In an optimally running hydraulic system, the temperature would remain consistent throughout the fluid’s duty cycle, the temperature of the fluid would never exceed 130 degrees, and the vented outside air would always be clean and dry. In these conditions your hydraulic oil could last forever and you would never have any equipment failures. In reality, none of these conditions are easily achievable, if not impossible. In a single duty cycle, a hydraulic oil can see changes in temperature ranging from 100 degrees to 500 degrees and even higher in areas of extreme high pressure. This rapid change in temperature leads to high levels of condensation (water) in your system.
Additionally, depending on the application, it is difficult to keep your reservoir temperatures below 165 degrees, which is the point that regular mineral based hydraulic oil starts to break down and oxidation occurs leading to sludge and varnish build up. Top that off with dirty, humid outside air being drawn in through the breathers and you have dirt and water that are definitively not good lubricants for your pump. With these conditions you can start to better understand why doing proper filtration and using a quality hydraulic oil is the only way to achieve a long service life for your equipment and your hydraulic fluid.
Outside of using after treatment devices such as heat exchangers to assist in controlling heat, fluid filtration systems to remove particles of debris, and reservoir breather filters that help absorb incoming moisture and debris, it is your hydraulic oil that is ultimately responsible for protecting your hydraulic system and keeping it running to minimize down time. With so many different options in hydraulic oils as well as quality levels it is important to understand what makes one product better than the other.
When it comes to all lubricating oils, it starts with a base oil. Using high quality base oil in a hydraulic lubricant means it will have fewer waxes, which leads to better flow properties as well as enhanced ability to shed water readily. The base oil will also have a more stable hydrocarbon molecule structure that resists thermal break-down, a process that leads to oxidation and ultimately varnish and sludge build-up. And it will have a better “Stay in Grade” capability, which basically means the hydraulic oil won’t get too thin in high temperatures leading to wear at the pump. A top notch base oil will also resist thickening at cold temperatures, which leads to pump starvation and improper valve operation. Finally, the proper mix and concentration of additives provide the correct amount of protection to your pump without holding water droplets in suspension. This is very important in hydraulic systems that have condensation issues and that is why we don’t generally use motor oil in a hydraulic system. Because of the high level of additives in motor oils, they will hold water in suspension which is not a quality you want in a hydraulic system.
So how do you determine whether your hydraulic oil is a quality hydraulic fluid? Review the product data sheet. This will tell you all the typical properties of the product as well as how it performed on standardized tests. Let’s review BlueSky Lubricants PureBlu Hydraulic Oil as an example. To determine the quality of the base oil go to the Oxidation Stability Test, ASTM D-943. This test procedure, which is reported in hours of performance, measures the oils ability to resist oxidation at elevated temperatures. BlueSky’s PureBlu Hydraulic Oil reports 5000+ hours. You can be assured that this product uses a high quality base oil considering most hydraulic oils come in between 1000 to 3000 hours. If a product data sheet doesn’t report oxidation stability, then you can almost guarantee it doesn’t perform well. Another key to base oil quality is the hydraulic oils pour point, the lowest temperature at which the fluid stops flowing. BlueSky’s PureBlu Hydraulic Oil reports -27 degrees F. This is outstanding for a single grade product. Some lesser quality products don’t have the ability to flow at 0 degrees. Not that any of our hydraulic systems have to operate at these low temperatures, but this is just a great indicator of your products quality. Another quality characteristic is reported in the Viscosity Index (VI). This gets back to an oils ability to “Stay in Grade” as it is affected by temperature. BlueSky’s PureBlu Hydraulic Oil reports a VI of 100. This is very good being that the higher the number the better the expected performance of the oil. Lesser products report Viscosity Index’s in the 80’s and 90’s where synthetic oils have viscosity Index’s starting at 120.
Finally you have your additive levels which are reported as Sulfated Ash Content. In hydraulic oils, zinc weight, being zinc is your main anti-wear additive used to protect your pumps. This is the area that you are limited in how much additive you use because it will affect the oils ability to separate from water, which is critical in most hydraulic applications. If water is an issue, look at the hydraulic oils demulsibility test, ASTM D5182. This is the time in minutes that it takes for a mixed sample of oil and water to separate. BlueSky’s PureBlu reports a 10 minute pass time, which again is an outstanding performance for a hydraulic oil. Beyond looking at these standard performance levels, certain hydraulic oil incorporate additional enhancements to their product to assist in operation. For example PureBlu Oil is dyed blue to help quickly identify leaks as well as help determine what product is leaking on a piece of equipment that has multiple lubrication systems. This is no different than knowing whether or not a leak on your garage floor is from your transmission.
Bottom line, your hydraulic fluid is a very important purchase that should be taken seriously. Hopefully you now have some insight and useful tools to help make this important decision for your hydraulic system’s needs