Detailed Comparison of Standards and Abbreviations in Hydraulic Oils
Hydraulic oils are marketed internationally under different designations. While in Germany, the DIN 51524 standards (HL, HLP, HVLP, etc.) are primarily used, at the international level, ISO 11158 / ISO 6743/4 (with abbreviations such as HL, HM, HV, etc.) are also applied. In Italy, reference is often made to UNI EN ISO standards, which incorporate the same international classifications. Essentially, all these standards describe similar parameters: wear protection, corrosion protection, oxidation resistance, and viscosity-temperature behavior. To help you navigate these classifications, we present a comparative overview of the most common classifications, their abbreviations, and their key characteristics.
1. Basic Principles: What Do These Standards Regulate?
- DIN 51524 (Parts 1 to 3)
- Defines the properties, additives, and viscosity ranges of mineral-based hydraulic oils for the German market.
- Part 1: HL
- Part 2: HLP
- Part 3: HVLP
- (There are also specific variants such as HLPD or HVLPD, which are not detailed in the standard but are recognized in the market.)
- ISO 6743/4 and ISO 11158
- Classify hydraulic fluids into different categories (HL, HM, HV, HG, etc.).
- These standards are widely used internationally, especially in engineering and machinery for export.
2. Comparison of Main Abbreviations
The following table shows the most commonly used abbreviations in DIN and ISO/UNI standards, along with a brief description of their characteristics:
Abbreviation | DIN 51524 | ISO 11158 / ISO 6743/4 | UNI EN ISO | Main Characteristics |
---|---|---|---|---|
H | – | – | – | Mineral oil without special additives (historical, practically no longer in use). |
HL | DIN 51524-1 (HL) | HL | HL | Antioxidant and anti-corrosion additives; moderate pressures and low thermal requirements. |
HLP | DIN 51524-2 (HLP) | HM | HM | Anti-wear protection (AW/EP additives) + oxidation and corrosion protection; standard for pressures > 200 bar. |
HVLP | DIN 51524-3 (HVLP) | HV | HV | Equivalent to HLP/HM but with a high viscosity index (High VI); performs better with temperature variations. |
HLPD | (Special case, not included in DIN 51524) | – | – | Identical to HLP but with detergent and dispersing properties (D). Handles impurities and water better. (Only in Germany) |
HVLPD | (Special case) | – | – | Combines high viscosity index (HVLP) with detergent additives (HLPD); rare but available in the market. |
Note: UNI EN ISO standards closely follow the ISO classification (HL, HM, HV). Therefore, HM in ISO/UNI corresponds approximately to HLP in DIN, while HV corresponds to HVLP.
3. Key Differences Between HL, HLP, HVLP, and Others
- HL (DIN 51524-1 / ISO HL)
- Contains additives for corrosion and oxidation protection.
- Used up to approximately 200 bar with moderate thermal requirements.
- Becoming less common as many systems require greater wear protection.
- HLP (DIN 51524-2) / HM (ISO 11158 / ISO 6743/4)
- Includes anti-wear (AW) and high-pressure (EP) additives.
- Supports higher pressures (> 200 bar) and greater temperature ranges.
- Widely used and considered the „reference“ hydraulic oil.
- HVLP (DIN 51524-3) / HV (ISO 11158 / ISO 6743/4)
- Similar to HLP/HM but with a high viscosity index (VI).
- The viscosity changes less with temperature variations, facilitating cold starts while remaining stable at high temperatures.
- Ideal for mobile machinery exposed to large temperature fluctuations.
- HLPD
- Contains detergent and dispersing additives that help “capture” dirt and moisture within the fluid rather than letting them settle.
- Recommended for highly contaminated environments or where water infiltration is possible.
- Not standardized internationally; mainly used in Germany.
- HVLPD
- Combines the benefits of a high viscosity index (HVLP) with detergent properties (HLPD).
- Not included in DIN standards but offered by some manufacturers for specific applications.
4. Practical Applications
- Industrial Hydraulic Systems (presses, injection molding machines, machine tools)
- HLP (HM) is typically used when the temperature is stable.
- If there are large temperature variations or sensitive components → HVLP (HV).
- In very dusty environments (e.g., cement plants), HLPD/HVLPD oils are useful.
- Mobile Hydraulics (Construction Machinery, Forklifts, Forestry/Agricultural Equipment)
- HVLP oils are often recommended to handle external temperature variations.
- If temperature fluctuations are minimal and pressure is not extreme, HLP may be sufficient.
- Agricultural Machinery
- STOU or UTTO oils are often used, designed to simultaneously handle various functions (engine, transmissions, wet brakes, hydraulic systems).
- These products generally meet HLP/HVLP specifications equivalent to DIN or ISO, depending on manufacturer recommendations.
5. Other Standards and Special Cases
- Fire-Resistant Hydraulic Oils (HFA, HFB, HFC, HFDU)
- Defined by DIN EN ISO 12922 for use in steel mills, foundries, or mining.
- They are not strictly part of DIN 51524 but are included in ISO 6743/4 classifications.
- Biodegradable Hydraulic Oils (HEES, HETG, etc.)
- Required in areas with high environmental regulations (forests, areas near water bodies).
- These may comply with DIN 51524 and also have biodegradability certifications (e.g., VDMA 24568).
6. Criteria for Choosing the Right Oil
- Technical Standards: Always follow manufacturer recommendations (e.g., „DIN 51524-2 HLP 46“).
- Viscosity (ISO VG): Consider operating temperature, cold-start requirements, and pressure levels (ISO VG 15, 22, 32, 46, 68, etc.).
- Temperature Fluctuations: If there are extreme variations (cold/hot climates) or continuous outdoor use, choose an oil with a high VI → HVLP/HV.
- Operating Conditions:
- Highly contaminated environments → HLPD / HVLPD.
- Fire risk → Use fire-resistant hydraulic fluids.
- Environmental sensitivity → Biodegradable oils (HEES/HETG).
- Maintenance: Regularly check oil condition (contamination, water analysis), replace filters, and strictly adhere to oil change intervals.
7. Summary
- HL, HLP, HVLP (under DIN) or HL, HM, HV (under ISO/UNI) are the most common hydraulic oil categories.
- HL provides basic corrosion and oxidation protection. HLP/HM includes anti-wear additives and withstands higher pressures and thermal stress. HVLP/HV features a high viscosity index, optimizing resistance to temperature variations.
- HLPD/HVLPD add detergent and dispersing properties but are mainly used in the German market.
- In Italy, the UNI EN ISO standards adopt the same international classifications, with direct correspondences (e.g., HLP ↔ HM, HVLP ↔ HV).
For professionals working with hydraulic oils in agriculture, construction, industry, or logistics, understanding the different standards and abbreviations is essential to avoid costly mistakes and ensure the reliability and longevity of equipment. If in doubt, always consult machine manuals or seek expert advice to confirm the oil that meets the required standards.