Rexroth A4FM axial piston fixed displacement hydraulic motor

The A4FM is a fixed displacement axial piston motor built on a swashplate design, manufactured by Bosch Rexroth for use in open and closed hydrostatic circuits. Unlike variable displacement motors (like the A6VM or A2FM's variable siblings), the A4FM delivers a constant displacement per revolution — the swashplate angle is fixed at the factory and doesn't change during operation.

That single design decision shapes everything about how the motor behaves:

Output speed is directly proportional to input flow and inversely proportional to displacement.

Output torque rises and falls with the pressure differential between the high- and low-pressure ports.

There's no internal control mechanism adjusting displacement on the fly, which means fewer moving parts, fewer failure points, and a simpler control strategy at the system level.

Rexroth positions the A4FM as the go-to option when installation space is tight, and a bent-axis motor (like the A2FM) won't physically fit — the swashplate geometry gives it a more compact axial footprint for a given displacement.

Typical Applications

The A4FM shows up wherever a compact, robust, fixed-ratio hydraulic motor is needed:

Mobile machinery — travel drives, winches, and auxiliary drives on construction and forestry equipment

Industrial hydrostatic drives — conveyor drives, mixers, extruders, and rotary tables where a constant speed-to-flow ratio is desirable

Marine and offshore equipment — deck machinery and hydrostatic propulsion auxiliaries

Agricultural equipment — drive systems where space constraints rule out bent-axis alternatives

Installation and Circuit Considerations

Open vs. Closed Loop Circuits

The A4FM is built to work in both open and closed hydrostatic circuits:

Closed loop: oil returning from the motor goes directly back to the pump inlet, common in travel drives and winch systems where efficiency and compact plumbing matter.

Open loop: oil returns to a reservoir before being redrawn by the pump, more common in industrial fixed-installation settings.

Case Drain and Flushing

Like most axial piston motors, the A4FM has internal leakage that must be routed away from the motor housing through a case drain line. Undersized or restricted case drain lines are one of the most common (and preventable) causes of premature motor failure — excess case pressure degrades shaft seals and can force contamination back into the bearings.

Shaft, Mounting, and Valve Options

Rexroth offers the A4FM with multiple shaft configurations (splined per DIN 5480, keyed, etc.), mounting flange patterns (SAE and metric), and optional integrated valves (relief valves, flushing valves, speed sensors) depending on the application. When replacing a unit, these details matter as much as the displacement figure.

For more information on the A4FM hydraulic motor, please visit https://www.baolilaihydraulic.com/product/.

Specifications

Selection Table

Q1: What's the difference between the A4FM and the A2FM?

A1: The A2FM is a bent-axis fixed displacement motor, while the A4FM uses a swashplate design. The A4FM's geometry generally allows for a more compact axial installation, making it the preferred choice where space is limited and a bent-axis motor won't fit.

Q2: Why is my A4FM overheating?

A2: The most common causes are restricted case drain lines, fluid viscosity outside the recommended range, or sustained operation near peak pressure. Case drain flow and fluid condition should be the first things checked.

Q3: What is the price and delivery time of the A4FM hydraulic motor?

A3: The specific price and delivery time will be provided based on the specific model you provide.

Q4:How to purchase an A4FM hydraulic motor?

A4:You can visit https://www.baolilaihydraulic.com/contact/ to contact us and purchase our products through various methods.