Insane Hydraulics
Bold and audacious blog about fluid power
I stumbled into something kind of unexpected this week when I was helping a friend of mine set a Danfoss H1B160 motor with the DHMA control, and I thought I should share what I saw. The DHMA is the hydraulic proportional displacement control with pressure compensator override. This post will definitely be receiving an add-on as soon as I get my hands on an H1 motor with hydraulic proportional control for a more detailed inspection.
But first, I will digress for a minute, if you don't mind. When I was located in Lisbon, all I did was repair pumps and troubleshoot hydraulic systems 24/7, but when we moved down South, my range of duties exploded. So bad, in fact, that even this very blog was put on ice for a good five years! And since it is still just "me and the missus" and we still need to present numbers to stay alive, I pretty much pick the biggest and ripest fruit from the ever-overflowing platter and say no to the rest. This means that I refuse a lot of repairs and assistance calls because operating at 100% capacity forces us to prioritize stuff with the highest yield, and overhauls and assistance calls, sadly, aren't tasks that yield the most per hour in a single-tech operation.
I guess what I am trying to say is I haven't been doing as much of the field/shop work as I used to, and sometimes I miss the days when I had daily first-hand access to most strange hydraulic phenomena! I am not even sure that "strange" is the right word. Rare?.. Weird?... Odd?... Noteworthy?... I am all out of words now, but my point is - you have to be in the game to stay on top of the game, and since I can not do that anymore, it may very well be that the phenomenon I am about to describe is old news. Still - I haven't found the information on what I came across in any of the files available online (datasheet, serice manual, parts list), so even if it's old news - I'm documenting it, which should count for something, I reckon.
Anyhow - the hydraulic proportional displacement control of the H1B160 was piloted with the following control circuit (simplified):
The motor wasn't reaching minimum displacement (and thus not gaining max speed). So, what do you do when you see a situation like that? You measure the pilot pressure, of course! So that's exactly what I did - I teed into the pilot line right at the motor control and saw that the pilot pressure was not going past the 18-ish bar even with the relief valve controlling the pilot pressure completely closed.
It was a brand new motor, and the nameplate read DHMAIA, which meant the pilot pressure threshold was supposed to be set at 10 bar (145 psi), so it would need about 24 bar to get to the 20% displacement (according to the datasheet). Simple, right? It's a pilot pressure issue!
So - I began looking for problems in the pilot circuit. Take a look at the hydraulic diagram and tell me what can go wrong for it not to be able to supply sufficient pilot pressure to the motor displacement control? The orifice? We checked it and it was OK. The relief valve? - Also OK! Is the pilot pressure source too low? Nope - solid 40 bar! Hmm... Any more ideas?
Then I disconnected the pilot hose from the motor, plugged it, and measured the pilot pressure again - and lo and behold - we were getting the full 40 bar now, which could only mean one thing - there was a leak in the hydraulic control actuator assembly (part number 11146306 - an easily removable part common for all H1B sizes - which is, by the way, a great solution IMO).
The hydraulic diagram suggested the displacement control would be leakage-free (like most classic hydraulic proportional controls) but unfortunately, the good folks from Danfoss decided not to spoil their technical files with a cutaway view of the hydraulic actuator, and I didn't find any information about the "built-in" leakage, so the only thing we could do to confirm what was going on was disassemble it and have a look. And here's what I found:
Apparently, the control does come with a small built-in leakage in the form of a tiny, but very real orifice, connected to the hole that runs through the center of the pilot piston/spool:
When I saw this, I took a couple of quick shots, put it back together, and solved the problem by tweaking the control threshold. My apologies for the image quality - the lenses of my phones are permanently covered with oil for some reason, and when you are on a clock you can't stop for long to take detailed and nice-looking pictures anyway. I'll make sure I make good shots (and a detailed drawing) when I get such a motor in the shop, but for now, these are all I have got.
I think the fact that the control has a draining orifice should have been mentioned in the catalog. This is important to be aware of if you are planning to use this control in a system that has a limited pilot flow capacity, which was exactly the case with the orifice + relief valve pilot pressure arrangement. Had it been a pressure-reducing valve - this would never have been an issue, but with two (tiny) orifices in series - pressure issues tend to appear.
There's another thing that I believe should have been mentioned in the catalog - the fact that the threshold adjusting screw is different from the "classic" adjusting screws that go in and out when you turn them - the designers opted for the threaded sleeve design, which means the adjusting screw has always the same height when you turn it. Of course, this piece of information is not crucial (and once again - a cutaway view would have been totally self-explanatory, so shame on you, Danfoss, for not including it!), but it is always nice to know what to expect from a setting screw before you stick a tool in it - and the catalogs are supposed to help us with that, are they not?
So - now you know it too - the H1B hydraulic proportional controls, apparently, come with a built-in leakage. I wish the Danfoss "catalog gurus" did the following three things:
Danfoss, here's an example of how your "uncle" does it - this is a similar control (HD1D) from a Rexroth A6VM parts list. Doesn't it look nice?
P.S. The part Nº 18 in the view above is a plug, so no pilot pressure leakage for the HD1D.