Maintenance: Hydraulic fluid selection

It all starts with your operator’s manual. There, manufacturers list the hydraulic fluid spec required for a specific model, either by SAE viscosity grade for engine oils or ISO viscosity grade for hydraulic fluids. Manufacturers specify a variety of oil types for use in their hydraulic systems, including hydraulic fluids, engine oils, tractor hydraulic fluids and transmission drive train oils.

For example, Case Construction Equipment specs an SAE 10W-30 engine oil for its 450 skid steer. Volvo Construction Equipment, on the other hand, specifies an ISO 15 or an ISO 46 for its motor graders, depending on the ambient temperature.

The SAE grades of engine oils and the ISO grades of hydraulic fluids indicate an oil’s viscosity. For example, an SAE 20 engine oil has a similar viscosity as a ISO 46 or 68 hydraulic fluid (see chart on page 48).

Just The Beginning
But these numbers are just the beginning of the information you’ll need to choose the right hydraulic fluid. Temperatures on either end of the scale – for instance, working in International Falls, Minnesota, in January, or paving in Atlanta in July – make hydraulic fluid choice more critical. But even if your jobsite conditions register in the normal range, choosing the right fluid has an impact on equipment durability and wear protection – in particular, the minimizing hydraulic pump wear – and oil drain intervals.

“Sometimes you have to step up from what the manufacturer is recommending for normal conditions,” says John Sutherland, national accounts manager, Chevron.

What many contractors don’t realize, says Ronald LeBlanc, senior technical service advisor, Petro-Canada Lubricants, is how much hydraulic fluids have changed in the past 10 years. “Pressures have risen and hydraulic reservoirs have gotten smaller and so it’s likely you’ll need something quite different than what was accepted then,” he says. “There’s no one fluid that will do it all anymore. There are some exceptions to the rule, however.”

There’s a significant advantage, however, with today’s hydraulic fluids. Some of today’s Group II-plus and Group III base oils (see explanation next section) based on the ASTM D943 standard are seeing 6,000-plus hours of performance, according to LeBlanc, compared with the 1,000 hours of 10 years ago. “We’ve really come a long way,” he says.

And just because your machine is older doesn’t necessarily mean you can downgrade your hydraulic fluid. “It goes back to pay me now or pay me later,” Sutherland says. Too many times a downgrade in oil is accompanied by poor hydraulic maintenance practices that accelerate machine wear. “Once a machine is out of warranty, a contractor can think, ‘well, the manufacturer is not going to pay for anything anymore,'” he says. “But that’s backward thinking, because you’re the one who’s going to pay for anything that breaks out of warranty.”

“If you stay with the oil quality you started with in the machine,” says Ted Naman, technical coordinator for ConocoPhillips, “you’ll extend the life of your machine.”

One note before we continue: For this discussion, proper hydraulic system maintenance – including contamination control and oil analysis – is a given. (For more on hydraulic fluid contamination control, see the February 2008 issue of Equipment World.)

At the base of it all
Lubricant providers use a variety of base oil stocks mixed with additives to formulate hydraulic system fluids. The purity of these base oils, combined with the level and composition of the additive package, determines a significant portion of its retail cost.

The American Petroleum Institute puts base oils into groups, based on the amount of sulfur in the oil (more sulfur is typically found in lesser quality oils) and the level of refinement. Group I oils are at the lowest rung of the base oil food chain. Most higher quality mineral-based monograde hydraulic base oils use Group II base oils. Multigrade oils are usually made from Group II-plus and Group III base oils with viscosity index improvers. Synthetic formulations typically use Group III and Group IV synthetic base oils.

While these different groups of base oils are seldom known among users, LeBlanc says contractors should be familiar with their general meaning, “because it affects the performance of your machines.”

Viscosity is king
The most important property of hydraulic fluid is its viscosity, or the measurement of a fluid’s resistance to flow, which affects its ability to put a proper oil film between moving parts. Viscosity depends on temperature – the oil gets thicker as temperatures decrease and thins out when heated. “When oil can’t flow, pumps can’t push the fluid through the system,” explains Mark Betner, heavy duty lubricants manger, Citgo.

There’s a balance to be maintained. You must match the operating temperature of your hydraulic system with the viscosity grade of your hydraulic fluid. Use a lower viscosity hydraulic fluid than what your manufacturer or lubricant provider recommends, and you’ll tend to increase metal-to-metal contact under heavy loads. Use a higher viscosity hydraulic fluid than recommended, and you can generate excessive heat within the hydraulic system, which degrades the fluid more rapidly, Naman says.

A hydraulic fluid with a higher viscosity index – or the measure of a fluid’s viscosity change with temperature (see glossary on page 52) – acts the same over a wider temperature range. As Chevron puts it, the higher the viscosity index, the smaller the relative change in viscosity with temperature change.

Multigrade oils also contain viscosity index improvers, or polymer additives that increase the viscosity of the fluid throughout its useful temperature range. “Multigrades are more efficient over a wider range of temperatures,” Betner says.

