Josh Cosford • Contributing Editor
Dad was a lot of things, but he was no hydraulic expert. However, that doesn’t mean he (and everyone else) didn’t have an opinion on hydraulic oil. The problem with opinions is that they come from persons with enough knowledge that people listen—but enough knowledge to be potentially dangerous with misinformation. Here are five critical things your father never told you about mobile hydraulic fluids.
1. Hydraulic fluid serves more than one purpose.
Hydraulic fluid’s primary purpose is to transmit hydraulic energy in the combined form of pressure and flow. For all intents and purposes, hydraulic fluid can be considered solid and nearly incompressible as it acts upon pistons of cylinders and gears/pistons/vanes of motors. The hydraulic pump pushes on fluid, and the fluid pushes on the actuators; this is the easily understood nature of fluid in a hydraulic system. However, transmission of mechanical force is not the only trick in its bag.
Contamination removal, heat transfer, sealing and lubrication are all important secondary properties of hydraulic fluid. Although these properties are innately functional within hydraulic oil, considering them as part of the design or maintenance of mobile machinery will help you get the most out of your system.
Contamination removal in hydraulic fluid is the most commonly considered of the secondary properties. Everyone knows dirt needs to be removed from oil, which is why every mobile machine has at least one hydraulic filter on it (typically a poor-quality one, which is something your father never told you about filtration). Although estimates vary, everyone agrees contamination is the number one cause of failure of pumps, valves and actuators on mobile machines, even ahead of physical damage.
Hydraulic fluid helps control contamination pretty much by default, because it circulates through every component you’re trying to protect. As contamination is either externally- or internally generated, it will be picked up in the fluid stream and carried back to where it can be trapped by the filter(s). We can ensure we take advantage of the particle-transportation properties of fluid by trapping as much as possible in strategic locations. You should use a high quality return line filter and a pressure filter mounted after the pump. Because pumps are a common failure point, their degradation will not result in more contamination being transmitted to every other component in the system when a pressure filter is installed.
Heat transfers efficiently through a liquid, making it an excellent tool for convection of thermal energy. Anywhere in a hydraulic system where hydraulic energy is not being used to create useful work, the by-product is heat. The major heat generators are pumps, motors and relief valves. Because heat reduces the viscosity of the fluid, it needs to be carried away from components and cooled if it reaches high temperatures. Although the fluid carries heat with it by default, just as with contamination, you must take advantage by sizing reservoirs appropriately, and by adding a cooler when heat generation is high.
Sealing between static components is almost exclusively handled by soft seals, such as O-rings. Dynamic sealing is also most often done with soft, mechanical seals, but not only does hydraulic fluid aid in sealing, it can also be the seal medium itself. Lip seals, for example, use the pressure of the fluid to push the lips against the sealed surface, further enhancing seal efficiency. However, because of hydraulic oil’s resistance to shearing, the oil itself is used to seal close-tolerance moving bodies, such as the spool and housing of a directional valve, or the piston and cylinder block of a pump. By ensuring your hydraulic fluid is in its most effective viscosity range, you also ensure the fluid will properly seal in these components.
Lubrication is probably the second most important quality of hydraulic fluid. It enables all the various internal components of pumps, motors, valves and cylinders to move against one another efficiently and reliably. If your hydraulic fluid loses its lubricity, such as with overheating or excessive water saturation, component damage is not far behind. Most mobile hydraulic systems use oil, which has natural lubrication qualities. But when applying less common fluids such as “arctic” or bio-oils, these additive packages provide excellent lubricity.
2. Oil does not need to be changed very frequently.
Hydraulic oil in your backhoe is not the same as engine oil in your car. Hydraulic oil isn’t exposed to the same abusive conditions experienced by automotive engine oil, such as >2000° combustion temperatures, soot ingression from incomplete burn, and excessive contamination from wear particles, water or fuel.
Under normal conditions, a well-maintained hydraulic system’s oil can last indefinitely. Hydraulic oil breaks down with extreme heat, excessive water saturation and oxidation. Further, conditions such as high water content and heat also exacerbate the oxidation. High heat also reduces viscosity, which if low enough, will allow metal-to-metal contact, generating internal contamination.
