Selection principles and steps
The correct and reasonable selection of the working medium for the hydraulic system is essential, as it allows the system to adapt to various environmental conditions. This thoughtful choice not only helps extend the life of the system and its components but also significantly improves the reliability of hydraulic equipment operation. Furthermore, it plays a crucial role in preventing accidents.
For all types of hydraulic systems, the choice of working medium need to consider many factors, see Table 1-23.
Table 1-23 Principles of hydraulic system working medium selection
| Principles of Selection | Consideration |
| Environmental conditions of the hydraulic system | Whether flame resistance is required (flash point, spontaneous combustion point) Ability to eliminate noise (air solubility, defoamability) Toxicity and odour of waste liquid treatment and environmental pollution requirements |
| Operating conditions of the hydraulic system | Operating pressure range (lubricity, ultimate load capacity) Operating temperature limits (viscosity, viscosity-temperature characteristics, thermal stability, low-temperature fluidity) Rotation speed (cavitation, wetting force on bearing surfaces) |
| Quality of the working medium | Physico-chemical indicators Adaptability to metals and seals Rust and corrosion resistance Antioxidant stability Shear stability |
| economic | Price and service life Ease of maintenance |
Hydraulic fluids can be selected in three basic steps:
1) List the requirements of the hydraulic system on the working medium performance indicators change range: viscosity, density, temperature limit, pressure limit, vapour pressure, flammability, lubricity, air solubility, compressibility and toxicity.
2) As far as possible, select a variety of working media that either meets or closely approximates the above requirements. Additionally, obtain recommendations for the working medium from the manufacturer of the hydraulic parts and consult the product catalogue. By doing so, you can ensure that the chosen medium aligns with the system’s needs and enhances overall performance.
3) The final synthesis, weighing, adjusting the requirements of all aspects of the parameters, to determine the selection of the appropriate working medium.
Selection of fluid viscosity
Once the selection of the type of fluid for the hydraulic system has been completed, a judgement of the viscosity of the fluid to be used is required. The basis for the judgement is as follows.
1) The hydraulic unit operates within a range of regions and climatic conditions, so it is important to consider viscosity limits carefully. The lubrication limit, which is the lower viscosity limit, represents the lowest viscosity allowed for a short period of time at the highest temperature, such as the viscosity of a leaking fluid at 90°C. On the other hand, the cavitation limit defines the upper viscosity limit. The cavitation limit determines the highest viscosity you can allow. However, this is only for a short period, such as when starting outdoors in winter.
2) The operating temperature variation requirements for hydraulic systems are a crucial consideration. For mineral-based hydraulic fluid, the temperature range is -20 to 80°C. However, to achieve the best service life for both the fluid and the hydraulic system, you should ensure that the maximum temperature does not exceed 65°C. In contrast, water-containing hydraulic fluid has a narrower temperature range of 10 to 54°C. Therefore, when selecting hydraulic fluid, it is essential to pay particular attention to the viscosity-temperature index. This adaptation ensures system stability and effectiveness.
3) The heaviest working components are pumps and motors. The viscosity of the selected fluid is generally applicable to these components. Additionally, the same viscosity is usually suitable for valves. Note that different manufacturers recommend different viscosity ranges for the same type of product. Table 1-24 provides the viscosity ranges recommended by various manufacturers for hydraulic components.
Table 1-24 Hydraulic components with different viscosity ranges
| factory | component | Recommended Viscosity/(mm²/s) | ||
| Upper viscosity limit | Lower limit of viscosity | Normal Operating Range | ||
| Danfoss | Straight shaft piston pumps and motors | 220 | 13 | 13~54 |
| Gear, vane, curved shaft pumps, motors | 860 | 13 | 13~54 | |
| Low-speed, high-torque vane motors | 110 | 13 | 13~54 | |
| general valve | 500 | 13 | 13~54 | |
| stacking valve | 500 | 8 | 8~51 | |
| proportional valve | 500 | 13 | 13~54 | |
| servo valve | 220 | 13 | 13~54 | |
| Piston Pumps, Cycloidal Motors | 1600 | 7 | 20~15 | |
| Bosch Rexroth | Piston pumps, motors | 1000~1600 | 5~10 | 16~36 |
| gear pumps | 1000~2000 | 10 | 20~100 | |
| Variable vane pumps | 800(Fuel supply start-up) 200(current-carrying start) | 10 | 10~160(carrier pressure<6.3MPa) 25~160(carrier pressure>6.3MPa) | |
| general valve | 800 | 10 | 10~380 | |
| proportional valve | 380 | 2.8 | 10~380 | |
| hydraulic cylinder | 380 | 2.8 | 20~100 | |
| Components | 200~800 | 10 | 12~100 | |
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