Mechanical Hydraulic Efficiency

Mechanical-hydraulic efficiency describes the ratio between the energy output and the energy input in hydraulic displacement machines, taking into account mechanical and hydraulic losses. It is a key parameter for evaluating the efficiency of hydraulic pumps, hydraulic motors, and hydraulic cylinders.

Definition and significance

Mechanical-hydraulic efficiency (_ηhm) indicates the proportion of the mechanical energy supplied that is actually available as hydraulic energy. Losses arise due to friction on moving components, flow resistance, and internal leakage losses. The higher this efficiency, the more efficiently the hydraulic component works.

In practice, modern hydraulic pumps achieve mechanical-hydraulic efficiencies of between 85 and 95%, depending on the design, operating pressure, and speed. Hydraulic motors have similar values, while cylinders often achieve efficiencies of over 90% due to their simpler design.

Types of losses in hydraulic components

Friction losses in hydraulic pumps

Various loss mechanisms occur in hydraulic pumps. Bearing friction occurs at the roller bearings of the drive shaft and depends on the speed. Seal friction occurs at piston and shaft seals and increases with increasing pressure. In addition, flow resistance in the displacement chambers and channels causes hydraulic losses.

The required drive torque consists of the theoretical torque for displacing the fluid and the loss torque:

_Mdrive =_Mtheoretical +_Mloss

The loss torque includes constant losses due to assembly inaccuracies (_Mc), pressure-dependent losses due to friction and jamming forces (_MΔp), speed-dependent losses due to fluid friction (_Mν), and flow losses (_Mρ). Explanations of drive torque can be found in the glossary.

Friction losses in hydraulic motors

In hydraulic motors, the loss torque reduces the usable output torque. The same loss mechanisms occur as in pumps, but with the opposite effect:

_Mnutz =_Mtheoretical -_Mloss

This means that in a hydraulic motor, the theoretically available torque is reduced by friction, flow losses, and internal leakage. The mechanical-hydraulic efficiency decreases with decreasing operating pressure, as the relative friction losses increase.

Friction losses in hydraulic cylinders

Hydraulic cylinders exhibit different loss behavior. The largest proportion of friction occurs at the seals, especially at the piston seal and the piston rod seal. The frictional force (_FRe) depends on the seal geometry, the contact pressure, and the viscosity of the hydraulic oil.

The mechanical-hydraulic efficiency of a cylinder is calculated from the ratio of the effective force to the theoretical force:

_ηhm = (_Ftheoretical -_FRe) /_Ftheoretical

Modern sealing systems with optimized geometry minimize friction losses and enable efficiencies of over 95%. However, wear and aging of the seals can reduce efficiency during operation.

Influence of operating parameters

Mechanical-hydraulic efficiency is not a constant value, but depends on several operating parameters.

The viscosity of the hydraulic oil influences both friction and flow losses. At low temperatures, viscosity increases, leading to higher friction losses. At high temperatures, viscosity decreases, causing internal leakage to increase and volumetric efficiency to decrease.

The operating pressure has a direct influence on friction and jamming forces. As the pressure increases, pressure-dependent losses increase, but at the same time the ratio between useful power and power loss improves.

The speed or velocity determines the magnitude of viscosity-related losses. At low speeds, static friction dominates, while at high speeds, flow losses predominate.

Relationship to overall efficiency

The overall efficiency of a positive displacement machine is the product of volumetric and mechanical-hydraulic efficiency:

_ηtotal =_ηvol ×_ηhm

While volumetric efficiency describes internal leakage losses, mechanical-hydraulic efficiency captures friction and flow losses. Only when both parameters are high does the component achieve good overall efficiency.

In hydraulic systems, knowledge of the mechanical-hydraulic efficiency is important for the design of the cooling system, as the power loss must be dissipated as heat. A precise calculation of the losses enables efficient dimensioning of the cooling system.

Measures for improvement

Various approaches are used to optimize mechanical-hydraulic efficiency.

The selection of high-quality bearings with low friction reduces mechanical losses. Modern sealing systems with optimized profiles minimize friction while providing good sealing.

Precise manufacturing of the housings and displacement chambers reduces jamming forces and flow losses. Optimized channel geometries reduce hydraulic resistance.

The use of hydraulic oils with suitable viscosity-temperature behavior ensures that good efficiency levels are achieved over a wide operating range. Additives to improve lubrication properties reduce friction on highly stressed components.

Regular maintenance, including replacement of worn seals and inspection of bearings, maintains mechanical and hydraulic efficiency over the entire service life of the component.

How efficient are hydraulic systems?

The efficiency of hydraulic systems is typically between 80 and 90%. This value is the product of volumetric efficiency (losses due to internal leakage) and mechanical-hydraulic efficiency (losses due to friction). The exact level depends on the component: axial piston pumps achieve 85–90%, gear pumps 50–90%, hydraulic motors around 85–90%, and hydraulic cylinders 85–90%. An efficiency of 100% is impossible because energy losses occur in every system. These losses are caused by friction in bearings, seals, and gears, as well as by flow resistance.
What is mechanical efficiency and how is it calculated?

