Hydraulic oils gradually degrade over time due to oxidation, thermal stress, and contamination, all of which can negatively impact system performance and shorten equipment life. One of the most reliable ways to evaluate a hydraulic oil’s resistance to oxidation which thereby estimate its service life through the TOST (Total Oxidation Stability Test, ASTM D943).
Hydraulic oil is often considered the lifeblood of industrial and mobile equipment, from factory machinery to construction vehicles. A key factor in determining its reliability and durability is oxidation stability. This is commonly assessed using the TOST (Total Oxidation Stability Test), which provides valuable insight into how long a hydraulic fluid can retain its protective qualities when exposed to oxidative and thermal stress.
What is TOST Life?
TOST Life refers to the duration a hydraulic oil can resist oxidation under accelerated conditions, as measured by the TOST (Total Oxidation Stability Test), standardized by ASTM D943. This laboratory test simulates the aging process of hydraulic oil by exposing it to heat, oxygen, water, and metal catalysts (typically iron and copper).
During the test, the oil reacts with oxygen, gradually becoming acidic. The test continues until the oil’s Total Acid Number (TAN) increases by a defined amount, typically 2.0 mg KOH/g, which indicates that oxidation failure has occurred.
- TOST life is expressed in hours (e.g., 5,000 hours). A higher number means better resistance to oxidation.
- Longer TOST life translates to extended oil service intervals, reduced maintenance costs, and enhanced protection for critical hydraulic components.
Why Oxidation Stability is Crucial
Hydraulic oils operate under constant stress, exposed to heat, pressure, and environmental contaminants. Over time, these factors lead to oxidation, which can seriously compromise system performance. Oxidation causes:
- Acid formation, which corrodes internal components
- Sludge and varnish buildup, clogging valves and restricting fluid flow
- Increased viscosity, making it harder for the oil to flow efficiently
- Reduced equipment life, due to poor lubrication and increased wear
That’s why oxidation stability matters. A hydraulic oil with a high TOST life is far more capable of withstanding high temperatures and extended operating hours, making it ideal for demanding or long-life applications.
Why TOST Life Matters in Hydraulic Systems
- Predicts Oil Service Life
- A higher TOST life means the oil can perform effectively for a longer period before needing replacement.
- This helps in planning maintenance schedules more accurately and minimizing unplanned downtime.
- Indicates Oxidation Resistance
- Oxidation can lead to the formation of sludge, varnish, and acids, damaging seals, pumps, and valves.
- Oils with low TOST life tend to degrade faster, especially under high-temperature conditions.
- Impacts System Reliability
- As oil degrades, it increases component wear, reduces overall system efficiency, and may even lead to failures.
- This is particularly critical in high-pressure systems like those found in construction equipment and industrial machinery.
- Differentiates Oil Quality
- Premium hydraulic oils often have TOST lives exceeding 5,000 hours, offering long-lasting protection.
- In contrast, budget oils may start breaking down much earlier, sometimes in less than 2,000 hours.
How TOST Testing Works
The TOST (Total Oxidation Stability Test) is designed to replicate real-world oxidative stress in a controlled lab environment. In this test, a sample of hydraulic oil is heated to 95°C and exposed to oxygen, distilled water, and metal catalysts, typically iron and copper coils.
The process continues until the oil reaches a specific level of acidity, typically 2.0 mg KOH/g, which indicates oxidation failure.
Key parameters of the TOST test:
- Temperature: Maintained at 95°C
- Catalysts: Iron and copper metals are used to accelerate oxidation process.
- Oxygen: Introduced in a controlled flow to simulate operational conditions.
- Water Content: Added to reflect contamination common in field use
- Endpoint: Test concludes when the Total Acid Number (TAN) reaches the defined limit, indicating a significant level of oxidation.
This test helps determine how well a hydraulic oil can withstand prolonged exposure to oxidative stress, making it a crucial indicator of long-term performance and durability.
Typical TOST life values:
| Oil Type | TOST Life (Hours) |
| Mineral Hydraulic Oil | 1,000 – 2,000 |
| Premium Group II Oil | 3,000 – 5,000 |
| Synthetic Hydraulic Fluids | 5,000 – 10,000+ |
Factors Influencing TOST Life
Several key factors impact the oxidation stability, and therefore the TOST life of hydraulic oil:
- Base Oil Quality: Oils made from higher purity base stocks like Group II, Group III, or synthetic oils naturally offer better resistance to oxidation.
- Antioxidant Additives: These chemical additives are specifically formulated to slow down the oxidation process, extending the oil’s useful life.
- Contamination Control: The presence of water, air, or metal particles in the system accelerates oxidation and degrades oil performance.
- Operating Conditions: High temperatures and pressures put extra stress on the oil, speeding up the oxidation process and reducing service life.
- System Design: Poor filtration, inadequate reservoir sizing, or excessive aeration can lead to quicker oil degradation and shorter TOST life.
Practical Implications of TOST Life
A longer TOST life isn’t just a technical benchmark; it brings real-world benefits that can significantly impact your operations:
- Extended oil change intervals, reducing service frequency
- Reduced downtime and maintenance, keeping systems running smoothly
- Improved reliability of hydraulic equipment under demanding conditions
- Lower total cost of ownership (TCO) through longer fluid life and fewer failures
That said, even when using high-TOST-life fluids, it’s important to regularly monitor oil condition. Real-world operating environments often introduce variables that may not be reflected in lab testing, such as unexpected contamination, temperature spikes, or system design limitations
Extending Hydraulic Oil Life
To get the most out of your hydraulic oil and ensure optimal system performance, consider the following best practices:
- Use high-quality oils, such as synthetics or Group II/III base oils, which offer superior oxidation stability and longer TOST life.
- Control operating temperatures, keeping systems below 60°C helps slow down oxidation and extend oil life.
- Implement effective filtration to remove contaminants like particles and water, which accelerate oil degradation.
- Monitor the Total Acid Number (TAN) regularly. A rising TAN is a clear sign of oxidation and oil breakdown.
- Follow OEM-recommended oil change intervals, especially those aligned with TOST data and field conditions.
Therefore, TOST life serves as a key indicator of a hydraulic oil’s long-term oxidation stability. Selecting a fluid with a high TOST life, particularly for demanding or high-temperature applications, can greatly improve the performance, reliability, and service life of your hydraulic system.
However, even the best oil won’t perform at its peak without proper maintenance. For truly optimal results, high-quality fluid selection should always be paired with regular monitoring and sound maintenance practices.
For expert advice and high-quality products, don’t hesitate to visit Palco’s Official Website (http://www.palco.co.in) or contact directly at the Email: enquiry@palco.co.in & Phone: +91 93117 94519.
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