Insane Hydraulics
Bold and audacious blog about fluid power
Today I will be looking into the construction of the Danfoss series 4 PVEH electrohydraulic actuator, part number 157B4033. This is the "high proportionality" version that has reduced hysteresis (4-9%). I know well that it's "old news" and that it's all about series 7 now, but we've sold quite a few of these bad boys back in the day, and now, as they are being gradually replaced, I can finally cut one open and see what our friends from Danfoss engineering department decided to put inside. It's not every day that you get a chance to wreck a 600 euro part "for free"!
A few words about the theory of operation. This very nice diagram (from the technical manual) pretty much explains the function - in essence, it is an electro-hydraulic closed-loop positioning system in which a hydraulic bridge, consisting of two normally open and two normally closed 2/2 valves moves the spool, an LVDT-based linear displacement sensor (LVDT standing for "linear variable differential transformer") reads its position, and an electronic module does its best to match the spool's position to whatever is "requested" by the ratiometric input.
Before cutting in, let's explore the "externals" - i.e. parts you can remove without cutting into the plastic enclosure.
Important things to note:
First - there's a tiny oil filter in the pilot P line, and you should check it every time you remove one of these from a valve bank. It's a good indicator of how clean a system is. When it clogs - nothing works. If you find "stuff" in it - don't clean it. Show it to the owner of the machine - it's a good way to convince someone to install a pressure filter.
Then you have two anti-cavitation check valves. It is important to know that they are there, but I doubt you will ever have to take one out and do something with it. And finally - the tank line orifice, which is removable as well.
These parts don't require much attention, but if you ever scrap such an actuator, I'd encourage pulling them out and storing them. You never know when you'll need a tiny check valve or an orifice...
But let's continue taking things apart.
If you remove the single screw - the aluminum base comes off, and you can see the four solenoid valves that from the bridge. If you ever have to troubleshoot a leaking PVE actuator, that keeps dripping oil even after a seal kit has been applied, it is possible that these o'rings need to be replaced as well. And finally - if you remove the two screws that secure the spool position feedback pin - you will find another small o'ring underneath it.
The feedback pin is interesting too. If you look at it closely - you will see that it's made of two pieces, and only the core that goes inside the LVDT is ferromagnetic.
Then you can also unscrew the 2/2 solenoid cartridges and pull them out. The rest of the contraption is potted, but as you can see - the potting does not involve the valves, and moisture can get inside and corrode them pretty badly.
The last thing that you can "reach" without cutting stuff is the small plastic plug in the back of the coil. There's an adjusting set screw underneath it, and at this point, I believe that it most likely adjusts the neutral point of the LVDT. When I get another one of these and I manage to "revive it" (more on reviving at the end of this post) - I will test it "live" and see how exactly it affects the working actuator.
It's cutting time!
As you can see, further "disassembly" involved ripping out connectors and removing tons of potting compound. The controller PCB is placed directly underneath the DIN connector. I saw zero signs of corrosion, which means the potting stuff did its job well. I still hate it though!
I managed to remove most of it, and here's what lies underneath the yellowish goo:
Some of the components are marked with part numbers, but they are definitely OEM. I was hoping to find more "reusables". The main controller has what looks like a Danfoss marking (155U5002 - underneath the QR code sticker). No matches for this reference in any of the IC databases. All I can say is that it uses QFN52 housing. I guess when you manufacture stuff in these quantities you can afford custom-made chips.
The LVDT coils are arranged in a three-wire configuration. Maybe I'll have use for them someday. The same goes for the four coil assembly.
So, now you know what's inside a series 4 PVEH actuator. Not much stuff one could re-use, but at least now you know where all the seals are placed, even the hidden ones.
Now - about "reviving" these actuators. Their construction is not that complicated, and it allows you to get to the sensitive valves and orifices. From my experience - if you take a certain amount of "broken" PVE*s, disassemble them, clean, wash and blow through all the valves and orifices with compressed air, and then put them back together - some of them will be perfectly functional again.
Use this knowledge wisely (wink) - i.e. on the "friends only" basis. Don't forget that this business is about sales as much as about repairs.