It’s no secret that today’s iron is running at wider operating temperature ranges than ever for lubricants, and Tier 4 engines are heating up overall operation even more.
As a result, more hydraulic oils with higher viscosity indexes (VI) have come into play. “Oil with a higher viscosity index does not thicken as much at low temperatures, and does not thin as much at higher temperatures,” says Robert Profilet, global commercial manager, hydraulics, for Lubrizol.
Maintenance managers should be aware of the differences in viscosity and other properties when considering fluid replacement.
Hydraulic fluid’s most important attribute is its resistance to flow. If it’s too thin (low viscosity) there’s less of a seal and too much movement between parts, introducing the possibility of increased wear. High viscosity, thicker, fluid increases the seal and lessens the threat of wear. The catch-22 is that it can be more difficult to pump through the system and decrease efficiency, according to the HydraulicFacts website.
The happy medium for a manager may be in selecting a fluid with the right mix of overall properties to protect components and extend their lives.
Top 10 characteristics of hydraulic fluid
Lubrizol lists 10 characteristics to keep in mind beyond viscosity: compressibility; wear resistance; oxidation stability; thermal stability; filterability; rust and corrosion protection; foam resistance; demulsibility; hydrolytic stability; and seal compatibility.
Compressibility is a measure of the amount of volume reduction due to pressure. Although hydraulic oils are basically incompressible, slight volume reductions can occur under certain pressure ranges. Compressibility increases with pressure and temperature and has significant effects on high-pressure fluid systems. It causes servo failure, efficiency loss, and cavitation.
Wear resistance is a fluid’s ability to reduce the wear rate in frictional boundary contacts. Antiwear hydraulic fluids contain antiwear components that can form a protective film on metal surfaces to prevent abrasion, scuffing, and contact fatigue. These additives enhance lubricant performance and extend equipment life.
Oxidation stability is the hydraulic oil’s resistance to heat-induced degradation caused by a chemical reaction with oxygen. Hydraulic oils must contain additives that counteract the process of oxidation, improve the stability and extend the life of the fluid.
Thermal stability is the ability to resist breakdown at elevated temperatures. Antiwear additives naturally degrade over time and this process can be accelerated at higher temperatures. The result of poor thermal stability is the formation of sludge and varnish which can clog filters, minimize flow and increase downtime. In addition, as these antiwear agents decompose at high temperatures, acids are formed, which attack bronze and yellow metals in piston pumps and other hydraulic system components.
Water can react with additives in hydraulic fluids forming oil insoluble material. These contaminants can precipitate from the lubricant and block filters, valves and other components resulting in decreased oil flow or the system going on bypass. Blockage can eventually result in unplanned downtime. Hydraulic fluids are designed to be filtered with modern filtration systems without fear of the additive being depleted or removed from the system. This enables systems to stay clean without sacrificing critical performance requirements such as antiwear, rust protection or foam inhibition.
In many systems, water can enter as condensation or contamination and mix with the hydraulic oil. Water can cause rusting of hydraulic components. In addition, water can react with some additives to form chemical species, which can be aggressive to yellow metals. Hydraulic oil formulations contain rust and corrosion inhibitors, which prevent the interaction of water or other chemical species from attacking metal surfaces.
Foam results from air or other gases becoming entrained in the hydraulic fluid. Air enters a hydraulic system through the reservoir or through air leaks within the system. A hydraulic fluid under high pressure can contain a large volume of dissolved or dispersed air bubbles. When this fluid is depressurized, the air bubbles expand and produce foam. Because of its compressibility and poor lubricating properties, foam can seriously affect the operation and lubrication of machinery. Proper foam inhibitors modify the surface tension on air bubbles so they more easily break up.
Water that enters a hydraulic system can mix or emulsify with the hydraulic oil. If this ‘wet’ fluid is circulated through the system, it can promote rust and corrosion. Highly refined mineral oils permit water to separate or demulsify quickly. However, some of the additives used in hydraulic oils promote emulsion formation, preventing the water from separating and settling out of the fluid. Demulsifier additives are incorporated to promote water separation from hydraulic fluids.
When hydraulic fluids come into contact with water, the water can interact with the additive system of the hydraulic oil resulting in the formation of acids. Hydraulic fluids that lack hydrolytic stability hydrolyze in the presence of water to form oil insoluble inorganic salts that can block filters and valves inhibiting oil flow. This can result in hydraulic system failure. Properly formulated hydraulic fluids are designed to contain additives that are resistant to interactions with water.
Leaking hydraulic fluids can cause many issues, from spill problems to more serious safety concerns and lubrication failures. Most hydraulic systems utilize rubber seals and other elastomers to minimize or prevent hydraulic oil leakage. Exposure of the elastomer to the lubricant under high temperature conditions can cause the rubber seals to harden, crack and eventually leak. On the other hand, hydraulic oil exposure can cause seals to swell excessively preventing hydraulic valves and pistons from moving freely. Hydraulic oils are tested against a variety of seal materials to ensure that the fluid will be compatible with seals under various conditions.
Fluid selection tips
Start by answering three basic questions: what type of equipment is involved, how severe is the duty cycle, and what will the fluid’s temperature range be during use? In mobile equipment, the choice is more difficult because there will likely be high pressures, severe duty cycles, and a much wider range of temperatures. Here, a multi-grade fluid that flows like light oil at low temperatures and like a heavier oil at high temperatures may be the best choice.
Multi-grade hydraulic fluids behave much like the multi-grade engine oils used in a car. To achieve this performance they contain polymer additives called viscosity modifiers (VM) or viscosity index improvers. Multi-grade fluids are also known as high viscosity index fluids.
VI is a measure of how much a fluid’s viscosity changes with temperature; the higher the VI, the less the viscosity changes. Where a conventional fluid typically has a VI close to 100, a multi-grade fluid will have a VI of at least 140. Some special-purpose fluids formulated for very cold environments have a VI of 200 or more.
Multi-viscosity fluids have a number of benefits: System performance is more uniform as temperatures change because the fluid maintains a more consistent viscosity. Cold weather operation is improved because the fluid flows more freely to minimize cavitation, sluggishness, drift, and shudder. “Higher viscosity index fluids have also been linked with providing a higher level of efficiency for the equipment,” Profilet says.
System components are better protected at high temperatures because the fluid maintains its viscosity, keeping an effective lubrication barrier and minimizing metal-to-metal contact. Energy efficiency and fuel consumption are both improved because the fluid provides greater mechanical and volumetric effectiveness. Also, the same fluid can be used year-round, simplifying maintenance and inventory.
The fluid maker, your local dealer, and the OEM all can help you make the right selection.