Hydrostatic & Variable Displacement Pumps: Danfoss High-Efficiency HPU

Ask ten hydraulic engineers to explain the difference between a hydrostatic circuit and a standard open-circuit variable displacement system and you will get ten reasonably correct but meaningfully different answers. That is not a bad thing — the distinction is genuinely worth understanding in depth, because it determines whether a machine drives efficiently, responds predictably, and survives the kind of load reversals that destroy poorly matched systems.

Danfoss built their hydraulic power unit range around both architectures, and the choice between them is not arbitrary. It follows directly from what the application actually does.

Open Circuit Variable Displacement: When It Fits

A variable displacement pump in an open circuit draws from a reservoir, pressurizes fluid to circuit demand, and returns exhaust flow back to that same reservoir through a return line. The swashplate position adjusts continuously to match system demand — less flow when actuators are holding position or moving slowly, more when work is being done at speed.

Danfoss variable displacement designs in this configuration pair well with load-sensing valve blocks. The pump receives a signal representing the highest load pressure in the circuit and adjusts to maintain a pressure margin above it — typically 20 to 25 bar. The result is that no unnecessary energy is consumed maintaining pressure above what any actuator actually needs. For multi-function industrial machinery and mobile equipment with several simultaneous consumers, this is the architecture that keeps fuel consumption and heat generation under control.

Hydrostatic Circuits: A Different Problem Set

A hydrostatic pump works in a closed loop. Its output goes directly to a hydraulic motor — no reservoir in the main circuit, no directional control valve, no throttling stage. The pump controls everything: flow direction determines motor direction, displacement magnitude determines motor speed. A small charge circuit maintains minimum inlet pressure to prevent cavitation at the pump suction.

This closed architecture eliminates the throttling losses that are unavoidable in open circuits whenever a directional valve is modulating flow. It also enables energy recovery — a motor acting as a load brake can be made to function as a pump, returning energy to the system rather than dumping it as heat. In mobile equipment with frequent deceleration events, that recovery is measurable in fuel consumption.

The Danfoss HPU Approach

Danfoss hydraulic power units are not catalog assemblies of separately chosen components. The pump, motor drive, and control electronics are engineered together so that efficiency optimizations that require coordination across the full power chain are actually achievable in practice. A variable speed motor drive adjusting pump input speed in coordination with swashplate position can reach operating points that neither control layer could find independently.

For industrial plant engineers specifying hydraulic power units, this integration reduces commissioning time and removes the trial-and-error of making separately sourced components cooperate. For mobile OEMs, it produces a validated power train that meets efficiency and low emission targets without requiring in-house hydraulic system expertise at the depth the design would otherwise demand.

HOVOO / HOUFU supplies seal kits for Danfoss variable displacement and hydrostatic pump platforms. Internal seal condition is the foundation that all efficiency claims rest on — a pump with worn seals leaks back internally and cannot maintain the output the control system commands. HOUFU seal kits are dimensionally verified against Danfoss specifications. Details at hovooseal.com.

Source: www.hovooseal.com