Last time I tried to peek through a crack of CAT's fence, I swear I
felt a bullet pass by my left ear! You must understand that after
putting this information here I will change my residence as well as my
appearance... Just kidding, CAT folks are all right, all the
information about their hydraulic pumps and motors is readily available
online, all technical catalogues are free and downloadable, and all
technicians are super-competent and eagerly land you any diagnostic
equipment upon your request, oh yeah, and I NEVER EVER lie!!!
The following information is ONLY what I
(MYSELF) think this control is, which may be not accurate. I have
nothing against Caterpillar, nor am disclosing any secret information.
I just happen to have a thing about dismantling stuff to see how it
works.
Anyhow, CAT engineers seem to have an itch to
over-engineer their hydraulics. They take a simple control from a well
known brand and then they start to "improve" it by adding (or asking a
brand engineers to add) a thousand and one unnecessary functions which
all, strangely, CAN be technically justified but, well, are just like a
spare wheel on a sports motrcycle... In the end they end up with an
exclusive control mounted on a seemingly standard pump/motor unit which
can intimidate many by its apparent complexity, protected looks and
original threads, BUT not us, back-engineering connoisseurs, who
consider complexity a challenge and laugh in the face of
"protected" OEM hydraulics.
This pump is a Caterpillar/Rexroth closed
circuit pump, used for boom turning on an excavator. A pretty standard
AA4VG pump, on which you will easily find tons of information online,
but not for this control. The pump reference is AA4VG56DWD1/32R, and
you will not find the description of the DWD1 control in the standard
model catalog. You know what it means, don't you? Time to back-engineer!
Pic1
shows you the normal A4VG body. Upon closer inspection we see that
there is no servo-pressure supply, so the hole which normally supplies
servo pressure for, for example, proportional electrical EP control, is
not connected to charge pressure, and, consequently, is used only for
pressure cut-off function (pressure limiter). Let us inspect further,
the control has three pilot connections, marked as Y1, Y2, and
(surprisingly) Y3. It has no feedback link, two servo-cylinder and one
tank connection. This pretty much tells us that this is hydraulic
no-feedback control of some type. Some more digging reveals pilot
orifices in Y1 and Y2 (pic5), which make sense as this control has pressure cut-off option; a shuttle valve (pic6), which connects servo pressures to the cut-off valve, as well as to the Y3; and two valves like this (pic7), which look strange and sophisticated at a first glance, but on a closer look (pic8)
are nothing but a piloted two position flow restrictors. Add up a pair
of anti-cavitation check valves connecting servo-cylinders to tank (pic9) and here you have it (pic10),
an improved DG (hydraulic direct operated) control with remote servo
pressure control option, which more then surely is used by the
excavator's power management system to control boom rotation power
consumption. Which may be done by some external electrical proportional
pressure limiter connected to the Y3 port, for example. Not so tuff and
sophisticated now, is it?
Pump controls, both standard and OEM, are nothing
but devices to control pump's displacement in accordance to some
external input signal and internal logics. That's it! Controls are easy
when you know what to expect and don't get intimidated by the absence
of technical information. Assumptions that make sense are correct most
of the times.
I like CAT's pumps because they are always a
protected puzzle. I hate CAT's pumps because they are always a
protected puzzle.