Hi everyone, it's Megan again from Lifcos content team. If you want to learn more about hydraulics and more indepth analysis of pumps you came to the right place. I sat down with Fraser and asked questions from our video on the PV270. This pump came in for basic troubleshooting and upon inspection there was significant wear on certain parts. We talk about the trunnion bearing and the servo piston, with schematics!
Video: PV270 Video
Megan: Damien opens this PV270 pump upside down and we see a part with significant damage that comes off (Picture 1). What is this part called and where did it come off of?
Fraser: Trunnion bushing (sometimes called trunnion bearing). It is between the swashplate and the housing and is made of brass (remember you can't have two metals of equal hardness moving against each other). That hole in the middle is for a set pin.
Megan: Why or how is the wear only on one side of it?
Fraser: Piston pumps and motors are not symmetrical: half of the pistons have a suction and half have pressure. And this one is open loop, meaning the swashplate only goes one direction of center. In normal operation the piston pressure is balanced with the swashplate pressure so that there is always a thin film of oil between the wearing parts. At some point that film became a little unbalanced and started to wear. It could just happen with time or maybe something like a piece of dirt got in the way.
Megan: Okay that makes sense and that answered my next question I was going to ask. Its interesting how it's worn completely smooth. The servo piston also had some wear and needed to be replaced. What does a servo piston do? And why does this pump have one and some others don’t?
Fraser: The servo piston, which is controlled by the compensator/controller, moves the swashplate. In some pumps it moves the swashplate on (more pump flow), and on other designs it moves it off (less pump flow). For this unit here, the Parker PV+, it is pushing the swashplate off. (What is turning it on is another component called a bias spring, plus system pressure).
This part is a little more technical: Servo is a technical term for something that controls and provides feedback. Technically the servo piston is part of a servo system which includes the swashplate and pump controller. All these are fancy words saying the pump flow is determined by the controller which gets its input from the system pressure. This is no fancier of a system than your furnace (the swashplate) getting a signal from the thermostat (the controller) which is receiving feedback from the temperature in the room.
Megan: That analogy really helps alot. Do we by chance have a schematic of one to break it down because visuals help me a lot to understand.
Fraser: (See Diagram 1) Servo piston is the large front and center object
Megan: So from what you just told me about what the servo piston does, can you help me relate it to this schematic (Diagram 1) so I can understand it a little better?
Fraser: I don't want to explain too much because the key to understanding them is looking at them repeatedly and in different scenarios. The key explanation to this one is that if the piston moves to the left, the pump is turning OFF. If you ever need help remembering, just look at the max volume stop that prevents the pump from turning on completely. It just literally stops the piston from moving too far.
Megan: Okay yeah, the best way to learn things is to just look at it over and over. But I at least definitely understand the servo piston a little more. Thank you