The mechanical efficiency (_ηhm) of a hydraulic component indicates the proportion of the mechanical energy supplied that is actually available as hydraulic power. For hydraulic pumps, it is calculated by dividing the theoretical torque required for the drive by the actual torque. For hydraulic cylinders, the mechanical efficiency is calculated from the ratio of the effective force to the theoretical force. An efficiency of 100% would mean that there would be no friction and no driving force would be required at zero pressure — which is practically impossible. Modern high-quality components achieve efficiencies of 85–95%.
What types of losses cause poorer mechanical-hydraulic efficiency?

Mechanical-hydraulic efficiency is reduced by several types of losses: Friction losses occur at roller bearings (speed-dependent) and seals (pressure-dependent), flow losses occur in displacement chambers and channels, and jamming forces arise due to assembly inaccuracies. In hydraulic cylinders, friction at the piston seal and piston rod seal dominates. The losses consist of constant losses due to component tolerances, pressure-dependent losses due to friction, speed-dependent losses due to fluid friction, and flow losses. These losses are dissipated as heat and must be managed by the system’s cooling system.
How do operating conditions influence mechanical-hydraulic efficiency?

Mechanical-hydraulic efficiency is not constant, but depends heavily on operating parameters. Oil viscosity has a major influence: at low temperatures, viscosity increases and causes higher friction losses; at high temperatures, viscosity decreases and increases internal leakage. The operating pressure increases the pressure-dependent friction and jamming forces, but at the same time improves the ratio between useful power and power loss. The speed determines the viscosity-related losses: at low speeds, static friction dominates, while at high speeds, flow losses predominate. Hydraulic systems should therefore be operated within their optimum operating range.
Why do different pump types have different efficiencies?

Different pump types have very different efficiencies. Axial piston pumps achieve 85–90% efficiency thanks to precise manufacturing and low internal leakage — ideal for high-pressure applications. Gear pumps vary greatly between 50–90% depending on design and quality, but are inexpensive. Vane pumps achieve 65–85% with a compact design. Screw pumps achieve 60–80%. These differences are due to different design principles: axial piston pumps with precise components and modern manufacturing achieve higher efficiencies, while simple gear pumps with larger tolerances show lower values. For energy-efficient systems with high pressures and continuous loads, high-quality axial piston pumps are more economical, despite higher acquisition costs.
How does poor efficiency affect the system temperature?

Poor mechanical-hydraulic efficiency leads to significant temperature problems. Any power loss is dissipated as heat, which leads to increased system temperatures. This has several consequences: Oil viscosity decreases, which impairs the lubrication of components and leads to direct metal contact. The wear of seals, bearings, and gears accelerates significantly. Pressure fluctuations and sudden system failures become more likely. The hydraulic oil decomposes more quickly, which shortens the system’s service life. During continuous high-load operation with poor efficiency, system temperatures can rise uncontrollably, leading to operational downtime. Precise dimensioning of the cooling system is therefore essential and is based on calculations of the expected power loss.
How does mechanical-hydraulic efficiency differ from volumetric efficiency?

Mechanical-hydraulic efficiency (_ηhm) and volumetric efficiency (_ηvol) are two different variables whose product gives the total efficiency: _ηtotal =_ηvol ×_ηhm. Volumetric efficiency captures leakage losses – i. e. , the amount of fluid that flows internally without being available as useful power. Typically 90–95% in modern systems. Mechanical-hydraulic efficiency captures friction and flow losses. Both variables are independent: a pump can be mechanically efficient (high_ηhm) but have high leakage losses (low_ηvol). Only when both values are high does the component achieve high overall efficiency and thus low operating costs.
What measures improve mechanical-hydraulic efficiency?

Mechanical-hydraulic efficiency can be optimized through several design and operational measures. Material and manufacturing: High-quality rolling bearings with low friction and modern sealing systems with optimized profiles reduce mechanical losses. Precise housing manufacturing reduces jamming forces and flow losses. Design optimization: The geometries of displacement chambers and channels should minimize flow resistance. Fluid properties: Hydraulic oils with suitable viscosity-temperature behavior and friction-improving additives are essential. Operating conditions: Systems should be operated within the optimum pressure range and temperature window. Maintenance and service: Regular replacement of worn seals and inspection of bearings maintains high efficiency throughout the entire service life. These optimizations are crucial for economical operating costs, especially for high-pressure components.
How is the mechanical-hydraulic efficiency of a hydraulic motor calculated?

