pumps encode nominal displacement in the model string, usually as the trailing number before the series suffix. Reading it correctly saves time when cross-referencing replacements.
- Gear pumps. On a CB-B series gear pump such as the PrimeFlow PF-GP-CB-B16 (Rexroth-equivalent), the trailing "16" means 16 ml/rev (cc/rev). The CB-B family is a low-pressure gear pump rated around 2.5 MPa (25 bar).
- Piston pumps. An A10VSO 45 denotes a nominal 45 cc/rev; a K3V 112 denotes 112 cc/rev. PrimeFlow's cross-reference table maps PF-A10VSO to Rexroth A10VSO, PF-K3V to Kawasaki K3V, and PF-PVH to PVH / Danfoss 90 series, so the displacement figure carries directly across to the equivalent.
- Vane pumps. Displacement usually appears in the catalogue as a size index (e.g. 10, 20, 25) that corresponds to cc/rev at full displacement.
Always verify against the cast nameplate rather than relying on the catalogue alone — the physical tag reflects the exact build, including any reduced-displacement variant.
From Displacement to Flow: The Core Formula
Once you can read displacement, flow follows directly. Theoretical flow is:
Q = Vg × n / 1000
where Q is flow in L/min, Vg is displacement in cc/rev, and n is shaft speed in rev/min. The 1000 converts cm³ to litres.
Real pumps leak internally, so actual flow is lower. Multiply by volumetric efficiency ηv (typically 0.85–0.95 for a healthy pump at rated conditions):
Q_actual = Vg × n × ηv / 1000
Using the PF-GP-CB-B16 at 1500 rpm and ηv of about 0.9: Q = 16 × 1500 × 0.9 / 1000 = about 21.6 L/min (indicative). At 2.5 MPa this pump is sized for low-pressure circulation and lubrication duties, not high-pressure work circuits.
Sizing a Pump: Pressure, Flow, and Input Power
Hydraulic pump sizing comes down to three inputs you must fix before talking to any supplier:
- System pressure (p, in bar). The maximum continuous pressure the actuator or circuit needs, plus a sensible margin. Pressure sets the pump's rating and the shaft loading.
- Required flow (Q, in L/min). Derived from the speed and volume of your actuators across the duty cycle, not from the pump's maximum.
- Drive speed (n, in rpm). Set by your prime mover — typically a 1450 or 1750 rpm electric motor on 50/60 Hz, a PTO, or an engine.
From these, displacement is back-calculated as Vg = Q × 1000 / (n × ηv), then rounded up to the nearest standard size.
Input power is what your motor must deliver. Hydraulic power is:
P_h = p × Q / 600 (kW, p in bar, Q in L/min)
Input shaft power accounts for overall efficiency ηt (mechanical × volumetric, commonly 0.80–0.90):
P_in = p × Q / (600 × ηt)
Undersizing the driver is a common and expensive mistake — always size the motor for peak pressure and flow, with margin, not for the average duty.
Gear, Vane, and Piston Pumps: How Displacement Behaves
The three pump families use displacement differently, which is why "bigger cc/rev" alone never picks the right technology.
| Type | Typical pressure | Displacement | Behaviour |
|---|---|---|---|
| Gear (external) | up to ~250 bar; low-pressure series lower | Fixed | Cheap, compact, noisy; flow pulses with tooth meshing |
| Vane | 70–210 bar | Fixed or variable | Quieter; suits mid-pressure industrial circuits |
| Piston (axial) | 250–400+ bar | Variable (or fixed) | Highest efficiency and pressure; most controllable; highest cost |
Gear pumps such as the CB-B16 suit ancillary and low-pressure circuits where cost dominates. Vane pumps fit indoor industrial hydraulics where noise matters. Piston pumps are the default for high-pressure, variable-demand work circuits — excavators, presses, marine winches — where variable displacement pays back in energy savings.
Worked Example (Indicative)
Assume a circuit needs 250 bar continuous, 40 L/min at the actuator, and is driven by a 1500 rpm electric motor. Use ηv = 0.90 and ηt = 0.85.
- Required displacement: Vg = 40 × 1000 / (1500 × 0.90) = about 29.6 cc/rev, rounded up to a 30 cc/rev pump.
- Hydraulic power: P_h = 250 × 40 / 600 = about 16.7 kW.
- Required input power: P_in = 16.7 / 0.85 = about 19.6 kW, so specify a 22 kW standard motor.
Frequently Asked Questions
What does cc/rev mean on a hydraulic pump?
cc/rev (also cm³/rev or ml/rev) is the pump's geometric displacement — the fluid volume the internal chambers sweep per shaft revolution. A 16 cc/rev pump theoretically moves 16 cm³ of oil every turn.
How do I calculate pump flow from displacement?
Multiply displacement by shaft speed and volumetric efficiency: Q = Vg × n × ηv / 1000, giving L/min. A 30 cc/rev pump at 1500 rpm with ηv = 0.9 delivers about 40.5 L/min.
Fixed or variable displacement — which do I need?
Fixed pumps (most gear pumps, some vane) are simpler and cheaper when demand is constant. Variable-displacement piston pumps cost more but save energy and reduce heat when demand varies, which is why they dominate mobile and high-pressure industrial hydraulics.
Does higher displacement mean higher pressure?
No. Displacement sets flow per revolution; pressure is set by the pump's mechanical design and the system load. A large-displacement low-pressure gear pump and a small-displacement high-pressure piston pump can deliver the same flow at very different pressures.
How do I match a replacement pump to an OEM part?
Read the displacement and pressure rating off the original nameplate, then cross-reference the model code. PrimeFlow publishes equivalents for Rexroth A2FO / A10VSO, Kawasaki K3V, and Danfoss PVH / 90 series, so displacement carries directly across.
Next step
With a displacement, pressure, and speed in hand, the fastest route to a confirmed match is a short technical RFQ. Share your target system pressure, required flow, drive speed, and any OEM cross-reference — PrimeFlow's hydraulic engineers will verify sizing and quote an equivalent pump, alongside valves, fittings, and seals, as one consolidated order.
Request an RFQ → Hydraulic equivalents catalog