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    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 unplanned stops.

    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 recent interventions.

   P.S. I do know that good (from the point of a hydraulic machine's "health preservation") operators are NOT easy to find...