I have this small black book, you know (it is more dark blue than black), where I write down troubleshooting episodes in which different types of hydraulic machinery managed to "outplay" me. The cases aren't numerous, thank God, but they transcend my (likely limited) understanding of hydraulics. Either I wasn't able to come up with a plausible theory that could explain the malfunction in question, or I did come up with a theory, but wasn't able or didn't have the means to confirm it. The following case is one of the few I recall almost every day.
The protagonist of the "novel" is a John Deere 1210B forwarder, propelled by a Danfoss series 90 closed loop pump coupled to a series 51 motor (the stuff 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 acquiring a used but "good as new" forwarder on a, let's say, regional market - hand to hand, hence the reasonable price tag and zero warranty. And the predicaments start pouring down...
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.
The machine ran well for another month, and then one sunny morning refused to move at all - in both directions this time... Bollocks! The same mechanic was called in - and he found another shattered multifunction valve - with the poppet missing and lots of bronze shavings inside the loop. The complete transmission was dismounted from the machine and sent over to our shop for an overhaul. 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 parts were replaced, the pump got new multifunction valves, the units were tested, and the transmission was re-mounted on the machine. The working theory at the time was - the "second-hand" multifunction valves installed a month before were "a bit too used". The forwarder was as good as new again.
The machine ran well for another thirty-odd days, and then one sunny morning - bam! - it stopped. Bullocks! The first thing the mechanic checked was, of course, the multifunction valves. Shattered, again, and this time on the other side, although nobody could tell for sure. Once again the damaged transmission took a trip to our Lisbon shop. This time, since the damage was even more catastrophic than before, it was cheaper for the owner to buy a brand new Danfoss series 90 pump we had in stock. The motor got another overhaul, and, by the way, all the parts used for this and previous overhauls were original Danfoss parts (Sauer Danfoss at the time). The usual mechanic (the man with almost twenty years of experience) recommissioned the machine. The forwarder was as good as new again.
The machine ran well for two months, and we already started to forget about the unfortunate chain of failures when - kaboom! - one sunny morning the machine refused to move once again. Bollocks! This time I went along with the mechanic to check it out and we pulled out yet another multifunction valve with a shattered poppet (just as expected, given the history). The transmission had to be "re-born" once more, and this time, a manifold with two cross-port fast-acting relief valves was added to the loop via a couple of one-inch hoses. The idea was to cut the pressure spikes with the valves and absorb "any other pressure noise" with the elastic quality of the added hose segments. 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 type "one" multifunction valves were replaced with "fives" (the ones with the smaller poppet orifice). Then the loop was tested in field conditions with fast pressure logging - and no pressure spikes above 470-480 bar were recorded. The forwarder was as good as new again.
The machine ran well for another couple of months, and then one sunny morning it stopped... The operator tried to reverse - it went for five meters or so and stopped again. Bollocks! And then - the usual shattered valve, the usual overhaul, the usual recommission - with flushing, cleaning, and all possible and impossible precautions taken. The result - the forwarder was as good as new again.
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 it - 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 days, but had no means to diagnose and fix the mechanical part of the transmission. The latest 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.
Thirty-five work hours later the forwarder stopped! Again! Bollocks! This time not the owner, but a John Deere representative came to our shop with the pump and the 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.
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 the sophisticated test 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 me ever since.
It took me a little more than one page to describe the adventure, but believe me - it involved a lot more!
When I was visiting the Sauer Danfoss factory at Neumünster, I presented the case to the production engineers and workshop personnel - and nobody could imagine what could possibly cause such a chain of failures.
An alternative theory was suggested at a certain point - the theory of sabotage... But 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 knew 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. I would only like to know how exactly...
Still fancy becoming a hydraulic tech?