Insane Hydraulics
Bold and audacious blog about fluid power
This week, I had to super-urgently fix this nice little twin 700 bar (10000 psi) power pack from Enerpac (exact model unknown because, as is always the case, the documentation "could not be found"):
It's not every day that I get to work on high-pressure HPUs - so I decided to make a post about the directional part of the hydraulic circuit of this unit because I find it super elegant, and I also think that Enerpac engineers don't do it enough justice in their docs. If you are only used to "normal" industrial hydraulic systems - there's a good chance this circuit will be new to you. I apologize in advance for the quality (and scarcity) of pictures - it was one of those jobs that had to be finished fast, and I didn't have time to take detailed or thoughtfully planned shots.
So, this is the part that I am going to talk about - the 4/3 directional valve that looks like this and is often represented in diagrams by a simple directional symbol with a couple of check valves to underline its leak-free load-holding nature:
One might think that the directional valve interfaced on top of the manifold would be the one responsible for directly channeling the oil, but, of course, this is not the case because the CETOP 3 interface is not rated for 700 bar, so how does it work?
If you remove the side cover of the manifold - you will find 2+1 sealed pistons that "push around" some check valves (note how thick one of the springs is):
And while all this "abundance" of steel balls, seats, and pusher rods may seem convoluted at first, if you study the channels and give it a little thought - you will soon realize the genius behind this design. To Enerpac's credit - even though there isn't much information about this particular manifold, marked as "BD7C" - they do put lots of downloadable technical files up on their website, and if you dig into electrically operated control valves, you will sooner or later come about this repair parts sheet for VE33/43s, where they show three identical pistons in exploded views and even include a couple of cutaway views and hydraulic diagrams that kind of explain their function. Sigh... You came so-o-o close, Enerpac. You stopped "one cutaway view short" from perfect! Well - it's my turn now. So, this is how the three pistons "cooperate" with the directional valve on top of the manifold:
Here are the three things that I find elegant in this design.
First - a simple check valve is turned into a reducing valve by adding a large piston with an extremely thick spring behind it, that keeps it open until a certain pressure is reached.
Second - the very same reducing valve is bypassing the pump flow when the open-center NG06 is centered. This is very cool, even though this is, quite obviously, only good for relatively low flow rates. When the NG6 is in neutral - the reducing check is open and the oil flows freely through the open center to the tank, but as soon as the DCV is shifted - the pressure goes up and the piston moves out, closing the check valve and making the pressure gallery "available" for the required outlet.
The VE33/43 manual states the correct pilot pressure should be around 82-96 bar (1200-1400 psi). This particular system worked perfectly fine with 50 bar. In fact - raising the pressure caused some audible instabilities in one of the HPUs - I am not sure why, and I honestly hope that this isn't one of those things that's going to come back to "bite me in the .. you know what" - I had no time to figure this one out since I had to fix the other "twin" that refused to work completely, but the HPU should return for a proper overhaul after it is finished with its current job, and I'll have more time to play with it then. It is interesting to note that this is not an "ideal" reducing valve - in the sense that it has no relieving function and its setting is modulated to a degree by the high pressure pushing on the check valve - but for the piloting circuit, this simple solution is perfectly fine, even though it does require a super-thick spring.
And third - you have a single pusher piston working two check valves at the same time! Just look at the things one can get away with when the flow is tiny!
There's actually more to this manifold - because, like any respectable reducing-valve-based pilot circuit, it also has a safety relief valve - but I didn't take any pictures of it, unfortunately.
If I were to represent this circuit in a schematic, I would much rather prefer something like the drawing below to the simplified symbol. I think that such details make its function clear and the diagnostics a lot easier. In fact - even this diagram is a simplified version because it omits the pilot pressure relief valve (as well as the port relieves) - but the purpose of this drawing is to merely convey the principle I just discussed:
So, have you ever thought about using a reducing valve as your primary open-center by-pass flow path? I, for one, haven't, but I am glad I came across this solution because now I know it can be done!