Hydraulic Fluid Compatibility Raises Concern

April 25, 2016

An “Equipment Executive” column by Mike Vorster in our June 2015 issue, “The Case for a Fluids Czar,” suggests that managing fluids for today’s technically advanced machines might require some fleets to make that task a specialized, highly focused job handled by an experienced, knowledgeable individual. A fleet manager who read the column recently responded, agreeing with Vorster and also saying that, in his opinion, hydraulic-fluid formulations could become yet another fluid-related challenge (along with such others, he says, as diesel-fuel cleanliness and new engine oils coming this year.)

The manager essentially told us that a problem he sees coming to the forefront when considering new machines is that different machines types—even those from the same manufacturer in some instances—might be using incompatible hydraulic fluids, some formulated with zinc-based additives, others using zinc-free formulations. When he asks knowledgeable people whether these fluids can be mixed, some say yes, others say absolutely not. He’s of the opinion that “a new realm of conflicting information and confusion is upon us.”

The manager’s primary concern is to avoid doing harm to hydraulic systems by mixing incompatible fluids—but he’s also concerned about how the situation works against standardizating equipment brands and fluids in the interest of efficiency and cost savings.

“As the prospect of someone in the organization mixing incompatible hydraulic fluids and causing excess wear and heat in a system, my concerns about machine health and lubricant management rise exponentially.”

Larry Ludwig, chief chemist/technical director at Schaeffer Oil, a manufacturer of synthetic motor oils, hydraulic fluids, and diesel-fuel additives, sums up the challenge facing fleet managers dealing with the zinc-based versus zinc-free issue. “Trying to get one fluid to work across the board is difficult.”

The problem defined

That difficulty goes beyond the zinc/no-zinc issue, however, and applies to hydraulic fluids generally, says Dan Holdmeyer, industrial and coolants brand manager at Chevron. “It’s always a good idea to check compatibility of fluids when switching or topping off. Two zinc-containing fluids can be incompatible, as can two zinc-free fluids. We’ve found Chevron’s Clarity ashless, zinc-free hydraulic fluids to be compatible with specific zinc-containing hydraulic fluids—but that does not mean they are compatible with all fluids.”

Dr. Kenneth Chao, senior staff engineer, lubricants, for John Deere’s materials engineering group, is of the same opinion. “Different zinc-based fluids can be incompatible, as can different zinc-free fluids, if their chemistries are dissimilar. On the other hand, products within the same chemistry family, whether zinc-based or zinc-free, can be compatible.”

“Compatibility” generally means that two fluids can be mixed together without forming by-products harmful to the system—and also that the additive packages in mixed fluids won’t be diluted by each other’s chemistry and rendered too weak to perform their functions.

 “Compatibility depends on fluid chemistry,” says Ludwig. “You can choose an additive package from an additive supplier that has a zinc-type chemistry stabilized such that it can be mixed with a non-zinc fluid. Most fluid suppliers try to choose an additive package that is compatible across the board—but the fleet manager doesn’t know who’s using what.”

A potential consequence of incompatibility, says Ludwig, is formation of a gel-like precipitate that could plug filters and result in diminished hydraulic flow.

Nicolas Huber, senior materials engineer in John Deere’s Germany-based materials engineering group, raises yet another consideration. “We must also distinguish between the issue of fluid compatibility and the issue of whether a particular fluid is acceptable for use in a particular system or application. These are two different shoes. In some instances, [the selection of fluid] is a question of how robust are the components in a system—and how do these components interact with each other.”

Japanese equipment manufacturers generally favor zinc-free formulations, says Chao, both from a concern about the environment (zinc is a heavy metal linked to  aquatic toxicity) and also, reinforcing Huber’s point about fluid suitability, from a concern that zinc-based formulations might promote yellow-metal (brasss, copper, bronze) corrosion in certain of their systems.

“Some [hydraulic-system] designs are more prone to yellow-metal corrosion than other designs,” says Chao. “Yellow metal can act as a catalyst and be reactive. It’s a design-related issue.”

Schaeffer Oil’s Ludwig surmises that the influx of Japanese equipment here has raised awareness of the zinc-based/zinc-free issue, but he reminds machine owners that environmentally friendly, biodegradable hydraulic fluid has always been zinc-free.

Basic chemistry/system design

The excellent anti-wear and oxidation-inhibiting qualities of zinc dialkyl dithio phosphates, or ZDDPs, have long made these compounds staple additives in hydraulic fluid. According to chemists, three basic ZDDP compounds can be used in hydraulic fluid, each having a particular attribute that contributes to the fluid’s overall character, whether wear-resistance, thermal stability, or hydrolytic stability (resistance to chemical decomposition in the presence of water.)

If ZDDPs are not used, a common replacement is the more environmentally friendly tricresyl phosphate (TCP).

Mobile hydraulic systems generally have become increasingly “power-dense”—to use the term applied to systems designed to pull more power from smaller systems. In some measure, the necessity to control exhaust emissions has hastened this move to power-dense hydraulic systems; bulky exhaust after-treatment hardware often takes up valuable under-hood space, and machine manufacturers have called on hydraulic-component suppliers to conserve space.

And they have. In many instances, hydraulic pumps and motors are smaller, but running at much higher pressures (theoretically, doubling pressure allows displacement to be halved). Valve packages and cylinders can be smaller, as can fluid reservoirs.

Smaller tanks, however, mean that a reduced fluid volume must work harder by circulating more rapidly, reducing the fluid’s dwell time in the reservoir for cooling, for allowing water and contaminants to settle, and for entrained air to dissipate. The consequence is that systems can run hotter and be prone to viscosity loss and oxidation, which degrades fluid and produces sludge.

