Insane Hydraulics
Bold and audacious blog about fluid power
I should have called this article "Finding Zero - Part 4", because it is, in essence, an extension to my series on zeroing of closed loop pumps, but a title with the word "lasers" in it is way cooler!
I am about to tell you how one can set the null of a particular closed-loop pump - the Rexroth A4VSG500 with the EO2 control - with the help of a laser. I am sure that the "laser trick" can be used on other similarly built pumps, but I only tried it on A4VSG500 pumps (four pumps in total, and with great success, I'll have you know).
Safety first thought (very important!) - the information I am about to describe is for experienced techs and engineers only! If fooling with the null adjustment of a "normally sized" closed loop pump can be compared to playing with a handgun, then fiddling with the neutral of a 500cc A4VSG is like messing around with a cannon... loaded with another cannon! Read the disclaimer, please, it is there for a reason.
So, the A4VSG500EO2 - a 500cc closed-loop variable displacement pump with an electronic displacement control that uses an inductive swash-plate position transducer and a directly driven proportional directional valve in conjunction with a VT 5035-1X controller card:
How does one set the null of a "normal" closed-loop pump? First, you by-pass the hydraulic centering system, then you set the mechanical (spring-applied) zero, and then remove the by-pass and set the hydraulic zero (read Finding Zero series for more detail). So... Why wouldn't one apply the same procedure to the A4VSG500EO2? It appears to have a similar spring-based centering of the swashplate and symmetrical (equally-sized) servo-pistons:
Well, as it turns out, it's not that straightforward, because even these seemingly large coil springs "pale" in comparison with the forces required to move the massive swashplate, especially when the gigantic rotary group projects tons of force over it even at low system pressures. This is why you need an external control pressure supply of 100 bar for the EO2 control to work properly! And this means that when the rotary group is pressurized, you can't simply by-pass the servo-cylinders (or, in this case, disconnect the proportional DCV plugs, and rely on the built-in by-pass orifice), and then tweak the spring assembly back to zero - it won't work. Not reliably, anyway, because you will most likely be compressing the springs to some extent before the swashplate moves, which will lead to an incorrect setting.
If you look at the cutaway view of the servo system of the A4VSG500EO2, you will see that the adjusting screws "20" must be touching the rods 4 and 3, but they must not compress the springs inside the servo-cylinder, because if they do so - the servo-cylinder gains a play, which can lead to additional instability (just like the servo-systems I describe in Finding Zero- Part 2). Now you can see why you need to make sure that you are moving the servo piston into the correct neutral position without compressing the springs inside it, and this is only possible when the swashplate is not "stuck" under the pressure-induced load.
Ideally, you would be setting the mechanical neutral position of the swashplate with the pump stopped and all pressure vented from the loop - in this condition the force required to move the swashplate is minimal, and then you can indeed push it to the correct position and lock the screws so that there's is no play and no spring compression. This is easily done if you loosen both of the setting screws, insert an Allen key in each of them, place a hand on each key, and then turn one in, while rocking the other one back and forth - you will easily feel the exact moment the rod touches the "wiggling screw".
But this, naturally, begs the question of how the hell one would know if the swashplate is in the correct position when the pump is turned off?! I mean - this is easy to see on an active pump because you can read the uneven pressures in the loop legs, or maybe see unwanted movement of the actuator - but how on earth would you tell if the swashplate is exactly in the correct null position with a dead pump? You don't want us to disassemble the damned thing and take measurements now, do you?!
Of course, not. This is a field-applicable technique, I promise!
So, the riddle is - you can't set the mechanical zero when the pump is pumping, because there's a chance that it will end up off-center, and you can't set the mechanical zero when the pump is stopped because you can't tell exactly when the swashplate is at the correct zero. Or... can you? And here is the moment when I tell you that you... actually can! How? By cleverly amplifying the pump's built-in swash plate angle indicator. This thing:
That it! This guy's nuts! The stress and long working hours have finally gotten to him!
Yet I humbly ask you to hear me out, my appraising friend, for there is rationale in my madness.
The EO2 displacement controls are great, but they are also kind of flimsy - in the sense that they can easily get knocked off center with an "accidental mechanical intervention" to the swash-plate position transducer coil. Yes, it is protected with a "shield of sorts", but if you step on it "hard enough", or maybe pull on the wire "enthusiastically" (because you tripped over it) - the pump is off by 5% in a snap!
Now, obviously, one should never step on such a sensitive part (or design power packs in such a way that a simple maintenance operation, like a filter change, would require a technician to walk over the pump to get to the said filters) - but I am not discussing questionable HPU designs here - I am saying how sensitive this displacement control is to even the tiniest change of the position of the swash-plate positional transducer coils. But this also means that when such a pump is off-center - you can easily bring it back to its senses by shifting the position of the coil assembly (7) adjsuting the two nuts that secure it in place:
So, this part is easy. If your EO2 pump is acting out, tripping high-pressure alarms when it's supposed to be in neutral - you nudge it back to zero by repositioning the transducer coil with a couple of wrenches. But now you need to confirm if the mechanical zero is also in the same very spot - but the tiny swash-plate angle indicator can't really give you enough resolution, and this is exactly where lasers come into play because a laser beam can greatly extend the size of the swash-plate indicator arrow! Behold - the laser contraption:
This is a cheapo laser lever that projects a nice red line, bolted onto the swash plate angle indicator, and when you take the play of the indicator out by applying a small torque with an Allen key (like I do in the last picture) it becomes a very long arrow that you can use to read the position of the swash-plate with great precision!
And now all you have to do is turn the classic centering procedure (mechanical first -> hydraulic second) on its head and set the hydraulic (or better - electro-hydraulic) zero first, then mark the target zero position of the swash-plate using your enhanced laser arrow and a marking instrument of your choosing, and then power the pump down and check if the springs place the swash-plate in the right spot, and adjust if necessary. Very simple!
By the way, the long laser arrow is a great demonstrator of how the EO2 control "never sleeps", constantly introducing tiny corrections to the swash-plate position even when the control card "is told" to keep the pump in neutral:
Let us take a step back now. Why would you even want to mess with the mechanical setting of such a large pump, anyway? Shouldn't it, like, already be 100% set at the Rexroth factory? And the answer to this question is, actually, yes. In most cases. But sometimes even a factory setting can be off, the centering screws can gain play due to wear, and, of course, no system is ever protected from "intrepid interventions" of random maintenance crew members, which means that you must still have the knowledge and the instruments to verify and adjust this setting if necessary. I just gave you the knowledge, and you can build the "instruments" in half an hour from a 10-buck laser pointer (I do advise using one that projects a line rather than a spot) and bits of scrap.
The EO2 control does not need the spring centering, it is quite stable and it can definitely over-power it, but a well-adjusted pump is a well-adjusted pump, and the settings must be right, even if they may not make that much of a difference! I still believe that having even such a weak mechanical zero overlapping the electro-hydraulic adds to the stability of the neutral.