Hydraulic system failure due to oil problems

Hydraulic systems are among the unsung powerhouses of modern industry that power everything from the landing gear of aircraft to construction machinery. However, behind every major failure there's almost always one unnoticed cause: oil.

Hydraulic systems are based on using a remarkably simple concept—pressure in a fluid transmits force. However, that simplicity can be deceiving. The hydraulic oil that runs through these lines isn't just a passive fluid; it is the blood of the whole system. It seals, lubricates, regulates temperature, and transmits power in a single step. If something is wrong with an oil system, it is the whole system that is liable frequently, violently, and usually at a cost.

Understanding that the role oil issues play is a factor in the failure of hydraulic systems isn't just an educational exercise. Maintenance engineers, operators of equipment, or fleet administrators: it's an important distinction between routine service and unplanned downtime of hundreds of millions of dollars.

The role of oil is the hydraulic system

Before you can identify the issues before tackling them, it is helpful to understand what the actual function of hydraulic oil is. One liter of hydraulic fluid is required to carry out multiple functions at once: to transfer energy from the pump to the actuator; to lubricate any moving part it touches to remove heat created through friction and pressure loss; and to shield the metal surface from corrosion and ensure a seal is maintained within tight clearances.

There is no single fault in hydraulic engineering that is as widespread as the degradation of oil. If the oil is damaged, each of these functions suffers damage all at once, and the system is beginning to die from the inside out.

Research in industrial maintenance regularly demonstrates that the degradation and contamination of oil are responsible for as much as 80 percent of failures of hydraulic systems. Many of these issues can be prevented completely with effective fluid management.

Contamination is the number one killer

Contamination is by far the most prevalent and destructive oil-related issue that can be found within a hydraulic system. It is present in three varieties, such as particulate, water, and chemical. Each attack on the system is different, but they are all capable of causing complete failure.

Particulate contamination happens when solid particles such as dust, metal wear debris, dirt, or damaged seal material are introduced within the system of hydraulics. Even particles that are as small as 10-15 millimeters (invisible to the naked eye) result in abrasive wear on the faces of pumps, valve seals, and spools. When components wear, they produce more particles, which causes wear and tear—the cycle continues to grow exponentially to the point of failure.

Water contamination is also a major issue. The entry point for water into hydraulic systems is condensation in reservoirs, leaky cool circuits, and contaminated makeup oil. Even a concentration of 0.05 percent water may significantly weaken the lubricating film and encourage corrosion, stimulate the growth of microbes, and create the formation of steam that causes hydraulic shock in hot spots with high temperatures.

Viscosity issues: The problem is that it's too thick or thin

Viscosity is perhaps the most crucial characteristic of the hydraulic oil's physical properties. It is a measure of how the oil can maintain a lubricating layer when it is under pressure, how well it moves through the restrictions, and how well it seals the clearances of motors and pumps. If you don't have the right viscosity, the results are a range of poor efficiency up to rapid destruction of components.

If the oil is too thin—due to the incorrect grade being used or the oil having been thermally degraded—the lubricating layer gets thinner to the point that contact between metal and metal occurs. The vanes of the pump, as well as gear teeth and piston slippers, start to wear at a faster rate. In extreme situations, a devastating seizure can happen in a matter of minutes. Thin oil can also leak past seals more easily, decreasing volumetric efficiency, making the system appear sluggish and weak.

However, an oil that is too viscous—usually on cold start-ups and when the improper grade of oil has been employed—results in high pressure drops in lines and filters, which depletes the pump's inlets of fluid and triggers cavitation. The resultant explosion of vapor cavities degrades the surfaces of pumps with a speed that is similar to sandblasting at a microscopic scale.

Degradation of the oil and its additives, as well as depletion of the reserves

Even the most clean and properly specified hydraulic oil is not guaranteed to last for a long time of service. Oxygen, heat, and pressure cycling slowly reduce the oil base and eat away at any additives in the package. The anti-wear components, the oxidation inhibitors, the anti-rust additives, and the foam suppressants all diminish as time passes. After that the base oil has to manage itself, and it's not equipped to handle it.

Oxidation is the principal degradation mechanism. In normal operating temperatures of more than 60°C, the rate of oxidation approximately doubles with every 10-degree increase in temperature. This results in acidic compounds and viscous sludge that plates onto valve bores as well as filter components. This results in an element that slowly gets less responsive, is hotter, and runs longer and then eventually ceases to function, often in times of peak demand, where the effects are the most severe.

The degradation of thermal energy adds another level of difficulty. Zones of high temperatures—such as relief valves, poorly sized orifices, and circuits that are not properly cooled—may cause carbonization of the oil locally, leading to varnish deposits that clog the servo valves and trigger irregular, unpredictable behavior within precision equipment.

Aeration and foam the silent killers of performance

The presence of air that is present in the hydraulic fluid can be a challenge that is often ignored until it leads to an issue. Air that dissolves releases as free-floating bubbles as pressure decreases, and the bubbles cause the cavitation to break when they break when pressure is high. The entrained air decreases the bulk modulus of oil, making the system appear spongy and unpredictable. In precise motion control applications it can be fatal.

Foam on the reservoir's surface is a visible sign of a more serious aeration issue. It's typically the result of return oil discharged over the level of the fluid as well as a leaky seal on the pump shaft drawing in air or low fluid levels allowing return lines to penetrate the surface. If left untreated, foam increases the oxidation process, which reduces cooling efficiency and could result in dangerous pressure transients.

The price of not paying attention to the quality of oil

The financial argument for the proactive management of oil is convincing. A thorough oil analysis program generally costs around only a few hundred dollars per year for each machine. The motor, pump, or cylinder it protects from failing could cost anything from a few thousand to several hundred thousand dollars, and that's not counting unexpected downtime, urgent labor, expedited parts delivery, or the loss of production.

Beyond direct costs Beyond the direct cost, there is the security aspect. Hydraulic systems found in aircrafts and mobile equipment as well as marine vessels and heavy industry are operating under extreme pressures, typically between 200 to 400 bar. A catastrophic failure caused by deteriorated oil doesn't signal its presence in a polite manner. It could cause severe injuries from high-pressure injections or uncontrolled machine movement or even structural collapse.

Prevention: What does good oil management appear to be? 

Avoiding failure of hydraulic systems by oil management is about discipline and coherence. Regular oil analysis and sampling at least every 500 hours of operation will alert you to the presence of contamination, changes in viscosity, and depletion of additives prior to the damage being caused. Maintaining the cleanliness of the system at ISO 4406 levels appropriate for the system, ensuring the proper levels of fluid, and replacing filters according to schedule rather than prolonging intervals to reduce costs are the fundamentals that safeguard thousands of dollars worth of equipment.

Using only the manufacturer-specified fluid grade, never mixing oils from different suppliers, and inspecting shaft seals proactively before they begin ingesting air round out a solid fluid management program.

The basic idea is the bottom line is that failure of hydraulic systems caused by oil issues is virtually impossible to prevent. The technology to analyze oil is available, the maintenance procedures are in place, and the payback on investment is clear. The only thing to consider is whether the dedication to the cleanliness of fluids and routine monitoring is incorporated into the maintenance process or if it is ignored in the name of quick savings and then waits until it is a time when it is no longer left unnoticed.