Cavitation is a nightmare for any hydraulic system, leading to a cascading series of damaging issues that will eventually destroy critical (and costly) components within your hydraulic system. While most people associate cavitation issues with pumps and motors, cavitation can also be a problem for hydraulic cylinders. In order to minimize or eliminate cavitation damage, knowledge of how it works and what kind of damage it leads to are good starting points. However, the wisest approach to dealing with cavitation issues is to take preventative measures.
Definition of Cavitation
The simplest explanation of cavitation is the presence of gas entrapped within bubbles in a liquid. In the case of hydraulic systems and components like pumps or cylinders, the liquid involved is the hydraulic fluid transmitting pressure. In hydraulic cylinders, the pressure transmitted by the fluid is used for linear actuation that can move thousands of pounds at the press of a button. Obviously, very high pressures are involved.
Bubbles filled with dissolved gases or vapor begin to form and grow when the pressure is close to that of saturated vapor for the fluid’s temperature (e.g., there is localized boiling of the fluid). For a cylinder, all the fluid needs to do is drop below atmospheric pressure for bubbles to form.
When these bubbles collapse on themselves (or implode), the result is noise, heat, serious surface damage, and a negative impact on efficiency and productivity. Essentially, the bubbles generate a powerful mechanical shock that results in microjets impacting nearby metal. That implosion occurs when the bubbles reach an area of low pressure that causes the vapor within them to condense. The metal begins to wear away in a process known as cavitation erosion, cavitation wear, or cavitation pitting.
Cavitation can also occur when air is trapped within the hydraulic fluid (a phenomenon known as aeration). As pressure increases (e.g., as the rod closes in on the end cap in a hydraulic cylinder) the bubbles violently burst. In a hydraulic cylinder, cavitation most often occurs on the cap side of the piston when it is extending quickly, and especially when it is attempting to stop an over-running load. It also occurs when the rod is pointing downward while supporting a heavy tensile load on the end. Some experts would argue that this is not true cavitation, but its effects are the same.
One of the easiest ways to detect cavitation in a hydraulic cylinder is by sound: when cavitation bubbles implode, you can hear them in the form of an intense rattling sound. You will also notice pitting on the surface of parts affected by cavitation; in the case of hydraulic cylinders, it can be found on the surface of the rod and the inside surface of the cylinder.
Problems Caused by Cavitation
Cavitation can lead to multiple problems within a hydraulic system as a whole, as well as hydraulic cylinders. These include accelerated wear of critical surfaces, seal failure, generation of unwanted heat, reduction in hydraulic oil quality, and lubrication issues.
Accelerated Surface Wear
One of the most serious problems caused by cavitation is surface wear, sometimes known as pitting. Keep in mind that the vapor bubbles implode at an almost molecular level, releasing a tremendous amount of force over an incredibly small area. That leads to high stresses — stresses high enough to remove metal from surfaces adjacent to those bubbles. The result is highly accelerated wear in the form of surface pitting and internally generated contamination. The metal particles removed from the surface will stay within the system and can cause the formation of even more cavitation bubbles.
Premature Seal Failure
Also, keep in mind that surface finish can be an important factor in many applications, and especially with hydraulic cylinders. When selecting an appropriate seal for a hydraulic cylinder, the surface finish of the rod is critical. If the rod is experiencing pitting due to cavitation, it will cause premature wear of the seal. As the seal begins to wear, hydraulic fluid can leak out (reducing performance) and environmental contaminants such as dust and moisture can make their way inside.
Excessive Heat Generation
The bubble implosions also lead to high temperatures, sometimes reaching up to 5,000°F. In addition, as the surface is damaged, there will be increased friction as fluid flows over. That friction results in not only system losses and lower efficiency but also unwanted heat generation. This is one of the reasons why excessive heat can also be a strong indicator of cavitation.
Reduction in Fluid Quality
Because of the high temperatures that can result from cavitation, the hydraulic fluid is very likely to suffer degradation (an overall reduction in fluid quality). Excessive heat and high temperatures will cause the fluid to age more rapidly than normal, seriously affect the additives present, and reduce the viscosity of the fluid.
Lack of Lubrication
The presence of bubbles within the hydraulic fluid can also cause a lack of lubrication, resulting in metal-to-metal contact and premature wear. This type of wear takes on a different form than that resulting directly from cavitation implosion and explosion: it will look like abrasions in a pattern consistent with the movement of the metal parts rather than pitting.
One of the means of preventing cavitation lies in temperature control. If the hydraulic fluid is too viscous, pressure drops will become more of a problem; this increase in viscosity can be the result of hydraulic fluid at a less than ideal temperature. In addition, high temperatures combined with low pressures can also cause cavitation, so care needs to be taken to ensure that the hydraulic fluid is as close as possible to an ideal operating temperature. In some instances, this can be as simple as insulating hydraulic pipes against direct sunlight.
Piping losses can contribute to cavitation and result from issues such as the use of too many fittings, a collapsed pipe liner or suction pipe, a gasket protruding into the pipes, a replacement pump that has too much capacity for the system, or the buildup of solids on the interior surface of the pipes.
Another way to prevent cavitation that is especially applicable to hydraulic cylinders is to prevent air from entering the hydraulic system. For example, hydraulic systems and some components should be bled properly after they are filled (or refilled) to release the air trapped within the system. When additional fluid is added to the system, it should be introduced gently to prevent splashing and agitation — both of which can allow air to be entrained with the fluid. In addition, improperly designed hydraulic reservoirs can inadvertently encourage the entrapment of air within the hydraulic fluid.
The presence of contaminant particles within hydraulic fluid can serve as a starting point for cavitation bubbles to form, so keeping the hydraulic fluid clean and the system free of contaminants can be an excellent starting point. This involves changing hydraulic filters as needed, filtering any hydraulic fluid that enters the system, and addressing leaks as soon as they are detected.
Repairing the Damage Caused by Cavitation
When parts have experienced surface damage due to cavitation, repairs are often a lost cause. For parts with a circular geometry, such as rods and cylinders, the pitting damage is often too deep by the time it is discovered for polishing to work because rods and cylinders must meet very strict surface conditions and geometric tolerances. Unless the damage was discovered quickly, it would be wiser to replace the damaged components. For seals that have been damaged as a side-effect of cavitation, replacing the lip of the seal may be enough.
Cavitation causes so much damage: accelerated surface wear in the form of pitting, premature seal failure, generation of unwanted heat, compromised hydraulic fluid quality, and lack of lubrication leading to additional wear. The best way to deal with cavitation is to take measures to prevent it because once it wreaks havoc on the surfaces of equipment (including hydraulic cylinders), it can be extremely difficult if not impossible to repair. Preventative measures against cavitation are the wisest course of action and can often include basic maintenance principles (e.g., bleeding air out of the system, filtering hydraulic fluid). As soon as hydraulic cylinders start making rattling noises or become hotter than normal, it is time to get them checked out.
At MAC Hydraulics, our highly skilled team can maintain, troubleshoot, and repair hydraulic systems and components, including hydraulic cylinders. Our repair services include replacing seals, polishing rods, and honing tubes — and we also have a 24-hour resealing pick up and delivery service. MAC Hydraulics can manufacture custom cylinders, tubes, and rods with our state-of-the-art fabrication facilities and service any brand equipment from industries including construction, recycling, manufacturing, rental, aviation, and waste handling. If you are experiencing symptoms that are consistent with cavitation, we will help you track down the cause and keep it from happening again. Contact MAC Hydraulics today for all your hydraulic cylinder needs!