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    Everybody, who works with hydraulic systems, has heard of the so called "cavitation" damage, and I have seen many mechanics directly involved in pump/motor overhauls look at those pitted surfaces, nod their heads, and say "m-m-m, cavitaiton...", some of them pointing with the index finger at the ceiling in the process...  Which obviously means that most people connect the appearance of pitted surfaces on parts with that cavitation thing, but doesn't mean they understand how it happens.  As I often get asked cavitation related questions, I decided to make the following (very brief) post.

    First of all, let us call things by their names. Cavitation is the formation of vapor bubbles (cavities) inside the oil when the pressure drops below its vapor pressure at given temperature, or the formation of gas bubbles, when the pressure drops below the saturation pressure of the dissolved gas. In real world cavitation bubbles will contain oil vapor mixed with air.

    Aeration is the process of direct mixing of oil with air, which creates air bubbles. Almost all hydraulic systems will have aeration present to a certain degree. Air bubbles, created in the process of mixing, will also contain oil vapor, but less than cavitation bubbles.

    Those pitted surfaces, that often can be seen on valve plates, cylinder blocks, manifolds, and in many other interesting places (as you can see on the pictures) are the result of cavitaion and aeration erosion. It must be said that the erosion process is still a debated topic, but the most common explanation of the phenomenon is the destruction of material by violent shock waves originated from the implosion of the above mentioned bubbles, both cavitaion and aeration, when passing through a pressure differential zone. The bubbles in question are microscopic but countless, so the erosion is caused by the effect of a zillion pulse loads on the surface and is not an "instant" process. Cavitation bubbles implode more violently than aeration bubbles, but adiabatic compression of air bubbles causes extreme local temperatures, which also is not good for the oil. 

    On the pictures you see a AA11VLO250 Caterpillar pump (it's NOT Caterpillar, it's a Rexroth pump with a "unique" Caterpillar protected reference) back cover. The pics show back covers of two equal pumps from the same machine, which present the same type of erosion. Main purpose of the pictures is to show that cavitation and aeration erosion can emerge in unexpected places and can cause substantial damage, especially on relatively soft materials, like, for example, the cast back cover. In this particular case, the erosion was caused by the pulsing stream of oil coming from the side orifice of the valve plate, connected to the pressure/suction transition zone. Every time a pressurized cylinder block bore passed over the orifice, an oil pulse was generated, creating a super-fast jet of oil directed towards the back plate.

    Cavitation and aeration are very undesirable phenomenae, which aside from erosion bring along excessive noise, overheating, efficiency and controlability issues, oil deterioration and must be avoided at all costs.