Insane Hydraulics

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The Houdini Pump

A friend of mine from a Deere's forestry equipment dealership called the other day asking if I could take a look at the hydrostatic transmission I repaired for them a couple of years before.

I asked what the problem was and he said that he was running the transmission calibration routine, when he heard a "bang" and the machine stopped, with the charge pressure in the closed-loop dropping to 5 bar. "Sounds like a catastrophic failure to me" - said I - "send it over". He also mentioned that the owner had some issues with the hydraulic motor displacement control, and he eventually replaced the motor, and then the pressures of the pump were raised to about 470 bar to give the forwarder some extra "oomph".

I got both the pump and the motor. The motor had nothing but normal wear, however when I opened the pump... - I had to stare at it for a couple of minutes trying to figure out what exactly had happened and wondering if I was "seeing it right"...

And now - we've reached the point where I explain why this post is called "The Houdini Pump".

The pump performed a magic trick. An impossible magic trick!

When I opened the unit and removed the cylinder block with the pistons and the retainer plate - I saw the ball-guide resting on top of the swash-plate!

If you look at how the rotary group of an A4VG pump is assembled, you will see that it is impossible to pull the ball-guide (part Nº 25) through the retainer plate (part Nº24) without breaking the retainer plate! And, apparently, this pump somehow managed to pull it off by only mildly deforming the said plate.

I thought a lot about how this could happen, and the most plausible explanation I can think of would be the following:

1) First, one of the piston slippers cracked into three nice-looking pieces (I am still wondering if it was a pressure spike or a maybe the fact that the pressure was set a tiny bit higher than normal) and left one of the pistons headless.

2) Then it hammered the swash-plate for a little while, turning the spherical head into a mushroom.

3) And finally (with a "bang") the deformed piston got stuck in the barrel when the swash plate was tilted and during the next half-rotation picked the retainer plate up with the mushroom head and yanked it over the ball guide like a skilled magician whips off a tablecloth from under a set table.

It must have happened during a single turn, because the retainer plate got deformed, but did not break!

I tried pushing the ball-guide through the deformed plate - and it does pass through, albeit with a little help from Mr. Mallet.

So, there you have it folks - a pump that did a magic trick!