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     These are a few parts originated from a Sauer Danfoss series 90 closed loop pump that was brought over to our workshop for repair. The main symptom was - "the machine doesn't work". The reason why the pump had stopped pumping was discovered as soon as it was disassembled - everything inside was covered with a very rigid and very smelly varnish-like substance, which completely seized and glued all moving parts inside the control module, as well as clogged all the sensitive and not sensitive orifices. Clearly something very "chemical" had happened with the hydraulic fluid. Hydraulic oil degradation and the associated varnish and sludge deposits are no news to hydraulics people, but this particular case was different, or better - more uncommon, both due to the main reason that caused the appearance of the deposit and to the chemical that was used to remove the smelly stuff.

    Further investigation revealed that the misadventure had happened during the regular maintenance stop of an all hydraulic tomato harvester, in which both open and closed circuits were fed by the same 200 liter oil tank. As a logical part of the preventive maintenance plan the tank was topped up to the due level - after this the machine started, worked for about ten minutes an then "died"...

   It turned out that the twenty odd liters of "hydraulic oil", that was used to top up the tank, was in fact some agricultural bug-killing formula, that was very conveniently stored in an empty hydraulic oil drum right next to other drums with hydraulic oil inside. The mixture caused an incredibly smelly chemical reaction, that resulted in the appearance of the very hard varnish-like deposit that spread all over the hydraulic system!!! Everything that was a spool got stuck and everything that was an orifice got clogged. The culmination point was reached when the charge pressure relief valve got stuck-shut, and the spin-on charge oil filter was blown away from its place by the oil pressure!

    It was clear that all the parts needed to be thoroughly cleaned in order to make the pump function - so a series of solvents were tried to remove the deposit. We tried petrol, gas, kerosene, benzin, all sorts of thinners and spirits with no or very little result - the deposit was incredibly resistant to all those agents, and was very hard to be removed mechanically. You know what  solved the problem in the end (I mean - literally - solved)?. Simple tap water!!! The tuff gluey deposit was completely dissolved and removed by warm soapy water! Since the pump suffered no mechanical damage, the repair process resumed to washing the parts and assembling the pump (and the motor, of course...). As you can imagine, the similar procedure had to be performed to the rest of the machine's components, which was neither fast nor cheap...

    This case brought me back to the time when we had one of those parts washers that used hot water based solution - it didn't stay for long, but I'm sure it would've been extremely handy for this particular "cleaning".
Varnish deposit caused by hydraulic oil degradation
Varnish deposit caused by hydraulic oil degradation
Varnish deposit caused by hydraulic oil degradation
Varnish deposit caused by hydraulic oil degradation
Varnish deposit caused by hydraulic oil degradation
Varnish deposit caused by hydraulic oil degradation