Are all mineral-based hydraulic oils the same?

Nov. 21, 2007
These tips will help you choose the right hydraulic oil for your application.

By Rob Profilet Lubrizol Corp.

In this month’s MRO feature, Rob Profilet from Lubrizol Corp. answers pertinent questions about the differences in hydraulic oils and how choosing the right one results in peak operation. Profilet is commercial manager of hydraulics and gear oils and works at Lubrizol’s headquarters in Wickliffe, Ohio.

Why is it difficult to choose the right fluid? Approximately 200 million gal of hydraulic oils are used in the U.S. annually. About 165 million gal of that volume are mineral based. The end user has to sort through a multitude of suppliers, brands, and performance levels to determine which fluid is best for his operation.

A fluid supplier will help the end user by providing a product data sheet for the hydraulic oil, which will typically list its physical and performance properties and any original equipment manufacturer (OEM) approvals or specifications it meets. But even with performance data and pricing in hand, it can still be difficult to differentiate hydraulic oils and choose the fluid that provides the most value.

What performance properties are important?
To appreciate the differences in hydraulic oils, it helps to understand what performance properties are important. Hydraulic oil needs to last a long time (meaning extended oxidation life) and protect equipment by providing anti-wear properties while preventing rust and corrosion.

Most hydraulic oils are designed to have good demulsibility properties. This means they separate water quickly so it can be drained from the sump, which is important because water doesn’t lubricate as well as oil. Also, hydraulic oils should resist varnish formation, which can result in sticking servovalves. The fluid’s ability to release air quickly is important so pumps are protected from cavitation, where air in the oil is released and then redissolved, generating heat that can damage pumps.

What goes into hydraulic oil and how can formulations vary?
A typical non-viscosity modified hydraulic oil will be formulated with 99% base oil and 1% additive. There are a number of high-quality base oils available today. There also are some lower quality, low-cost base oils that find their way into hydraulic oil formulations. These base oils can cause significant performance problems, including emulsification of water, which can increase pump wear, corrosion, and poor filtration. Fluids formulated with poor base oils also may have a shorter effective life, resulting in more frequent oil drains. It is important for end users to ask their lubricant suppliers about the base oil in their fluid and its impact on performance properties such as oxidation life, water separation, filtration, and anti-wear. Ask if the base oil is from a consistent source. Ask if the properties of the base oil vary from batch to batch, potentially influencing the finished hydraulic fluid.

Although hydraulic oil will typically have less than 1% additive, the composition of that additive is critical to protecting the hydraulic system.

How do you protect against wear and maintain thermal stability?
The heart of the additive system is the anti-wear component. Zinc-containing and zinc-free formulations both exist in the marketplace, and both provide anti-wear protection. Most fluids will contain zinc chemistry, generally in the form of zinc dithiophosphate, for anti-wear. It can be tempting to compare fluids based on zinc content alone, simply choosing the fluid with more zinc. Although zinc helps protect against wear, too much or not the right balance of zinc can result in corrosion of yellow metals.

There also are different types of zinc anti-wear molecules. Some are derived from secondary alcohols and are less thermally stable than those derived from primary alcohols. Often, a supplier can provide pump test data and thermal stability testing to validate the anti-wear performance and thermal stability of a fluid.

What about rust, corrosion, and filtration protection?
Another key component in protecting equipment is selecting the right balance of corrosion inhibitors. The fluid supplier can easily demonstrate this with ASTM test method D665A or the more severe D665B test for steel and D130 for copper. Some fluids may contain corrosion inhibitors that can lead to filtration problems because they react with potential contaminants. Filtration test data will generally be available for high-quality fluids to validate performance. Filtration tests can be performed with or without water to ensure there is not a dramatic decrease in filtration performance when the hydraulic fluid is contaminated with water.

Can you protect against oxidation and formation of varnish?
Hydraulic fluids contain antioxidants to help prevent the oxidation of hydrocarbon molecules. Oxidation rates for hydraulic oil can double for every 10° C increase in operating temperature. Hydraulic oils will still operate at temperatures above 180° F; however, the oxidation rate will have accelerated, decreasing the life of the oil. As hydraulic oil becomes oxidized and thermally degraded, varnish particles may form. Over time, varnish will deposit on servovalves, causing them to stick. Varnish is sticky and attracts other wear particles in the system, forming an abrasive residue that can result in more wear. The soft deposit eventually becomes a hard lacquer, which is difficult to remove. Fluid suppliers generally measure the oxidation performance of their hydraulic oil based on the ASTM D943 test.

How durable is the fluid?
Hydraulic oils are carefully formulated to provide the right balance of all performance requirements. Formulators will combine the different additives in the laboratory with a chosen base oil and perform tests on the new, unused fluid. To determine durability and performance retention, tests are conducted on used fluids after extended use. This helps to ensure that even as oil ages, acceptable performance is maintained in your equipment.

What role does cost play?
When choosing a fluid, be sure to consider the performance factors previously discussed. Some fluids may cost more per gallon but provide a much greater benefit than those costing less. In a machine with a 100-gal sump, which fluid would be more economical?
• Fluid A is $5/gal, lasts three months, and causes servovalves to stick.
• Fluid B is $8/gal, lasts 12 months, and doesn’t make servovalves stick.

In your selection process, be sure not only to consider the purchase price but also fluid longevity, protection of the equipment, and potential downtime plus labor in case of failures.

What OEM approvals are important?
It always is important to understand if your equipment manufacturer designates a certain type of oil for use in your equipment. OEMs have carefully developed specifications and approval processes for lubricant manufacturers to follow to have their fluid approved. Be sure whatever fluid you select fits your equipment manufacturer’s guidelines.

For more information, e-mail [email protected] or call (440) 943-4200.

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