Insane Hydraulics
Bold and audacious blog about fluid power
If you passed by Castro Verde fairground this Monday and overheard loud evil laughter coming from inside our shop - you weren't imagining it. It was me appreciating this "present" I'd been waiting for for a long time:
Ever since I had my first encounter with the "mysterious" flow consumption of the hydraulic proportional displacement control of a Danfoss H1B motor I wanted to lay my hands on such a unit with enough time to back-engineer and document its control actuator properly, because our friends from Danfoss, apparently, don't have enough time for that, for some reason. Finally, fate smiled at me!
I wasn't even supposed to be in the shop that day - I was supposed to be in the mine making dirty HPUs a little less dirty during a scheduled maintenance stop (on a side note - in case you are aspiring to a career in hydraulics, know that hydraulic technicians spend 90% of their time cleaning parts/installations and replacing hoses - have no illusions about that) but I ended up going to the shop anyway, because I had a couple closed loops that needed complete hose replacement - and I prefer making sensitive hoses in our shop because I can clean them with projectiles and cap the ends properly - something that is next to impossible to do in a real-life underground mine. But that's not important. The important part is - I enter the shop, and there it is - a Danfoss H1B160 bent-axis motor with a hydraulic proportional displacement control in all its glory!
I just had to verify right there and then if I could see the built-in leakage with my own eyes, so I immediately removed the actuator assembly (part number 11146306):
...and hooked it up to out test bench to see if any oil would come out from the orifice located at the end of the actuator piston (as I was expecting):
Just a quick reminder about why I am looking into all of this hydraulic displacement control actuator leakage thing. Some weeks ago, I ran into a hydraulic system that used a relief valve and a 0.6mm orifice as a pilot pressure source for controlling the displacement of a hydraulic motor - this arrangement worked flawlessly with Rexroth A6VM motors with HD1D displacement controls, but it presented issues with the Danfoss H1B motors with DHMA controls. I attributed the issues to the apparent flow demand of the Danfoss displacement control actuator piston that could not be met due to the limitations imposed by the 0.6 mm orifice of the piloting circuit - but I didn't have time to study the actuator in detail then - and I promised myself that it would be the first thing I would do the next time I'd get to work on such control (check out this post for more details).
I pressurized the pilot line and... saw but a couple of drips of oil, even with the pilot pressure at 40 bar! Where's the so-expected leakage? Is the orifice obstructed, or am I wrong in my "built-in leakage assumptions?" So, I disassembled the actuator:
The tiny hole at the end of the spool was obstructed - so I cleaned it with a small wire and blew through it with compressed air, reassembled the actuator, and...
Now we are cooking! It freaking leaks, I tell you! Here's how much oil I got at 20 bar after one minute, and then at 40 bar after a second minute:
So, yes - it is official now - the hydraulic proportional displacement control actuator of Danfross H1B bent-axis motor has built-in leakage in the form of a 0.6 mm orifice located in the nose of the "pusher spool." I am honestly surprised that Danfoss neither mentions this fact in the catalog nor represents it in the hydraulic diagram of the motor:
Now, the last thing to do is, of course, make a proper model of the actuator assembly, so that there are no doubts about its function:
The cutaway view also clearly shows why, when you adjust the threshold pressure of this control, the adjusting screw stays always the same height - something that may feel "strange" if you are not expecting it.
P.S.
I feel I need to mention something else. A small tip, if you will. Here's a cheap drill bit crafting kit I got from Lidl for 2 euros and 99 cents:
If you think 2.99 is cheap - AliExpress retails similar kits at less than 2 euros - but the price is not the point. Obviously, these bits suck as drilling bits - but they make perfect tools for measuring orifices, because the kit carries bits from 0.6 mm to 2.5 mm in 0.1 mm increments!
P.P.S.
Take that, Danfoss! My cutaway view beats yours! Oh, wait... My bad. You don't even have a cutaway view!