Strange Malfunction of Closed Loop Transmission

    I have this small black book, you know (to tell the truth it is more dark blue than black...), where I write down troubleshooting episodes in which different types of hydraulic machinery turned out to be stronger than me. The cases aren't numerous, thank God, but mysterious in the sense that they transcend my (most likely limited) understanding of hydraulics, and until now I wasn't able to come up with a plausible theory that could explain the malfunction in question, or alternatively, I came up with a theory, but wasn't able or didn't have means to confirm it. The following case is one of the few I recall almost every day...

   The main character of the novel is a John Deere 1210B forwarder driven by a standard Sauer Danfoss series 90 plus series 51 closed loop transmission (the type that I repaired and troubleshot a zillion times before). Since the malfunction I am about to describe gave me so much gray hair, I was diligent enough to combine a more or less accurate list of the events in chronological order (written down in the "black book", of course) which I am now sharing with you in the form of a free-style tale.

   The story begins with a good client of ours buying a used but "good as new" forwarder on a , let's say, regional market - hand to hand, hence reasonable price but no warranty whatsoever, and the predicaments start pouring down...

    Predicament one:

    The machine ran well for a couple of months, and then one sunny morning refused to move in one direction. Bollocks! The usual mechanic was called in - and upon inspection discovered a broken multifunction valve. The man happened to have a couple of used multifunction valves in his toolbox which allowed him to solve the problem on the spot. The forwarder was as good as new again.

    Predicament two:

    The machine ran well for another month, and then one sunny morning refused to move at all - this time in both directions... Bollocks! The usual mechanic was called in - and upon inspection discovered a shattered multifunction valve -with the poppet missing and lots of brass scraps, bits and shavings inside the loop. The complete transmission was dismounted from the machine, and sent over to our shop for repair. When we opened the units, we discovered that the poppet bits had done serious damage to the rotary groups both in the pump and the motor. The damaged parts were replaced, the pump got new multifunction valves, the units were tested, and then the transmission was re-mounted on the machine. The working theory at the time was - the "second hand" multifunction valves mounted a month before were "too used". The forwarder was as good as new again.

    Predicament three:

    The machine ran well for another thirty odd days, and then one sunny morning - bam! - it stopped. Bullocks! The first thing the usual mechanic checked was, of course, the multifunction valves. Shattered, again, and apparently this time on the other side, although nobody could tell for sure. Once more the damaged transmission took a trip to Lisbon (we're in Lisbon, you know). This time, since the damage was even more catastrophic than before, it was cheaper for the owner to buy a brand new Sauer Danfoss series 90 pump we had in our stock, plus the motor got overhauled as well, and all the parts used for this and previous overhauls were original Sauer Danfoss parts, by the way. The usual mechanic (the man with almost twenty years of experience) remounted and recommissioned the machine. The forwarder was as good as new again.

   Predicament four:

   The machine ran well for two months, and we already started to forget about the unfortunate set of failures when - kaboom! - one sunny morning the machine refused to move yet once again. Bollocks! This time I went along with the mechanic to check it out and, as we were already suspecting - one of the multifunction valves had a shattered poppet. The transmission had to be "re-born" once more, and that time, a specially adapted manifold with two cross-port fast acting relief valves was mounted in the legs of the loop via two one inch hoses. The block was supposed to cut the pressure spikes, as well as the additional hose segments were supposed to absorb them. The pressures that time were set a little lower than the recommended John Deere's settings - the pressure limiting system of the pump was set to 420 bar above charge pressure, and the fast acting relieves were set to 450. This time, by the way, the standard number one multifunction valves were replaced with number fives (the ones with the smaller poppet orifice). The loop then was tested in the field conditions with fast acting pressure gauges - and no pressure spikes above 470-480 bar were recorded. The forwarder was as good as new again.

    Predicament five:

   The machine ran well for another couple of months, and then one sunny morning it stopped... The operator tried reverse - it went for five meters or so and stopped again. Bollocks! And then - the usual shattered valve, the usual overhaul, the usual assisted start up - with complete flushing, cleaning and all possible and impossible precautions taken. The result - the forwarder was as good as new again.

   Predicament six:

    As we were trying to figure out what to do with the machine and what more tests to make - the owner called (and yes - it was a sunny morning that day)! After but a few hours the machine stopped and - you guessed - one of the new multifunction valves was shattered again! Bollocks! This time we agreed to let mr. John Deere himself do the work, since we could fix and test pumps till the end of the world, but had no means to diagnose and fix the mechanical part of the transmission. The last working theory was - the gearbox managed to create certain unfavorable conditions, and possibly vibrations or load fluctuations, which were causing resonance effects inside the high pressure part of the pump. The John Deere guys checked the machine, found no apparent faults in the mechanical part of the transmission, and since they had no pumps or motors in stock, they took out a complete closed loop from another used machine they had for sale. The machine had some 7000 hours, and the hydraulic transmission had never caused problems before. The forwarder was as good as new again.

    Predicament seven:

    Thirty five work hours later the forwarder stopped! Again! Bollocks! This time not the owner, but the John Deere's representative came to our shop with this pump and this motor, once again the units were repaired, and the consecutive commission was performed by the official John Deere workshop with original spares, service proceedings, high quality hydraulic oil and all... The forwarder was as good as new again.

    Predicament eight:

   Three to four months later - another catastrophic failure - with exactly the same pattern! Bollocks! If I could - I would take this machine to a laboratory and study it to the slightest detail with all sophisticated gear I could get my hands on to find out what exactly was causing the f#cking valves to shatter, but... The machine was taken again by the Joh Deere specialists, who worked "behind closed doors", and after a long time delivered the machine to the client, and it never broke again! This time I had no inside information about what exactly they did - so it has been a mystery to myself ever since.

    It took me a little more than one page to describe the adventure, but believe me - it involved A LOT MORE!

   Last time when I was in Neumunster at the Sauer Danfoss factory - I presented the case to the Sauer Danfoss production engineers and workshop personnel - and nobody could even imagine what could possibly cause this set of failures.

   An alternative theory was suggested at certain point - the theory of sabotage... But I, personally, find it even more impossible to happen in that case than anything else. It was a family business, and the owner and his son were the operators, who, by the way, had a lot of experience with alike machines. I know the men well enough to believe that they would never shoot their own feet, if you know what I mean.

So, there you have it - apparently it is possible to cause premature failure of multifunction valves in a Sauer Danfoss series 90 pump by conditioning the load alone, without surpassing the maximum pressure or speed levels. I would only like to know how exactly..