Insane Hydraulics
Bold and audacious blog about fluid power
People tell me that I "disclose" too much of a "valuable secret information" in my blog. I disagree. Do you know who is the main reader of my blog? I am. I write these "secret information" articles for myself, because when I forget about things (and I forget about things all the time) - I have a convenient place online that I can visit to refresh my memory.
I'll be definitely coming back to this one! Several of these manifolds (marked with Rexroth part number R978035707) have already passed through our shop, always with the same "malfunction" (more on that later), and none of the clients had anything even remotely resembling technical documentation (and, strangely, there seems to be nothing on the web on these manifolds either). So, when I received another one this week, I decided to document its schematic (and the peculiar malfunction, as well, which is a very good example of a very bad example - something I love posting about):
This manifold is direct-mounted on Helac L20 rotary actuators for motion control and anti-spike protection, and the actuators, in their turn, are used to rotate buckets (buckets as aerial platforms where people work) on tele-handlers and boom lifts.
It appears that a lot of mechanics find these manifolds "intimidating" as soon as they see that they carry eight (!) cartridge valves, and opt for sending them out to shops like ours instead of attempting any repairs on the spot. I'll show you that the manifold is very simple, and its operation is quite interesting.
Now - about the recurring malfunction. It's always the same - the bucket stops (or almost stops) rotating in one direction. Let me show you why it happens in 99 cases out of 100. It happens because there are two 0.5mm orifices in the work ports, and they get clogged very easily:
Maybe the OEMs should consider using fittings with mesh filters for these? The tiny gigglers are extremely "particle-sensitive".
There's another 0.5mm orifice in the manifold:
This one is not that "dangerous" though (meaning that nothing will stop rotating if it gets clogged). The orifice acts as a "softening" bypass between the V1 and V2, and its purpose is to remove the "stiffness" of the actuator control (a solid way to fight self-oscillations).
And now, before diving into the operation and the schematic of the manifold, I want to tell you the very educational story behind this particular specimen.
The techs brought it over with the usual complaint that the bucket wouldn't turn in one direction, and what made things even worse was the fact that it was a brand-new manifold! Furthermore - they had already gone through the trouble of removing all the valves from it, and even checked the orifices - and guaranteed that they were unobstructed! They tried swapping the hoses and turning the manifold around - and pinpointed the failure to the manifold, because swapping the hoses didn't do anything, but turning the manifold inverted the direction of the malfunction. Very intriguing, right?
Let me say something that may sound harsh - when a tech that I don't know tells me that he checked an orifice and is 100% sure that it's unobstructed - I will only believe it after I've personally seen and verified the orifice! And here's why.
Quite often people verify orifices "with their eyes", i.e. - they put a light source behind it and check if they can see through, like so:
Looks OK, doesn't it? But it's completely blocked! When I checked the orifice with a piece of wire - it couldn't get through!
And, sure enough, with quite some effort, I managed to remove what looks like a grain of silica sand, which is translucent (look at the tip of the tool):
Pretty cool, right? This tiny thing stopped a big machine and its crew for a couple of days! So here's a tip for you - checking orifices for obstruction must be done "mechanically", and not "visually".
Now - for the function of the manifold, in other words - let's answer the question "Why so many valves?". After a short "back-engineering session" I came up with the following diagram (here's a PDF file in case you need one):
A very clever arrangement, that allows for the relief function of the over-center valves to function even with a closed center spool, which is very important when you want to make sure that your (equal area!) actuator stays 100% filled with oil. Here's another (simplified) diagram that I made, with the "classic" over-center arrangement side-by-side with this one. Hopefully, it explains the principle:
And there you have it - another successful back-engineering/learning session (and a reference for my future self!)