Hydraulic oil is a critical medium in hydraulic systems for power transmission, component lubrication, corrosion prevention and heat dissipation. Virtually all hydraulic systems require hydraulic oil. For many users of hydraulic equipment, proper selection of hydraulic oil is the most effective measure to prevent faults and extend equipment service life. Therefore, classification and selection of hydraulic oil are important topics.

Classification of Hydraulic Oils

1. Classification by Base Composition

There is a wide variety of hydraulic oils. Based on base stock, they fall into three categories: mineral oil-based hydraulic oil, synthetic hydraulic oil and biodegradable hydraulic oil, covering most industrial applications.

2. Classification by Viscosity Grade

Hydraulic oils are classified into six grades according to kinematic viscosity at 40°C: ISO VG 15, 22, 32, 46, 68 and 100. Larger numbers indicate higher viscosity.

Note: 32#, 46# and 68# are the most commonly used hydraulic oil grades.

3. Classification by Operating Temperature Range

Four types are available: general-purpose, high-temperature, low-temperature and wide-temperature hydraulic oils.

4. Classification by Additive Type

Specific additives are blended into hydraulic oils to enhance performance. According to additive formulations, hydraulic oils can be divided into non-additive type, anti-wear type, anti-oxidation & anti-rust type, etc.

5. Classification in accordance with GB/T 7631.2-2003

Seven main categories are defined: L-HH, L-HL, L-HM, L-HR, L-HV, L-HS and L-HG.

Guidelines for Selecting Suitable Hydraulic Oil

Multiple factors shall be comprehensively considered to ensure efficient and stable operation of hydraulic systems when selecting hydraulic oil.

1. Follow recommendations from equipment manufacturers. Equipment manuals normally specify the required oil type and viscosity grade.

2. Take operating temperature into consideration. If equipment starts at extremely low temperatures, such as construction machinery operating outdoors in winter, low-pour-point hydraulic oil shall be selected. HV hydraulic oil suits environments above -30℃ in cold regions, while HS hydraulic oil is suitable for extremely cold regions above -40℃.

3. Match hydraulic oil type with working conditions. For general working conditions, select cost-effective mineral oil-based HL and HM oils. Synthetic hydraulic oil is adopted for extreme temperatures and precision equipment to guarantee viscosity stability. Biodegradable hydraulic oil is chosen for scenarios with strict environmental requirements to reduce pollution.

4. Pay attention to fluid compatibility. Hydraulic oil must be compatible with equipment seals and metal components. For example, mineral oil is compatible with nitrile rubber seals, while phosphate ester hydraulic oil matches fluorocarbon rubber seals to avoid corrosion and aging.

Important note: Hydraulic oils of different brands and types must not be mixed arbitrarily. Differences in additive formulations may lead to oil deterioration and blockage of components.