There is a downside to using viscosity index improvers, however. As system temperatures increase, the improver can be made ineffective, eliminating the multigrade advantage. And these improvers can also downgrade the oil’s air separation properties. Because of this, some manufacturers only allow the use of multigrade hydraulic fluids when a machine is working in extreme temperatures.

“If you work in Phoenix, you may think you don’t need a multigrade, since you have no cold temperature start-up requirements,” LeBlanc says. “And although a monograde won’t work on a skid steer in the Detroit winter, there are other advantages of using a multigrade, such as energy efficiencies.

Pumpability and pour point – or the lowest temperature at which a product will pump and flow – can be critical in some construction applications, such as utility bucket trucks that move workers up to the top of electric power poles. These operations are dead in the water if the truck can’t pump the oil and lift crew members to the work zone. Manufacturers of these types of products tend to favor multigrade hydraulic fluids since they flow better at temperature extremes, Naman says.

Heat – either caused by ambient temperatures or a system that’s generating its own heat from a variety of causes – is the natural enemy of hydraulic systems. “Anytime you can take the heat out of a hydraulic system you’re doing good,” Sutherland says. Hydraulic systems should ideally run in the 130-to-140-degree Fahrenheit (54 to 60 degrees Celsius) range, LeBlanc adds.

Additives, additives
A number of additives supplement the base oil to handle a variety of negative traits that could affect hydraulic fluid performance:

Antiwear: “Antiwear is an essential part of the additive package,” Naman says. It protects the pump and valves from metal-to-metal contact and acts as an oxidation inhibitor. Most manufacturers will recommend a minimum level of antiwear additive. “There are a number of fluids that might have the right viscosity grade, but they don’t have the right level of antiwear additives,” Betner says. “You won’t typically see the antiwear level on a product information sheet so ask your lubricant provider for it and know what level of antiwear your manufacturer is requiring.”

One typical antiwear additive found in hydraulic fluid is zinc dialkyl dithiophosphate, or ZDDP. There are also products available that opt for an environmentally sensitive additive instead of ZDDP. Be aware that lower cost lubricants may not have the amount of antiwear additives you need in your machine.

Anti-foaming: “When operators say the controls feel squishy or spongy, you have air in the hydraulic system,” Sutherland says. The reason: hydraulic systems resist compression, while air does not. This means your pumps have to work harder. Entrained air also creates more heat.

There are two causes behind entrained air, Sutherland explains. One, the foam resistant additive in your machine oil has been used up. You can fix this with a partial drain and addition of new oil. Or two, the system is introducing air at some point, perhaps with a leaky cylinder seal. The solution: investigate how the air is getting into the system, manually bleed the air and supplement the hydraulic system either with fresh hydraulic fluid or additional foam suppressant.

“If the hydraulic system is fairly new and there are no leaks, yet you’re still getting foam, you can get a small dose of foam suppressant from your lubricants supplier and add it to the system,” Naman says. Also investigate whether you should step up the quality of your hydraulic fluid.

Rust protection: “You need to have the fluid fortified against corrosion,” Betner says. Without this protection, idled equipment will rust, since a hydraulic reservoir breathes in air – and its attendant moisture – whether pressurized or not. If you don’t have a rust inhibitor in the fluid to counteract the moisture the resulting rust particles will make the fluid work harder and could lead to a breakdown.

Demulsibility/Emulsibility: Hydraulic fluids also need demulsibility, or the ability of the oil to separate from water. This feature becomes critical in applications where you are working around water – for example, an excavator taking gravel out of a river where the bucket cylinder goes into the water and water gets into the hydraulic system. The water stays suspended in the hydraulic fluid until the machine stops. Once a machine is turned off, any water in a hydraulic system should drop out quickly and go to the bottom of the reservoir to be drained. If water stays suspended in a hydraulic fluid it accelerates its aging and thus reduces its lubricity and filterability. If water cannot be drained, it is better to emulsify the water so it doesn’t build up in the sump, according to Petro-Canada.

The base oils used to formulate hydraulic fluids have a role in this water separation ability, Naman says. “Group II or III base oils have the ability to separate water quickly,” he says. “Group I oils need additives to improve water separation. Demulsibility will also affect rust and corrosion resistance.”

Oxidation inhibitors: Oxidation – or when oxygen attacks hydraulic fluid – causes oil to thicken up and darken, thus introducing more heat into the system.

“Oxidation happens because of heat,” LeBlanc says. “Every 18 degrees above 140 Fahrenheit, you can theoretically cut the life of your oil in half, so it becomes important to keep temperatures within operating conditions. Anyone’s oil will break down when you elevate the temperature, it’s just a matter of how soon. Higher quality multigrade hydraulic oils will maintain their optimum viscosity and allow operations to run more efficiently under wide operating temperatures giving you increased energy efficiencies.”

Detergents: There’s some disagreement on detergents – which maintain hydraulic system cleanliness – as an additive. Naman calls them an important part of the additive package, although he says the detergent used in a hydraulic fluid is neutral and much less active than what’s found in engine oil. In off-road equipment hydraulic systems, the oil temperature can reach 140-150 degrees Fahrenheit under heavy loads in hot weather, and that’s where the combination of oxidation inhibitor and detergent is effective at controlling deposits.

