Bosch Rexroth A4CSG axial piston variable hydraulic pump

The A4CSG is a variable-displacement axial piston pump built in swashplate design, manufactured by Bosch Rexroth for hydrostatic drives operating in closed circuits. It's part of Rexroth's broader A4 axial piston family, which also includes the A4VSO (industrial, open circuit) and A4VG (mobile, closed circuit) — but the A4CSG is purpose-built as a compact, self-contained unit for heavy-duty closed-loop applications.

Unlike a fixed-displacement pump, the A4CSG can vary its output flow while running at constant speed, simply by changing the swivel angle of its swashplate. That's the core of what makes it valuable: one pump, infinitely variable flow, without touching engine or motor RPM.

Key Design Features

Swashplate, variable-displacement design rated for closed-circuit hydrostatic drives

Nominal pressure of 350 bar (5,100 psi), with a peak pressure rating of 400 bar (5,800 psi)

Integrated boost pump and valving — the charge pump, high-pressure relief valves, and flushing valve all live inside the housing, so you're not bolting on a separate charge circuit

Reversible flow direction, which lets the same unit drive a motor forward and reverse without external switching valves

Compact envelope relative to its output, with a short installation length that matters a lot in mobile equipment where space is at a premium

Through-drive capability, so a second pump (another A4CSG, an A4VSO, a gear pump, or similar) can be tandem-mounted off the back of the primary unit

How the A4CSG Works

Swashplate Displacement Control

Inside the A4CSG, a cylinder barrel carrying multiple pistons rotates against a swashplate set at a variable angle. As the barrel turns, each piston is forced in and out of its bore, drawing in fluid on one stroke and discharging it under pressure on the next. Tilt the swashplate further, and each piston travels a longer stroke — more displacement per revolution, more flow out. Bring the swashplate back toward zero angle, and displacement — and flow — drops toward nothing, even with the shaft still spinning at full speed.

This is the fundamental advantage over a fixed pump: you get flow control without governing engine speed, and because the pump is reversible, the swashplate can swing past center to reverse the direction of flow entirely — which is what lets a single A4CSG drive a hydraulic motor forward, neutral, and reverse.

Closed-Circuit Operation and the Integrated Boost Pump

In a closed-circuit hydrostatic transmission, fluid doesn't return to a reservoir after every pass — it flows from pump to motor and back to the pump in a continuous loop. The catch is that closed loops lose a small volume of fluid to internal leakage and case drain, so the loop needs constant "topping off" to stay full and to replace the oil that's carrying heat away.

That's the job of the integrated boost (charge) pump built into the A4CSG housing. It draws fluid from a reservoir, keeps the low-pressure side of the loop charged (typically in the 20–30 bar range depending on configuration), and works with the flushing valve to continuously exchange a portion of loop oil for cooler, filtered fluid. Because this is all built into the pump body rather than bolted on separately, the A4CSG stays compact, and the plumbing stays simple.

Control and Adjustment Options

The A4CSG is offered with several different control devices depending on how the application needs the swashplate angle to be commanded:

Hydraulic control (HD/HM) — pilot-pressure or control-volume dependent adjustment

Hydraulic control with servo/proportional valve (HS) — for more precise, remotely piloted control

Electro-hydraulic control (EP) — proportional solenoid-driven, ideal where a controller (like a machine ECU) needs to command displacement electronically

Electro-hydraulic pump control with hitch control valve — used in more integrated mobile applications

Fluid, Filtration, and Maintenance Requirements

Getting long service life out of an A4CSG comes down to fluid cleanliness and correct fluid selection — this is true of essentially all axial piston equipment, but it matters especially here given the pressures and tight internal clearances involved.

Hydraulic Fluid Selection

The A4CSG is designed to run on HLP mineral oil per DIN 51524, though Rexroth also publishes guidance (data sheets 90220, 90221, 90222) for environmentally acceptable fluids and fire-resistant HFDR/HFDU fluids where required. Fluid should be selected so the operating viscosity stays within Rexroth's optimum viscosity range (νopt) across the expected temperature envelope — running too thin or too thick both accelerate wear and reduce efficiency.

Filtration and Cleanliness

Rexroth specifies a minimum cleanliness level of ISO 4406 20/18/15 for the A4CSG, and recommends filter cartridges rated at β20 ≥ 100 for the boost circuit. In practice, this means:

Install filtration in the charge/boost line, either via a threaded filter port or a mounted in-line filter

Check and change filters on a defined interval, not just "when the light comes on"

Take particle counts periodically on high-value or critical machines — cleanliness drift is often the earliest warning sign of an approaching failure

Temperature Limits

Component temperature should never exceed 90°C at any point on the unit. Persistent high case-drain or loop temperatures usually point to inadequate cooling, a degraded boost pump, or a developing internal leakage path — worth investigating before it becomes a catastrophic failure.

Bearing Flushing

Certain high-speed or high-power operating conditions require active bearing flushing to remove heat from the shaft bearing area — this is specified per application in the Rexroth data sheet and shouldn't be skipped on units that call for it.

Ordering Code and Identification

Bosch Rexroth uses a structured type code to describe exactly which A4CSG variant you're looking at — displacement size, control device, direction of rotation, seal material, shaft configuration, and through-drive options are all encoded in the part number (for example, something like `A4CSG 355 EPG/30R-VSD85F994N`). When ordering a replacement or a spare, always work from the full nameplate designation and the Rexroth material number (R9... part number) rather than assuming interchangeability based on displacement size alone — control device and port configuration differences are common sources of ordering mistakes.

For more information on the A4CSG hydraulic pump, please visit https://www.baolilaihydraulic.com/product/.

Specifications

Selection Table

Q1: How do I know if my A4CSG needs the boost pump/charge pressure checked?

A1: Symptoms like sluggish response, cavitation noise, overheating, or inconsistent motor speed under load are common signs that charge pressure has dropped below spec — often due to a worn boost pump, clogged filter, or relief valve issue. A pressure gauge check at the charge port is the fastest diagnostic step.

Q2: Is the A4CSG still in production, or has it been superseded?

A2: Rexroth continues to produce and support the A4CSG in its current Series 30 and 33 form (per the RE 92105 data sheet, most recently revised in 2023), alongside related closed-circuit pumps like the A4VG. Always check the current Rexroth documentation for the latest series and availability, since product lines are periodically revised.

Q3: How to purchase an A4CSG 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 A4CSG hydraulic pump?

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