For a hydraulic motor, the calculation is the reverse of that for a pump. While for a pump the loss torque is added to the required drive torque (_Mdrive =_Mtheoretical +_Mloss), for a hydraulic motor the loss torque is subtracted from the theoretical torque: _Museful =_Mtheoretical –_Mloss. This means that in a hydraulic motor, the theoretically available torque is reduced by friction, flow losses, and internal leakage. The mechanical-hydraulic efficiency decreases with decreasing operating pressure, as the relative friction losses increase. A hydraulic motor with 90% efficiency at rated load can perform significantly worse at partial load. This must be taken into account when designing hydraulic motors for variable working scenarios.
What role does efficiency play in the overall assessment of a hydraulic system?

Mechanical-hydraulic efficiency is central to the overall efficiency and cost-effectiveness of a hydraulic system. It directly influences the required motor power and thus energy costs, power consumption, and_{CO2 emissions}. The power loss (difference between input and output power) must be dissipated 100% by the cooling system – poor efficiency requires disproportionately greater cooling. In systems with long operating times or high-load operation, poor efficiency quickly becomes a cost factor. Selecting high-efficiency components leads to reduced operating costs, longer component life, smaller cooling systems, and higher reliability. For systems that run continuously under load, it is necessary to calculate the costs over the entire life cycle, which quickly offsets the higher purchase price of more efficient components.
How does hydraulic oil affect mechanical-hydraulic efficiency?

Hydraulic oil has a major influence on mechanical-hydraulic efficiency. Viscosity is crucial: the right viscosity class (typically ISO VG 46) minimizes friction losses, while too high a viscosity increases friction losses and too low a viscosity increases internal leakage. The viscosity-temperature behavior must be suitable across the entire operating range – modern oils with an improved VI index keep the viscosity more stable. Additives to improve lubricity (e. g. , anti-friction additives) reduce friction on highly stressed components such as seals and bearings. Demulsification behavior and oxidation stability are important, as water and oxidation products increase the coefficient of friction. Regular fluid analysis with measurement of viscosity, acid number, and wear metals helps to maintain optimal conditions. Choosing a high-quality hydraulic oil with optimized additives can improve overall efficiency by 5–10%.
When does optimizing efficiency become economically viable?

Optimizing efficiency is economically viable when the annual operating time and drive power are high. For a hydraulic system with 1000 operating hours per year and 10 kW drive power, every percentage point of improved efficiency can save around 100–150 euros/year in energy costs. High-quality components with 2–3% improved efficiency pay for themselves after just 3–5 years of operation. With 5000 operating hours/year (continuous operation), the savings are four to five times higher. In addition, non-energy factors must be taken into account: longer component life, reduced downtime, smaller cooling systems (investment cost savings), and lower maintenance costs are often more economically significant than pure energy savings. A total cost of ownership (TCO) analysis over the entire life cycle usually shows that an investment in efficient components quickly pays for itself – especially in modern, energy-efficiency-driven industries.
How is efficiency measured and verified in practice?

Mechanical-hydraulic efficiency is determined in practice by measuring input and output variables. On a pump test bench, the drive torque/power at the motor is recorded using a pressure transducer and flow meter. In a hydraulic motor, the pressure difference, volume flow, and output torque are measured. During operation, indicators can point to efficiency problems: unexpected temperature increases, excessive noise, unusually high energy consumption, or deteriorated system performance. Diagnosis is performed by oil analysis (viscosity, wear metals), pressure testing, and speed control. Modern hydraulic systems have temperature and pressure sensors to detect anomalies. For optimization purposes, thermal camera inspections are also carried out to identify local overheating points. Regular measurements help to detect wear at an early stage and optimize maintenance intervals.
Are there differences in efficiency between single-acting and double-acting cylinders?

The mechanical-hydraulic efficiency of cylinders differs between single-acting and double-acting cylinders. In single-acting cylinders, the stroke movement is performed by hydraulic pressure, while the return stroke is performed by a return spring or external force. The efficiency here is relatively high (90–95%) as there is only one working volume. In double-acting cylinders, pressure acts on both sides of the piston, which means more complex sealing systems and higher friction. The efficiency is also 85–92%, but the hydraulic power required to displace the outflowing fluid must be taken into account. The friction at the piston rod seal must also be taken into account in double-acting cylinders, as the piston rod must be sealed on both sides. Wear and aging of seals have a greater impact on efficiency in double-acting cylinders. Regular maintenance is therefore essential for high efficiency in double-acting cylinders.
How does the condition of the seals affect efficiency?

The condition of the seals has a direct and significant impact on mechanical and hydraulic efficiency. New sealing systems with optimized geometries are a key factor in achieving high efficiency (> 95%), as they minimize friction while providing a good seal. Friction losses increase continuously with age and wear: a slightly worn seal can reduce efficiency by 2–3%, while heavily worn seals can reduce it by 5–10%. Worn seals lead to increased heat generation due to friction, which drives up the system temperature and in turn accelerates oil aging. Damaged or deformed seals also cause internal leaks, which also impair volumetric efficiency. This makes it clear that regular maintenance with replacement of worn seals is not only a safety measure but also an efficiency measure. In high-performance systems, seals are critical components whose condition should be checked regularly to maintain optimal operating conditions.