OT a litte,
Years ago had to have been 30 I guess a customer called me to come out and repair a hydraulic pump on a Heald ID grinder. As I walked into the bigger shop I could hear it. It was a Vickers vane pump and it is very common when the pump is loud it is sucking air. It's a high pitched whine. The only place it can suck air is on the suction side of the pump or around the motor shaft where there is a seal or back plate seal. An old trick my Dad showed me was to squirt oil on the shaft as it enters the pump and the pipes in the suction side as it goes into the oil holding tank. You listen for the noise to change pitch. I did that and no change, Then I said to the maintenance man who was standing there.
Have you checked the oil level? He said he had just added more oil before I came and it was full. Hmmmmm...sounded good, but I had him remove the cover on top of the tank to have a look for myself. They shut off the machine, pulled the cover and it looked more like milk then light brown hydraulic oil. I said that air in the oil, Then I rolled up my sleeve and reached in the goo....As I felt for the oil strainer in the bottom of the tank I felt a layer of thick goo and when my hand reached the strainer (some call it a oil filter) which was around 3" diameter and 10" long and it screws onto a pipe that leads to the suction side of the pump.
It was covered with a thick layer of goo and that was the problem the pump was cavitating. I said to the maintenance man when is the last time you changed the oil? He gave me this dumb look and said he had never changed the oil, just added it when it was low. I laughed and said "try that with your car engine and see how long it lasts. Moral to the story is change the oil in your machines depending on how often you use them.
This also is so important to clean and change the way oil pump which on many of the Knee Mill is a BiJur style one shot pump and there is a strainer felt and screens on the suction side of the pump. :-) Rich