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This article discusses the demands new machinery is placing on hydraulic oils and offers advice on what properties to look for when purchasing an oil to meet the challenge.
As original equipment manufacturers (OEMs) continue to make improvements to modern machinery, many of the new hydraulic systems are now smaller, more compact, and operate at faster speeds, higher pressures and have tighter component tolerances. This offers advantages to engineering and manufacturing companies needing to fit more machinery into a factory and wanting to substantially increase productivity. However, such machine improvements place additional stress on hydraulic oils.
New machines have a reduced reservoir capacity compared to larger, traditional machines. Their smaller pumps place greater thermal stress on the oil used to keep the hydraulic system operating efficiently. In other words, with more power being transferred by less oil, the hydraulic oil may operate at higher temperatures, which causes most lubricants to oxidize at faster rates.
What's Happening to My System?
In the author's experience, temperature significantly affects hydraulic oil performance and system lifetime. When oil oxidizes, it can create sludge and varnish that clogs filters and control valves, resulting in system operational problems and premature component failure. This oil oxidation can accelerate with extended operation at high temperatures and thermal cycling. In addition, the viscosity (thickness) of the oil increases, which can result in increased inefficient transfer of power and possibly more pump wear, adding up to additional maintenance costs and production losses.
With the oil capacity of machines now smaller, yet operating at faster speeds, this means more oil pumping per minute to generate the required work, which in turn puts greater thermal stress on the oil. As a result, oil retention time is significantly reduced; the oil gets hotter and can break down much more quickly. To illustrate this point, consider an older hydraulic system with an oil capacity of 150 gallons and a pump capacity of 25 gallons. This rate yielded a six-minute oil retention time. By contrast, in one new system, the oil capacity was 80 gallons with a pump capacity of 40 gallons, which reduced the oil retention time to one-third of the old time - to only two minutes.
The improvements of modern machinery put increased stress on the oil and magnified the importance of eliminating impurities, including entrained air and water contamination picked up during the cycle. While many hydraulic systems have filters, if the oil is not relieved of the impurities, these will be circulated through the pump and around the system in a continuous cycle, with the oil picking up still more impurities each time it cycles through the reservoir. The oil does not get sufficient time to recover. This failure to adequately filter out impurities not only affects the performance of the hydraulic oil, but also ultimately reduces productivity and causes wear damage to the machinery, because wear particles can serve as destructive contaminants in a hydraulic system.
When buying a new machine, it is important to check the operating manual for the oil requirements. In order to meet today's more demanding operational conditions, newer machines may require a different viscosity than what has traditionally been used.
When water, air and/or other contaminants enter the system, the result can cause serious, costly effects on the hydraulic system, application, and the finished work or manufactured part. If excessive amounts of water get into the system, several problems can occur. The water may emulsify or mix into the oil and continue to circulate throughout the system. This emulsion can promote rust and corrosion, adversely affect the proper function of valves and actuators, increase wear of all components, promote oil oxidation, deplete additives and plug fine filters.
Hydrolytic stability is the measure of the reactivity of additives with water, and the resulting corrosion on yellow metals such as copper and brass. When water comes into contact with additives, acids can form and copper corrosion can occur. Because some water will contaminate a hydraulic system from time to time - usually through condensation, wash down or heat exchanger leaks - it is important for the hydraulic oil to have good hydrolytic stability to protect all copper-containing alloys in pumps and components when even small amounts of water enter and contaminate the hydraulic system.
Air in hydraulic fluids is another serious issue. If air bubbles generated during the operating cycle are not given enough time to clear the surface, the pump will circulate the air bubbles throughout the system - just like water. Air in hydraulic fluids can cause a number of problems including spongy response and cavitation, which result when air bubbles within the oil are compressed in the hydraulic pump. This can lead to a rapid spike in temperature as the air bubble implodes with violent force, causing the surrounding oil to oxidize, which then turns the oil dark and forms oil degradation by-products that can damage pumps and affect system operation.
If unchecked, cavitation can lead to pump damage and premature failure, and in addition to changing the pump, it will be necessary to change the oil and clean the system of all deposits and oil degradation by- products. Industry experience shows that the lifetime of a pump will continue to decrease - unless not only the pump and oil are changed under such circumstances, but also the system is cleaned of all degradation by-products. Other problems caused by air in hydraulic fluids include spongy controls, horsepower loss, foaming, poor temperature control, noise, poor lubrication and oxidation.
With all the demands placed on the lubricant to perform effectively, hydraulic oils today need to excel in many different capacities. These include antiwear, air release, demulsibility, filterability, thermal stability, antifoam, hydrolytic stability, seal compatibility, and rust and corrosion resistance. There are many organizations and OEMs which provide standards and specifications for hydraulic oils and set guidelines for true premium hydraulic oils.
It is important to consider specific OEM lubrication guidelines and standards that can meet the individual hydraulic system and production requirements. Industry-leading hydraulic oil suppliers generally will have their oils tested or data reviewed by independent OEM laboratories, such as Eaton Vickers, Denison, Cincinnati Lamb and General Motors, to name a few.
Many lubricant suppliers may claim they meet the requirements of the specifications, but they have not actually had their oil product approved by the OEM.
A recent comparison by Shell of major competitor oils in Cincinnati Lamb thermal stability tests showed the following results (Figures 1 and 2). These photos are from a thermal stability - CM heat test conducted over a seven-day period using steel and copper rods in oil; the images compare Shell Tellus® Premium versus a competitor's oil.
Key performance areas of a hydraulic oil to give maximum system protection include wear protection, thermal stability, oxidation stability, hydrolytic stability and filterability. If your oil fails in one of these areas, you run the risk of compromising hydraulic oil and system performance.
Hydraulic oil is the lifeblood of any hydraulic system. Sound maintenance dictates the ability of a lubricating oil to handle the stresses and pressures of all hydraulic applications, new and old. Shell Tellus and Shell Tellus Premium meet that challenge.