Insane Hydraulics
Bold and audacious blog about fluid power
I deal with pumps much more often than I deal with the respective machines, and sometimes I encounter pump features that I find super-cool and super-smart, but if you were to ask me how big of an impact they have on the performance or behavior of that particular piece of machinery - my answer would be "an educated guess" at best, because I lack the specialized knowledge and experience. Nonetheless, when I come across something like that, it goes in the blog for my and everybody else's reference, and so today I want to show you a special "blog-worthy" control feature of this very little, but very neat tandem pump from the Komatsu PC30MR-2 excavator:
This aggregate is composed of only two pumps - a single open-loop variable-displacement axial-piston pump (LPV30) at the front, and a gear pump (SBR8.5) at the tail, which is a common pump arrangement for small diggers. The gear pump is for the independent swing, and the piston pump is for everything else.
This piston pump, like pretty much every other excavator-driving pump in the world, is equipped with a summation torque limiter control, and its displacement control is of the closed-center load-sensing type with a dedicated (remote) pressure port for the LS spool, which is very typical for Komatsu pumps (as opposed to the classic LS control, in which the upstream pressure side of the delta-P spool is internally connected to the pump outlet).
Nothing too special so far, but before I get to the promised "secret feature", I would like to document a couple of important practical points that may be interesting to the eventual "hands-on users" of this pump model (as well as my future self), namely - the losables and the cloggables.
Let us begin with the cloggables.
The two long fittings on the side of the displacement control look identical, don't they?:
But if you inspect them closer, you will find that only one of the fittings is "normal" - the other one is actually a tiny orifice:
To Komatsu's credit - there is a clear "0.5" marking on the side of the orifice fitting. These two ports are the LS and the remote pressure port, and the orifice fitting goes into the LS port.
Then there's only one more orifice to "worry about", and it is located in the hole that feeds the servo piston:
And now for the losables.
You can see in this picture that Komatsu engineers installed removable mesh screens in every single hole leading to the control module (how very preventive of them), and when you remove the screens, you are left with this:
If you look closely, you'll see your first losable - the spacer that goes under one of the screens. It looks like the body, but is actually a small part that sits loose in the hole, and it can easily fall out and get lost:
The next losable is not very obvious, because you need to disassemble the torque limiter part of the control module to get to it, but it is super sneaky. The torque limiter setting is adjusted by the eccentric shaft of the feedback lever (a very Komatsu design), and if you remove the shaft - there's a tiny spacer on the other side of the feedback lever, and if you don't know it's there - it can very easily "drop out of sight":
Now, with the practical bit out of the way, let us get back to our special secret control feature. You can see all of the internals of the control in this picture:
The torque limiter part is "relatively classic" - you have the double area piston on the left (for the summation of torques of the gear pump and the piston pump), then the oil directing spool, and then the variable-pitch spring riding the swashplate feedback lever:
But the most interesting part of this control is the LS spool. Quite an interesting configuration, don't you think?:
Apparently, it balances out not one, but two pairs of equal areas - one for the externally supplied delta-P, which would be the load-sensing part, and the other for the two channels that go somewhere into the body, and one of them - the one that's closer to the spring - also seems to be connected to the double piston of the torque limiter. Hmm... Let us see if we can figure out where the holes are leading.
It turns out that both of the holes lead to the outlet of the gear pump, and if you inspect further, you will see that there's an internal thread in the pressure port of the gear pump, and one of the holes channels upstream of the thread, while the other channels downstream!
Now, an internal thread inside an oil channel suggests that an orifice can be mounted there, and if you look at these connections considering the orifice possibility, and the fact that this line is an outlet of a fixed displacement pump, you will realize that this arrangement gives this control an rpm-dependent delta-P setting! In other words - the LS setting is dynamic - you get a lower delta-P setting, and therefore a better fine control, at lower rpm. Here's a drawing showing how the spool fits onto the control body and the complete pump diagram:
There was no orifice, though, and with the hole itself being some 10 mm wide, the delta-P override, probably, has no noticeable effect (I surely would love to find out if this is true or not) but boy do I find this solution super smart! My heart melts every time I see a hydraulic control that does not need electricity to function!