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    Once upon a time a client from far far away sent over an A2FM32/61W Rexroth hydraulic motor to repair. The motor was used in a simple open circuit to run a big wood cutting saw. Local mechanics had already opened the motor and verified that the unit was in bad shape, so they joined all the pieces and reached for "professional" help.

    Quick verification showed that, in fact, the motor was pretty much destroyed, and so the complete rotary group had to be replaced - the cylinder block,  the pistons, the central pin, the retainer plate and the valve plate, topped up with a seal kit. The motor was assembled, tested and sent to the client.

     Not a day passed when the man called claming the motor was not working properly - it would run fine for about a minute, sometimes less, and then would stop.  To set it again in motion the machine had to be stopped and restarted. Suspecting bad start-up, contamination, mounting error, component failure, etc... he was advised to dismount the motor and send it over (the guy opted for a more than 350 km long drive and brought the motor himself, because he really needed the saw running!)  for verification.

    When the motor was disassembled, the rotary group appeared to be in perfect condition, apart from some minor scoring of the valve plate and the cylinder block, all the parts looked as if they'd just come from the shelf. Even the client got puzzled, because he was expecting to see a major failure. He was claiming that the motor had stopped rotating completely, and was also positive that the rest of the hydraulic circuit was working fine. As no damage was detected, the motor was reassembled, retested in front of the client, showed perfect efficiency and the client was advised to look for a problem elsewhere in the circuit of the saw.

    The man took the motor back (another 350 km stretch) prepared to look for a malfunction. He was calling the next day to say the motor was stopping the same way as before. A couple of minutes of service - that's all he could get from the saw, then the motor would stop. The man had a happy idea to check the motor drain line, and when the motor stopped the line presented HUGE increase in flow, so it had to be the damned motor!

    Although the poor fellow was not happy about the hours he'd spent behind the wheel, he had no choice but to bring the motor over once again. The motor went through the same procedure as before - it was disassembled, no wear was detected, it was assembled, tested - and worked OK. At this point yours truly was asked to give it a glance. It would be really ugly for the man to have to go through the same trouble again...

    There's nothing I like more than "puzzle solving", especially when it has something to do with hydraulics. Not only the problem had to be nailed, a way to convince the client that the problem was  detected and solved had to be found, which could only mean one thing - creative testing! (Hurray!!!...)

     A few words about how the previous tests were made. It was a fixed displacement motor with no loop flushing block, so the first time only a very basic test was performed - the motor was connected to a test bench distributor, and then a needle valve was used to restrict the flow coming out of the motor to raise the pressure inside the rotary group, then the case drain flow was evaluated to come to a conclusion if the rotary group had excessive leakage (simplified schematics). Although this test is not perfect, it is a fast way to check for rotary group excessive case leakage and, also, verify the existence of external leaks. The second time the motor was mounted on a test bench, which used another hydraulic motor to create braking torque, again the case drain, entering flow and the RPMs were being monitored, different speeds and pressures were applied and yet again the motor was performing perfectly - as any decent motor that'd just gotten a rotary group replaced would.  Obviously the workshop test was not recreating the real life working conditions.

    At the machine, the motor was spinning a massive wood saw, which had tons of inertia, there even was a system of anti-cavitation  check valves installed to compensate for the blade over-run when the distributor would decrease oil supply. To simulate that condition, I coupled the motor to a 30 KW electric motor, to use the electric motor massive rotor to simulate the heavy blade overrun conditions. The hydraulic motor was still being run by the first test bench.

    During the previous tests the case drain had always been insignificant, very low, so it had to be evaluated in a very scientific "by eye" manner. This time was no different, the case drain hose was hanging freely from side of the motor. I started the pump, and slowly moved the distributor handle to set the hydraulic motor/electric motor assembly in motion. As I was increasing and decreasing the speed of the motor, there was a moment when it suddenly lost all the torque and the drain line, instead of giving out a match thick trickle was pouring out a serious jet, having immediately set the standers-by fleeing. There it was, the malfunction, staring us in the face.

    I like to say that successful remake of a problem in workshop equals solving the problem. There it was - all the oil entering the motor and ending up in the casing. However, stopping the flow and the motor, and then starting it over again slowly  made it run fine. And then when I decelerated it abruptly, and then accelerated - the case drain boosted up once again... The first and most obvious explanation would be the cylinder block lift, but how? With new original parts? Well, let's tear it down once again!

    All the parts were checked, the valve plate was a perfect match to the cylinder block, the disc springs (or belleville washers) were there, wait a minute, what's this? It became clear at once what the problem was! The mechanic who'd mounted the motor had mounted only four bellville washers (the nasty A6VM habit), while this motor required six! As you can see from the picture, four washers, even when stacked in an alternating direction, are lower than the guide washer. Even with new parts, the cylinder block was not sufficiently pre-loaded against the valve plate, It was merely touching it, allowing it to pass all the tests, as the steady system pressure would hold the hydrostatically compensated cylinder block compressed against the valve plate. 

    When the massive saw blade was over-running the oil supply, low pressure was being created in the motor line ( the motor working as a pump), and as the cylinder block was not sufficiently pre-loaded, the case pressure was enough to cause the cylinder block lift. Once lifted,  the block would stay separated due to the passing oil flow and wouldn't settle until the the oil supply stop.

    The test was a classic cylinder block lift situation. It was so classic that I even made a video of it (for future didactic purposes), which was then successfully  erased from the camera by a colleague of mine (thanks a lot!). The problem was solved by adding the two missing washers. By the way, there are different ways you can stack the washers. Rexroth advises to stack them in the following manner. I know from experience that these motors run fine when all the six washers are stacked in an alternating direction, so if you mounted one like this, don't beat yourself about it. Stacking the washers in parallel does create a stiffer spring, though. Belleville washers are the old design, all newer series use coil springs, which are better.

     What does this prove? It proves that pre-loading of a cylinder block is not a mandatory condition for high volumetric efficiency. A well designed cylinder block is held compressed against the valve plate by the system pressure, so the whole pre-load spring system is only necessary to guarantee that it  stays compressed when there is no hydraulic force condition, which happens at start-up, or, as we've just seen, during the overrun, when a lower than the case pressure develops in working lines. Of course, there are also vibrations, pressure surges and peaks, extreme velocities, temporary low oil viscosity, and many other things contributing to block lift and making the pre-load system necessary. But at a steady speed and pressure you can completely remove the pre-load springs, and the cylinder block will remain adequately compressed against the valve plate by means of the system pressure only!

    Understanding this phenomenon as well as understanding what a cylinder block lift is and how it can manifest itself is very important for correct troubleshooting. It is also critical to be aware of the fact that standard workshop hydraulic tests are nothing but a good approximation to real life conditions, therefore some malfunctions are very difficult to recreate in a workshop. In these situations a good technician should be able to think outside the box and be ready to perform "creative testing" to nail the problem.