How well do you know your hydraulic fluid?
Keep your hydraulic fluid running healthy with these tips
By Rob Profilet
What's different now from 20 years ago?
It's no secret demands on hydraulic oil have increased in the past 20 years. Systems are generally smaller, so less oil is used. Pumps have been tasked to produce as much or more output, resulting in higher operating pressures. Smaller oil volume and higher pressures result in higher oil temperatures. Hotter oil temperatures mean an increase in oxidation rates and more thermal stress on the additive system. At the same time, end users want to spend as little as possible for fluid that lasts as long as possible and maintains a high level of protection for the equipment.
How long should hydraulic oil last?
The answer depends on many factors. Although the life of a hydraulic oil really depends on many variables, including quality of oil, operating conditions, and potential contamination, a good quality hydraulic oil should last at least 6 months if conditions are not severe. With monitoring and a good oil analysis program, the hydraulic fluid could last much longer.
In the simplest case, some OEMs recommend when to drain the oil. In other situations, the answers are not as clear.
How do temperatures impact oil life?
The operating temperature of an application provides good information about the potential life of the oil. Fluid temperatures in most industrial applications are around 60° C, and fluctuations or spikes to 85° C are not uncommon.
However, when you operate at temperatures above 85° C for extended periods, you definitely will shorten the life of the oil because the oxidation process is accelerated. Every 10° C increase in temperature doubles the oxidation rate and cuts the life of the oil in half.
What does oxidation do?
Oxidation occurs when hydrocarbon molecules are converted to carboxylic acids. The rate of this process increases with higher temperatures and the presence of certain metals, including wear particles. Over time, oxidation byproducts can form varnish, which can result in sticking servovalves or sludge, which can in turn clog filters and suction strainers. In situations where oxidation is severe, it is best to change the fluid immediately.
What types of fluids are available?
Many different hydraulic fluids are on the market. The base fluid comprises about 99% of the formulation in most non-viscosity modified formulations and has a great impact on the lifetime of the fluid. The base fluid, in combination with antioxidants, determines the oxidation resistance of a new, clean fluid. Hydraulic fluids are tested for resistance to oxidation by the Turbine Oil Oxidation Test ASTM D943. The test measures the time in hours for the acid number of the fluid to increase to 2.0 when exposed to water, heat, and oxygen with a metal catalyst present. Some OEMs or end users require that fluids last at least 1500 hours in this test. Some fluids last over 5000 hours in this test.
What role does contamination play?
Contamination in hydraulic fluids is very common and can cause numerous problems. Water, air, dirt, fuel, and other hydraulic oils or lubricants can all act as forms of contamination.
Water contamination is very common. Used oil analysis often will show levels of 500-1000 ppm water in hydraulic fluid taken from an industrial application. Water is detrimental because it does not lubricate as well as oil and can cause wear. In some cases, it also can react with the additive system to form acids that can cause yellow metal corrosion. Of course, water also leads to rust of component surfaces and can increase the rate of oxidation, decreasing the oil life.
Air contamination also can result in oxidation, which can increase oil viscosity. Over time, oxidation can lead to varnish, a lacquer that coats surfaces and causes servovalves to stick. Even worse, air contamination can cause pumps to cavitate as air in the oil rapidly dissolves from heat generation, which can lead to pump loss.
Dirt, metal particles, and soot are common hydraulic oil contaminants. Metal particles can accelerate the oxidation process, and all of these solid contaminants can cause abrasion or surface fatigue on machine surfaces.
Fuel contamination introduces several changes, including lower flash point, lower viscosity, and increased vapor pressure. The viscosity change alone creates lower film strength, hampering lubrication efficiency.
How can oil life be maximized?
Operating conditions, the type of fluid used, and potential sources of contamination all can influence the lifetime of the oil. Although there may not be a single answer to how long your oil may last, here are three tips on extending the life of your oil:
- Choose a good quality hydraulic oil from a reputable lubricant manufacturer who can provide good technical support as needed.
- Monitor operating conditions. Keep the fluid clean, cool, and dry. Maintain filtration, and use clean lines to transfer hydraulic fluids into your equipment.
- Analyze your used oil. How often will depend on your system. Keep a watchful eye on your system and change the oil when oxidation and/ or contamination become excessive. Monitor the particle counts and make adjustments to the filtration system as needed to keep particle counts low.