Hydraulic Oil Maintenance Guide — Field Practice

Oil is the Blood of the System

In a hydraulic system, oil is not merely a fluid — it carries pressure, distributes heat, provides sealing and protects metal surfaces. If it cannot perform these four duties, failure is inevitable. In our field experience, the root of 70% of hydraulic failures lies in contamination, heat or viscosity errors.

1) Knowing the Oil: Specification

Every hydraulic oil has three basic identities:

• Viscosity grade (e.g. ISO VG 32, 46, 68): selected according to operating temperature
• Type (HLP, HVLP, HFC, HFDU): toxicity, fire resistance, food compatibility
• Additive package: anti-wear, oxidation inhibitor, demulsifier

Failing to comply with the grade stated in the system manual reduces warranty cover and degrades performance.

2) Preventing Contamination

Even freshly purchased oil is usually contaminated at the ISO 4406 class 20/18/15 level. Servo valves, by contrast, require the 17/15/12 level. In other words, flushing or cartridge filtration may be mandatory before the oil is poured into the system.

Sources of contamination:

• Open filling (dust from the atmosphere)
• Worn seals and leakage
• Metallic particles left behind by a worn component
• Water mixing into the oil (condensation, maintenance error)

Countermeasures: closed transfer pump, magnetic filter, breather filter, regular sampling.

3) Temperature Control

The ideal operating temperature is 40–55 °C. Above 60 °C, every 10 °C halves the oil life (the Arrhenius rule). If the temperature is rising:

• Check the cooler capacity
• Measure internal pump leakage (efficiency may have dropped)
• Check the ventilation of the surrounding environment

4) Oil Analysis: At Least Twice a Year

Six parameters to review in a sample analysis:

1. Viscosity (40 °C): a candidate for replacement if it exceeds the ±10% limit
2. Particle count (ISO 4406): flushing if it has degraded from the target class
3. Water content (ppm): above 200 ppm is critical
4. TAN (Total Acid Number): indicator of acidification
5. Metallic elements (Fe, Cu, Al): wear source analysis
6. Colour + odour: quick field check

Changing oil blindly, without an analysis report, is both a cost and environmental waste.

5) Change or Flushing?

• If viscosity and TAN are within the normal range: flushing is sufficient (the oil is not discarded, particles are removed)
• If viscosity has changed by more than 10% or TAN is above the critical threshold: a full change
• If water content is high: analysis after vacuum dehydration, then a decision

For details of our flushing service, see our oil filtration service.

6) Practical Field Rules

• Do not store oil in a plastic drum for long periods (UV + moisture)
• When changing a filter, open and inspect the old filter — metallic particles mean there is a root cause
• Colour change, foaming or abnormal odour in the system = take a sample immediately
• A system without an annual analysis schedule is a system open to surprise failures

Remember

Oil is the cheapest part of the system but the most expensive cause of failure. The right oil + the right maintenance = a long-lived, efficient, planned system. When in doubt, send the sample — the analysis report states the condition of the field clearly.