External gear pumps
They are pumps with a simpler design compared to other hydraulic pumps. They are used for applications operating at medium/high pressure.
Typical applications include:
- hydraulic power units
- mobile equipment and construction
- metal forming and stamping
- compactors for static and mobile waste
- agricultural machinery
- earthmoving equipment
- forklift trucks
In external gear pumps, two gears mesh inside a closed housing. When the gears rotate, the fluid fills the space between the gear teeth and is transported from the suction side to the discharge side, passing around the outer circumference of the gears. In the areas where the teeth mesh, the fluid is unable to leak and is therefore pushed towards the outlet.
Thanks to their characteristics, they are commonly used in applications where pressurizing or metering fluids with high precision and very low pressure fluctuations are required.
Available external gear pumps
| Brand | Series | Operating pressure (bar) | Link |
|---|---|---|---|
 | ALP, GHP, 1P, 0.25 & 0.5 | 250 | Read more |
How an external gear pump works
An external gear pump uses two meshing gears inside a closed housing: one is driven by the shaft, the other is driven by the first. As the gears rotate, oil fills the spaces between the teeth on the suction side and is carried around the outside of the gears to the delivery side, where the meshing teeth push it towards the outlet. Each revolution moves a fixed volume of oil: the displacement, expressed in cm³/rev.
In modern pumps such as Marzocchi and Bucher, the gears are supported by bushings that are pushed against the gear faces by the delivery pressure. This axial compensation limits internal leakage and keeps good efficiency up to the maximum working pressure, while the tooth profile keeps pulsation and noise low.
How to calculate the flow: flow (l/min) = displacement (cm³/rev) × speed (rpm) ÷ 1,000 × volumetric efficiency. For example, an 8 cm³/rev pump at 1,500 rpm delivers about 12 l/min, slightly less in practice depending on pressure and efficiency.
External vs internal gear pumps
External gear pumps are simpler, compact and cost-effective, and work at medium pressures (up to 250 bar with Marzocchi, up to 300 bar with the Bucher cast iron AP series). Internal gear pumps reach higher pressures (up to 400 bar), are quieter and are the reference choice for servo pumps and injection moulding machines. For mobile hydraulics, agricultural machinery and standard power units, an external gear pump is usually the most efficient choice in terms of cost.
Types and versions
- Single rotation or bidirectional: single rotation pumps work in one direction of rotation; bidirectional (reversible) pumps can reverse the flow, for example to drive actuators in both directions.
- Multiple pumps: two or more pumps combined on the same shaft, with independent circuits (Marzocchi GHP with independent driving systems).
- With built-in relief valve: available on the Marzocchi ALP series.
- Low noise: for noise-sensitive applications, the Marzocchi Elika helical gear pumps.
- Micro pumps: very small displacements (0.25 and 0.5 series) for micro hydraulics.
- Aluminium or cast iron housing: aluminium pumps (Marzocchi, Bucher AP) are light and cost-effective for most applications; cast iron pumps (Bucher AP212HP, AP250HP and AP312HP) withstand higher pressures, heavier shaft loads and harsh working conditions, typical of earthmoving machines, tractors, agricultural machinery and lift trucks.
Marzocchi external gear pump series
| Series | Main features | Typical uses |
|---|---|---|
| 0.25 and 0.5 | Very small displacements | Micro hydraulics, compact power units |
| 1P | Compact pumps for small flows | Small power units, automotive |
| ALP1, ALP2, ALP3, ALP4 | Reversible version or with relief valve available | Mobile and industrial hydraulics |
| GHP | Choice of shaft, flange and ports; multiple pumps with independent circuits; reversible version | Mobile and industrial hydraulics, agricultural and earthmoving machinery |
| Elika | Helical gears, very low noise | Power units in noise-sensitive environments |
Range of the ALP, GHP, 1P, 0.25 and 0.5 series: displacements from 1.4 to 200 cm³/rev, working pressure up to 250 bar, European, SAE and German standard flanges and shafts. Matching Marzocchi ALM and GHM external gear motors are also available.
Bucher AP external gear pumps
The Bucher Hydraulics AP series covers single pumps in aluminium and cast iron, unidirectional and reversible, plus double and triple pumps that can be configured by displacement, flange and auxiliary valves. Optimised hydraulic compensation gives high overall efficiency. The cast iron HP versions extend the range where aluminium pumps stop: they reach 300 bar continuous and are designed for the most demanding mobile applications. Custom versions are available with front covers for radial and axial loads and back covers with integrated pressure and flow control circuits.
| Series | Housing | Displacement (cm³/rev) | Operating / peak pressure (bar) | Max speed (rpm) |
|---|---|---|---|---|
| Bucher AP/APR05 | Aluminium | 0.25–1.6 | 190 / 230 | 7,000 |
| Bucher AP100 | Aluminium | 1.2–10.0 | 210 / 280 | 5,000 |
| Bucher AP/APR212 | Aluminium | 4.4–27.1 | 250 / 300 | 4,000 |
| Bucher AP/APR212HP | Cast iron | 15.1–34.1 | 250 / 300 | 3,500 |
| Bucher AP250HP / AP250FP | Cast iron | 15.2–54.0 | 300 / 320 | 3,500 |
| Bucher AP312HP | Cast iron | 40–75 | 300 / 320 | 3,000 |
Data from the Bucher Hydraulics catalogue. Available on request: send us the application data or the code of the pump to replace.
Replacing a pump from another brand
Marzocchi ALP pumps are interchangeable with the Casappa PLP series and GHP pumps with the Casappa PHP and HD series. Send us the code on the nameplate of the pump you need to replace and we will confirm the equivalent Marzocchi model, including flange, shaft and ports.
How to choose an external gear pump
- Flow: calculate the displacement from the required flow and the motor speed.
- Pressure: continuous and peak working pressure of the circuit; above 250 bar, the Bucher cast iron HP versions.
- Direction of rotation: clockwise, anticlockwise or bidirectional.
- Mounting: flange, shaft and ports to European, SAE or German standards, or matching the pump to be replaced.
- Housing material: aluminium for standard duty, cast iron (Bucher AP HP) for heavy-duty mobile machines.
- Circuits: single or multiple pump.
- Noise: for indoor applications close to operators, consider the Elika helical gear pumps.
External gear pump FAQ
How does an external gear pump work?
Two meshing gears carry the oil around the outside of the gears from the suction side to the delivery side; each revolution moves a fixed volume of oil, the displacement.
What is the maximum pressure of an external gear pump?
Up to 250 bar with Marzocchi external gear pumps and up to 300 bar continuous (320 bar peak) with the Bucher AP250HP and AP312HP cast iron series.
How do I calculate the flow of a gear pump?
Flow (l/min) = displacement (cm³/rev) × speed (rpm) ÷ 1,000, multiplied by the volumetric efficiency.
Is there an alternative to Casappa gear pumps?
Yes: Marzocchi ALP pumps are interchangeable with Casappa PLP and GHP pumps with Casappa PHP and HD.
Why choose Interfluid
- Official Marzocchi Pompe and Bucher Hydraulics distributor
- ALP, GHP, 1P, 0.25 and 0.5 series usually in stock
- Double and multiple pumps assembled quickly
- Online ordering on Fluid-Hub.net
Tell us the code of the pump to replace or the flow, pressure and speed of your application – request a free consultation.
To learn more
Hydraulic • 10 October, 2023
How to choose the most suitable hydraulic pump
In other words, how to choose the hydraulic pump with the most suitable technology for each specific application, taking into account flow rate, pressure, cost, efficiency, energy savings, reduced noise emissions and other essential factors.