Rexroth A4FO Fixed Displacement Axial Piston Hydraulic Pump

The A4FO is a swashplate design axial piston pump with a fixed displacement. That means the angle of the swashplate is machined into the housing—permanently. Result: every rotation of the input shaft moves the same volume of oil from inlet to outlet.

Because there are no control pistons or servo valves, you eliminate a whole layer of potential failure. That’s why you’ll find this pump in open-circuit systems where simplicity is a feature, not a cost-cutting measure.

Technical Breakdown

Volumetric Efficiency > 95%

Thanks to precise cylinder-to-valve plate mating and hydrostatic bearing balancing, the A4FO maintains high efficiency even at low viscosities or high temperatures. A typical gear pump might drop to 85–88% under similar loads.

Pressure Capacity

Where a vane pump might max out at 210 bar, the A4FO runs daylong at 350 bar. That matters for:

Injection molding machines (holding pressure)

Marine steering gears (continuous high load)

Test benches (precise flow control)

Flanges and Shafts

ISO 3019/2 (two-hole or four-hole)

Splined shafts to ISO 14 (metric) or SAE J744

Optional tapered roller bearings for heavy radial loads (belt drive applications)

Installation Best Practices (Avoid Common Failures)

Over a decade of field data, 70% of A4FO failures trace back to three causes: cavitation, contamination, or case drain overpressure.

1.Inlet Line Must Be Oversized

The A4FO hates starvation. Because it’s fixed displacement, it tries to pull full flow even if the inlet is restricted.

Minimum inlet pressure: 0.8 bar absolute (for standard NBR seals)

Solution: Use a suction strainer, not a filter (max 100 µm). Add a vacuum gauge on inlet.

2.Case Drain Pressure

This is the most overlooked spec. The A4FO’s shaft seal fails at just 2 bar continuous case drain pressure. Yes, only 2 bar.

Always run a separate case drain line direct to tank (no backpressure).

For multi-pump stacks, the rear pump’s case drain must not pressurize the front pump.

3.Fluid Cleanliness

Target ISO 4406 18/16/13 (NAS 7–8). The A4FO has close internal clearances (~5–15 µm). Dirt causes:

Valves plate scoring → efficiency drop

Piston slipper bearing failure → metal contamination

For more information on the A4FO hydraulic pump, please visit https://www.baolilaihydraulic.com/product/.

Specifications

Selection Table

Q1: Is the A4FO variable displacement?

A1:No. That’s the A4VSG series. The A4FO is fixed. If you need pressure or flow control, use a separate relief valve or variable pump.

Q2:Can I use it in a closed-loop (hydrostatic) circuit?

A2:Not recommended. The A4FO is designed for open circuits. Closed-loop pumps (like A4VG) include charge pumps and boost valves.

Q3:How to purchase an A4FO 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 A4FO hydraulic pump?

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