English 
Spanish 
The classification of hydraulic oil is a very important topic because choosing the right hydraulic oil is key to ensuring the stable, efficient, and long-term operation of a hydraulic system. The main ways to classify hydraulic oil are as follows:
I.Classification by Base Oil Type
This classification is based on the fundamental components used in manufacturing hydraulic oils, directly determining the oil’s performance, lifespan, and environmental characteristics.
1. Mineral Oil-Based Hydraulic Oils
This is currently the most widely used hydraulic oil, accounting for over 85% of global consumption. It is refined from petroleum distillates as base oils with various additives.
Characteristics:
- Advantages: Mature technology, low cost, balanced performance, meeting the needs of most industrial applications.
- Disadvantages: Poor biodegradability, unfriendly to the environment and ecology; inherent limitations in properties such as flash point and pour point.
Main Types:
- HH Type: Additive-free refined mineral oil. Essentially obsolete.
- HL Type: Refined mineral oil with rust and oxidation prevention properties. Used in hydraulic systems of general low-pressure machine tools.
- HM Type: Anti-wear hydraulic oil. Based on HL, anti-wear agents are added. This is the most commonly used type in modern industrial equipment. Effectively protects pumps and motors, extending equipment life. Further subdivisions are available based on anti-wear performance (e.g., zinc-containing/zinc-free formulations).
- HV Type: Low-temperature hydraulic oil. Based on HM, improved viscosity-temperature properties (high viscosity index) and a lower pour point make it suitable for cold regions or equipment with large outdoor temperature differences.
- HG Type: Based on HM, improved anti-creep (anti-stick-slip) properties make it suitable for systems combining hydraulic and guideway lubrication, such as some grinding machines.
2. Synthetic Hydraulic Oils
Base oils prepared through chemical synthesis have a designed molecular structure that surpasses the performance of mineral oils.
Features:
- Advantages: Excellent high and low temperature performance (wide operating temperature range), high viscosity index, good oxidation stability, long service life, and low volatility.
- Disadvantages: Expensive, and has compatibility issues with certain sealing materials and coatings.
Main Types:
Phosphate Ester Type: Excellent flame retardancy, commonly used in hydraulic systems near high temperatures and open flames in metallurgy, power plants, etc. However, it has poor environmental performance and requires special sealing materials.
Water-Glycol Type: Flame-resistant hydraulic fluid with good flame retardancy and low-temperature fluidity. However, it has poor lubricity, a low viscosity index, and can affect certain metals and seals.
Polyalpha-Olefin Type: Wide operating temperature range, high viscosity index, and excellent thermal oxidation stability. Commonly used in aerospace and high-end industrial fields.
Synthetic ester type: It has the characteristics of being biodegradable, having low toxicity, and having excellent lubricity. It is often used in fields with high environmental protection requirements (such as forestry and marine industries).
3. Biodegradable Hydraulic Fluids (Environmentally Friendly)
Based on synthetic esters or vegetable oils (such as rapeseed oil), these fluids can be rapidly decomposed by microorganisms in the natural environment.
Features:
- Advantages: Extremely low toxicity, biodegradable, and minimal environmental pollution.
- Disadvantages: Higher price, generally less oxidative stability than mineral oils, requiring more frequent monitoring and replacement.
Applications: Primarily used in environmentally sensitive areas such as forests, farmland, reservoirs, and ports.
II.Classification according to International Standards (ISO)
The International Organization for Standardization (ISO) has systematically classified hydraulic fluids according to their main applications, composition, and properties, as defined in standard ISO 6743-4. This is one of the most authoritative and widely used classification methods.
III.Classification by Viscosity Grade (ISO VG)
Viscosity is one of the most important indicators of hydraulic fluid. ISO has established viscosity grades (VG) based on the kinematic viscosity range at 40°C.
Common viscosity grades: ISO VG 15, 22, 32, 46, 68, 100, etc.
Selection Principles:
- VG 32 / VG 46: The most commonly used grades, suitable for most indoor industrial and mobile equipment.
- VG 68: For equipment with heavy loads and significant system leakage.
- VG 22 / VG 15: For precision servo systems or low-temperature environments.
Core Idea: The selected oil viscosity should ensure optimal lubrication (viscosity should not be too low) and highest volumetric efficiency (viscosity should not be too high) at the system’s operating temperature.
IV.How to choose the right hydraulic oil?
1.Follow the equipment manufacturer’s (OEM) recommendations:
This is the first principle. The manual usually specifies the required ISO type (e.g., HM, HV) and viscosity grade (e.g., ISO VG 46).
2.Consider the operating environment:
- Temperature: Use HM oil for indoor use at normal temperatures; HV oil for cold regions or outdoor use; use fire-resistant hydraulic oils (HFC/HFDR, etc.) for high temperatures or fire hazards.
- Environmental sensitivity: Consider biodegradable oils (HFDU) in situations where leakage is likely and the environment is harmful.
3.Consider system parameters:
- Pressure: Medium and high pressure systems must use anti-wear hydraulic oil (HM).
- Pump type: Vane pumps and piston pumps have high requirements for oil anti-wear properties; gear pumps have relatively less stringent requirements.
4.Consider oil compatibility:
When changing oils, ensure that the new oil is compatible with the old oil, seals, hoses, and filters in the system.
In short, hydraulic oil classification is a multi-dimensional system. In practical applications, it is usually necessary to combine ISO type (such as HM) and viscosity grade (such as VG 46) to describe and select a hydraulic oil, such as “L-HM 46 anti-wear hydraulic oil”.
★★★Review of previous selected articles:
- Hydraulic cylinder oil port types and selection
- Analysis of the causes of the hydraulic cylinder’s self-descent in the hydraulic system and its solution
- Working principle, composition and structure of hydraulic system
- Technical analysis of the three core issues of hydraulic cylinder exhaust devices
- Hydraulic cylinder selection，Determination of stroke S and selection of mounting method
- Explanation of common hydraulic basic terms
- A question: Why do hydraulic cylinder seals fail?
- Complete guide to hydraulic cylinder failure repair!
- Six basic laws of hydraulic systems: from pressure formation to flow distribution
- Causes and solutions for the creeping phenomenon of hydraulic cylinders during operation