Machinery Lubrication

Machinery Lubrication March April 2017

Machinery Lubrication magazine published by Noria Corporation

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14 | March - April 2017 | HYDRAULICS Cavitation or Aeration? By Al Smile y, GPm HydrAulic conSultinG Many maintenance technicians confuse cavitation and aeration. In fact, aeration is sometimes referred to as pseudo cavitation. While these two conditions have similar symp- toms, their causes are entirely different. What Is Cavitation? Cavitation is the formation and collapse of air cavities in liquid. When hydraulic fluid is pumped from a reservoir, a low-pressure drop occurs in the suction side of the pump. Despite what many people believe, the fluid is not sucked into the pump but rather pushed into it by atmospheric pressure, as shown in the left illustration below. The movement of the rotating gears leads to a drop in pressure at the suction line. The resulting pressure difference between the reservoir and the pump inlet causes the fluid to move from the higher pressure to the lower pressure. As long as the pressure difference is sufficient and the flow path is clear, the oper - ation goes smoothly, but anything that reduces the inlet flow can create problems. Whenever the pump cannot get as much fluid as it is trying to deliver, cavitation occurs, as shown in the right illustration below. Hydraulic oil contains approximately 9 percent dissolved air. When a pump does not get enough oil, air is pulled out of the oil. These air bubbles travel into the pump and eventually collapse and implode when they reach an area of relatively high pressure. The ensuing shock - waves produce a steady, high-pitched whining sound and damage to the inside of the pump. Causes of Cavitation Any increase in fluid velocity can lead to cavitation. Fluid velocity is inversely propor- tional to the size of the hydraulic line. Most pumps have a suction line that is larger than the pressure line. This is to keep inlet velocity low, making it very easy for oil to enter the pump. Any blockage, such as a plugged suction strainer or filter, can result in the pump cavitating. A contaminated suction strainer is the most common cause of cavitation simply because it is under- neath the oil level in the reservoir. One of our consultants was recently called to a plant in Georgia that had changed five pumps on a machine within a week. The first thing that was noticed was a How to Tell the Difference Sufficient available pressure Low available pressure — vapor cavities form and implode inside pump

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