A technical paper published by the Society of Automotive Engineers (SAE), “The Development of Zinc-Free Anti-Wear Hydraulic Oil: Doubling Useful Service Life” (number 982000, dated 9/14/98), states that the development of “increasingly compact hydraulic systems utilizing high service-pressure pumps (> 300 kg/cm2) (4,200 psi) has lead to operating temperatures (> 250C) (482F) that have been shown to thermally degrade ZDDP into insoluble sludge.” Cited as the source for this observation are Y. Matsuyama and M. Takesue, Idemitsu TriBio Review (No.22-1996) pp. 1345-6.

The paper “chronicles the development of a non-ZDDP AW [anti-wear] hydraulic oil, and evaluates its performance relative to a ZDDP oil,” concluding that the zinc-free formulation “reduces sludge by 85 to 99.5 percent, retains more viscosity, reduces metal wear by 04-06 percent, and extends useful service life to at least twice that of ZDDP-based hydraulic oil.”

Other than saying that the zinc-free fluid employed a phosphorous/sulfur anti-wear agent and a “phenyl amine” oxidation inhibitor, and that it was made from a hydro-cracked base stock (considered a “synthetic” in some areas of the world), investigators were tight-lipped about the exact formulation. Although no one is questioning the paper’s conclusions, one might assume from the results that zinc-free formulations are the only choice for today’s power-dense hydraulic systems and are better in all ways than those with zinc-based additives.

Huber takes issue with any such general assumption. “Some will say that zinc-free is the future and zinc-based is the past. That’s not true; for the next decade, zinc-based and zinc-free technologies will exist side-by-side. The question is always, ‘what fluid best fits the application.’”

Huber and his colleague Chao say that zinc-free formulations should be considered as another tool in the hydraulic-system designer’s toolbox. “With available new zinc-free technology,” says Huber, “designers have added tools to address issues related to specific applications.”

Huber makes a further observation. “Some assume that the service life of a zinc-free formulation is longer by virtue of its being zinc free. The reality is, however, that fluids with longer service lives are those using more highly refined or synthetic base stocks, which many zinc-free formulations employ. Longer life does not come from zinc-free additives, but from the base oil.”

Fluid-choice considerations

“I can understand the dilemma when an equipment owner has a mixed fleet, with one manufacturer calling for one fluid, and another manufacturer calling for a different fluid,” says Huber. “The best practice, of course, is to follow each manufacturer’s recommendation, but if reducing the number of fluids is the goal, then to avoid fluid incompatibility, the system should not only be drained, but also flushed—in some instances, several times—to minimize the concentration of old fluid.”

Schaeffer Oil’s Ludwig adds that those thinking about switching fluids must be cautious to ensure that the particular system’s viscosity requirements are met.

Jeffrey Bauer, staff engineer, hydraulic product verification, John Deere Construction & Forestry, adds still another caution. “For those fleet owners thinking about switching fluids, the durability of the fluid is an important consideration. No matter whether zinc-free or zinc-based, the key is how long the substitute fluid’s anti-wear and anti-oxidation protection will last in a particular system.”

Jonathan Miller, product line marketing manager, John Deere Merchandise, says that the machine manufacturer knows the answer to the additive-longevity question for the machine’s factory-fill, because that fluid has been selected and tested by the manufacturer for that particular machine’s hydraulic-system design and application. The recommended drain interval, says Miller, is predicated on testing of the chosen fluid.

Also important to keep in mind is that hydraulic-fluid formulations vary widely in their base-stock quality and specific additive content. Fluids can range from those containing only additives that inhibit corrosion and oxidation, but with no specific anti-wear additives, to those that contain anti-wear additives, detergents, water and air dispersants, and viscosity-index improvers that maintain viscosity over wide temperature ranges. Choices must conform to the equipment manufacturer’s requirements.

Questions remain

One might question what trade-offs might be required when a system designer specifies a zinc-based fluid for a particular machine, only to have the machine ordered with a manufacturer-sanctioned biodegradable fluid that is zinc-free. The manufacturer obviously is confident that the additive package in either fluid renders adequate protection.

In the interest of standardization, could the switch be made the other way, substituting a zinc-based fluid for zinc-free? One equipment manufacturer seems to imply that possibility, stating “a system flush of factory-fill zinc-free fluid is required before use of any hydraulic fluid containing zinc anti-wear additives.” Can one assume that the manufacturer has considered the implications for yellow-metal protection in certain systems if such a switch is made? Would recommended drain intervals remain the same?

Chemists say that specific zinc-containing fluids can be perfectly compatible with specific zinc-free formulations, creating no harmful by-products and not interfering with each other’s additive package. In the interest of simplifying fluid logistics for fleets with a predominant population of zinc-based systems, could an appropriate zinc-based fluid be used to top off all systems in a mixed fleet?

Or, considering the equipment investment that’s on the line in any size fleet, is it better to use the fluid specified by the manufacturer for each particular machine—even though in large fleets the lube-truck diver might need a separate laptop to keep things sorted out?

No definitive answers seem to be forthcoming for equipment owners attempting to bring some measure of standardization to their fluids programs. We leave you with the perspective of the fleet manager responding to the Fluid Czar column.

“The issue about zinc-based versus zinc-free hydraulic fluid is a growing conundrum. In order to make a well-informed choice, a fleet manager may actually have no choice—except to take the word of the OEM [Original Equipment Manufacturer] that everything’s going to be all right if you use their lubricant products. Is it possible, due to the technical advancements of products and the lubricants that have been engineered especially for them, that OEMs have literally closed the door on end-users with respect to providing alternative lubricant specifications for specialty items?”

Has the Fluids Czar arrived yet?