A Simple Guide to Hydraulic Fluid Selection

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Selecting the appropriate hydraulic fluid can sometimes be overwhelming due to the numerous available options. Here we will simplify the process by discussing a framework for understanding the factors behind selecting the ideal hydraulic oil for your needs. We will focus on viscosity and zinc content as primary performance dimensions and discuss how these properties affect hydraulic oil performance in real-world applications.

Important Performance Dimensions

Viscosity is an essential factor to consider when selecting hydraulic oil. The viscosity index (VI) generally ranges from 0 to 250, but for simplicity, we will classify hydraulic oils as standard VI and high VI products. To achieve a high VI, either the base oil must be replaced, such as moving from group two to group three or from group one to group three, or more viscosity index modifiers must be added. Unfortunately, both options increase the cost of hydraulic oil.

On the other hand, we can group hydraulic oils based on their zinc content as either zinc-based or zinc-free. Zinc dialkyldithiophosphate (ZDDP) is a widely-used, cost-effective anti-wear additive in zinc-based hydraulic oils. When choosing a zinc-free oil with comparable performance, alternative additives are required, typically at a higher cost. It’s important to note that while zinc is not toxic, the phosphorus and sulphur in ZDDP are harmful to the environment and aquatic life.

A simple matrix to inform hydraulic oil selection. This is by no means comprehensive, but designed to be used as a starting point.

As you move up and to the right on the matrix, considering both viscosity and zinc content, the cost of the hydraulic oil increases. Balancing these factors will help you decide when to select the appropriate hydraulic fluid for your specific needs.

Real-World Implications of Viscosity and Zinc Content

In practical applications, viscosity and zinc content play crucial roles in hydraulic oil performance. The viscosity index affects the temperature range where a hydraulic system can safely operate and directly influences hydraulic efficiency. Higher VI oils allow for better efficiency, which translates into energy savings and the ability to work across a broader temperature range.

Zinc content, on the other hand, is related to anti-wear and keep-clean performance. Zinc-based hydraulic oils have higher anti-wear properties but may increase sludge formation as the anti-wear additives break down. In contrast, zinc-free hydraulic oils help keep the system cleaner because they lower the propensity for sludge formation.

Selecting the right hydraulic oil for your scenario involves considering the viscosity index and zinc content balance. In addition, factors like temperature ranges, efficiency requirements, environmental impact, and equipment specifications can all influence your decision. For example, standard VI zinc-based products might suit systems with mild temperatures and large reservoirs. In contrast, high VI zinc-free products could be more appropriate for mobile equipment with tighter tolerances and higher power density.

Selecting Hydraulic Oil Based on Specific Scenarios

The selection of hydraulic oil depends on various factors, including the system’s specific needs and operational conditions. Here are some examples of scenarios that can help guide your choice:

Performance requirements of hydraulic oils.
  1. Zinc-based standard VI hydraulic oils: These are the most affordable options and are suitable for high-loss systems with leaks, large reservoirs relative to the hydraulic pack size, and operations in mild temperature conditions. Investing in more expensive hydraulic oils may yield little performance benefit.
  2. High VI zinc-based hydraulic oils: These products are standard in industrial hydraulic applications and offer a more comprehensive temperature range, making them suitable for operations with varying ambient conditions. They are also beneficial for those seeking improved efficiency. However, larger reservoirs and stationary equipment often make zinc-free solutions unnecessary in typical industrial settings.
  3. Standard VI zinc-free hydraulic oils are ideal for situations with low toxicity requirements, such as operations near water sources or environments where environmental regulations demand reduced toxicity. While zinc is not toxic, the phosphorus and sulphur in ZDDP can harm the environment and aquatic life. Therefore, in certain circumstances, environmentally friendly lubricants may contain zinc but not ZDDP.
  4. Zinc-free High VI hydraulic oils: These top-of-the-line products are perfect for applications requiring low toxicity, a wide temperature range, and excellent keep-clean performance. They are well-suited for systems with close-tolerance servo valves or proportional-integral (PI) controllers. Mobile equipment, which often has smaller hydraulic packages and higher power density, can also benefit from these high-quality oils, as they help prevent thermal and oxidative breakdown.

In addition to these scenarios, environmentally aware lubricants (EALs) and synthetic hydraulic oils may be considered for ecologically sensitive applications or extreme operating conditions. By understanding the specific requirements of your hydraulic system and considering factors such as temperature, efficiency, environmental impact, and equipment specifications, you can make an informed decision about the most suitable hydraulic oil.

Additional Hydraulic Oil Categories to Consider

Apart from the primary categories discussed earlier, two more hydraulic oil categories are worth considering based on specific applications and requirements.

  1. Environmentally Aware Lubricants (EALs): These lubricants are designed for environmentally sensitive applications and adhere to strict regulations and guidelines. EALs are suitable for situations with a high risk of environmental contamination or when operating in protected areas. While a detailed discussion of EAL specifications is beyond the scope of this article, it’s essential to know that these lubricants play a crucial role in minimising the ecological impact of hydraulic systems.
  2. Synthetic Hydraulic Oils: Although rare, synthetic hydraulic oils can offer unique performance advantages in extreme operating conditions. High VI zinc-free hydraulic oils perform exceptionally well in most cases, but synthetic oils can further improve performance in challenging environments. For instance, synthetic hydraulic oils have been used in wind turbines in Canada, where temperatures can drop as low as -40 or -45 degrees Celsius. In such conditions, synthetic oils can maintain viscosity and perform better than high VI zinc-free hydraulic oils.

By evaluating the specific needs of your hydraulic system and considering factors like environmental regulations, extreme operating conditions, and equipment requirements, you can select the most suitable hydraulic oil from these additional categories. Remember, the right choice of hydraulic oil can significantly impact your hydraulic system’s efficiency, longevity, and overall performance.

Taking Performance Dimensions, International Standards, and OEM Specifications into Account

When selecting hydraulic oil, it’s crucial to consider various performance dimensions to ensure the optimal operation of your hydraulic system components, such as pumps, filters, valves, actuators, and reservoirs. Many standardised tests exist to evaluate hydraulic oil performance, and while these tests may not fully replicate real-world scenarios, they provide a valuable benchmark for comparison.

Basic list of international hydraulic oil standards.

International standards, such as DIN (Deutsches Institut für Normung) and ISO (International Organization for Standardization), offer definitions and specifications for different types of hydraulic oils, including R&O (Rust and Oxidation Inhibited) hydraulic oils, anti-wear hydraulic oils, and multi-grade hydraulic oils. Adhering to these standards can be beneficial, especially if your organisation is required to comply with specific international regulations.

Original Equipment Manufacturer (OEM) specifications often build upon international standards, incorporating additional specialised tests tailored to their equipment. Common OEM standards include Denison, Vickers, Bosch Rexroth, and Cincinnati. If your hydraulic system relies on components from a specific manufacturer, it’s essential to ensure the chosen hydraulic oil conforms to its OEM criteria.

Comparison of common OEM tests. Up and right indicates increasing severity of test.

Comparing OEM test parameters can help you understand how various hydraulic oils perform under different pressures and speeds. For example, the Bosch Rexroth test is considered the most stringent OEM test, with the highest pressure and speed requirements. Hydraulic oils that pass this test demonstrate exceptional performance and durability, making them the gold standard for hydraulic applications.

By considering performance dimensions, international standards, and OEM specifications, you can make a more informed decision when selecting hydraulic oil for your application. This comprehensive approach will help optimise your hydraulic system’s performance, efficiency, and longevity.