Overall Efficiency

In hydraulics, overall efficiency describes the ratio of the usable output power to the input power of a hydraulic component or an entire system. It accounts for all energy losses and indicates how efficiently a hydraulic pump, a hydraulic motor, or a system operates. A high overall efficiency means lower power loss and lower energy consumption.

Composition of Overall Efficiency

The overall efficiency of a hydraulic machine is calculated as the product of the volumetric efficiency and the hydromechanical efficiency. Both partial efficiencies account for different loss mechanisms that occur in every positive-displacement machine. Only when both partial efficiencies are at a high level does the overall efficiency also achieve satisfactory values.

Volumetric efficiency

Volumetric efficiency describes the ratio between the actual delivered or received flow rate and the theoretically possible flow rate. The difference arises from internal leakage flows that flow from the high-pressure to the low-pressure area or to the outside. Clearance geometries on pistons, control discs, and sealing surfaces form the main leakage paths. A volumetric efficiency of 90 percent means that 10 percent of the theoretical flow rate is lost as leakage.

Leakage losses depend heavily on the viscosity of the hydraulic oil. At low viscosity—such as due to high operating temperatures—gap flows increase and volumetric efficiency decreases. Conversely, higher viscosity reduces leakage but simultaneously increases friction losses, which negatively impacts hydromechanical efficiency. This trade-off makes the selection of the right oil and operating temperature a central aspect of efficiency optimization.

Hydromechanical efficiency

Hydromechanical efficiency accounts for all losses caused by friction within the machine. These include solid-to-solid friction at bearings and seals, fluid friction in the gaps, and flow losses at transitions and control grooves. The so-called no-load power, which a pump must generate even during pressureless operation, is also classified as a hydromechanical loss.

As pressure increases, hydromechanical efficiency initially rises because the net power output grows in proportion to the friction losses. At very low pressures, such as below 10 bar, friction dominates the power balance, and the hydromechanical efficiency drops significantly. This effect explains why hydraulic pumps often exhibit significantly lower overall efficiencies in partial-load operation than in rated operation.

Typical overall efficiencies of hydraulic components

The achievable overall efficiencies vary considerably between different designs. Designers must take these differences into account when selecting pumps and motors, as they have a direct impact on the system’s energy consumption and heat generation.

Overall efficiencies of hydraulic pumps

Axial piston pumps achieve the highest overall efficiencies because their piston-control surface geometry limits both leakage and friction losses. At the optimal operating point, at pressures around 200 to 280 bar and speeds in the range of 1500 to 1800 rpm, efficiency values exceed 90 percent. Gear pumps exhibit lower efficiency due to their comparatively large clearance areas and the lack of a variable displacement option. Their strength lies more in their robustness and cost-effectiveness than in their efficiency.

Overall efficiency of hydraulic motors

Hydraulic motors often achieve slightly higher overall efficiencies than pumps of comparable design. The reason is that motors do not have to actively displace fluid against pressure resistance, but rather convert the pressure into rotational motion. The loss mechanisms are similar, but the emphasis shifts: In motors, flow losses at the inlet channels are less significant than delivery losses in pumps.

Factors influencing overall efficiency

Overall efficiency is not a fixed figure but depends heavily on operating conditions. The same axial piston motor can achieve efficiencies ranging from 60 to 94 percent depending on the operating point. Anyone planning or operating systems must be aware of these dependencies to operate the components within the most efficient operating range.

Pressure and Speed

System pressure influences overall efficiency through both partial efficiencies. As pressure increases, leakage flows rise, which reduces volumetric efficiency. At the same time, hydromechanical efficiency increases because friction losses decrease relative to the transmitted power. Overall, for most pumps and motors, maximum efficiency is achieved in the range of the rated pressure. Below 10 bar, the overall efficiency drops drastically.

Speed also affects both partial efficiencies, but in opposite directions. Higher speeds improve volumetric efficiency because leakage per revolution remains constant, while the delivered flow rate increases. At the same time, friction losses increase, particularly fluid friction in the gaps. Here, too, there is an optimal speed range that varies depending on the size and design.