However, if oil is clean, cool and free from water, the chemistry to break it down does not exist, and it remains in the same state as it was poured into the reservoir. If anything, some of the additives, such as zinc, can deplete over time, so keeping an eye on your fluid through an oil-analysis program will ensure you’re within operating parameters.
Manufacturers of tractors and other off-highway machinery will publish required hydraulic oil change periods, which are the longest of any of the fluid in the machine. However, some newer and larger tractors have no published hydraulic oil change interval, which could be related to the overall design of the machine, including measures to ensure heat, water and particle contamination are kept to a minimum or avoided altogether.
3. Not all hydraulic fluid is the same.
Hydraulic fluid varies just as much as the fluids in your car. Hydraulic fluid can come from conventional oil, but it can also be formulated from synthetic stock. Hydraulic fluid can also be glycol-based, a water/glycol mix, or even nearly 100% water in some applications.
Luckily, most hydraulic fluid used in mobile machinery is the standard “decomposed dinosaur” type, made from the same stock as your 10w30 engine oil, but with a slightly different additive package. However, hydraulic oil varies enough that you should be careful selecting the appropriate type for your application.
Viscosity is the most important consideration, and is important enough to have a noticeable effect on machine performance. If you had an oil-spill on a winter morning and sent your farm-hand down to the Co-op to grab a pail, and he comes back with AW68, you could see sluggish operation because of the high viscosity.
The quality of oil differs greatly as well, and just like engine oils, synthetic hydraulic oil is far superior. Synthetic oil has higher viscosity index, better lubricity and a lower pour point than regular oil. Synthetic oil will perform better in a wider range of conditions, last longer and generally provides peace of mind knowing it’s providing extra protection for your expensive machine.
4. ATF makes a fantastic hydraulic oil.
Automatic Transmission Fluid is used in everything from a Fiat 500 to a Ford F-150, and because of the astounding service life required by vehicle transmissions, the fluid used is some of the most high-quality lubrication available. ATF experiences such extremes in temperature, humidity and viscosity, yet can last 30,000-60,000 miles or more.
ATF is formulated for high viscosity index (the ability to maintain test viscosity over a wide temperature range), and has excellent water control properties, allowing the fluid to hold more water in saturation before being released to create corrosion. Also, the additives in ATF are formulated to improve lubricity, anti-foaming, shear stability and resistance to oxidation.
ATF is a premium hydraulic fluid, but also comes at a premium price, which is its major downside. A gallon of ATF is hard to find for less than $20, but five gallons of hydraulic oil can be had for less than $40. However, ATF is closer in quality to synthetic hydraulic fluid, and with this consideration, ATF can be seen as fairly reasonable in price. Also, many companies with large vehicle fleets stock bulk ATF purchased at reduced prices, allowing them to use one fluid for both applications.
5. Bio-oil spills still need to be reported and/or cleaned up.
Bio-oil is quickly becoming a popular fluid choice for mobile hydraulic machinery, and for good reason. Biologically safe oils are often both biodegradable and non-toxic, so leaks and spills are more readily processed by Mother Nature with little or no harm to flora and fauna. While this may be true, the use of bio-oil in place of standard hydraulic fluid does not provide a hall pass for institutions to ignore their spills.
While a slow leak at a fitting dripping vegetable-based hydraulic oil will cause little concern, large spills still need to be reported under the Resource Conservation and Recovery Act. The advantage with using bio-oil is that environmental fines and clean-up costs are often reduced. The disadvantage is in the cost of vegetable-based oils, which is high, especially when formulated for high performance.
However, one must consider that oil spill cleanups and fines can cost hundreds of thousands of dollars. The extra cost of the bio-oil could be offset by a single future spill, and besides, protecting the environment from damaging and toxic hydrocarbons is a no-brainer. High effort should be expended in reducing the frequency of oil-spills, especially near water. But if a regrettable spill does occur, you’ll be glad you were using bio-oil.
If you are involved with mobile hydraulic machinery, you should have a minimum level of familiarity with the fluids you add to your noninexpensive equipment. Know what options you have for hydraulic fluids and keep in mind that your system’s well being is depending on your choices. Dad would be so proud.