Insane Hydraulics

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An Oscillating Closed Loop Transmission (and More)

This Friday I had to "urgently compose" a "more or less usable" motor out of three scrapped A6VM55s with HD1D controls, and seeing the pressure control valve brought back the memories of a rainy evening when I had to diagnose an oscillating hydrostatic transmission on a John Deere forwarder.

Rexroth A6VM55 HD1D control body
Rexroth A6VM55 HD1D control body

By the way, in a minute I'll explain why the "rainy" part was especially important to make sure I never forget about that troubleshooting episode.

So, as I was recalling the transmission case, a thought crossed my mind - have I ever written about that case in the blog? I am not sure. I looked through the headlines, and it seems I have never mentioned it. It's a good one, and self-excited oscillations in hydraulic systems are ubiquitous and pretty challenging to diagnose, which is why today I want to:

a) tell a troubleshooting story that is burned in my memory,

b) share a small and easy tip that "fixed" the oscillating transmission syndrome many times in my practice, and

c) now that we talk about self-excited oscillations - share a small list of things that I ever had to do to fix a "shaky" hydraulic system.

And yes - since it is the 25th of December 2022, Merry Christmas to everyone!

Generally, any control system can become prone to instabilities and self-excited oscillations. Hydraulic motors with proportional and pressure controls are not an exception. With all the tests and optimizations that engineers run on the pump and motor controls at the respective production facilities - they can't predict all the variables of a target system, and sometimes closed-loop hydrostatic transmissions become "wobbly", and instead of driving the vehicles at nice and steady speeds or predictable accelerations and decelerations, they start doing the "whow - whow - whow" kind of driving.

I can't say that an oscillating closed loop is an everyday failure, but I've seen at least a dozen over the years. I confess that whenever I am going to troubleshoot something hydraulic that is self-oscillating - I know that I am in for a treat... not... because the diagnostics of self-excited oscillations is often a painful process of "fiddling" with everything without getting any results.

Let us go back to the oscillating closed loop now. I was asked to have a look at a forwarder that could not finish its transmission calibration routine because the motor displacement control was unstable, and the machine was periodically accelerating and decelerating on its own.

Checking the motor's servo-pressures confirmed that the "wobbliness" was being caused by the hydraulic motor displacement changes. The motor had the EP2D control (electric proportional displacement control with a direct high-pressure control), and it was modified for an external high-pressure supply from a 3/2 electric valve that was flanged on top of the motor work ports (electric travel direction valve). This is a pretty common arrangement, that for some reason is not mentioned in the standard catalog (most likely due to OEM reasons we all know and love... not!). Here's the control's cutaway:

Rexroth A6VM160 EP2D control cutaway view

So, I started with the "small and easy wobbly hydrostatic motor fixing tip" I said I'd be revealing here, and here it is: I can't say how many times, but definitely more than once, I managed to fix a self-oscillating closed-loop drive (especially when the motor has been recently overhauled) by simply changing the displacement control threshold setting. In this motor's case, this would be adjusting the set-screw number 19.

There can be a number of reasons for this to work. Sometimes the screw can come back in a completely different setting after an overhaul. Sometimes simply shifting the control into a different current region makes it more stable (once again - for many possible reasons). It's hard to say without a proper investigation, but I can tell you that I've seen this simple setting change solve the instability problem more than once. This is why, when I hear the words "unstable displacement" - seeing how the transmission responds to the displacement threshold setting change is one of the first things I do. Plus it is very easy to do as well, and a good troubleshooter always does easy things first!

So - I tried increasing and decreasing the setting to see how the transmission would behave - and saw myself in one of the scariest situations I've ever been in!

Let me set the scene now. When a forwarder does its transmission calibration routine - it has to drive back and forward for a certain distance, and it needs a preferably straight stretch of road for that. The shop was located in front of a quiet public street, that ran straight for several hundred meters and had two unoccupied lanes and very little traffic. This is why, very often, the transmissions were calibrated right there. And, under normal circumstances, it has always worked, because it doesn't take that long to calibrate a good transmission anyway.

So - that's what we did. Only that time I had to fiddle with the motor's threshold setting, so we had to make the pass several times. It was going more or less as planned, but then it started raining. Not raining - pouring! This didn't seem like a big problem, though - obviously, the machine could drive in the rain, and it could even go back in the shop so that we could work in a sheltered place to change the setting, but... as it is often the case - something "unexpected" happened.

Picture this: I am looking at the forwarder accelerate down the street (under the pouring rain) and then I see it stop abruptly, and I also see the driver wave to us (frantically). OK, then, I guess a little bit of rain can't hurt a seasoned tech, right? So I cover myself up and walk to the forwarder to see what happened, and as I come close to it, I realize, to my horror, that I can't walk or even keep myself up straight!

Yes - you heard me right. The street, so freshly and abundantly watered with rain, got also very abundantly covered with hydraulic oil so that it was virtually impossible to get any grip at all! I've seen ice rinks less slippery than that road, I kid you not! And then I realize - this is a public street! A freaking public street! This means that the next car to pass by will surely end up straight in the ditch (or worse), and my imagination immediately began drawing gruesome headlines involving the words "tragic traffic accident" and a picture of myself staring into the camera from behind the bars in a striped uniform.

We were really, and I mean really, lucky that no cars were passing by at the moment and we managed to close off the street and clean the spill. But I can still remember how scared I got!

So, where did the spill come from? Since I was experimenting with the setting, I thought it would be OK to change the setting screw's 19 position without securing it with the nut NÂș21. Seemed like a good idea at the time. But it was not. The screw fell out, then pin 20 fell out, and then a jet of hot oil from the slightly pressurized motor case came out as well! Damned vibrations!

I guess this is another piece of hydraulic advice for you right there - always lock your setting screws when running tests on actual machinery, even when "it is only for a little while"!

Changing the setting didn't work that time, by the way, and in the end, I had to replace the complete control assembly, which did solve the unstable displacement problem for good.

And, now, as promised, a list of things that helped me solve self-oscillating hydraulic stuff in the past (Note that this only includes purely "hardware-related" situations, self-excited oscillations can also be caused (and fixed) fixed by software or parameter changes in PLC-controlled systems, which belong to a class of their own):