Rexroth AA4FO open circuit fixed displacement hydraulic pump

The Bosch Rexroth AA4FO is a fixed displacement axial piston pump designed specifically for open circuit hydraulic systems. Unlike variable displacement pumps that adjust their output based on system demand, the AA4FO delivers a constant flow volume per shaft revolution, making it inherently simple, predictable, and mechanically robust.

Key Technical Characteristics

The AA4FO is available across a range of displacement sizes, commonly from 28 cc/rev up to 250 cc/rev and beyond, depending on the series variant. Core performance parameters include:

Maximum operating pressure: up to 400 bar (5,800 psi), depending on the frame size

Maximum continuous pressure: typically 350 bar

Speed range: up to 4,500 RPM for smaller displacements; derated for larger frames

Fluid compatibility: mineral hydraulic oil (ISO VG 46/68 typical); compatibility with HF fluids varies by configuration

Mounting: SAE and ISO flange options; through-drive capability for tandem configurations

Common Application Areas

The AA4FO's combination of high-pressure capability, fixed displacement simplicity, and robust bent-axis construction makes it a natural fit in several categories of equipment.

Industrial Machine Tools

CNC machining centers, broaching machines, and hydraulic presses frequently use fixed displacement pumps on dedicated circuits where flow demand is relatively constant during the working cycle. A hydraulic press, for example, may use an AA4FO on a high-pressure intensifier circuit where the pump runs at a consistent speed, and the system pressure is managed through sequence valves.

In one common configuration, a machine tool builder uses an AA4FO-28 (28 cc/rev) running at 1,450 RPM on a 50 Hz motor, delivering approximately 40 liters per minute to a clamping circuit. The fixed displacement matches the fixed-speed electric motor drive directly, eliminating the complexity of variable displacement controls.

Mobile Construction Equipment

Excavators, wheel loaders, and telehandlers often use open-circuit fixed-displacement pumps for implement circuits, particularly on secondary functions like blade tilting, outrigger extension, or auxiliary attachment drives. These are functions where the operator demands full flow intermittently, and a simple, direct-drive fixed pump serves the purpose reliably.

On a compact wheel loader, an AA4FO-56 through-drive configuration might handle both the steering circuit and a front-end loader implement circuit in tandem, two pumps coupled off a common PTO, each feeding an independent open circuit.

Marine and Offshore Deck Equipment

The AA4FO's pressure tolerance and availability in stainless-compatible sealing packages make it suitable for deck machinery-winches, cranes, and mooring systems-where contaminated environments and demanding load cycles are the norm. Marine classification societies typically require documentation of component ratings, and Rexroth's established product documentation and approvals support this process.

Test Stands and Fluid Power Research

Hydraulic test benches used in component development or quality control frequently require a pump with well-characterized, consistent output. The fixed displacement AA4FO's predictable flow-speed relationship makes flow measurement and test repeatability straightforward-there are no variable displacement controls to drift or require recalibration between test runs.

Selecting the Right AA4FO Configuration

Getting the most from the AA4FO requires matching the unit to the application carefully. The following parameters drive the selection process.

Displacement Size

Start with the required flow rate and the available drive speed:

Q (L/min) = Displacement (cc/rev) × Speed (RPM) / 1000

Factor in volumetric efficiency (typically 0.92–0.96 at rated conditions) to get the actual delivered flow. Always verify against the rated speed for the frame size-overspeeding a large displacement unit significantly reduces service life.

Pressure Rating Selection

The AA4FO series spans several pressure tiers. Select a frame with a continuous pressure rating at least 10–15% above your expected maximum working pressure, not just your relief valve setting. Systems with pressure spikes-shock loads, valve closing transients-can briefly exceed the relief setting, and derating the pump for continuous use at or near its maximum rating accelerates wear.

Mounting and Drive Interface

SAE flange mounting is standard for most industrial configurations; ISO flanges are common in European machine designs. Shaft options include SAE spline, keyed shaft, and tapered shaft variants. Confirm compatibility with the drive coupling or PTO interface early in the design phase.

For through-drive applications, verify the torque rating of the through-drive shaft. A common error is selecting the base pump correctly but ignoring that the through-drive must carry the combined torque load of both pumps.

Sealing and Fluid Compatibility

The standard AA4FO ships with NBR (nitrile) seals suited for mineral oil. Applications involving phosphate ester fluids, water-glycol mixtures, or vegetable-based fluids require alternative seal packages (typically FKM/Viton). Confirm fluid compatibility before specifying-seal incompatibility is a common and avoidable cause of early pump failure.

For more information on the AA4FO hydraulic pump, please visit https://www.baolilaihydraulic.com/product/.

Specifications

Selection Table

Q1: Is the AA4FO available with a servo or proportional control interface?

A1: No — the AA4FO is fixed displacement and has no displacement control mechanism. If variable flow control is required, consider the Rexroth A10VO (swashplate variable) or AA4VG (bent-axis variable) series. The AA4FO can be unloaded via an external solenoid unloading valve if intermittent flow demand is the application requirement.

Q2: Can the AA4FO be used in a closed circuit system?

A2: No. The AA4FO is designed specifically for open circuit operation. Its valve plate geometry, internal porting, and the absence of a charge pump interface make it incompatible with closed-circuit (hydrostatic) configurations. Rexroth's AA6VM and A4VG series are the appropriate choices for closed-circuit hydrostatic drives.

Q3:How to purchase an AA4FO hydraulic pump?

A3:You can visit https://www.baolilaihydraulic.com/contact/ to contact us and purchase our products through various methods.

Q4:What is the price and delivery time of the AA4FO hydraulic pump?

A4:The specific price and delivery time will be provided based on the specific model you provide.