Insane Hydraulics
Bold and audacious blog about fluid power
Despite the fact that the main purpose of a charge pump in a closed loop hydraulic system is to keep the circuit filled and provide servo pressure for the displacement control, there are still people who, for some reason, are convinced that the higher the charge pressure, the better is the machine's performance. On these pics you can see a charge pump from a very old A4V Hydromatik unit. The pump had been in perfect condition a week before the pics were taken. The client claimed warranty and said the machine was running OK and then suddenly stopped for no apparent reason (gee, I wonder why?). However a closer inspection revealed a COMPLETELY BLOCKED charge pressure valve - and it was not damaged, it had been blocked on purpose, by screwing in the regulating screw till it stopped.
These simple internal gear pumps are not hydrostatically balanced, and the pressure inside the pump pushes the outer ring towards the housing and the central gear in the opposite direction, creating radial load. There is no need to balance them because their working pressure is relatively low. However if the pressure goes beyond certain level, the disbalance will cause excessive wear to both the bushings and the housing (especially the housing). In this particular case the gear-ring "machined" through the housing as far as the o'ring groove before breaking. This wear coupled with the "blocked" charge pressure relief left no doubts about the cause of the failure.
Sorry, guys, no warranty here. I can only guess how high the charge pressure went...