Like most pump manufacturers, Rexroth makes great pumps, good pumps, and "me-e-h" pumps. Take the extremely common 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 a good pump.
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 that "bad"). And 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 "lost" swash-plate orifice can escape detection, and this is why I believe it's important to describe this failure.
A few words about this type of servo-mechanism. From the engineering standpoint this design is perfect. I love the idea, I really do. If you look at the cut-view - you see nothing but advantages:
- The fact that the swash plate is off-centered, and the sum of the piston slipper forces projected on the swash-plate is asymmetrical and directed towards maximum displacement, means this solution doesn't require neither a bias piston nor a strong bias spring.
- Only one servo-piston and a simple link is needed to tilt the swash-plate.
- Swash plate liners lubrication is performed through the piston slippers via the orifice in the face of the swash-plate - so no extra lubrication channels are required
- Reduction of the mass of the swash-plate mechanism gives it a better dynamic response
- Much less material is required to manufacture this design, which increases power to weight ratio of the pump and makes manufacturing process more economic.
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 "lost orifice" problem. Am I starting to see a pattern here?
Take a look at the cut-view again - notice that the swash plate is marked with two numbers - N7 and Nº32 - the Nº 32 is the 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.
I can't explain why, but 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 swash-plate 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 swash-plate - and so even if you find it inside the casing, you will not associate it with its "swash-plate 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 the near future.
In case you're wondering - if you don't manage to coax the loose orifice out - these break-off plugs are easy to remove if you have a MIG welder around. Just apply a decent amount of anti spatter spray to the swash-plate, 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 is more than enough to undo the thread glue and the plug should come out without a fight.
So, if you ever happen to work on a Rexroth A10VO series 52 pump:
a) if you find a semi-bashed orifice inside of the pump case - you will know where it came from
b) if you don't find any, still, make sure to inspect the swash-plate and shake it vigorously after you've cleaned and dried it. Hear any clinking noises inside? You know what to do.
Add-On from 19 May 2019 - Rexroth A10VO Series 53 - Failure Analysis