The principle behind the oil cup in centrifugal pumps is not convincing based on the principle of communication vessels
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When the oil level in the bearing housing drops, a gap is created between the oil drain port of the oil cup and the oil surface in the bearing housing. Air enters the oil cup, and changes in pressure inside the oil cup cause the oil to flow out. This phenomenon is described as a communicating vessel, and that’s what is stated in books. Can someone explain to me why this is the case?
The oil in the bearing housing and the oil in the oil cup form a communicating vessel. The external forces present are atmospheric pressure, and they are all under atmospheric pressure
If you fill a bottle or jar 3/4 full with water and turn it upside down in water, will the water in the bottle flow out? It won’t! When you remove the bottle opening from the water surface, the water will flow out. The oil level in the bearing housing is the same as that in the reservoir located below the level control cup. When the oil level in the bearing housing drops, the oil level in the reservoir beneath the level control cup also decreases. When it drops to a certain level and the bottom of the cup becomes exposed, air will enter the cup, causing the oil inside it to flow out. Once the oil level covers the bottom of the cup, no more oil will be supplied to it
I had the same doubt before. If it’s a communicating vessel, then the liquid levels on both sides should be the same, right? That’s what a level gauge is for. It was only later that I understood that the air at the top of the oil level in the oil cup is not the same as the atmospheric pressure, whereas the liquid level in the bearing box is in contact with the atmosphere. When oil is added, the level in the oil cup drops, causing the volume of air at the top to increase and the pressure to decrease. The gas pressure is then not sufficient to force oil into the bearing box, so the liquid level remains constant.
The bearing housing and the seat of the constant-level oil cup form a communicating vessel, so the liquid level in both is at the same height