Viscosity and Temperature

The viscosity of the hydraulic oil is one of the most significant factors influencing overall efficiency. At optimal viscosity—which for most industrial applications lies in the range of 30 to 50 mm²/s—pumps and motors achieve their best performance. If the viscosity deviates above or below this range, one of the two partial efficiencies will inevitably deteriorate.

High operating temperatures lower viscosity and increase leakage, while low temperatures raise viscosity and increase friction. A temperature-controlled oil supply that keeps the hydraulic oil within the optimal viscosity range sustainably improves overall efficiency. In practice, systems with oil temperature control often achieve efficiency levels several percentage points higher than non-tempered systems.

Part-load operation

In part-load operation—that is, at reduced flow rate or speed—overall efficiency drops significantly in nearly all designs. Variable-displacement pumps that reduce their displacement volume to a small fraction operate with greatly increased relative leakage losses. Zero-displacement power also becomes a significant factor at low displacement. For swashplate motors that reduce to less than 30 percent of their maximum displacement, the overall efficiency can fall 10 to 20 percentage points below the rated value.

This relationship is particularly relevant for applications with highly fluctuating power requirements. Here, multi-pump systems or load-dependent controls offer advantages because they can keep the active pumps within an operating range that maximizes efficiency at all times.

Measurement and Standardization of Overall Efficiency

The central standard for efficiency measurement on hydraulic positive-displacement machines is ISO 4409. It defines the test procedures for hydraulic pumps, motors, and compact gear units, including test bench configuration, sensor placement, and test procedure. Measurement accuracy is classified into grades, with Grade B being standard for most industrial applications.

On a test bench compliant with ISO 4409, pressure, torque, and flow rate are measured simultaneously. From these measured variables, both the volumetric and hydromechanical efficiency can be determined, and the overall efficiency calculated from them. The standard ensures that results from different manufacturers are comparable.

DIN EN ISO 4413, on the other hand, regulates the safety requirements for hydraulic systems and contains no direct specifications regarding efficiency measurement. Nevertheless, it is relevant for system operators because it sets design requirements that indirectly influence efficiency, such as the stipulation that systems must be designed so as not to generate unnecessary heat.

Significance of Overall Efficiency for System Design

Overall efficiency has direct consequences for the design of a hydraulic system. Every percentage point of power lost in the pump or motor is converted into heat. This heat must be dissipated by coolers, which requires additional energy and installation space. A pump with an overall efficiency of 85 percent converts 15 percent of the drive power into heat. With a drive power of 100 kW, this amounts to 15 kW of power loss that places a load on the oil cooler.

Design engineers who optimize overall efficiency as early as the design phase not only reduce the system’s energy consumption but also minimize the costs associated with cooling and filtration. In industrial continuous-operation systems, such as injection molding machines or presses, the use of more efficient components often pays for itself within a few years through the energy costs saved.

Measures to Improve Overall Efficiency

Various approaches help improve the overall efficiency of a hydraulic system or maintain it at a high level:

• Component selection: Choosing the right design for the specific application is the most important factor. Axial piston pumps and motors offer the best conditions for high efficiency under varying loads.
• Operating point optimization: Pumps and motors should be operated as close as possible to their maximum efficiency. Load-dependent variable displacement controls help adjust the operating point.
• Oil temperature management: A temperature-controlled oil supply keeps viscosity within the optimal range and prevents efficiency losses due to excessively high or low temperatures.
• Maintenance and condition monitoring: Wear on pistons, control discs, and seals increases leakage and reduces volumetric efficiency. Regular oil analyses and pressure-flow measurements detect efficiency losses early on.
• System architecture: Multi-pump systems, load-sensing controls, and speed-controlled drives reduce partial-load losses and improve the overall efficiency of the system.

Development Trends

The energy efficiency requirements for hydraulic systems are continuously rising, driven by regulatory mandates and economic pressures. Manufacturers are developing pumps and motors with optimized clearance geometries that reduce leakage without increasing friction. New coating processes for pistons and control discs reduce wear and stabilize overall efficiency over the service life.

In addition, speed-controlled drive concepts are gaining importance. They allow the flow rate to be controlled not by adjusting the pump, but by adjusting the motor speed. This eliminates adjustment losses, and overall efficiency improves significantly, particularly during partial-load operation. Sensor technology that measures pressure, temperature, and flow rate directly at the component also enables real-time control optimized for the operating point.