But LeBlanc says hydraulic fluid usually does not operate at high enough temperatures to cause the sludging and varnishing that detergents take care of. But some detergents will neutralize acids that form during the oxidation process.

Synthetics and biodegradables
Synthetic oils offer a solution for extreme temperature conditions, such as working in the arctic, or on machines that operate continuously for days without shutting down. Since most construction machines are not used in these conditions, synthetic oils are a specialty item in this industry. Another huge factor is cost, since these oils can be two to three times higher than conventional oils. And, says Naman, certain seals in your machine may not be compatible with synthetic oil, which can cause leaks.

Another option is biodegradable hydraulic fluids. Because of cost, contractors usually use this type of oil only at the direct request of a project owner. Typically made out of vegetable oil, they can be used on projects that are environmentally sensitive, such as working close to water or on national park land. Certain full synthetic oils can also be classified as biodegradable.

But there are downsides. The cost can be up to six times more than conventional oils. Because vegetable-based oil biodegrades over time, it also has about half the life of standard hydraulic fluid. Vegetable-based oils are not oxidative stable and so have a tendency to form deposits inside hydraulic systems, Naman says. Compatibility with some seal materials may also pose a problem.

And be aware that there are different biodegradable formulas on the market, LeBlanc says, including some made specifically for use around water. Just because it’s labeled ‘biodegradable” doesn’t mean it will meet the true need of the application.

Also, some biodegradable oils are mineral based, including those with zinc-free antiwear additives. As opposed to vegetable-based oils, which are considered readily biodegradable, these mineral-based oils are inherently biodegradable. “These degrade,” LeBlanc says, “but they degrade over a longer period of time. Plus they tend to have much better performance than vegetable-based oils.”

“Biodegradable oils are much more predominant in Europe,” Naman says. “I would put them at 1 percent of the U.S. market now, and don’t see them growing beyond 2 percent in the next 10 years because of their higher cost.” Still, Betner says about half the inquiries Citgo received at its 2005 ConExpo-Con/Agg booth were from those seeking information on biodegradable oils.

Ask what your lubricant supplier can do for you
A number of questions need to be addressed when consulting with your lubricants provider. First off, they need to know the makeup of your fleet, how many hours its working per day on average and the ambient conditions – including cold, heat, dry or wet. Also under consideration:

  • The hydraulic fluid you’ve been using.
  • Any problems you’ve had.
  • Whether it needs any special properties, such as biodegradability or dielectric strength (the ability to resist electrical charges).

According to Betner, perhaps the most important thing is determining what you want to get out of a hydraulic fluid. Do you need to overcome temperature extremes? Are you having hydraulic pump problems? Do you want to optimize your fluid inventory? Are you trying to find an oil with the best economic life cycle? “The best economic deal will be a hydraulic fluid that improves the performance of your hydraulic system, plus the service life of your pumps and valves,” Betner says. “That’s where the dollars are.”

Lubricant providers know fluid inventories can be problematic since service trucks have limited space to carry product. In addition when you carry additional products, you have to warehouse each type and there’s the danger of putting the wrong product into a machine. Sutherland says this can be solved many times by stepping up a grade of fluid. “It may cost you 5 percent more, but when compared with the other issues involved, it’s worth the cost,” he says.

Lubricant suppliers – including OEMs who brand their own products – have a wealth of information available for the asking: field test data, bench test data, manufacturer-required hydraulic fluid specs and testimonials from other contractors. These suppliers can also be a conduit for best practices – telling a contractor in California, for instance, what has worked for an unnamed contractor in Texas. “It’s easy to put a dollar on any savings resulting from a saved machine or additional hours before overhaul,” Sutherland says.

“When you’re looking for real economic solutions,” Betner says, “you should realize that lubricants are only a few pennies on an operational budget. Even if we gave lubricants away it wouldn’t mean much to your operational budget, and yet they contribute so much to component life.”

Quick glossary
Antiwear additives

Agents that form a protective coating to minimize metal-to-metal contact and wear.

The ability of a hydraulic fluid that is insoluble in water to separate from water with which it may be mixed in the form of an emulsion.

In hydraulic fluids, these are neutral calcium, magnesium or barium compounds that keep deposits from building up on internal hydraulic system parts such as pumps, control valves and reservoirs.

Multigrade oil
An oil meeting the requirements of more than one SAE viscosity grade classification, and may therefore be suitable for use over a wider temperature range than a single-grade oil.

Occurs when oxygen attacks hydraulic fluids. The process is accelerated by heat, copper and steel catalysts and the presence of water, acids or contaminants. It leads to higher viscosity and acidity, plus deposit formation.

Oxidation inhibitor
Chemical added to hydraulic fluids to increase their oxidation resistance, thereby lengthening their service life and minimizing their degradation; also called anti-oxidant.

Pour point
Lowest temperature at which an oil or distillate fuel is observed to flow, when cooled under conditions prescribed by test method ASTM D97. The pour point is 3