Can you mix different brands of hydraulic fluid?

In the majority of cases mixing different kinds of hydraulic fluid is not advised regardless of whether their viscosity grades and the base oil type seem to be compatible. Different manufacturers have their own additives -- anti-wear compounds, anti-foam substances corrosion inhibitors, anti-foam compounds, and detergents, which aren't always chemically compatible with each other. Mixing brands could trigger foaming, additive dropout in sludge, foaming or reduced lubricity even if both are labeled with the identical ISO quality of viscosity (such as ISO 46 or ISO 68. ISO 46 or ISO 68). If a brand change is not possible, the most secure option is to perform a complete system flush, not an over-off, and confirmation from both companies of the information sheets on technical specifications that both base stock and additives are compatible.

The brand of hydraulic fluid is more important than most people would think

It's tempting, however, to view liquid hydraulics as something that's a good deal. So long as the viscosity on the jug is in line with that number, it's going to be fine, you think? Not quite. Two liquids may share the same ISO viscosity grade but behave differently when mixed, as viscosity grade refers to the fluid's characteristics at a certain temperature. It doesn't provide any information regarding the additive package beneath.

Hydraulic fluids are made up of the base oil (mineral, synthetic, or bio-based) and a mixture of additives that are specifically designed to function together. They typically comprise:

• Anti-wear (AW) additives—usually zinc dialkyldithiophosphate (ZDDP)—that form a protective film on metal surfaces under boundary lubrication conditions
• Anti-foams that block air entrainment within the reservoir
• The rust inhibitors and corrosion inhibitors safeguard internal components during shutdown times
• Oxidation inhibitors that reduce the process of thermal breakdown and prolong the service life
• Dispersants and detergents, which keep contaminants suspended instead of settling into sludge

Every manufacturer develops its additive package in order to function as a unit. If you add a different brand's additive package to the mix, both chemicals aren't guaranteed to coexist. This is why cross-brand mixing is risky even though the specifications are similar on paper.

What can go wrong when you mix brands?

Sludge and dropouts from the addition of water

Certain additives are surfactants—chemicals that are made to remain in solution and then migrate to metallic surfaces or walls of reservoirs. If two different surfactant packs come into contact, they will be neutralized or even disappear completely from the solution. This is known as additive dropout. It usually manifests as sticky residue that is found in the middle of the reservoir or an appearance that is hazy within the fluid. When additives are removed, the fluid ceases to have its protective properties that it was created to provide and can be able to flow and look like normal oil.

Air entrainment and foaming

Anti-foam products from various suppliers are able to work against one and can be in conflict with each other. Instead of suppressing bubbles, incompatible defoamers could cause excessive foaming of the reservoir, leading to air entrainment within the pump's intake. Aerated fluid expands instead of transferring force efficiently which results in spongy and irregular actuator movement, and increased cavitation damage to the pump.

Reducing anti-wear performance

If the anti-wear film made by two different additive packages are competing for the same metal surface, neither can make a full, continuous protective layer. This results in an entire system that appears filled with fluid; however, it is operating quietly with worn-out wear protection, which is a fault that is usually not noticed until a valve or pump displays the signs of premature scoring.

Accelerated oxygenation

Certain additive combinations may cause oxidation, rather than preventing it, which can reduce the service life of the fluid. This is evident in the form of darkening of the fluid, a burning smell, and rising acid levels on analysis of fluid reports, which is well ahead of the normal interval for change.

Mixing brands has less risk?

There are many cross-brand mixes that are not the perfect recipe for disaster; however, the factors that make the mix "safer" are narrow:

• Similar kind of base oil. Mixing two mineral oil-based fluids poses a lower risk than mixing synthetic and mineral or bio-based (e.g., vegetable esters) fluids, which may be chemically incompatible but fundamentally different.
• Similar additive technology. Certain manufacturers license or utilize similar additive chemistry (for instance, certain zinc-based AW packages are used extensively across different brands). Technical data sheets may reveal compatibility with other brands' formulations.
• Small emergency top-offs, and then monitoring. A one-time top-up of a modest amount and followed by an analysis of the fluid within a brief period has a lower risk than a regular practice of switching brands every time you service.

Even in the less risky situations, it's still a compromise instead of the best method. One way to determine the truth is to ask for documents on compatibility for both companies or let a lubricant provider conduct a bench test for compatibility prior to committing.

How do you handle an unintended brand change the right way?

Step 1: Drain and flush; do not top off

If you're switching suppliers for the long term, it is recommended to do a complete system drain, followed by a flushing cycle with your new liquid (or the flushing fluid that is compatible) to get rid of any remaining additive film from the internal surfaces and hoses, as well as the walls of the reservoir. By draining the reservoir and refilling it, it often results in 10-15 percent of the original fluid in the lower points and cylinders and hose runs, which is enough remaining volume to trigger interactions between additives.

Step 2: Replace or cleanse the filter system

Filter elements from the past may hold the residual fluid as well as any dropout of additives from the prior fluid. Change filters during the fluid changeover, rather than reuse them.

Step 3: Run a short-interval fluid analysis.

After changing brands, plan an oil analysis with a shorter time frame -- typically between 250 and 500 hours following the changeto identify indications of early signs of incompatibility such as a rise in particulate count or a sudden change in viscosity or changes in acid number.

Step 4: Record the switch.

Keep track of the date, fluid type, batch/lot number, and the reason behind the change in the maintenance documents. If a component malfunction occurs in the future, it is much easier for the engineer who is a lubricant to determine whether the change in fluid caused the problem.

Step 5: If you are unsure, call the manufacturer.

Major hydraulic fluid producers offer technical support services specifically designed to address compatibility concerns. Supplying both product data sheets (old as well as the latest) is typically enough to allow their labs to provide a compatibility assessment or, in some cases, referring to prior tests of compatibility they've run against rival products.

Matching the grade of viscosity is not the same thing as chemical compatibility. Different additives can be in contact with each other, causing degradation of the anti-wear protection, encouraging foaming, or accelerating the process of oxidation. These issues usually go unnoticed until the component fails. If a brand-new switch is required, think of it as a complete fluid swap with a proper flush, not just a top-off. You should also use an analysis of the fluid to ensure that the new fluid's performance is exactly as you would expect.

Can you safely mix 2 hydraulic fluids of identical ISO quality of viscosity?

Not necessarily. A match with the ISO viscosity class only indicates the same flow characteristics at a specific temperature, but it doesn't assure that the packages of additives of different manufacturers can be chemically compatible.

How do I deal with it if I mix hydraulic fluids that are not compatible?

The system is likely to keep operating in the short term. However, you might notice indications of fogging or cloudy fluid or sludge accumulation in the reservoir or an increase in wear on valves and pumps as time passes.

Do I have the ability to mix mineral-based and synthetic hydraulic fluid?

This type of mix is more risky as compared to mixing 2 mineral oils since mineral and synthetic base stocks may have distinct solvency characteristics that impact sealing compatibility and stability of additives. It is highly recommended to flush prior to changing base oil types.

How can I tell whether my hydraulic fluid is added or dropout?

Common signs include a blurred or cloudy appearance and visible sludge or sediment in the bottom of the reservoir and abnormal results in the report on fluid analysis in particular for additive metals and particle counts.

Do all brands of hydraulic fluid utilize the same additive chemical?

No. Additive packaging is a proprietary formulation as well as fluids that meet the same specifications of industry (such like DIN 51524 and ISO 11158) can use various anti-wear, anti-foam, and corrosion-inhibiting chemicals.