Insane Hydraulics
Bold and audacious blog about fluid power
Like most pump manufacturers, Rexroth makes great pumps, good pumps, and "me-e-h" pumps. Take the ubiqutous A10VO series 32, for example. These pumps make use of the classic opposed servo-cylinder design, and are a perfect choice for medium duty/medium pressure applications, respectably delivering 5 to 10 thousand hours of reliable operation before needing an overhaul. We repair them all the time, and I consider this series to be good pumps.
Unfortunately, I can't say the same about their "younger brother" - the A10VO series 52, which employs a much more compact single servo-piston and off-centered swash-plate arrangement. For me, it's only a "me-e-eh" pump (which, I remind you, is still better than bad). The reason I am saying this is that when I happen to overhaul this series, I find the following two issues with the swash plate mechanism more often than I would like to: either the housing gets worn out and ovalized in the servo-piston area, or the swash-plate blows out the internal orifice, which then damages the bearing liner and often the swash-plate itself.
While spotting an ovalized housing is easy, the loose swash-plate orifice can escape detection during an overhaul, and this is why I want to give this "sneaky" failure a detailed description.
A few words about this servo-mechanism. From an engineering standpoint, this design is perfect. I love the idea, I really do. If you look at the cutaway view - you see nothing but advantages:
To recap - this design is smaller, lighter, faster, and cheaper. My only issue with it is that I have been finding it to be not as reliable as the older series was... I've just received two more A10VO85s, and both of them had the same "loose orifice" problem. Am I starting to see a pattern here?
Take a look at the cutaway view again - notice that the swash plate is marked with two numbers - N7 and Nº32 - the Nº 32 is the hidden orifice. The orifice is of grub-screw type, and it is mounted inside of the threaded hole in the swash-plate, blanked by a break-off plug.
Threadlocker or not, the orifice eventually unthreads itself and becomes loose inside the drilled hole, bashing itself around until it finds its way to the area between the swashplate and liner, where it gets to do the most damage. When you open such a pump, the deformed orifice (or pieces of it) fall off the liner when you remove the swashplate - and so even if you find it inside the casing, you will not associate it with its "swashplate origins" unless you already know it's from there.
Furthermore - if you inspect the rest of the components, looking for a place this orifice could have originated from - you won't find any, and so you may think it's from somewhere else and eventually leave the problem uncorrected.
Another thing that may happen (and you can see this in this picture) - the orifice may already be loose, but still inside the swash-plate, and if you don't take it out and replace it - it will damage the recently overhauled unit.
In case you're wondering - if you don't manage to "coax" the loose orifice out - these break-off plugs are easy to remove with a MIG welder. Apply a decent amount of anti-spatter spray to the swashplate, put a small shim on top of the plug, and weld it, then find a nut, put it on top of the shim, and fill the inside of it with more weld. The heat of the welding process will undo the thread glue and the plug will come out without a fight:
