Hydraulic System Lubrication - Basic Technical Knowledge

1. Hydraulic System and the Role of Lubrication

Hydraulic systems use hydraulic oil to transmit power, lubricate pumps - valves - cylinders, provide cooling, and prevent corrosion.
Hydraulic oil accounts for 70 - 80% of system efficiency and durability - selecting the right oil is critical for system reliability.

2. Types of Hydraulic Pumps

Gear Pump: simple, durable, medium pressure (≤ 200 bar).

Vane Pump: medium pressure (140-180 bar), requires clean oil and stable viscosity.

Piston Pump: high efficiency, high pressure (250-350 bar, up to 420 bar), highly sensitive to oil cleanliness.

Quick Reference Table: Pump - Pressure - Viscosity - Oil Cleanliness

Table meaning:

• Higher pressure - stricter oil cleanliness and anti-wear performance.
• Viscosity selection depends on temperature - load - OEM recommendation.
• ISO 4406 code: the lower the numbers, the cleaner the oil.

3. Determining Hydraulic Oil Viscosity

Hydraulic oils are classified by ISO VG (ISO 3448).

• ISO VG 32: cold climate, high-speed systems.
• ISO VG 46: most common, suitable for general industrial use.
• ISO VG 68: hot climate, heavy load, high pressure.

Principle:

• Oil too thin - metal-to-metal wear.
• Oil too thick - poor pumpability, overheating.

4. Types of Hydraulic Oils

• Mineral Oil: widely used, cost-effective.
• Fire-resistant oils (HFDU, HFC): steel, mining, aviation applications.
• Synthetic Oils: excellent thermal & oxidation stability, long service life.
• Biodegradable Oils (Bio-hydraulic oils): eco-friendly, for agriculture and marine use.

5. Hydraulic Oil Cleanliness

Measured by ISO 4406 particle count code.

• Gear pumps: ≥ 18/16/13
• Precision piston pumps: 16/14/11
  Contamination causes 60-80% of hydraulic failures.

6. Hydraulic Oil Quality Grades & Standards

• ISO 11158: HH, HL, HM, HV, HR.
• DIN 51524:

Part 1 (HH, HL): basic lubricating oils.

Part 2 (HLP): anti-wear oils.

Part 3 (HVLP): anti-wear + viscosity index improved oils.

• Denison HF-0, HF-1, HF-2: piston & vane pump applications.
• Eaton, Bosch Rexroth approvals: specific OEM requirements.

7. Summary

Hydraulic systems require the correct oil viscosity (ISO VG 32- 68) and standards (DIN 51524, ISO 11158).
The more advanced the pump, the cleaner the oil required (ISO 4406).
Hydraulic oil not only transmits power but also lubricates, cools, and protects equipment.
Using the wrong oil = reduced pump/valve/cylinder life + higher maintenance costs.