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
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
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.