Bosch Rexroth A4VG closed-circuit hydraulic pump for the Hamm road roller 1242806 1255665 2057209 2057219 2149052 01255630 01297449 02057225

The A4VG is a variable-displacement axial piston pump built specifically for closed-circuit hydrostatic transmissions — the "G" in the model code marks it as a closed-loop design, as opposed to the open-circuit A4VO used for implement or auxiliary functions. It uses a variable swashplate design with a nominal pressure of 400 bar and a peak pressure of 450 bar, and later Series 40 variants push that further, with closed-circuit ratings up to 500 bar.

Unlike a fixed-displacement pump, the A4VG's swashplate angle can be adjusted continuously. Flow is proportional to drive speed and displacement angle, and because the swashplate can swing through a neutral midpoint, the pump can reverse the direction of oil flow smoothly — letting the swashplate change flow direction gradually rather than through an abrupt mechanical shift. That's the mechanical trick behind a roller's seamless forward-neutral-reverse transitions.

Why Hamm Chose the A4VG for Roller Propulsion

Hamm's tandem and single-drum rollers rely on hydrostatic drive because it needs to do things a mechanical gearbox can't: creep at walking pace for fine finishing passes, reverse instantly at the end of a strip, and hold a steady drum speed on a grade without the operator constantly correcting throttle. A closed-circuit piston pump paired with an axial piston or bent-axis travel motor is the standard architecture across the compaction equipment industry for exactly this reason.

Smooth operation, especially at low speeds, is essential for uniform compaction — it ensures the drum applies consistent pressure to the surface, which is a direct prerequisite for a high-quality, even finish. A pump that hunts, surges, or lags in response destroys that consistency and shows up as visible banding or waviness in the finished mat.

The A4VG family is also chosen for its high power density and durability against shock loads and contamination — both realities on a compaction jobsite, where the pump sits close to the engine bay, runs continuously through a full shift, and has to tolerate less-than-pristine hydraulic fluid over the machine's service life.

Maintenance and Failure Points Worth Knowing

Filtration matters more than most owners assume. A minimum fluid purity grade of 20/18/15 per ISO 4406 is required for safe operation, tightening to 19/17/14 at high fluid temperatures. Contaminated charge oil is one of the fastest ways to shorten piston and swashplate bearing life.

Seal temperature range is limited. Standard FKM shaft seals are rated from -25°C to +115°C; below that, an NBR seal rated to -40°C is required — relevant for rollers that work in cold climates or are stored outdoors through winter.

Radial shaft loading has hard limits. Each pump size has a maximum permissible radial force on the drive shaft depending on mounting distance, so improper coupling alignment during a pump swap can shorten bearing life even when the part number is correct.

Charge pressure is the first diagnostic check. Because the integrated auxiliary pump feeds both the boost circuit and the servo control, a drop in charge pressure often shows up first as sluggish direction changes before it becomes an audible cavitation noise.

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Q1: What's the typical service life of an A4VG pump in a wheel loader application?

A1: This varies enormously with duty cycle, maintenance quality, and operating environment, but with proper filtration and fluid maintenance, many units run well beyond 10,000 hours before requiring major internal work. Harsh, dusty, high-cycle applications will shorten this considerably.

Q2: Does the A4VG pump control travel speed directly, or does the engine RPM also matter?

A2: Both. Output flow is a function of engine (input) speed multiplied by swashplate displacement angle, so travel speed is influenced by throttle position and displacement command together — which is part of why hydrostatic-drive loaders feel so responsive to pedal input.

Q3: What is the price and delivery time of the A4VG hydraulic pump for the Hamm road roller?

A3: The specific price and delivery time will depend on the equipment model or specific product model you provide.

Q4: How to purchase an A4VG hydraulic pump for the Hamm road roller?

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