One hot summer I was called to troubleshoot an agricultural machine
that was, according to the client, making excessive noise. When I
came to the site, the machine turned out to be a tomato harvester, and
indeed was making a rather loud buzzing sound. Apart from the annoying
noise the machine seemed to operate fine.
A word must be said about the owner. It was one of
those rare cases when owner actually took preventive measures to make
sure the machine would last. The machine was kept in clean condition,
oil leaks were taken care of as soon as they appeared, maintenance was
done "by the book" and, as an enormous plus, the machine had always
worked with the same operator. The operator was no mechanic, but had
"an ear" for things, and was very familiar with the machine, so he knew
when something was not normal. Whenever an unusual situation popped up,
a professional assistance was requested. I think it was one of very few
Portuguese tomato harvesters, that made harvesting campaigns without
A quick glance at pressures confirmed that hydraulic
controls part (it was a simple open circuit load sensing system)
was OK. The operator also said, that in the morning and after the lunch
break, the pump would operate with its usual noise, which then would
transform into the annoying buzz in a matter of half an hour. This
symptom alone suggested oil aeration as the most probable cause
of the noise. The fact that the noise would disappear could be
explained by air bubbles coming out of the oil during the breaks. To
confirm the theory an oil sample was taken from the oil tank at the
time of "noisy" operation. As I was expecting, it turned out foamy and
yellowish. All I needed to do now was to find out how the air was
entering the circuit.
Fortunately for me, the operator remembered that an
additional piece of equipment had been recently mounted on the machine,
and wondered if it could be causing the problem. The equipment was a
hydraulic motor and a three way flow regulator to control its
speed. The motor was quite small and was requiring little flow, so the
flow regulator was adjusted to send most of the oil to the excess flow
port. Following the hose revealed that the mechanic, who had installed
the circuit, had mounted an additional fitting to the oil tank to
connect the excess flow line. Peeking inside the oil tank with a
flashlight revealed that the oil was being discharged ABOVE the fluid
level in a needle-like jet form. To top it up the fitting was mounted
directly above the suction inlet. One couldn't possibly create a better
layout to inject air directly into the suction line!
Connecting the excess flow line to the return line, as it should have been, solved the problem.
I am describing this case here not because it was a tricky
troubleshooting case, but because although this type of failure (with
variations) is very common, there are still those, who find it
difficult to believe that presence of oil bubbles in oil can change
notably the sound of a hydraulic system, as well as reduce the life
span of a pump. It is very important to pay attention to all unusual
noises in a hydraulic system to detect aeration at an early stage.
Another thing I learned from the experience is huge benefits of a "single operator use" (single as one, not as not married).
I find that, as a rule, the fewer operators a machine has, the better
it is for its health, as unusual situations get detected at early
stages. Before making any troubleshooting decisions, it is very
important to gather as much information as you can get, and the usual
operator is the first person to ask about the machine's service history and
P.S. I do know that good (from the point of a hydraulic
machine's "health preservation") operators are NOT easy to find...