Mixing Zinc-Free Hydraulic Oils

[MikePhua]

Introduction
Hydraulic systems are integral to various industries, powering machinery from construction equipment to manufacturing tools. The performance and longevity of these systems heavily depend on the quality and compatibility of the hydraulic fluids used. Hydraulic oils are typically classified into two categories: those containing zinc additives and those that are zinc-free. Understanding the implications of mixing these two types is crucial for maintaining system integrity and performance.
Zinc-Containing Hydraulic Oils
Zinc additives, specifically zinc dialkyldithiophosphate (ZDDP), have been traditionally used in hydraulic oils due to their excellent anti-wear properties. These additives form a protective film on metal surfaces, reducing friction and wear. However, ZDDP can be corrosive to certain metals, such as yellow metals (brass, bronze, copper), and is toxic to aquatic life, raising environmental concerns.
Zinc-Free Hydraulic Oils
Zinc-free hydraulic oils, also known as ashless or biodegradable oils, utilize alternative additives to provide anti-wear and anti-oxidation properties. These oils are designed to be more environmentally friendly and are less likely to cause corrosion in sensitive components. They are particularly favored in applications near water bodies or in industries with stringent environmental regulations.
Compatibility Issues
Mixing zinc-containing and zinc-free hydraulic oils is generally not recommended. The different additive chemistries can interact negatively, leading to several potential issues:

• Additive Neutralization: The additives in each oil can neutralize each other, diminishing the overall effectiveness of the hydraulic fluid.
  
• Deposit Formation: Incompatible additives can lead to the formation of sludge or varnish, which can clog filters and reduce system efficiency.
  
• Seal Compatibility: The interaction between different additives can affect the compatibility of seals, leading to leaks or premature seal failure.
  
• Performance Degradation: Mixing oils can alter key properties such as air release time, water separation, foaming tendency, and filterability, compromising the hydraulic system's performance.
  

Case Study: Hitachi ZX35U
A practical example involves the Hitachi ZX35U mini-excavator. The manufacturer recommends using zinc-free hydraulic oil, such as Hitachi Super Ex HN46. However, users have reported mixing this with other zinc-free oils like Phillips Powerflow NZ 46 without apparent immediate issues. Despite these observations, mixing oils from different brands or formulations is not advisable due to the potential for long-term compatibility problems.
Best Practices
To ensure the optimal performance and longevity of hydraulic systems:

• Avoid Mixing Oils: Do not mix zinc-containing and zinc-free hydraulic oils.
  
• Use Manufacturer-Recommended Fluids: Always use hydraulic fluids specified by the equipment manufacturer.
  
• Perform System Flushing: Before changing oil types, thoroughly flush the hydraulic system to remove any residual fluid.
  
• Regular Monitoring: Regularly monitor the condition of the hydraulic fluid and the performance of the system to detect any potential issues early.
  

Conclusion
While it may be tempting to mix different hydraulic oils to save costs or simplify inventory, the potential risks outweigh the benefits. Maintaining the integrity of hydraulic systems requires careful selection and management of hydraulic fluids. By adhering to manufacturer recommendations and avoiding the mixing of incompatible oils, operators can ensure the reliability and longevity of their equipment.